Cement mortar with enhanced flexural strength and durability-related properties using in situ polymerized interpenetration network

Abstract

The low flexural strength and high brittleness of cementitious materials impair their service life in building structures. In this study, we developed a new polymer-modified mortar by in situ polymerization of acrylamide (AM) monomers during the cement setting, which enhanced the flexural and durable performances of mortars. The mechanical properties, micro-and-pore structures, hydrated products, interactions between cement hydrates and polyacrylamide (PAM), and durability-related properties of the mortars were investigated comprehensively. Mortars with 5% PAM exhibited the best performance in terms of flexural strength among all the mixtures. The mechanical strength of cement pastes modified by in situ polymerization of AM monomers was significantly superior to those modified by PAM. The chemical interactions between the polymer molecules and cement hydrates together with the formation of polymer films glued the cement hydrates and polymers and resulted in an interpenetrating network structure, which strengthened the flexural strength. Reductions in porosity and calcium hydroxide content and improvement in capillary water absorption were achieved with the addition of PAM. Finally, the chloride resistance was significantly enhanced with the incorporation of PAM.

Keywords

acrylamide / in situ polymerization / interaction / porosity / durability

Introduction

Concrete as an extensively used building material has many advantages, such as high compressive strength, good durability and fire resistance, superior bonding strength between concrete and steel bar, and relatively low cost. However, concrete also has some distinct disadvantages, including low flexural strength, low toughness, and high brittleness. To overcome these limitations, fibers (steel fiber, PVA fiber, etc.) and polymers (polyvinyl alcohol, polyacrylamide (PAM), SBR latex, polyethylene glycol, etc.) have been applied in cement-based materials to reinforce their physical and durability-related performance. Fiber is an ideal raw material for enhancing the mechanical performance of cementitious materials. By controlling the formation and advancement of cracks, providing toughness, and offering flexural strength, cement-based materials can consume and absorb significantly more energy under dynamic loads (such as impact and blast) with the assistance of fibers in comparison with the materials without fibers [¹–⁴].

In contrast to fibers, polymers can also act as a flexible material for reinforcing cementitious materials. Polymers are appealing to many researchers owing to their superior properties and superior compatibility with cement. Polymers typically possess excellent properties, such as high flexible capacity, good adhesion, good self-healing, and strong corrosion resistance. Combinations of cement and polymers can adequately take advantage of the exceptional properties of both and obtain organic/inorganic composites with outstanding mechanical performance. Chen et al [⁵]. incorporated polyethylene glycol, PAM, and sodium polyacrylate in a slag-based geopolymer and found that the bending toughness coefficient of specimens with 0.6 wt% sodium polyacrylate increased by 53.7%, demonstrating a better modified effect in comparison with that of other polymers. The polypropylene was added to concrete to ameliorate the flexural strength, bonding strength between the aggregate and cement matrix, water, and shrinkage resistance [⁶,⁷]. Nevertheless, the polymer tends to aggregate with the addition of excess polymer in the cement [⁸–¹⁰], thereby resulting in defects in the cementitious materials. Cracks generate and advance rapidly toward these faults in the materials when exposed to a load. As a result, the mechanical performance of cement-based materials is significantly degraded owing to the existence of these drawbacks. Reductions in the compressive strength were observed in polymer-modified cement composites with the incorporation of superfluous polymers [¹¹–¹⁴].

Apart from the mechanical properties, the durable performances are important factors that affect the potential applications of polymer-modified cementitious materials. Polymer-modified cement-based materials can be utilized in marine concrete constructions owing to their high water resistance properties. While plain mortars exhibit poor resistance to aggressive media (chloride, sulfate, etc.), polymer-modified mortars exhibit excellent resistance to them. Therefore, polymer-modified mortars can effectively hinder the invasion of harmful media when exposed to an aggressive environment. Polymer latexes reduce water absorption and permeability and improve the resistance to carbonation and sulfate attack of cementitious materials [¹⁵–¹⁷]. The concrete composition with the styrene-acrylic ester polymer exhibits a higher resistance to sulfuric acid than the control mixture [¹⁸]. However, the aforementioned studies employed only the polymer to the cementitious materials in the form of an aqueous solution or a dispersible powder, while ignoring the synergistic effect between cement hydration and the polymer. The objective of this study is to disperse acrylamide (AM) monomers into the mortar to establish an interpenetrating network structure through in situ polymerization and the simultaneous hydration of cement for the sake of improving the mechanical and durable performances of mortars. The mechanical strength of cement pastes modified with AM monomers and PAM was determined. Different properties of mortars were compared with variations in PAM concentration. The microstructure, porosity, and chemical interactions between the cement hydration products and PAM chains were studied. The compressive strength and flexural strength were measured to assess the effect of the PAM fraction on the mechanical properties of the mortars. Furthermore, the durable properties of capillary water absorption and chloride penetration were investigated experimentally.

