Effects of nanosilica application in Portland cement concrete

Abstract

For development of the potential of nanosilica application in Portland cement concrete the effect in compressive strength R_com increase was studied in the age from 1 to 360 d. Sol of hydrothermal nanosilica (HNS) solution produced by technology developed was used with SiO₂ particles Brunauer-Emmett-Teller surface area of 500 m²/g. Concretes compositions were tested with liquid/binder ratio 0.5 and SiO₂ doses of cement replacement 0.5, 1, 2, and 3 wt.%. Modification of concrete with HNS sol provided a significant increase in compressive strength R_com up to 86.4% (14.8%–43.9% at the age of 360 d) and high values of efficiency coefficient k_ес 19.5–60.9 at the age of 28 d calculated using the relative increase of R_com. Increase in compressive strength is characterized by an early gain in R_com at the age of 1–7 d: the ratios R_{\mathrm{c}\mathrm{o}\mathrm{m}}^{1,7}/R_{\mathrm{c}\mathrm{o}\mathrm{m}}^{28} and R_{\mathrm{c}\mathrm{o}\mathrm{m}}^{28}/R_{\mathrm{c}\mathrm{o}\mathrm{m}}^{360} are higher in modified concrete. The obtained results can be used to model the hardening and predict the strength of modified concrete at the age of 2, 4, and 10 y. Results of the thermogravimetry, X-ray diffraction, scanning electron microscopy and water absorption methods provided explanations of pozzolanic reaction effects on compressive strength, water impermeability, abrasion and frost resistance, impact viscosity and durability were discussed. The dependence of k_ec coefficient on SiO₂ dose and concrete’s age was analyzed. Using the coefficient k_ec for different doses of SiO₂ at the age of 28 d, the corresponding clinker’s savings coefficient (23.4%–37.6%) and reduction of CO₂ emissions to the atmosphere during the production of Portland cement were calculated. Economic evaluations of the efficiency of using HNS in Portland cement concrete were made in comparison with different materials: commercial nanosilicas and pozzolans such as condensed silica fume, metakaolin, fly ash; metal oxides TiO₂, Al₂O₃, Fe₂O₃, ZrO₂ and multi-walled carbon nanotubes nanopowders.

Graphical abstract

Keywords

HNS / concrete / compressive strength increase / pozzolanic reaction / durability / economic efficiency

1 Introduction

Nanoparticles of SiO₂ have been used in studies of modified concrete in various forms produced by different technological methods: pyrogenic nanopowders (flame silica), nanopowders precipitated from the Na₂SiO₃ solution (precipitated silica), colloid SiO₂ sol based on the precursor of Na₂SiO₃, nanosilica with pozzolanic materials, such as condensed silica fume or metakaolin [1–3]. Nanosilica produced from a hydrothermal nanosilica (HNS) solution has a number of significant effects in the modification of concrete [4–6] due to the high specific surface and surface density of silanol groups Si-OH and low production cost, which are provided by the technological process.

Nanosilica was applied in different types of concrete’s compositions: heavyweight and lightweight concretes [7–9], self-compacting [10] and high-performance concretes [11], Portland cement concretes with pozzolanic additives [12–14]. Effects on improving concrete’s performance were obtained: the fresh rheology, mechanical strengths and elastic modulus, pore structure of the concrete [15–18], increasing fracture toughness and impact viscosity [7,19–21], water impenetrability, abrasive, frost, chemical, thermal resistances and durability and other effects [4,5,10]. Due to the use of nanosilica, savings in the clinker component of Portland cement can be achieved and a corresponding reduction the amount of carbon dioxide emissions to the atmosphere in cement production [7,22].

Most of the works in the field of application of nanoparticles for improvement of concrete properties refer to nanoparticles of SiO₂ and TiO₂ [23,24]. There are studies of Al₂O₃ [25], Fe₂O₃ [26,27], and ZrO₂ [28] nanoparticles and multi-walled carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs) [29–31]. The fundamentals of nanoengineering of Portland cement materials were developed [32,33] based on nano-core effect of nanoparticles combined the effects of great specific surface area, nucleation and filling effects, core-shell proportions for different chemical compositions and geometric shapes: 0-Dimensions (SiO₂, TiO₂, Al₂O₃, ZrO₂, CaCO₃), 1-Dimensions (SWCNT, MWCNT, carbon nanofibers (CNFs)) and 2- Dimensions (graphen plates).

Nanoparticles affect the structure of calcium silicate hydrates gel (CSH). The mechanisms of these effects were studied by using a number of methods [34–38].

SiO₂ nanoparticles accelerate the hydration of alite C₃S and other clinker minerals [39–43]. The acceleration of kinetics can be explained due to the high specific surface area and energy, which allows the surface of nanoparticles to serve as additional crystallization centers for the formation of CSH gel, portlandite and other hydrates. The data also showed the formation of a more ordered CSH gel structure [44], which was characterized by a higher volume fraction of gel phases with a denser packing of gel granules and improved mechanical properties [41–43]. The pozzolanic reaction reduces the content of portlandite in the cement-water matrix and increases content of CSH gel, resulting in a decrease in the total pore volume [45,46]. The change in pore structure leads to the reduction in the volume fraction of macropores and mesopores, which significantly reduces the permeability of concrete [45].

Modification by SiO₂ nanoparticles can provide the improvement of pore structure of concrete, increase of compressive strength, water impermeability, abrasion, frost and corrosion resistance [47]. The effect of nanoparticles influences on the structure of CSH-gel, its modulus of elasticity and hardness was investigated by the method of nanoindentation [39,41,42].

The effects of nanosilica in different forms of sol and pyrogenic (flame) nanopowder and condensed silica fume(CSF) were compared in the works [48–51].

The effects of SiO₂ nanoparticles on the characteristics of cement-based materials are explained primarily by the nucleation and pozzolanic effects. A certain role is played by the compaction of the water-cement matrix around the surface of the nanoparticles due to the nano-core effect and the reinforcement due to the higher mechanical characteristics of nanoparticles in comparison to the water-cement matrix. Nanoparticles of metal oxides and MWCNTs do not have pozzolanic activity but they also accelerate the kinetics of cement hydration. The effects of their action in concrete are explained by nucleation, compaction, reinforcement and pores filling mechanisms.

There is a need for large-scale use of nanosilica with a high specific surface area of 400–600 m²/g and high pozzolanic activity for Portland cement concretes. To expand the potential for the use of nanosilica in Portland cement concretes, one of the possible approaches is to apply to the nanosilica the concept of the efficiency coefficient of the pozzolanic additive. Such coefficient was widely used to analyze the efficiency of various pozzolanic materials: CSF, metakaolin (MK), fly ash (FA), slag, volcanic glass and tuff and others [22,52–55]. A similar approach can be developed in relation to HNS. There is a lack of publications on the application of the efficiency coefficient for nanosilica action in concrete. In the work [22], the efficiency coefficient was used to analyze the results of increasing the compressive strength of cement-sand mortars modified with colloid nanosilica (S_BET = 150 m²/g), metakaolin (S_BET = 28.6 m²/g), and combination of nanosilica and metakaolin at the SiO₂ doses of 1.5%, 3%, 4.5%, and MK doses 10, 15, 20 wt.%, liquid-to-binder ratio (L/B) = 0.46, at the age of 3–56 d. The Bolomey’s model was used for the efficiency coefficient (k_e). With the help of k_e, clinker savings and reduction of CO₂ emissions into the atmosphere during clinker production were calculated.

The objective of this research was to evaluate the economic efficiency of HNS in Portland cement concrete. For this purpose, the actions of HNS on the concrete’s compressive strength, water impermeability, abrasion and frost resistance, impact viscosity and durability were discussed. Results of the thermogravimetry (TG), X-ray diffraction (XRD), scanning electron microscopy (SEM), and water absorption methods provided explanations of pozzolanic reaction action on these performance concrete’s characteristics. The efficiency coefficient of the pozzolan additive k_ec is proposed, calculated by the relative increase in compressive strength according to the author's model. The values of k_ec and clinker’s savings coefficient (CLS) for HNS, commercial nanosilicas, pozzolans, metal oxides TiO₂, Al₂O₃, Fe₂O₃, ZrO₂, and MWCNTs were compared. Economic evaluations for the additives were made for several possible ways of using in Portland cement concrete to obtain benefits.

2 Materials and methods

2.1 Materials

2.1.1 Ordinary Portland Cement (OPC)

The ordinary Portland cement CEM I 42.5N was manufactured by Spassky Cement Plant Ltd. (Spassk-Dalny, Primorsky state, Russia). The content of basic chemical compounds and minerals is presented in Table 1, the density of cement grains was 3.1 g/cm³, the fineness of cement grinding −0.32 m²/g.

2.1.2 Coarse and fine aggregate

The coarse aggregate was crushed stone from Pervorechensky deposite (Primorsky region, Russia) of two fractions from 5 to 10 mm and from 10 to 20 mm with the density of 2.81 g/cm³, bulk density 1340 kg/m³. The fine aggregate was sand from Novoshahtinsky deposite (Primorsky region, Russia) with the density of 2.66 g/cm³, bulk density 1430 kg/m³, the fineness modulus of 2.86.

2.1.3 Superplasticizer (SP)

Tap water was used with pH = 7.5 and total hardness 0.41–0.60 mg-eq/l. To maintain the workability of raw concrete’s mix at the uniform level the polyanionic polycarboxylate SP Sika Visco Crete 5-Neu (Sika AG, Switzerland) with high molecular weight and chain’s length of 10 nm was used, water reduction ability of 29%, dry polymer’s content 40 wt.%, density of 1.072 g/cm³. The SP consumption at liquid/binder ratio L/B = 0.5 was near 0.2 wt.% for the reference composition, 0.75, 1.3, 2.6, and 4.0 wt.% for the doses of nanosilica relatively to binder [SiO₂] = 0.5, 1.0, 2.0, and 3.0 wt.%.

2.1.4 Hydrothermal nanosilica sol

HNS sol was characterized by low production cost and high values of specific surface area S_BET up to 500 m²/g to obtain significant increments of the mechanical strength and efficiency factor.

