Quantification of hydration products in cementitious materials incorporating silica nanoparticles

Abstract

In the present work, silica nanoparticles (30-70nm) were supplemented into cement paste to study their influence on degree of hydration, porosity and formation of different type of calcium-silicate-hydrate (C-S-H) gel. As the hydration time proceeds, the degree of hydration reach to 76% in nano-modified cement paste whereas plain cement achieve up to 63% at 28 days. An influence of degree of hydration on the porosity was also determined. In plain cement paste, the capillary porosity at 1hr is ~48%, whereas in silica nanoparticles added cement is ~35 % only, it revealed that silica nanoparticles refines the pore structure due to accelerated hydration mechanism leading to denser microstructure. Similarly, increasing gel porosity reveals the formation of more C-S-H gel. Furthermore, C-S-H gel of different Ca/Si ratio in hydrated cement paste was quantified using X-ray diffractometer and thermogravimetry. The results show that in presence of silica nanoparticles, ~24% C-S-H (Ca/Si<1.0) forms, leading to the formation of polymerised and compact C-S-H. In case of plain cement this type of C-S-H was completely absent at 28 days. These studies reveal that the hydration mechanism of the cement can be tuned with the incorporation of silica nanoparticles and thus, producing more durable cementitious materials.

Keywords

degree of hydration / porosity / calcium-silicate-hydrate (C-S-H) / silica nanoparticles

Introduction

The hydration of Portland cement is a complex phenomenon. It needs to be estimated as being related to porosity, heat of hydration, strength development and chemical shrinkage [ 1]. Powers and Brownyard developed a model that can be used to describe hardened cement paste (hcp) at different w/c ratios and degrees of hydration [ 2]. This model is largely based on non-evaporable water measurements and water vapor sorption isotherms. From the experiments they estimated densities for several components in hcp and were able to describe it from the volumetric point of view. According to Powers and Brownyard about 23% of water (by weight of cement) is required for complete chemical reactions [ 3]. Earlier studies showed that the determination of non-evaporable water was most accurate method to better understand the hydration chemistry of the cementitious materials. The hydrated cement is a porous mass containing both hydrated and unhydrated cement particles along with several different types of water. The bound (combined or non-evaporable) water is chemically combined within C-S-H and forms a definite part of hydrated compounds. Microstructure of cement paste consists of capillary and gel pores. Capillary pores are long continuous pores that exist within the unhydrated cement paste and gel pores are of very small dimension that occur within the reaction products i.e., calcium-silicate-hydrate (C-S-H) gel [ 4- 6].

For producing High Performance Concrete, the use of supplementary cementitious materials (SCM), such as silica fume, ground granulated glass blast-furnace slag and fly ash is well recognized [ 7]. The performance of today’s Ultra High Performance Concrete can be further improved by nanotechnological approaches. The most commonly used nanomaterials in cement are nano-silica, nano-titania, nano-alumina, carbon nano-tubes etc [ 8]. Among all, nanosilica due to its high reactivity has been proved as an effective additive for accelerating cement hydration [ 9]. Several studies [ 10, 11] show that the application of nano-silica in cementitious system increase the strength peoperties, especially at early age of hydration. Thomas et al. [ 12]. reported that hydration of tricalcium silicate is accelerated by addition of nanosilica, based on the assumption that nucleation process took place on silica surface, which serves as C-S-H seeds for further acceleration in hydration mechanism. The microstructure examinations have verified the finer structure and the drastically reduced porosity for the specimen containing nano-silica powder. Nano-silica has much finer particle size and greater pozzolanic reactivity as compared to silica fume [ 13], thus act as seeds and pozzolan more effectively [ 13, 14]. The accelerating effects of colloidal nano-silica on the rate of C₃S phase dissolution; C-S-H gel formation and silica polymerisation in cement paste hydration were also studied [ 15]. In cement-based materials, fine pores and very large pores cannot be observed by image analysis and mercury porosimetry correctly [ 16, 17]. From the measurements of the evaporable and nonevaporable water contents in the cement paste, the degree of hydration and porosity are estimated using relations based on known reaction stoichiometry and volumetric proportions of the reaction products [ 18]. The addition of nano-silica cause a refinement of the microstructure and induced the precipitation of small-sized C-S-H gel, probably having a higher stiffness and lower Ca/Si ratio [ 19]. C-S-H is a principal binding agent in the cement paste and responsible for strength and shrinkage. It is generally accepted that the Ca/Si ratio of C-S-H in mature pastes varies between 1.4 and 2.0, has an average about 1.7 [ 20]. Addition of additives to Portland cement results in the formation of secondary C-S-H, which could possess a lower Ca/Si ratio [ 21]. In the present paper the influence of silica nanoparticles on degree of hydration and porosity was estimated through non-evaporable water and compared with silica fume. For the durability point of view a new approach for the quantification of C-S-H of variable Ca/Si ratio was conducted using Thermogravimetry and X-ray diffraction techniques.