Materials and methods

Materials and mix proportions

In this work, normal Portland cement type P·I 52.5 was used for cement mortars, and the chemical compositions are presented in Table 1. The maximum grain size of the local river sand was 5 mm. AM, ammonium persulfate (APS), and N, N’, N', N'-tetramethyl-ethylenediamine (TEMED) were purchased from Macklin Chemical Reagent Co., Ltd. During the experiment, tap water was used to prepare the AM solution and mixing mortar.

Four batches of mortars were prepared to study the effects of PAM concentration on the mechanical and durability of the material, as listed in Table 2. RF denotes the reference mortar without PAM, and PAMX denotes that the concentration of PAM added to the samples is X%; for example, PAM1 indicates that the concentration of PAM is 1%. The monomer AM was added to cement mortar at 1%, 3%, and 5%. The APS and TEMED were used to initiate and accelerate the polymerization of AM at dosages of 0.2 wt% and 0.32 wt%. The amount of AM added is based on the cement content.

Preparation of mortar test specimens

The AM, APS, and TEMED were first added to the water and stirred for 5 min to form the AM solution. The cement and sand were dry mixed in a Hobart mixer for 1 min and then mixed with the prepared AM solution for another 3 min to obtain well-distributed mixtures. During cement hydration, a polymerization reaction occurred with the assistance of the initiator APS and accelerator TEMED, guaranteeing the formation of cement hydrates and PAM simultaneously. Thereafter, the fresh mixtures were cast in 40 mm × 40 mm × 160 mm, 40 mm × 40 mm × 40 mm, and 100 mm × 100 mm × 100 mm plastic molds and compacted on a vibration table. Cement paste (W/C = 0.6) with different dosages of PAM (0%, 1%, 3%, and 5%) was also manufactured to perform a Fourier transform infrared spectroscopy (FTIR) test. After casting, the samples were sealed with a plastic film membrane at room temperature (25°C) for 24 h and then demolded. Afterward, the specimens were cured in a curing box at a temperature of 20±1°C and relative humidity≥95% for 7 and 28 d. Three samples were tested to determine the flexural strength, while three samples were subjected to a compressive strength test.

Test samples

Compressive and flexural strength tests

The compressive and flexural strengths of each batch of mortars were measured at 7 and 28 d using 40 mm cubes and 40 mm × 40 mm × 160 mm prisms, respectively. Compressive strength testing was performed using a universal testing machine (INSTRON 3300) at a loading rate of 1 mm/min. The flexural strength testing was performed using a Mechanical Testing & Simulation (MTS) machine with a capacity of 50 kN, subjected to a loading rate of 0.01 mm/s. The corresponding stress can be calculated based on the ASTM D790-10 standard [¹⁹].

Scanning electron microscopy

Samples of 28 d curing were used for scanning electron microscopy using Zeiss Zigma field emission scanning electron microscope (FESEM). The specimen preparation included immersing the crushed samples in ethanol to repress cement hydration, along with drying at 60°C in a vacuum drying oven until the water was exhausted. Then, the samples were polished to a relatively smooth surface and coated in gold.

Brunner-Emmet-Teller measurements

Brunner-Emmet-Teller (BET) measurements were employed to measure the pore structures of the mortars. Before the crushed specimens were dried at 60°C in a vacuum drying chamber for 2 d, the hydration of the samples was terminated by using anhydrous ethanol at 28 d. After the samples were prepared, an examination was performed.