The sample of HNS sol was prepared from the hydrothermal solution from produced by the wells of geothermal power plants (GeoPP) used technology developed in our previous works [48–51]. The hydrothermal solution as the liquid phase of steam-water heat carrier was obtained in separators of GeoPP at the pressure of 0.6–0.8 MPa and the temperature of 150–170 °С. The vapor was fed to the turbines to generate cheap electricity need to drive the ultrafiltration capillary membrane unit pumps. Hydrothermal solution with the total content of SiO₂ of 600–700 mg/kg was cooled in heat exchanger to 20 °C for rapid orthosilicic acid (OSA, H₄SiO₄) polymerization and SiO₂ nanoparticles formation during 120 min.

The temperature of the hydrothermal solution at the stage of polymerization of OSA is constant regulated within 20–80 °C. With a decrease of this polymerization temperature, the degree of solution supersaturation relatively to amorphous silica solubility increases and the rate of nucleation and polymerization of OSA molecules increases and, accordingly, the final size of SiO₂ nanoparticles decreases with increase the specific surface area.

OSA polymerization occurs in the aqueous medium at pH = 8–9.4 and its mechanism differs from those in the technology of obtaining pyrogenic (flame) nanosilica and the technology of precipitated silica based on the precursor Na₂SiO₃. This provides high values of density of surface silanol groups Si-OH for HNS. The kinetics of the nucleation of the CSH gel increases with an increase in the specific surface area of SiO₂ nanoparticles and the density of surface SiOH groups. The kinetics of the pozzolanic reaction is proportional to S_BET and the density of surface silanols. This ensured high values of k_ec when using HNS.

The density of Si-OH groups on the hydrophilic surface of HNS reached the high limiting value 4.9 nm⁻² [56,57] that is important for pozzolanic activity and nucleation effect. Hydrothermal sols and nanopowders demonstrated low level of toxicity [57]. The high values of specific surface area, density of surface silanols and its high chemical activity, high share of amorphous phase provide the advantage of HNS over commercially available nanosilicas.

The sol sample was used with SiO₂ content 125 g/dm³, density 1075 g/dm³, pH = 9.0, electrolytes content −650 mg/kg, total amount of impurities about 1.6 wt.%, high share of amorphous SiO₂ in HNS, more than 98.3 wt.% as compare to CSF with higher content of crystalline phase > 3.2 wt.% (Figs. 1(a) and 1(b), diffractometer ARL X’tra, Switzerland). Average by volume diameter of SiO₂ nanoparticles in sol determined by the method of dynamic light scattering was 5.3 nm (Zetasizer, Malvern, UK). Stability of sol was provided by average zeta potential of the particle surface determined by the method of electrophoretic light scattering, ξ_m = −32.3 mV, specific surface area of powder extracted from the sol’s sample determined by low temperature nitrogen adsorption (Brunauer−Emmet−Teller method)––S_BET = 500 m²/g.

Spherical shape of SiO₂ nanoparticles after sol sublimation was identified by the method tunneling electron microscopy (transmission electron microscope JEM-100CX (JEOL, Japan, Fig. 2).

2.2 Composition of concrete mixes and its fresh characteristics

The amounts of added nanosilica and SP satisfied the conditions of 1) uniform workability of the fresh concrete mix and 2) equal L/B ratios. To meet the first condition, the SP dose was adjusted so that the slump and slump flow were approximately 20 and 50 cm, respectively. To meet the second condition, the consumption of solid nanosilica SiO₂ was subtracted from the Portland cement consumption, and the amount of water added by the HNS sol and SP solution was subtracted from the water consumption, so that the L/B ratios remained constant for all concrete mixes (Tables 2 and 3).

Concrete curing time was increased up to 360 d, enabling analysis of the time dependence of the efficiency coefficient over a wide range.

Concrete samples of the control and modified compositions were prepared and tested for compressive strength in accordance with Russian state standard GOST 10180-2012. The dry mix of Portland cement, fine and coarse aggregate was premixed, the liquid containing HNS sol and a SP was added, and the mixing was repeated. The concrete mixes were placed in metal forms and vibrated, demolded, and placed in the normal curing chamber at the temperature of 20 °C and the relative humidity of 99%. Compressive strength tests were conducted on cubic concrete samples with the sizes of 100 mm × 100 mm × 100 mm at the ages of 1, 2, 7, 28, and 360 d.

The density values of fresh mix for different doses of SiO₂ and hardening concrete depending on the age are presented in Table 3. The density of amorphous silica nanoparticles (2.2 g/cm³) is lower than the density of cement particles (3.1 g/cm³). Therefore, the density of the fresh mix should decrease with increasing dose of Portland cement substitution with SiO₂. However, at relatively high doses of SiO₂ (2 and 3 wt.%), the density of the fresh mix increases compared to the reference composition HNS0. This indicates the influence of SiO₂ nanoparticles on the rheology of the mix after vibration, reducing the adhesion of cement particles, the formation of voids between sand and crushed stone particles, and reducing air entrainment. The density of the solid concrete was reduced at the age 1 d by 1%–3% relatively to the raw mix due to the evaporation of a part of chemically unbound water in conditions of water-air hardening. However, at the age 2–28 d the decrease in density stopped and slightly increase was observed due to the growth of volume of cement hydration products and porosity decrease. But the density of the reference composition decreased due to further evaporation of unbound water to the age of 28 d. In compositions modified with nanosilica, more complete hydration was noted, which is confirmed by an increase in density between 1 and 28 d to 1.0% for HNS3 composition. At the age of 28 d the concrete’s density of the compositions HNS2, HNS3 modified with high doses of SiO₂ 2 and 3 wt.% (0.33%–0.5% relatively to the concrete’s volume) increased by 1%–1.5% as compare with HNS0 due to the pozzolanic reaction, more rapid hydration, production of addition quantity of CSH gel with chemically bound water and decrease in the total pore volume.

2.3 Calculation of efficiency coefficients and clinker savings coefficient by mathematical models

If the reference mix was prepared with the amount of cement C₀, then another mix is with a pozzolan using replacement amount of p × C₀ and (1 – p) × C₀ of the remaining cement. This pozzolan additive quantity is equivalent to the quantity k_e × p × C₀ of cement. The efficiency coefficient k_e reflects the influence of the additive on compressive strength, depending on the dose of the pozzolanic additive and the age of concrete (curing time). The quantity of the equivalent binder is (1 – p) × C₀ + k_e × p × C₀, and the same result can be obtained with p × C₀ quantity of the pozzolanic additive and (1 – p) × C₀ of cement [52–55].

According to the Feret model, the compressive strength S₀ can be expressed through the volumes of cement c, water w, and entrained air v in the fresh mix using the following Eq. (1), which contains the coefficient K_F depends on the concrete’s age and the grade of cement [52–55]:

(1)S_0=K_{\mathrm{F}}{\left(\frac{c}{c+w+v}\right)}^2.

The volumes of cement, water, and air can be expressed through their mass rates of cement and water C₀ and W, densities ρ_c and ρ_w, and the mass fraction of entrained air y, and proceed to Eq. (2):

(2)\begin{array}{rl}{S}_0=& K_{\mathrm{F}}{\left(\frac{C_0/{\rho }_{\mathrm{c}}}{C_0/{\rho }_{\mathrm{c}}+W/{\rho }_{\mathrm{w}}+yW/{\rho }_{\mathrm{w}}}\right)}^2\\ =& K_{\mathrm{F}}{\left(\frac{1}{1+\left(W/C_0\right)d_{\mathrm{c}}\times \left(1+y\right)}\right)}^2,\end{array}

where d_с = ρ_c/ρ_w, v = yW/ρ_w, y = 0.015–0.02 for standard concrete consistency [55]. The coefficient K_F can be expressed by Eq. (3):

(3)K_{\mathrm{F}}{=S_0/\left(1+\left(W/C_0\right)d_{\mathrm{c}}\times \left(1+y\right)\right)}^2.

Taking into account the equivalent quantity of cement (1 – p) × C₀ + k_e × p × C₀, the dependence of concrete compressive strength on the dose p of replacement of Portland cement with the pozzolanic additive can be expressed in the form of Eq. (4) [55]:

(4)S(p)=K_{\mathrm{F}}{\left(\frac{\left(1-p\right)+k_{\mathrm{e}}p}{\left(1-p\right)+k_{\mathrm{e}}p+\left(W/C_0\right)d_{\mathrm{c}}\times \left(1+y\right)}\right)}^2.

The efficiency coefficient based on Feret model can be calculated using Eq. (5), which takes into account the strength of concrete S(p) at a certain dose p of replacement of the pozzolanic, the age of the concrete with K_F value and W/C ratio [55]:

(5)k_{\mathrm{e}}^{\mathrm{F}}=1-1/p\left(1+\frac{{\left(S\left(p\right)/K_{\mathrm{F}}\right)}^{0.5}\times \left(W/C_0\right)d_{\mathrm{c}}\times \left(1+y\right)}{{\left(S\left(p\right)/K_{\mathrm{F}}\right)}^{0,5}-1}\right).

In the Bolomey model, the efficiency coefficient k_e can be expressed using Eq. (6) [52–55]:

(6)S=K_{\mathrm{b}}\left(\frac{C_0}{W+V}-a\right),

where C₀, W, and V are the masses of cement, water, and the mass of water in the volume occupied by the air, parameters K_b and a depend on the cement type and on the concrete age, respectively; in the studies [52,53] the calculated values were K_b = 38.8 MPa, a = 1.06, 0.72, 0.5, 0.23 for 2, 7, 28, and 90 d. Eq. (7) for calculating the efficiency coefficient using the Bolomey model is as follows [52,53]:

(7)k_{\mathrm{e}}=\frac{1}{p}\left(\left(\frac{S\left(p\right)}{K_{\mathrm{b}}}+a\right)\times \left(W/C_0\right)\times \left(1+y\right)-\left(1-p\right)\right).

Coefficients calculated using Feret and Bolomey models usually gave values close to each other [52–55].