Experimental

Materials and methods

In the present study, 43 grade OPC, Type I cement and a high quality commercial grade silica fume were used (Table 1). Further, silica nanoparticles (30-70nm), prepared in laboratory having specific surface area 116 m²/g were used [ 22]. 3% silica nanoparticles (NS) and silica fume (SF) were added to cement paste with w/c 0.35 (constant for all mix.). Prior to mixing NS with cement matrix, NS and water were sonicated for 30 min using a bath sonicator till the solution became milky, followed by addition into cement and mixed thoroughly. The cement-NS slurry was then mixed for 5 min before casting into small balls. Water curing of specimens was continued at 20±1°C up to 28 days.

To evaluate non-evaporable water (NEW) content, the core of the samples was crushed and immediately immersed into acetone for 2 h to arrest the hydration at different time intervals. The powdered samples were dried in oven at 110°C for 1h. Out of this dried sample about 2g was taken for ignition at 1000°C for 1h. All chemically bound water was assumed to be lost on ignition at 1000°C and corrected for loss on ignition of unhydrated cement. NEW was calculated as difference in weight between 110 and 1000°C. For each experiment, triplicate samples were tested and their average value is reported. The degree of hydration (DOH) is assumed to be proportional to the amount of chemically combined water into hydration products and was calculated by using following Eq. (1) [ 23].

(1) DOH(\%)={\displaystyle \frac{NEW(t)}{W_c\times \mathrm{0.23}}}\times 100,

where, W_c is the weight of cement content taken. The pore structure of cement-based materials is one of the most important characteristics and strongly influences the mechanical behavior and transport properties. The durability of cement-based materials (depends on permeability properties) is affected by the pore structure. Power and Brownyard defined the total porosity as a sum of capillary and gel porosities. Therefore, the total porosity of cement paste at different time intervals was calculated using the following Eq. (2) [ 23]:

(2) P_{\mathrm{Tot}}(\%)={\displaystyle \frac{{\displaystyle \raisebox{1ex}{$w$}\!\left/ \!\raisebox{-1ex}{$c$}\right.}-\mathrm{0.17.}\alpha }{{\displaystyle \raisebox{1ex}{$w$}\!\left/ \!\raisebox{-1ex}{$c$}\right.}+\mathrm{0.32}}}\times 100,

where, P_Tot is the total porosity of cement matrix, w/c is water-to-cement ratio, a is degree of hydration and 0.32 is specific volume of cement (cm³/g). The expression for calculating gel porosity is given in Eq. (3) [ 23]:

(3) P_{\mathrm{g}\mathrm{e}\mathrm{l}}(\%)={\displaystyle \frac{\mathrm{0.19}\cdot \alpha }{{\displaystyle \raisebox{1ex}{$w$}\!\left/ \!\raisebox{-1ex}{$c$}\right.}+\mathrm{0.32}}}\times 100.

Further, the C-S-H quantification of different Ca/Si ratio was carried out by using thermogravimetric (TGA) and X-Ray diffraction (XRD) techniques. Powder X-ray diffraction (Rigaku, DMax-2200) with a X-ray source of Cu Ka radiation (l = 1.5418 Å) was used. The scan step size was 0.02°, and in the 5 to 80° (2q) range. The X-ray tube voltage and current were fixed at 40 kV and 40 mA, respectively. A standard database (JCPDS) was used to identify different type of C-S-H in cement paste. Thermogravimetry analysis was performed on Perkin Elmer analyzer at heating rate of 10°C min^-1 from the temperature range of mass loss between 110°C and the temperature at which CH loss begins (400°C) was considered as indicator of the loss of water from various types of C-S-H formed in hydrated portland cement. The quantification of C-S-H content through TGA was calculated by using the Eq. (4) [ 24, 25]:

(4) W\left(\mathrm{C}-\mathrm{S}-\mathrm{H}\right)\left(\mathrm{\%}\right)=WL\left(\mathrm{C}-\mathrm{S}-\mathrm{H}\right)\times {\displaystyle \frac{MW\left(\mathrm{C}-\mathrm{S}-\mathrm{H}\right)}{molesofwater\times MW\left(\mathit{H}\right)}}\mathrm{.}

where WL (C-S-H) is the weight loss corresponding to C-S-H dehydration, MW (C-S-H) and MW (H) are the molecular weight of variable CSH and water, respectively.

Results and discussion

Non-evaporable water content

To study the influence of silica nanoparticles and silica fume on cement hydration, NEW was determined on ignition basis. Cement without any additive combines 0.04 g of water at 1h, whereas 0.08 and 0.13 g of water combined with SF and NS due to accelerating effect on hydration. As water comes in contact to cement grains, dissolution of ions starts and these ions combine water to form hydration products during pre-induction period. NS react readily within the first hours and provide more sites for condensation of SiO₄ monomers in the formation of C-S-H network due to high specific surface area. Therefore, NEW is found to be greater in NS incorporated cement paste. As the induction period starts hydration becomes slow as evident with the amount of NEW in cement at 2h, 3h and 4h is 0.05, 0.04 and 0.05 g, while with NS is 0.14, 0.15 and 0.15, respectively (Fig. 1). The non-evaporable water is almost constant during this period but still higher in NS modified cement paste. Further, during acceleration period, the amount of NEW increased with the hydration time. Thereafter, at 1d NEW in plain cement, cement with SF and NS was 0.13, 0.23 and 0.29 g respectively. The percentage rise of NEW in NS incorporated cement paste was found higher (~55%), while in SF admixed cement it was up to ~43%, thereby, confirming to our earlier studies that NS accelerates the hydration mechanism, especially at early ages [ 26]. With the progress of hydration time, NEW increases up to 0.29 g in plain cement, 0.32 and 0.35 g in SF and NS incorporated cement paste at 28d, respectively. In presence of NS more hydration products were formed, therefore more NEW was observed. Further, for the quantification of hydration products degree of hydration was determined.

Degree of hydration

DOH is the fraction of cement completely reacted with water relative to total amount of cement in sample and quantitatively determined the total amount of hydration products formed including CH, C-S-H gel, ettringite etc. in cement. For determining the influence of NS at early age hydration, DOH was calculated from 1h to 6h continuously at the intervals of an hour. Figure 2, shows that at 1h, DOH in plain cement is 8.6%, whereas in SF and NS added cement, it is 17.3% and 28%, respectively. This initial rise in hydration product quantity is due to early hydration effect of NS. Further, with the increase on hydration time, DOH increased at all the ages. During induction period hydration become slow due to the layer of initial hydration products form around the cement grains, which prevents further dissolution of cement grains. Therefore, in plain cement, DOH becomes constant (~10.8%) from 2h to 5h, whereas in NS incorporated cement the DOH increase significantly (30%) at 2h and 39% at 5h in induction period. This increase in DOH continuously attributed to the nucleating effect of NS, where NS act as nucleation sites for the deposition of more hydration products. The hydration process accelerates in the presence of NS and more hydration products are quantified as 63% and 76% at 1d and 28d, respectively.

Porosity measurement in plain and blended cement paste

The volumes of capillary and gel pores are calculated on the basis of degree of hydration. Figure 3, illustrates that in plain cement at 1d the total porosity is 44.6%, whereas in SF and NS added cement, it is 38.7% and 35.2%, respectively. NS reduces porosity of cement matrix due to the formation of additional hydration products leading to a dense microstructure. Gel pores are associated within interlayer of C-S-H and consist the water adsorbed over the surface of C-S-H. As the hydration progresses, the gel porosity in plain and NS modified cement was calculated as 8.7% and 19.7%, respectively, at 1 day (Fig. 4). This increase in gel porosity at early stage of hydration is of significance as more amounts of hydration products i.e., C-S-H gel are formed. On the other hand, the volume of capillary pores reduces (15%) more significantly in NS incorporated samples i.e., 15% due to acceleration rate of hydration of cement grains. In plain cement it is found 35% at 1 day. This observation revealed that NS accelerate the hydration mechanism and nucleate more hydration sites producing more hydration products are forming and occupying the capillary pore volume. At the same time, during pozzolanic reaction, CH is observed to be consumed regularly by NS and converted into additional C-S-H. Hence more C-S-H or more gel porosity was observed [ 26, 27].