Fourier transform infrared spectra and thermogravimetric analysis

Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), and X-ray diffraction (XRD) tests were conducted on powder specimens. The cement paste after 28 d curing was milled to powder and passed through a sieve with 300 mesh, followed by drying at 60°C for 24 h before testing.

Capillary water absorption and chloride diffusion

After 28 d curing, mortars of 100 mm × 100 mm × 100 mm were dried in a ventilated oven at 60°C until they reached a constant weight. When the weight of the samples was constant, they were removed and cooled to ambient temperature. Then, the mortars were sealed with wax, except from the top and bottom sides. The bottom surface was placed in direct contact with water, where the water level was approximately 3–5 mm above the bottom of the specimens. The amount of water uptake was determined by weighing the samples after 0.5, 1, 2, 4, 8, 12, 24 h, 2, 3, 7, 14, and 28 d. The capillary water absorption coefficient could be obtained by fitting the profiles of water absorption.

Chloride diffusion in the mortar was measured on three samples for 7, 14, and 28 d. The prisms were sealed with paraffin on five sides except for the bottom side after 28 d curing and were immersed in 3% sodium chloride solution at an ambient temperature of 25°C. The depth of chloride invasion was determined using the digital image process by taking photographs of the cross-sections of the mortars that were sprayed with 0.1 mol/L silver nitrate solution after 7, 14, and 28 d. Thereafter, the photographs were analyzed by Image J, and the penetration depth h could be calculated by h = S/d, where S is the central area of white precipitation of AgCl and d is the half width of the sample, as illustrated in Fig. 1.

Results and discussion

Mechanical properties

Figure 2(a) depicts the profiles of the compressive strength with the variation of the PAM fraction. A descending trend of the compressive strength is observed as the PAM content in the specimens increases. The control group manufactured without introducing PAM has a compressive strength of 63.43 and 82.08 MPa for 7 and 28 d, respectively. The compressive strength of the mixture consisting of 1% PAM is 59.85 and 76.25 MPa for 7 and 28 d, which are approximately 5% and 7% lower than those of the reference group, respectively. The slight reduction in the compressive strength indicates that incorporating a small amount of PAM has no adverse effect on the development of the compressive strength for mortars. However, with the further introduction of PAM, a sharp drop in compressive strength is observed, particularly at a content of 5% with a 28 d compressive strength of 50.58 MPa, which corresponds to a 38% reduction in comparison with that of the control mixture. The reduction in the compressive strength can be attributed to the delayed cement hydration in comparison with that of the control mixture and the relatively low stiffness of the PAM.

The addition of PAM also has a negative influence on cement hydration. From Fig. 2(a), it is notable that the compressive strength has a lower growth ratio for specimens with PAM in comparison with that of mortars without PAM after 28 d curing. The increasing ratio of compressive strength in the control mortar is approximately 29.4% from 7 to 28 d, whereas for the samples with 1%, 3%, and 5% PAM, the increasing ratios are 27.4%, 28.8%, and 23.3%, respectively. The reduction in the increasing ratio of compressive strength indicates that introducing PAM in the cement matrix inhibits the hydration of cement particles. After the prepared AM solution is added to the mortar slurry, polymerization of AM also occurs to produce considerable PAM, which wraps around the surface of cement grains and restrains the cement grains from coming in contact with external water [²⁰]. Therefore, the hydration of cement is restricted, leading to a relatively low compressive strength for the PAM-modified cement mortar.

The influence of the PAM content on the flexural strength of the mortar is depicted in Fig. 2(b). As the PAM content increases from 0% to 5%, the flexural strength demonstrates a growing trend. For example, mortars fabricated with 5% PAM have a flexural strength of 9.28 MPa at 28 d and that of the plain mortar is 6.77 MPa, where the growth of flexural strength is more than 37%. The polymer of PAM can react with metallic cations from cement hydration to generate gel-like polymer films that cover the cement hydrates and unhydrated cement particles to create a continuously and consistently interweaving network structure [²¹], which is confirmed by SEM images in Section 3.2. The short polymer chain formed by the polymerization of AM is inlaid between the hydrated products and cement particles, and connects the hydrated products and cement particles to construct an interpenetrating network structure (seen in SEM images), making a remarkable contribution to the enhancement of flexural strength. Samples with 1% PAM and without PAM have a relatively equal flexural strength as the curing age ranges from 7 to 28 d, suggesting that the addition of 1% PAM has a limited impact on the flexural strength. Moreover, the filling effect of the polymer compacts the interfacial transition zone (ITZ) [²²,²³] and the content of calcium hydroxide at ITZ is reduced owing to the chemical interaction, modifying the ITZ’s performance [²⁰]. Accordingly, the interface bonding strength is improved, leading to reinforcement of the flexural strength.