Another way to introduce the efficiency coefficient k_ec proposed by us [58] is to use the relative increase in compressive strength and the additive dose p in Eq. (8a), which allows the coefficient to be calculated by a quick and convenient method:

(8a)\begin{array}{c}{k}_{\text{ec}}=\frac{(S\left(p\right)-S_0\left(p=0\right))\times 100\mathrm{\%}}{{pS}_0(p=0)\times 100\mathrm{\%}}.\end{array}

The connection between k_ec and k_e can be easily deduced in Eq. (8a):

(8b)\begin{array}{c}{k}_{\mathrm{e}}=k_{\text{ec}}\left(1-aW/C_0\right)+1.\end{array}

Calculations of k_ec according to Eq. (8a) provide a simpler and faster way to evaluate the efficiency of the pozzolanic additive compared to the Bolomey and Feret models.

For tensile bending strength of cement composites, the linear equations similar to those used for compressive strength can be applied to derive expressions for the efficiency coefficients for the Bolomey models and the relative strength increment.

The increase in the compressive strength of concrete and the increase in the efficiency coefficient k_ec reflects the possibility of saving clinker, which can be evaluated using the CLS under condition of equal R_com values of control and modified compositions according to Eq. (9):

(9)CLS=k_{\text{ec}}\times p/1-p+k_{\text{ec}}\times p.

By increasing the compressive strength of concrete, clinker savings are achieved and carbon dioxide CO₂ emissions into the atmosphere can be reduced during the production of Portland cement according to Eq. (10):

(10)CDR=CLS\times C_{\mathrm{P}\mathrm{C}}\times CDE,

where CDR is the reduction in the amount of CO₂ per unit volume of concrete produced, kg/m³, which is proportional to the part of saved clinker CLS, the mass consumption of Portland cement per unit volume of concrete C_PC, kg/m³, the specific amount of CO₂ released into the atmosphere during the production of Portland cement, which depends on the type of cement, the consumption of electricity and fuel for clinker firing and the transport of raw materials and clinker, which averages 0.7 kg CO₂/kg OPC.

3 Effect of hydrothermal nanosilica solution on the concrete compressive strength and the efficiency coefficients values

On Fig. 3 the curves with the concrete’s compressive strength at the ages 1–360 d and the different doses [SiO₂] = 0.5, 1, 2, 3 wt.% are presented. The early age gain of concrete’s strength is reflected by the ratios R_{\mathrm{c}\mathrm{o}\mathrm{m}}^1/R_{\mathrm{c}\mathrm{o}\mathrm{m}}^{28}, R_{\mathrm{c}\mathrm{o}\mathrm{m}}^7/R_{\mathrm{c}\mathrm{o}\mathrm{m}}^{28} and R_{\mathrm{c}\mathrm{o}\mathrm{m}}^{28}/R_{\mathrm{c}\mathrm{o}\mathrm{m}}^{360}: at the doses [SiO₂] 0 and 3 wt.% the ratios are 0.396–0.453, 0.782–0.899, and 0.641–0.706. For the doses [SiO₂] 0.5, 1.0, and 2 wt.% the ratios R_{\mathrm{c}\mathrm{o}\mathrm{m}}^{28}/R_{\mathrm{c}\mathrm{o}\mathrm{m}}^{360} are 0.728, 0.786, 0.766. Its ratios are higher in modified concrete compared to unmodified concrete. This reflects the kinetics of the pozzolanic reaction at the high specific surface area of nanosilica, which appears at the age of 1–28 d, and the contribution of the nucleation effect in the first hours of hardening.

In Fig. 3(b), the compressive strength curves are shown in the logarithmic scale of concrete curing time t_cur. The trend lines corresponding to the linear dependence of R_com on the logarithm of time, lnt_cur (d) are shown (Fig. 3(b)):

(11){\mathit{R}}_{\mathrm{c}\mathrm{o}\mathrm{m}}={\mathit{A}}_{\mathrm{R}}\times \ln {\mathit{t}}_{\mathrm{c}\mathrm{u}\mathrm{r}}+{\mathit{B}}_{\mathrm{R}}.

The coefficients A_R, B_R of the linear dependence were found for each dose of SiO₂. The coefficients A_R, which characterize the slope of the straight lines and the rate of strength gain by concrete from lnt_cur, regularly increased with an increase in the SiO₂ dose compared to the reference composition, with the exception of the composition with the dose [SiO₂] of 2.0 wt.%: 7.4709 (0 wt.%, coefficient of determination (R²) = 0.9973), 8.8382 (0.5 wt.%, R² = 0.9689), 8.4795 (1.0 wt.%, R² = 0.9502), 8.0727 (2 wt.%, R² = 0.9586), 9.5146 (3 wt.%, R² = 0.9794) (Fig. 4). The coefficients B_R, corresponding to the strength at the age of 1 d (B_R = R_com¹(t_cur = 1 d)) also increased with the increase in the SiO₂ dose (Fig. 3(b)). The dependences of the coefficients A_R and B_R on the SiO₂ dose allow us to predict the compressive strength of concrete SiO₂ and the efficiency coefficient k_ec = (R_com([SiO₂], t)/R_com(0, t) – 1) × 100%/[SiO₂] for the age of 2, 4, 6, 10 y, using Eq. (12):

(12)k_{\text{ec}}=\frac{\left({\displaystyle \frac{A_{\mathrm{R}}\left(\left[{\text{SiO}}_2\right]\right)\ln \left(t_{\text{cur}}\right)+B_{\mathrm{R}}\left(\left[{\text{SiO}}_2\right]\right)}{A_{\mathrm{R}}\left(\left[0\right]\right)\ln \left(t_{\text{cur}}\right)+B_{\mathrm{R}}\left(\left[0\right]\right)}}-1\right)\times 100\mathrm{\%}}{{[\text{SiO}}_2]}.

In the Table 4 results of calculations of efficiency coefficients values k_ec (Eq. (8a)), k_e (Eq. (7)) and k_e^F (Eq. (5)) are presented. The range of k_ec values for concrete at L/B = 0.50 was from 11.95 to 89.60, the same for k_e, from 10.1 to 59.9. The efficiency coefficient k_e^F differed slightly from the coefficient k_ec within the range of 1.2%–7.8% (Table 4). The dependences of the coefficient k_e^F on the nanosilica dose and on the age 1–360 d were almost the same with the coefficient k_ec. The linear variation y(x) = (0.9191x + 1.5807) between k_ec and k_e^F coefficients with high value of statistical correlation coefficient R² = 0.9986 proved the validity of the model based on the relative increase in compressive strength [58].

The dependence of k_ec on the dose [SiO₂] (wt.%) was falling function due to limitation of nucleation and pozzolanic effect by the rate of clinker minerals hydration and transfer Ca²⁺ cations to the surface of nanosilica particles. Trend lines, equations and statistical factor R² for the trend lines are presented in Fig. 5. The trend line was well approximated by a power-law with coefficients A and exponents n: k_ec = A_p/[SiO₂]ⁿ. The values of the coefficient A were in the range of 19.3, 39.9, 41.8, 50.4, the exponent of the power function n from 0.43, 0.45 to 0.66–0.76. The highest statistical factor R² was achieved for the concrete’s age of 28 d R² = 0.9885 (Fig. 5).

The coefficient k_ec demonstrated the tendency to decrease with increasing age from 1 to 360 d (Fig. 6). In our previous research [4] k_ec for concrete decreased with the age at high dose of HNS [SiO₂] = 2.0 wt.% and L/B = 0.713: k_ec = 43.0 (1 d), k_ec = 32.5 (2 d), and k_ec = 13.0 (28 d).

Results of researches [7,15,16,22,54,59,60], on nanosilica in the form of nanopowder, colloid sol, HNS, CSF, MK, FA, volcanic tuff with different specific surface area S_BET from 0.4 to 10, 20, 100, 200, 350, 560 m²/g can concluded that coefficients k_ec (0.1, 2, 3, 4 to 10, 30, 61) is proportional to the S_BET with well accuracy [58]. The coefficient k_ecs can introduce that takes into account the effect of parameter S_BET (m²/g) on the mechanical strength of concrete: k_ecs = (k_ec/S_BET). In the wide range of values of the effective surface area S_BET from 0.4 to 10, 20, 100, 200, 350, 560 m²/g at the age 28 d, the values of the efficiency coefficient k_ec for different pozzolanic additives can be approximated by a linear function of the type y(x) = (0.0609x + 1.4089) with high statistical correlation factor R² = 0.97 [58].

Another way to compare concretes modified with different pozzolanic additives is to take into account the influence of the water-cement ratio and introduce a coefficient k_ecw multiplying the known coefficient by the (L/B) = 0.5–0.7 according to equation k_ecw = k_ec × (L/B) [58].

4 Discussion

4.1 Hydrothermal nanosilica and commerсial nanosilicas

High level of k_ec values for the concretes with OPC replacement by HNS at different L/B ratios was obtained in our previous research: for L/B = 0.61–0.71 and doses [SiO₂] = 2.0 wt.% k_ec values were in the range 13, 20, 30, 40, 64 [4]. In the study [10] for L/B = 0.379 and dose [SiO₂] = 1.67 wt.% k_ec was 19.64.

The results obtained in the work of Chithra et al. [61] are most fully coincide with the results of the present research: the efficiency coefficient k_ec for concrete modified with colloid nanosilica sol with specific surface area of nanoparticles about 150 m²/g at [SiO₂] doses of 0.5–3.0 wt.% at the age from 1 to 90 d was in the range from 5.86 to 80.2. The coefficient k_ec decreased with an increase in the [SiO₂] dose and demonstrated the general tendency to decrease with an increase in the age of concrete from 1 to 90 d, decreasing at the transition from 1 to 3 d, then increasing at the transition from 3 to 7 d and further decreasing at the transition from 7 to 28, 56, 90 d [61]. For example, at the dose of [SiO₂] = 1.0 wt.% [61] the coefficient k_ec changed as follows: k_ec = 56.4 (1 d), 15.4 (3 d), 21.6 (7 d), 8.5 (28 d), 12.4 (56 d), 13.7 (90 d). The values of the coefficient k_ec corresponding to the results of the research [61], at different doses of [SiO₂] can also be approximated by the power-law equation: at the age of 1 d, the coefficients are A = 56.85, n = 0.6–0.62, R² = 0.89.