Quantification of C-S-H through XRD/TGA

The influence of silica nanoparticles on Ca/Si ratio of C-S-H was observed through XRD and TGA studies. Various types of C-S-H were identified by XRD followed by their quantification was carried out using TGA data. In plain cement, C-S-H of Ca/Si ratio 1.5 (C_1.5SH), 2.0 (C₂SH_0.35, C₂SH) and 0.8 (tobermorite) were formed at 1d of hydration (Fig. 5(a)). As the hydration proceeds, the composition of C-S-H change at 28d and new peaks of C-S-H are observed, corresponding to the C-S-H of Ca/Si ratio 2.0 (C₂SH₃) and 1.5 (C₅S₃H₂, C_1.5SH), respectively (Fig. 5(b)). Other than the C-S-H peaks, an intense peak of CH are also observed at 28d, revealing the crystallization of calcium hydroxide. Similarly, C-S-H of different Ca/Si ratio is identified with NS & SF. A noticeable change in Ca/Si of C-S-H is also observed. With the addition of SF, few more peaks of C-S-H are observed (Fig. 6). In NS added cement paste, the peaks of C-S-H (C_1.5SH, C₄S₅H₂, Rosenhahnite), as illustrated in Fig. 6(a) are observed. In the presence of additives, different reactions (pozzolanic, nucleation and polymerization) take place concurrently. These reactions are responsible for the change in Ca/Si of C-S-H, therefore at 28d, the C-S-H with lower Ca/Si ratio is identified as tobermorite, C_1.5SH, Jennite and C₂SH₃ etc (Fig. 6(b)). Further, these C-S-H are categorised in three ranges<1.0, 1.0-1.5 and 1.5-2.0, on the basis of Ca/Si ratio. From TGA data, quantification results show that in plain cement paste, the amount of C-S-H is 7%, 10% and 83% with Ca/Si ratio<1.0, 1.0-1.5 and 1.5-2.0, respectively. Whereas in nano-modified cement the amount of C-S-H varies as 51% and 49% with<1.0 and 1.0-1.5,respectively, at 1d (Table 2). These results showed that NS favors the formation of C-S-H of lower Ca/Si ratio, which is desirable for high strength and durable structures.

Finally, as the hydration proceeds up to 28 days, in cement pastes with ~73% and ~24% of C-S-H are formed in the range of Ca/Si ratio 1.0-1.5 and 1.5-2.0 (Table 2). Similarly, with SF it is 79% and 21%. On the other hand, in NS incorporated cement 24%, 60% and 15% of C-S-H with Ca/Si ratio<1.0, 1.0-1.5 and 1.5-2.0 is formed. It is observed that in the nanomodified cement paste, C-S-H gel of lower Ca/Si ratio is higher as compared to the plain cement, which attributes the formation of more polymerized and compact C-S-H. Therefore, the hydration mechanism of the cement can be tuned with the incorporation of nanoparticles and thus, producing more durable and sustainable cementitious materials.

Conclusion

Results of the present study reveal that NS accelerate the hydration mechanism, thus more hydration products were formed which results in higher combined water. NEW in plain cement sample is observed 0.29 g whereas in 3% SF and NS incorporated cement as 0.34 and 0.35 g, respectively at 28 days. At 3% addition of NS degrees of hydration is achieved as 76% at 28d. The growth of the hydration products reduces the size of the capillary pores. More gel pores are found within NS incorporated samples, which signify the formation of additional C-S-H. Results of quantification of C-S-H of variable Ca/Si ratio highlight that in the presence of NS C-S-H of lower Ca/Si ratio obtained as ~24% with Ca/Si ratio (<1.0). In case of plain and SF added cement this type of C-S-H was almost absent at 28 days, which attributes the seeding effect of the NS and/or by the pozzolanic reaction, leading to the formation of polymerised and compact C-S-H. Overall, these results attributed that the hydration mechanism of the cement can be tuned with the incorporation of nanoparticles to produce more durable and sustainable cementitious materials.

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