Results were published [22,59,60] for testing of Portland cement concretes, mortars, pastes with additive of nanosilica in the forms of pyrogenic (flame) nanopowder and colloid sol on mechanical characteristics: compressive, tensile bending and split tensile strength, modulus of elasticity.

4.2 Other pozzolanic materials

The level of k_e values for mortars with OPC replacement by nanosilica was much higher than for mortars with metakaolin (10, 15, 20 wt.%) and with combination of metakaolin and nanosilica in 2, 3, 4 times [22].

The range of k_ec values (Table 4) is in 2–40 times higher than for concretes with replacement of Portland cement with CSF and MK (k_ec = 1.5, 2.0, 2.5, 3.0, 3.5 [52–54]) at the same level of L/B and much higher than for FA (k_e = 0.5, 0.8, 0.8, 1.2 [23,24]), slag (SL) (k_e = 0.0, 0.1 [52,53]) and natural pozzolanic materials, such as volcanic glass, tuff, trass, pumice, diatomite and others (k_e = 0.2, 0.3, 0.4 [52,53]).

The dependence of k_e on the age for pozzolanic additives differed from nanosilica. The increasing was observed in the research [52] of k_e up to 3.0 for the concretes with CSF to the age 28 d and then decreasing to 2.4 to the age 90 d at L/B = 0.5. For the type of FA with low Ca content, k_e increased with the age from 2 to 90 d: k_e = 0.8 (2 d), k_e = 1.0 (7 d), k_e = 1.1 (28 d), k_e = 1.2 (90 d) [52]. The values of k_e for volcanic and diatomite rocks were about at low constant level 0.3–0.4 and 0.2 [23,24]. For the concrete with slag from nickel furnace k_e was in the range 0–0.1 [52,53]. For CSF and MK [54] with the doses of OPC replacement 5, 10, and 15 wt.% the tendency was obtained of increasing k_e with the age to 90–180 d up to 2.0, 2.5, 3.0, 3.5, 4.0, and k_e decreasing with the dose of CSF and MK from 3.5–4.0 to 2.0–2.5. For mortars with OPC replacement by MK [22] at the dose 10 wt.%, k_e increased with the age from 3 to 56 d, for the doses 15 and 20 wt.% k_e increased at the age from 3 to 7 d then decreased from 7 to 28 and 56 d age.

The coefficient k_e (−0.7, 0.1, 1.2, 1.9) in the concretes [55] with OPC replacement by steel blast furnace slag 5–50 wt.% with specific surface area S_BET = 0.25–0.42 m²/g and curing temperature t_cur = 20–60 °C changed the dependence on replacement with the age: increased at the age 1–3 d, were near constant at the age 7–28 d and decreased at the age 28–90 d. Coefficient k_e increased with the age 1–90 d for all doses 5–50 wt.% of replacement by slag. The statistical correlations were obtained for k_e = A + B × lnt_cur with dependences of A and B coefficients on the dose of replacement and with logarithmic dependence of A and B on S_BET – ln(S_BET/S_c), S_c – specific surface area of cement, 0.3–0.4 m²/g.

4.3 Metal oxides and carbon multi-walled carbon nanotubes

Metal oxide nanopowders showed an increase in concrete’s compressive strength of about 10%, 20%, 30% at Portland cement replacement doses of 0.5–1.0 wt.%, S_BET = 160 m²/g, L/B = 0.4: TiO₂ at 0.5 wt.%––13.85%, k_ec = 27.7 [23], 1.0 wt.%––17.9%, k_ec = 17.9 [23] (in Ref. [24], 1.0 wt.%––8.4%, k_ec = 8.4); Al₂O₃ at 0.5 wt.%––11.7%, k_ec = 23.4 [25], 1.0 wt.%––14.9%, k_ec = 14.9 [25]; Fe₂O₃ at 0.5 wt.%––8.7%, k_ec = 17.4, 1.0 wt.%––15.4%, k_ec = 17.4 [26] (in Ref. [27], 1.0 wt.%––56.4%, k_ec = 56.4, and 3.0 wt.%––74%, k_ec = 24.6); ZrO₂ at 0.5 wt.%––16%, k_ec = 32, 1.0 wt.%––18,4%, k_ec = 18.4 [28].

In the study [29], it was shown that MWCNTs in combination with polyacrylic acid polymers contributed to an increase in the strength of concrete by 50% with the dose of 0.1%, k_ec = 500. In the study [30], at the MWCNTs dose of 0.1 wt.%, the tensile bending strength of cement composite was increased by 8%, which corresponds to the value of k_ec = 80. The study for different combinations of MWCNTs and CNTs [31] shows a significantly smaller effect – 5% with the dose of 0.2 wt.%, k_ec = 25.

Nanopowders of metal oxides and MWCNTs do not have a pozzolanic effect. The action of these additives in Portland cement composite is explained by the nucleation effect of accelerating the polymerization of the CSH gel, which leads to an increase in the volume of the gel, acceleration of cement hydration and an increase in the portlandite content with decrease of portlandite crystals sizes. These nanoparticles are also able to fill the pores of the cement-water matrix, reducing the pore volume, and to densify cement-water matrix around of particle surface. Reinforcement of the cement-water matrix volume with nanoparticles also contributes to the increase in mechanical strength. When using carbon nanotubes MWCNTs and CNTs, it is necessary to take measures to prevent their aggregation and coagulation in the volume of hardening concrete, which requires the preparation of its stable aqueous suspension with the addition of a SP.

4.4 Effects of hydrothermal nanosilica on concrete’s microstructure

4.4.1 Thermogravimetry

The effect of HNS of increasing concrete strength at the early age of hardening is largely explained by the high specific surface area up to 400–500 m²/g, amorphous structure and high density of surface silanol groups up to 4.9 nm⁻² [4,62]. These factors lead to the high ability of the nanosilica to absorb CaO. SiO₂ nanoparticles in the lime CaO solution of pore water have the high chemisorption activity for Ca²⁺ cations with the formation of nanodispersed particles of the calcium silicate hydrate with the compositions xCaO·ySiO₂·zH₂O (Energy-dispersive X-ray spectroscopy method). The ability of nanosilica to adsorb CaO is much higher than CSF, opal, sand [4,62].

Data were obtained by titrometric determination of Ca²⁺ concentration in the calcium hydroxide solution on the sorption of Ca(OH)₂ by different samples of amorphous silica with different specific surface area: powder of HNS (S_BET = 410 m²/g), CSF (20 m²/g) and opal (0.6 m²/g) [4,62]. CaOH₂ binding was more pronounced for the HNS sample, especially during the first hours of reaction. At the time point of 24 h, the CaO binding for hydrothermal nano-SiO₂ was 206 mg CaO/ 1 g SiO₂, compared with 31 mg CaO/1 g SiO₂ for CSF and 33 mg CaO/1 g SiO₂ for opal. At the time point of 200 h, the CaO binding for hydrothermal nano-SiO₂ was about 680 mg CaO/1 g SiO₂, compared with 325 mg CaO/1 g SiO₂ for CSF and 200 mg CaO/1 g SiO₂ for opal. After 1000 h of reaction, the absorption rate for HNS and CSF slowed significantly, and the slope of the absorption curves became small. The HNS precipitate showed the 2.25-times mass increase due to the pozzolanic reaction, compared with 2.21 times (the CSF sample) and 1.7 times (the opal).

In cement paste samples modified with HNS sol, the chemisorption activity δCaO of SiO₂ nanoparticles was determined by the TG method based on the decrease in portlandite Ca(OH)₂ content [4,62]. At the dose [SiO₂] = 1.15 wt.% and L/B = 0.39: at the age of 1 d, δCaO = 750 mg CaO/g SiO₂, the percent of SiO₂ in pozzolanic reaction was 60%–70%, Ca(OH)₂ content decreased by 20%, the rate of cement hydration increased by 20%–30%; at the age of 28 d, δCaO = 1000 mg CaO/g SiO₂; at the age of 720 d, δCaO = 1360 mg CaO/g SiO₂ and Ca(OH)₂ content reduction (40%) [4,62].

The chemisorption activity δCaO in mortars and cement pastes of other pozzolanic materials, obtained by titrometric determination of Ca²⁺ concentration in Ca(OH)₂ solution, is much lower than nanosilica (mg CaO/g SiO₂, 28 d) [22,63]: MK, CSF (1000, 1230), FA (200), volcanic glass, tuff and other natural pozzolanic materials (50, 100, 200).

The decrease in the k_ec coefficient with increasing SiO₂ dose occurs mainly because the rate of the pozzolanic reaction at the high surface of SiO₂ nanoparticles is limited by the kinetics of clinker minerals hydration and the rate of diffusion of Ca²⁺ cations to the surface of nanoparticles.

4.4.2 X-ray diffraction

X-Ray analysis of cement pastes modified with HNS sol confirmed the decrease in the content of portlandite Ca(OH)₂ during the pozzolanic reaction [4], Fig. 7 ([SiO₂] = 1.74 wt.%, L/B = 0.36, the age of 1 d). The height of the portlandite peaks increased with increasing cement hydration time, but the peaks heights in the pastes modified with NNS were significantly lower than those in the reference composition for all basic crystal distances at all times from 1 to 1100 d. The heights of the main peaks corresponding to clinker minerals were lower for the compositions containing HNS sol due to its accelerated hydration.

4.4.3 Scanning electron microscopy

SEM allows to draw conclusions on the changes in the shape and volume of mineral phases in the microstructure of cement pastes modified with HNS [4] (Fig. 8). The microstructure of the cement paste without the addition of SiO₂ nanoparticles shows the presence of portlandite crystals in the form of hexagonal prismatic plates with submicron sizes (Fig. 8(a)). The microstructure of the cement paste modified with SiO₂ nanoparticles contains submicron sized needle-like CSH particles, characterized by smaller dimensions, denser volume packing, and lower Ca/Si ratios compared to CSH particles in unmodified compositions (Fig. 8(b)). These differences in the microstructure are the consequence of the pozzolanic and nucleation effects of nanosilica, which lead to increased mechanical strength and fracture resistance of concrete.

4.4.4 Kinetics of water absorption and capillary pore volume of cement material

The relative change in mass W(t) of HNS modified concrete over time t during water absorption was determined in accordance with the Russian Federation state standard GOST 12730.4-78 (Fig. 9) [4]. Using these data and Eq. (13) with the constant W_max as maximum mass increase, the open capillary pore structure parameters λ and α, corresponding to the average pore diameter d_p and the uniformity of pore diameter distribution, respectively, were calculated:

(13)W_{\mathrm{t}}(t)=W_{max}\left[1-e^{-(\overline{\lambda }{t)}^{\alpha }}\right].

During the first hours of water absorption, the mass increase and total pore volume of the modified concrete was lower, depending on the nanosilica SiO₂ dose. The parameters α and λ were calculated using Eqs. (14) and (15) based on the values of mass increment at times t₁ and t₂:

(14)\begin{array}{rl}\propto =& \ln \left[\ln \left(1/\left(1-W(t_2)/W_{max}\right)\right)/\ln \left(1/\left(1-W(t_1)/W_{max}\right)\right)\right]/\\ & \ln \left(t_2/t_1\right),\end{array}

(15)\lambda ={\frac{\left[\ln \left(1/\left(1-W(t_2)/W_{max}\right)\right)\right]}{t_2}}^{1/\alpha }.

The relative weight loss during drying was lower for modified concrete and decreased with an increase in the SiO₂ dose. Upon completion of drying, the weight loss for unmodified concrete was W_max = 6.25%, for modified concrete at the dose of SiO₂ 0.5 wt.% (W_max = 6.2%), 2.0 wt.% (W_max = 5.7%) and 3.0 wt.% (W_max = 5.4%). The parameters λ and α changed depending on the dose of the SiO₂ nanoadditive as follows: 0 wt.% (reference composition) (λ = 1.93, α = 0.263); 0.5 wt.% (λ = 0.223, α = 0.577); 2.0 wt.% (λ = 0.07645, α = 0.7049); 3.0 wt.% (λ = 0.0552, α = 0.896). With an increase in the nanosilica dose [SiO₂] from 0 to 3 wt.%, the W_max decreased by 1.157 times, the parameter λ decreased by 28–35 times, and the parameter α increased by 3.4 times. Parameter λ, proportional to d_p³, decreased, while parameter α of pores structure uniformity increased with increasing SiO₂ dose. At the dose of SiO₂ = 3%, λ decreased by 28.4 times compared to the reference composition, and the average pore diameter accordingly decreased by 3.05 times.

Taking into account the concrete’s density (28 d), the porosity relative to the volume of concrete was: SiO₂ 0 wt.% (14.75%), 0.5 wt.% (14.60); 2.0 wt.% (13.59%); 3.0 wt.% (12.93%). Taking into account the consumption of cement and water per the concrete’s volume 1 m³, the proportion of the open capillary pores volume V_cp, recalculated to the volume of cement stone in concrete, was as follows: [SiO₂] = 0 wt.% (44.59%), 0.5 wt.% (43.66); 2.0 wt.% (40.53%); 3.0 wt.% (38.55%).

The λ parameter decreased with an increase in the [SiO₂] dose along with a decrease in the total porosity determined by W_max. The increase of α parameter demonstrated the transition of the pore structure to a more uniform pore diameter distribution. Such changes in the λ and α parameters can be explained by the effect of the pozzolanic reaction involving nanosilica and Ca²⁺ cations in the pore water led to formation of additional amount of calcium silicate hydrate gel CSH and decrease in the total volume of capillary pores. In this case, the volume of large capillary pores with diameters greater than 1 μm decreased to a greater extent, which increased the uniformity of the pore diameter distribution.

4.5 Durability

4.5.1 Effect on water impermeability of concrete

As the result of the pozzolanic reaction and the reduction of the total porosity and average pore diameter and the increase in the uniformity of the pore size distribution, the water impermeability of concrete has increased significantly [4]. According to the Russian Federation state standard GOST 12730.5-84, the concrete water impermeability grade WN is determined by the number of stages (N/2) taking into account the time at every stage (no more than 16 h) for the appearance of drops or a wet spot of water on the surface of the cylindrical sample with diameter of 150 mm and height of 150 mm opposite to the filtration direction (0.1 MPa) under the pressure of (N/10) MPa applied to the surface in filtration direction. The pressure upon transition to the next stage increases by 0.2 MPa, the duration of each passed stage is 16 h, the stage is considered passed if the appearance of water drops or a wet spot is not observed within 16 h. The compositions of modified Portland cement concrete with the liquid-binder ratio L/B of 0.5 corresponded to the concrete water impermeability grade W18 at the nanosilica doses SiO₂ of 1–3 wt.%, the control composition corresponded to the grade W10.

The compositions of modified Portland cement mortar with a ratio of L/B = 0.4 [64] showed a consistent increase in water impermeability grade with increasing SiO₂ dose: 0–0.01 wt.% (W10), 0.05–0.1 wt.% (W12), 0.25–1.0 wt.% (W14), 2 wt.% (W16), 3 wt.% (W18). Prismatic samples with face dimensions of 100 mm × 100 mm and the thickness of 30 mm were tested for the stage duration of 4 h. The filtration coefficient K_f was determined by the volume of water filtered under a certain pressure difference applied to the opposite surfaces of the sample for a fixed time through a certain surface area. The filtration K_f consistently decreased with an increase in the nanosilica dose from 0 to 3 wt.% by 4.0 times. In this case, the compressive R_com and tensile bending R_tb strengths consistently increased with increasing the dose of SiO₂, showing respectively the quadratic and linear correlation with the coefficients K_f with high values of statistical factors R² 0.9624 and 0.9342. Correlations (R_com, R_tb) – K_f in cement mortar samples indicated the similarity of the mechanism of the effects of increasing mechanical strength and water impermeability when modified with nanosilica, based on the action of the pozzolanic reaction, the decrease in the volume and average diameter of capillary pores.

4.5.2 Effect on resistance to abrasion

When modifying concrete or cement mortar with nanosilica, the reduction in pore volume and average diameter, along with a significant reduction in the proportion of pores with diameters greater than 1 μm, leads to the reduction in the height and uniformity of roughness protrusions on the material’s surface. Its mechanical strength and hardness also increase. Improving these characteristics allows for increased abrasion resistance of the cement material. In the study [5] presents the results of tests to determine the abrasion resistance of cement mortar compositions modified with HNS sol or nanopowder at SiO₂ doses of 0.01–3.0 wt.% and at the L/B ratio of 0.4. The mass losses (m²/g) of cubic samples with dimensions of 100 mm × 100 mm × 100 mm were obtained during abrasion on the abrasive disk with silicon carbide SiC grains with dimensions of 4.75–45 μm at the fixed load on the sample surface of 300 N and a pressure of 60 kPa, the total abrasion path of 600 m. The mass losses due to abrasion rapidly decreased with the increase in the SiO₂ dose in the range of 0.01, 0.25, 0.5 wt.% from 0.9 to 0.73–0.7 g/m², in the range of SiO₂ doses of 0.5–3.0 wt.% the decrease was relatively slow, from 0.7 to 0.71 to 0.67–0.68 g/m². Cement materials modified with HNS sol or nanopowder at SiO₂ doses of 0.25–0.5 wt.% and higher can be classified as a low-abrasion grade material.

4.5.3 Effect on freeze resistance

The reduction in the volume and average diameter of pores and an increase in the abrasive resistance of the cement material lead to an increase in frost resistance. Samples of cement mortar compositions modified with NNS sol or nanopowder at the SiO₂ dose of 0.01–3.0 wt.% at L/B = 0.4, were tested to determine the material grade for frost resistance according to the Russian Federation state standard GOST 10060-2012. Samples of modified cubic compositions with dimensions of 100 mm × 100 mm × 100 mm after reaching the age of 28 d were subjected to freeze–thaw cycles at a temperature of −18 °C for 2.5 h and 20 °C for 2.0 h, and losses in compressive strength and mass of the samples were determined. The frost resistance grade of the samples FN corresponded to the number of cycles at which the relative losses in compressive strength did not exceed 10%, and mass losses were less than 2%. With the increase in the dose of SiO₂, there was the decrease in the relative losses of compressive strength and mass of the samples (Fig. 10). With the high number of freeze–thaw cycles 300 and 350, the decrease in the losses of mechanical strength and samples mass in the range of SiO₂ doses from 0.1 to 0.25 to 3.0 wt.% was more pronounced, which is consistent with the effects of nanosilica on water absorption and abrasion resistance characteristics, and indicates the increase in the freeze resistance effect with increasing the number of cycles. At 300 and 350 cycles with the increase in the SiO₂ dose to 3.0 wt.%, compressive strength losses decreased to 30%, and mass losses to 25%. The frost resistance grade of cement mortar compositions modified with HNS sol or nanopowder at the SiO₂ dose of 0.25 to 3.0 wt.% was reliably increased from F200 to F300.

4.5.4 Effect on of hydrothermal nanosilica and microfiber on impact toughness, dissipative capacity and fracture resistance of cement composite

The reduction in the volume of capillary pores and CSH gel pores due to the nucleation and pozzolanic effect of HNS leads to the increase in the mechanical properties and fracture resistance of the cement-water matrix, as well as the impact toughness and dissipative properties of the cement material. The impact toughness of cement mortars modified with HNS sol or nanopowder at SiO₂ doses of 0.01–3.0 wt.% and basalt microfiber with the length 12 mm, diameter 12 μm and the length/diameter ratio 1000 at L/B = 0.4, was determined in accordance with the ACI Committee 544 standard [6]. The impact toughness was determined on prismatic plate samples with dimensions of 600 mm × 600 mm × 50 mm using the falling striker with the mass of 10 kg with the tip made of the steel ball with the diameter of 40 mm, the drop height of 0.6 m and the frequency of strikes, 1 impact every 5 s.

When modifying with HNS sols or nanopowders, significant increases in the mechanical and dissipative properties of cement mortars with the increase of SiO₂ dose were obtained [6.64]: compressive strength R_com (up to 13.9%–25.6%), tensile bending strength R_tb (3.35%–19,0%), number of impacts to the first crack N_ff (50%–180%), number of impacts to complete destruction N_cd (150%), energy of complete destruction E_im (12000 J), impact toughness N_iv = N_cd/N_ff (60%). Modification with basalt microfiber did not significantly affect R_com, but the increment in R_tb and impact toughness indicators was significant –– R_tb (up to 59%), N_cd (776.3%), E_im (71980 J), N_ff (280%), N_iv (127.2%). When modifying with the combination of SiO₂ nanoparticles and basalt microfiber, the strong synergistic effect was established [6], expressed in the fact that when using the combination, the increase in R_tb and impact toughness indicators was significantly higher than the simple arithmetic sum of the increments after separate use of SiO₂ nanoparticles and microfiber (Figs. 11(a)–11(d)): 2.2 times (R_tb), 2.4 times (N_cd), 2.1 times (E_im), 1.7 times (N_ff), 1.6 times (N_iv). The synergistic effect can be explained by the pozzolanic and nucleation activities of nanosilica, which lead to the increase in the volume fraction of the HD –phase of CSH gel phase with the high packing density of nanogranules and the increase in the shear stress between the lateral surface of the microfiber and the CSH gel.

The increments of mechanical and dissipative characteristics of impact toughness were related between the coefficients, statistical correlations with high values of the R² factor were obtained [6] (Figs. 11(e) and 11(f)): linear correlations (N_ff–R_com) and (N_ff–R_tb), R² = 0.9962 and 0.9817; exponential correlations (N_cd–R_com) and (N_cd–R_tb), R² = 0.9364 and 0.9654. Fracture resistance parameters, such as fracture toughness K_IC and specific fracture formation energy G_f, statistically correlate with mechanical characteristics R_com and R_tb [6]. Fracture resistance parameters also correlate with dissipative properties [6], for example, according to the well-known relationship K_IC = A + B·ln(E_im/E_{\mathrm{i}\mathrm{m}}^0), where A and B are coefficients dependent on the fiber length/diameter ratio, and E_{\mathrm{i}\mathrm{m}}^0 is the specific fracture energy in the absence of fiber. Given these statistical relationships, the increments in mechanical and dissipative properties achieved by modification with HNS and microfiber provide the increase in the fracture resistance characteristics K_IC and G_f.

4.5.5 Durability of concrete modified with hydrothermal nanosilica

The pozzolanic and nucleation activity of nanosilica enhances the mechanical properties of concrete, increasing fracture resistance, dissipative and plastic properties, water impermeability, abrasion resistance, and frost resistance, thereby improving the durability of concrete. The decrease in the pore volume and diameter can reduce the diffusion coefficients of CO₂, sulfate and chloride ions in the cementitious material, and increase the resistance of concrete to atmospheric and se corrosion.

In the study [5] the example was analyzed of the experimental production of curbstones for edging asphalt roads at the reinforced concrete plant using concrete modified with HNS sol at the SiO₂ dose of 0.05 wt.%, Portland cement consumption of 520 kg/m³, and the liquid-binder ratio of L/B = 0.33. The sol and SP’s solution were fed through the devises of liquid dosing into the tank with mixing water, and then mixed with the cement, sand, and coarse aggregate in the plant's process units. The addition of sol and SP solution to the mixing water, followed by mixing the water with cement and aggregates, ensures the uniform distribution of SiO₂ nanoparticles throughout the concrete. Concrete was poured into curbstone molds (Fig. 12) with the dimensions of 3000 mm × 320 mm × 300 mm, volume of concrete = 0.188 m³, overall dimensions volume = 0.288 m³ and weight = 470 kg. For durability testing, the products were installed in the continuous line for edging the asphalt road with the total length of 45 m.

Durability was determined based on the time it took for peeling and cracking to appear on the concrete surface, loss of shape, and splitting. In cold climates with long periods of subzero temperatures, high levels of rain and snow, intense ice melt with large water flows, and possible contact with cold seawater, seismic activity curbstone corner lines require replacement every 1–2 y. The line of products made from concrete modified with HNS was replaced after 6 y of operation, which corresponded to a 3–6-fold increase in durability.

4.6 Economic evaluations of clinker savings and nanosilica efficiency

Table 5 shows the values of the clinker savings coefficient CLS calculated by Eq. (9) for different doses of HNS for the age of concrete of 28 and 360 d. The reduction in CO₂ emissions calculated by Eq. (10) was proportional to the corresponding values of the CLS coefficient. The CLS coefficient slowly increased with the increase in the dose of nanosilica, which is important for evaluation the economic efficiency of nanosilica: at the age of 28 d, with an increase in the SiO₂ dose from 0.05 to 3 wt.%, the CLS coefficient increased from 0.234 to 0.376. Such a weak dependence of the CLS coefficient on the nanosilica dose is due to the fact that the product k_ec on p (k_ec·p) changes slightly from the parameter p according to the power-law equation k_ec = A_p/[SiO₂]ⁿ for the age of 28 d: k_ec − A·[SiO₂]¹⁻ⁿ, n = 0.66, A = 39.88. It is possible to evaluate the k_ec and CLS coefficients at low doses of nanosilica: at the age of 28 d and the dose of [SiO₂] = 0.1 wt.%, k_ec = 182.3 and CLS = 0.154 = 15.4%.

The economic efficiency of modifying concrete with nanosilica additive is primarily determined by the market prices of nanosilica sol and polycarboxylate SP used for modifying concrete and the saved amount of Portland cement, concrete products and structures evaluated using the CLS coefficient and experimental results on compressive strength increment. It should be taken into account that market prices for nanoparticles and SPs with mass proportion in about 1:1 in wholesales are significantly lower by 5–10 or more times than retail prices.

The technology of HNS sol production can ensure low cost and market price for wholesale. The process flow chart excludes chemical raw materials required for the production of traditional nanosilicas: during flame production of pyrogenic silica powder, gas-phase reagents FeCl₃ and H₂, O₂ are consumed, while the production of nanosilica sol requires the consumption of sodium silicate precursor Na₂SiO₃ (Na₂O·mSiO₂) and sulfuric acid H₂SO₄, hydrochloric acid HCl for regeneration of cation exchange resin, and electricity for sol concentration by evaporation or ultrafiltration. In the case of HNS, OSA molecules H₄SiO₄ enter the hydrothermal solution due to the dissolution of rock minerals at temperatures of 250–300 °C and higher. The main stages of the HNS technology include the lifting of hydrothermal solution from the porous environment of rocks to the surface through productive wells of GeoPP, polymerization of orthosiliсic solution molecules and the growth of SiO₂ nanoparticles and ultrafiltration membrane concentration of nanoparticles. High selectivity of ultrafiltration membranes with respect to SiO₂ nanoparticles ensures low specific energy consumption of no more than (0.5–2.0) (kW·h)/kg of solid SiO₂ in the final product ash with a mass content of 30 wt.% at a zero tariff for electricity at the GeoPP. Hydrothermal deposits are the renewable source of energy and mineral raw materials. Taking into account the cost of electricity for driving pumps, periodic washings of the capillary membrane layer and replacement of membrane cartridges, the cost of producing HNS in the form of sol is no more than $(1–1.5)/kg. The market price for wholesale sales can be set at $(1.5–3.0)/kg. The market price level for traditional commercial sodium silicate-based sols (Ludox, etc.) is at the level of $(6–10)/kg of solid SiO₂, for nanopowders based on sodium silicate −$(2–8)/kg, for pyrogenic nanopowders of the Aerosil trade mark −$(5–10)/kg.

The use of nanopowders and puzzolans such as CSF that create significant amounts of dust faces the problems of uniform mixing the nanopowder with Portland cement to achieve the uniform distribution of SiO₂ nanoparticles or to prepare stable aqueous suspensions.

The use of traditional commercial pozzolans requires their consumption 5–10 times higher (10, 20, 30 wt.%) compared to nanosilica while achieving the comparable effect of compressive strength increasing. For CSF at the dose of 10 wt.%, the increase in concrete compressive strength at the age of 28 d is about 20%, which corresponds to the value of the coefficient k_ec = 2, clinker retention coefficient CLS = 0.181 = 18.1%. The market price of CSF is within the range of $(0.06, 0.125, 0.30)/kg.

The wholesale market prices for metal oxide nanopowders are higher, and the production level (t/y) is lower compared to SiO₂: TiO₂, Al₂O₃, Fe₂O₃––$(10, 20, 30)/kg, Zr₂O₃––$(20–30)/kg. The market price of carbon nanotubes MWCNT is within the range of $(500–1000)/kg.

Market wholesale prices for large-tonnage sales of polycarboxylate SPs in liquid form with a dry matter content of 40 wt.% are within the range of $(0.25–0.5)/kg.

The market price of Portland cements increases depending on the compressive strength that it provides at the age of 28 d and is within $ (70–90)/t. Prices for concrete for monolithic construction depend on the type of structure and the maximum load required by the project and are within the range of $(60–75)/m³, and prices for steel reinforced concrete are $(75–120)/m³.

There are several possible ways of using nanosilica in combination with polycarboxylate SP to obtain economic benefits.

1) Saving the consumption of Portland cement per 1 m³ of concrete due to the effect of increasing the compressive strength when using nanosilica; at HNS doses of 0.5–1.0 wt.% and higher, such an approach does not provide economic benefit since the cost of the consumed nanosilica and SP exceeds the cost of saving Portland cement; at HNS doses of about 0.1–0.2 wt.%, the economic benefit per 1 m³ of concrete is about $3.3/m³.

2) Replacing Portland cement with another cheaper type, providing lower compressive strength of concrete; while maintaining cement consumption and reducing its price, the economic benefit can reach $3.3/m³ at HNS doses of 0.1–0.2 wt.%.

3) Saving the volume of concrete and reinforced concrete due to reducing the cross-sectional area of the concrete structures, which is determined by the effect of increasing the compressive strength; such the approach can be implemented at relatively small HNS doses 0.1–0.2 wt.% and high ones 0.5, 1.0, 2.0 wt.% with the economic benefit of $(6–14)/m³ and higher.

4) Reducing the duration of heat and moisture treatment with accelerated hardening of concrete; the 20% reduction in the duration of heat and moisture treatment can provide energy savings of up to 40 kW/m³, which corresponds to an economic benefit of about $1.0/m³, it can be implemented only at small HNS doses of 0.1–0.2 wt.%.

The use of traditional commercial nanosilica products significantly reduces the economic benefit if act according to the options considered, since their production costs and market prices are higher than those of HNS.

When using CSF according to options 1) and 2), the economic benefit decreases compared to HNS to $2/m³ due to the high consumption of pozzolan. When reducing the volume of concrete according to the option 3), the economic benefit from using these materials is comparable, but the pozzolanic additive requires significant costs for transport of large mass of material, uniform mixing or preparation of the stabilized aqueous suspension.

The use of metal oxide nanopowders with economic benefit is possible only at small doses of 0.1–0.2 wt.% with a significant reduction in economic effect compared to HNS due to higher prices. For MWCNTs, the promising method of application in Portland cement composites may be the using at small doses of about 0.01–0.001 wt.% to improve the CSH gel nanostructure.

The most promising option, in our opinion, is the option 3) with the decrease in the cross-section of structures. It can be implemented in various ways. With the statistically reliable increase in the compressive strength of concrete by the factor of λ, it is possible to reduce the cross-sectional area and the consumption of Portland cement for the structural concrete by the factor of λ⁻¹ for compressible elements, such as columns, perfect arches and shells, by the factor of λ^−2.3 for bending beams and λ^−1/2 for slabs. The increase in the fracture toughness K_IC = µK_0IC will reduce the cross-sectional area by the factor of µ⁻¹ for columns and by the factor of µ^–4/5 for beams in bending or torsion.

5 Conclusions

Based on the obtained results, the following conclusions can be made regarding the development of the use of nanosilica for modifying Portland cement concrete.

1) The use of HNS sol additive in Portland cement concrete can provide significant increases in compressive strength up to the age of 360 d (14.8%–43.9%) and an increase in the efficiency coefficient k_ес (19.5, 60.9 at the age of 28 d. The obtained results can be used to model the hardening and predict the strength of modified concrete at the age of 2, 4, 10 y.

2) The result of the additive action is characterized by an early gain in strength at the age of 1–7 d. In this case, the ratios R_com¹/R_com²⁸ and R_com⁷/R_com²⁸, R_com²⁸/R_com³⁶⁰ are higher in modified concrete compared to unmodified concrete. This reflects the kinetics of the pozzolanic reaction at a high specific surface area of nanosilica, which appears at the age of 1–28 d, and the contribution of the nucleation effect in the first hours of hardening.

3) According to the results of the TG, XRD and SEM methods, the pozzolanic reaction leads to the decrease in the portlandite content, the increase in the volume of calcium hydrosilicates CSH (I) and acceleration of cement hydration. As the result, the volumes of capillary and CSH gel pores decreases, water absorption decreases, water permeability, abrasion and frost resistance, fracture and impact toughness increase. The modified concrete has increased durability and can be used in cold climates with the large amount of rain, snow and ice, contact with sea water and seismic activity.

4) The efficiency coefficient k_ec is verified as the result of comparison with calculations using the Ferré’s and Bolomey’s models in wide ranges of SiO₂ doses and ranges. The value of the coefficient k_ec decreases with the age of concrete along with the decrease in the effect of the relative gain in compressive strength due to the consumption of nanosilica during the pozzolanic reaction and the formation of the lay of CSH gel around cement grains and SiO₂ nanoparticles. The coefficient k_ec decreases with increasing SiO₂ dose according to the power dependence at all ages of the period from 1 to 360 d, since the total nanoparticle’s surface increases linearly with the dose of SiO₂, and the concentration of Ca²⁺ cations in the pore water decreases due to the limitation of the kinetics of hydration of cement minerals (C₃S, C₂S) and transport of Ca²⁺ cations. The coefficient k_ec weakly depends on the grade of Portland cement and significantly depends on the specific surface area of nanosilica and the water–cement ratio.

5) High values of the k_ec coefficient can provide the significant reduction in Portland cement consumption and the reduction in carbon dioxide emissions during its production. The clinker savings coefficient of CLS slowly decreases with decreasing nanosilica dose, which can be used to apply HNS at small doses of 0.1–0.25 wt.% taking into account the low production cost of the developed technology compared to traditional commercial nanosilicas. The level of the k_ec coefficient for commercial pozzolans (CSF, MK, etc.) is significantly lower than that of HNS, and the consumption per unit volume is significantly higher, which compensates for their low market prices. Metal oxide nanopowders are characterized by similar levels of the k_ec coefficient at the same doses of Portland cement substitution, but higher market prices compared to HNS and commercial nanosilicas. Carbon nanotubes MWCNTs, CNTs show high k_ec values at relatively small doses at high market prices. One of the promising options for using HNS is to reduce the volume of concrete and thin structures based on the effect of increasing mechanical strength.

6) Nanosilica is promising for use in ultra-high-strength concrete UHPC compositions, which are characterized by the low liquid-to-binder ratio L/B of 0.08, 0.1, 0.15, 0.2, 0.25. The high specific surface area S_BET, due to the pozzolanic and nucleation activity of nanosilica, can provide significant increases in compressive strength, tensile bending strength and other characteristics of UHPS concrete at the early age of 1–28 d in combination with fiber. The increase in the efficiency coefficient of nanosilica k_ec is inversely proportional to the decrease in the L/B ratio, which allows to expect the significant effect when replacing nanosilica with Portland cement, CSF, MK or FA.

References

[1] Aggarwal P , Singh R P , Aggarwal Y . Use of nano-silica in cement based materials—A review. Cogent Engineering, 2015, 2(1): 1078018

[2] Zhang P , Wan J , Wang K , Li Q . Influence of nano-SiO₂ on properties of fresh and hardened high performance concrete: A state-of-the-art review. Construction & Building Materials, 2017, 148: 648–658

[3] Khaloo A , Mobini M H , Hosseini P . Influence of different types of nano-SiO₂ particles on properties of high-performance concrete. Construction & Building Materials, 2016, 113: 188–201

[4] Potapov V , Efimenko Y , Fediuk R , Gorev D . Effect of hydrothermal nanosilica on the performances of cement concrete. Construction & Building Materials, 2021, 269: 121307

[5] Potapov V , Efimenko Y , Fediuk R , Gorev D , Kozin A , Liseitsev Y . Modification of cement composites with hydrothermal nano-SiO₂. Journal of Materials in Civil Engineering, 2021, 33(12): 04021339

[6] Potapov V , Efimenko Y , Fediuk R , Gorev D . Impact resistance of the cement–mortar composite modified with SiO₂ nanoparticles and microfiber. Journal of Materials in Civil Engineering, 2022, 34(7): 04022135

[7] Abhilash P PNayak D KSangoju BKumar RKumar V. Effect of nano-silica in concrete: A review. Construction and Building Materials, 2021, 278: 122347

[8] Singh L P , Karade S R , Bhattacharyya S K , Yousuf M M , Ahalawat S . Beneficial role of nanosilica in cement based materials—A review. Construction & Building Materials, 2013, 47: 1069–1077

[9] Barbhuiya G , Moiz M , Hasan S , Zaheer M . Effects of the nanosilica addition on cement concrete: A review. Materials Today: Proceedings, 2020, 32: 560–566

[10] Quercia G , Spiesz P , Hüsken G , Brouwers H J H . SCC modification by use of amorphous nano-silica. Cement and Concrete Composites, 2014, 45: 69–81

[11] Yu R , Spiesz P , Brouwers H J H . Effect of nano-silica on the hydration and microstructure development of Ultra-High Performance Concrete (UHPC) with a low binder amount. Construction & Building Materials, 2014, 65: 140–150

[12] Zhang M H , Islam J . Use of nano-silica to reduce setting time and increase early strength of concretes with high volumes of fly ash or slag. Construction & Building Materials, 2012, 29: 573–580

[13] Nazari A , Riahi S . The role of SiO₂ nanoparticles and ground granulated blast furnace slag admixtures on physical, thermal and mechanical properties of self-compacting concrete. Materials Science and Engineering A, 2011, 528(4–5): 2149–2157

[14] Nazari A , Riahi S . The effects of SiO₂ nanoparticles on physical and mechanical properties of high strength compacting concrete. Composites. Part B: Engineering, 2011, 42(3): 570–578

[15] Oltulu M , Şahin R . Effect of nano-SiO₂, nano-Al₂O₃ and nano-Fe₂O₃ powders on compressive strengths and capillary water absorption of cement mortar containing fly ash: A comparative study. Energy and Building, 2013, 58: 292–301

[16] Wu Z , Shi C , Khayat K H , Wan S . Effects of different nanomaterials on hardening and performance of ultra-high strength concrete (UHSC). Cement and Concrete Composites, 2016, 70: 24–34

[17] Givi A , Rashid S , Aziz F , Salleh M . Experimental investigation of the size effects of SiO₂ nano-particles on the mechanical properties of binary blended concrete. Composites. Part B: Engineering, 2010, 41(8): 673–677

[18] Durgun M Y , Atahan H N . Strength, elastic and microstructural properties of SCCs’ with colloidal nano silica addition. Construction & Building Materials, 2018, 158: 295–307

[19] Afzali-Naniz O , Mazloom M . Fracture behavior of self-compacting semilightweight concrete containing nano-silica. Advances in Structural Engineering, 2019, 22(10): 2264–2277

[20] Erdem S , Hanbay S , Güler Z . Micromechanical damage analysis and engineering performance of concrete with colloidal nano-silica and demolished concrete aggregates. Construction & Building Materials, 2018, 171: 634–642

[21] Fediuk R , Timokhin R , Mochalov A , Otsokov K , Lashina I . Performance properties of high-density impermeable cementitious paste. Journal of Materials in Civil Engineering, 2019, 31(4): 04019013

[22] Abhilash P P , Potapov V , Kumar R , Kumar V , Gupta U . Integrated effects of metakaolin and nano-silica in superplasticizer-free mortar: An analysis of mortar compressive strength with relative strength, K-factor and clinker savings. Civil Engineering and Architecture., 2024, 12(3): 1540–1561

[23] Nazari A , Riahi S , Riahi S , Shamekhim S F , Khademno A . Assessment of the effects of the cement paste composite in presence TiO₂ nanoparticles. Journal of American Science, 2010, 6(4): 43–46

[24] Suneel MRao G V R. Effect of nano-TiO₂ at macro and micro level of concrete by partial substitution of cement. Research on Engineering Structures and Materials. 2024, 11(4): 1545–1559

[25] Nazari A , Riahi S , Riahi S , Shamekhi S F , Khademno A . Influence of Al₂O₃ nanoparticles on the compressive strength and workability of blended concrete. Journal of American Science, 2010, 6(5): 5–9

[26] Nazari A , Riahi S , Riahi S , Shamekhi S F , Khademno A . Benefits of Fe₂O₃ nanoparticles in concrete mixing matrix. Journal of American Science, 2010, 6(4): 102–106

[27] Abdoli Yazdi N , Arefi M R , Mollaahmadi E , Abdollahi N B . To study the effect of adding Fe₂O₃ nanoparticles on the morphology properties and microstructure of cement mortar. Life Science Journal, 2011, 8(4): 550–554

[28] Nazari A , Riahi S , Riahi S , Shamekhi S F , Khademno A . An investigation on the Strength and workability of cement based concrete performance by using ZrO₂ nanoparticles. Journal of American Science, 2010, 6(4): 29–33

[29] Konsta-Gdoutos M S , Metaxa Z S , Shah S P . Highly dispersed carbon nanotube reinforced cement based materials. Cement and Concrete Research, 2010, 40(7): 1052–1059

[30] Shah S PKonsta-Gdoutos SM Z SMetaxa . Highly dispersed carbon nanotube-reinforced cement-based materials. US Patent, 9 499 439. 2016-11-22

[31] Brenner AMariputtana Kavi ALi M G Y. Carbon nanotube reinforced concrete composites and methods of making same, US Patent, US 2008/0134942 A1 (43). 2008-06-12

[32] Han B , Zhang L , Zeng S , Dong S , Yu X , Yang R , Ou J . Nano-core effect in nano-engineered cementitious composites. Composites Part A: Applied Science and Manufacturing, 2017, 95: 100–109

[33] Han BDing SWang JOu J. Nano-Engineered Cementitious Composites: Principles and Practices. Singapore: Springer, 2019

[34] Kawashima S , Hou P , Corr D J , Shah S P . Modification of cement-based materials with nanoparticles. Cement and Concrete Composites, 2013, 36: 8–15

[35] Hou P K , Kawashima S , Wang K J , Corr D J , Qian J S , Shah S P . Effects of colloidal nanosilica on rheological and mechanical properties of fly ash-cement mortar. Cement and Concrete Composites, 2013, 35(1): 12–22

[36] Mondal P , Shah S P , Marks L D , Gaitero J J . Comparative study of the effects of microsilica and nanosilica in concrete. Transportation Research Record, 2010, 2141(1): 6–9

[37] Hou P , Kawashima S , Kong D , Corr D J , Qian J , Shah S P . Modification effects of colloidal nano-SiO₂ on cement hydration and its gel property. Composites Part B: Engineering, 2013, 45(1): 440–448

[38] Martin G FGhooray GHo R HXu X M. The origin of serotoninergic projections to the lumbosacral spinal cord at different stages of development in the North American opossum. Developmental brain research, 1991, 58(2): 203–213

[39] Sharma U , Singh L P , Zhan B , Poon C S . Effect of particle size of nanosilica on microstructure of C-S-H and its impact on mechanical strength. Cement and Concrete Composites, 2019, 97: 312–321

[40] Sharma U , Singh L P , Ali D , Poon C S . Effect of particle size of silica nanoparticles on hydration reactivity and microstructure of C-S-H gel. Advances in Civil Engineering Materials, 2019, 8(3): 346–360

[41] Sharma U , Ali D , Singh L P . Formation of C-S-H nuclei using silica nanoparticles during early age hydration of cementitious system. European Journal of Environmental and Civil Engineering, 2019, 25(8): 1491–1502

[42] Singh L P , Zhu W , Howind T , Sharma U . Quantification and characterization of C-S-H in silica nanoparticles incorporated cementitious system. Cement and Concrete Composites, 2017, 79: 106–116

[43] Singh L P , Bhattacharyya S K , Shah S P , Mishra G , Sharma U . Studies on early stage hydration of tricalcium silicate incorporating silica nanoparticles: Part II. Construction & Building Materials, 2016, 102: 943–949

[44] Alhawat MAshour AEl-Khoja A. Influence of using different surface areas of nano silica on concrete properties. In: AIP Conference Proceedings. Melville, NY: AIP Publishing, 2019

[45] Maddalena R , Hall C , Hamilton A . Effect of silica particle size on the formation of calcium silicate hydrate [C-S-H] using thermal analysis. Thermochimica Acta, 2019, 672: 142–149

[46] Ltifi M , Guefrech A , Mounanga P , Khelidj A . Experimental study of the effect of addition of nano-silica on the behaviour of cement mortars. Procedia Engineering, 2011, 10: 900–905

[47] Ardalan R B , Jamshidi N , Arabameri H , Joshaghani A , Mehrinejad M , Sharafi P . Enhancing the permeability and abrasion resistance of concrete using colloidal nano-SiO₂ oxide and spraying nanosilicon practices. Construction & Building Materials, 2017, 146: 128–135

[48] Ibrahim K I M , Al-Tersawy S H . The hybrid effect of micro and nano silica on the properties of normal and high strength concrete. Journal of Mechanical and Civil Engineering, 2017, 14(4): 62–72

[49] Jalal M , Pouladkhan A R , Ramezanianpour A A , Norouzi H . Effects of silica nano powder and silica fume on rheology and strength of high strength self compacting concrete. Journal of American Science, 2012, 8(4): 270–277

[50] Kong D , Pan H , Wang L , David J , Corr Y Y , Shah S P , Sheng J . Effect and mechanism of colloidal silica sol on properties and microstructure of the hardened cement-based materials as compared to nano-silica powder with agglomerates in micron-scale. Cement and Concrete Composites, 2019, 98: 137–149

[51] Tobón J I , Mendoza Reales O , Restrepo O J , Borrachero M V , Payá J . Effect of pyrogenic silica and nanosilica on Portland cement matrices. Journal of Materials in Civil Engineering, 2018, 30(10): 04018266

[52] Papadakis V G , Tsimas S . Supplementary cementing materials in concrete Part I: Efficiency and design. Cement and Concrete Research, 2002, 32(10): 1525–1532

[53] Papadakis V G , Antiohos S , Tsimas S . Supplementary cementing materials in concrete Part II: A fundamental estimation of the efficiency factor. Cement and Concrete Research, 2002, 32(10): 1533–1538

[54] Wong H S , Razak H . Efficiency of calcined kaolin and silica fume as cement replacement material for strength performance. Cement and Concrete Research, 2005, 35(4): 696–702

[55] Ezziane K , Kadri E H , Siddique R . Investigation of slag cement quality through the analysis of its efficiency coefficient. European Journal of Environmental and Civil Engineering, 2011, 15(10): 1393–1411

[56] Potapov V , Fediuk R , Gorev D . Hydrothermal SiO₂ nanopowders: Obtaining them and their characteristics. Nanomaterials, 2020, 10(4): 624

[57] Potapov V VFediuk R S. Polymer nanocomposites based on nanosilica. In: Myasoedova V V, Thomas S, Maria H J. eds. Chemical Physics of Polymers Nanocomposites: Processing, Morphology, Structure, Thermodynamics, Rheology. Weinheim: Wiley-VCH GmbH, 2024

[58] Potapov V , Efimenko Yu , Fediuk R , Abhilash P . Efficiency of hydrothermal nanosilica for Portland cement concrete in comparison with other pozzolanic materials. Journal of Sustainable Cement-Based Materials, 2025, 14(11): 2349–2368

[59] Al-Hagri M , Döndüren M . Effect and optimization of incorporation of nano-SiO₂ into cement-based materials—A review. Challenge Journal of Concrete Research Letters., 2022, 13(1): 36–53

[60] Givi A , Rashid S , Aziz F N A , Salleh M A M . Influence of 15 and 80 nano-SiO₂ particles addition on mechanical and physical properties of ternary blended concrete incorporating rice husk ash. Journal of Experimental Nanoscience, 2013, 8(1): 1–18

[61] Chithra S , Kumar S S , Chinnaraju K . The effect of colloidal nano-silica on workability, mechanical and durability properties of high performance concrete with copper slag as partial fine aggregate. Construction & Building Materials, 2016, 113: 794–804

[62] Potapov V V , Efimenko Y V , Gorev D S . Determination of the amount of Ca(OH)₂ bound by additive nano-SiO₂ in cement matrices. Nanotechnologies in Construction, 2019, 11(4): 415–432

[63] Malhotra V.M. Supplementary Cementing Materials for Concrete. Ottawa: Canadian Government Publishing Centre, 1987

[64] Potapov V V , Efimenko Yu V , Fediuk R S , Gorev D S . Performances of concrete modified with hydrothermal SiO₂ nanoparticles and basalt microfiber. ACI Materials Journal, 2022, 119(5): 139–151

RIGHTS & PERMISSIONS

Higher Education Press