Figure 3 depicts the comparison of the mechanical performances of cement pastes modified by in situ polymerization of AM monomers and by PAM. We can observe that the mechanical strength of the cement pastes modified by in situ polymerization of AM monomers is significantly superior to that of samples modified by PAM, demonstrating that in situ polymerization of AM monomers in the cement matrix is more conducive to reinforcing the mechanical performance of cementitious materials. The AM polymerizes to generate an interpenetrating polymer-network system under cement hydration. The synergy of the cement hydration and the polymerization reaction of AM significantly enhances the interaction between the cement hydrates and the PAM and combines the stiffness of the cement and flexibility of the polymer, thereby making the polymer-modified cement stiff and tough. The formation of the polymer-network system by in situ polymerization endows the cement with additional deformation capacity, which reinforces its ability to bear bending load and finally improves its flexural strength. The uniform distribution of PAM and the strong interaction between the cement hydrates and the PAM effectively limit the decrease in the compressive strength caused by the softness of PAM.

Microstructure of mortar

The microstructures of the blank mortar and PAM-modified mortar are illustrated in Fig. 4. It is apparent that the surface of the cement hydrates of blank mortar has more edges and corners and is smoother, as depicted in Figs. 4(a) and 4(d). However, the PAM-modified mortar has hydrated products with a rough surface that is covered by the thin film of PAM, and the edges and corners of the hydrated products are obscure, as depicted in Figs. 4(b) and 4(e). A polymer-like film was detected in the cement matrix, and this film proved to be the PAM film owing to the existence of C through energy dispersive spectroscopy (EDS) (see Figs. 4(c) and 4(f)), confirming that the polymerization reaction of AM occurs during the process of cement hydration. More importantly, the needle-like hydrated products were covered by flexible PAM gel to build the fiber-like products, bridging the separate hydrated products. PAM molecules and calcium cations produced from cement hydration can react with each other to form the chemical bonds of COO-Ca-OOC and HO-Ca-OOC, which produces crosslinked PAM gels [²⁴]. Therefore, the polymer gel film formed by the chemical interaction between cement hydrates and PAM adheres to hydrated products and cement grains, establishing an interpenetration network structure, which promotes the improvement of the mechanical performance of mortars, particularly the flexural strength.

FTIR analysis results

Figure 5 depicts the FTIR analysis results of the polymer-modified cement pastes after 28 d. In comparison with the neat paste, a few new absorption bands are observed in the specimen incorporating PAM. The band at 3643 cm⁻¹ corresponds to the stretching vibrations of the OH group, which disappears in the PAM-modified cement mix. The variations in the absorption band at approximately 3450 cm⁻¹ are attributed to the chemical interaction between PAM and the ettringite phase of the cement hydrates [²⁵]. These bands move toward the right in contrast to the plain paste, which may result from the formation of new crosslinks between the hydrated products of cement and organic phase. The existence of these new crosslinks is conducive to enhancing the mechanical performance of samples, particularly with regard to flexural strength. The wavenumber at approximately 1639 cm⁻¹ corresponds to the stretching of C= C, and appears with the introduction of PAM. In comparison with the neat paste, the appearance of the wavenumber at 2025 cm⁻¹ may be due to the vibration of S= O in the PAM-modified cement, which can be ascribed to the introduction of APS. The wavenumber at approximately 871 cm⁻¹ may be derived from the Al-O stretching vibration of the aluminate phase. The wavenumber at 619 cm⁻¹ is the N-H absorption peak and emerges with the addition of PAM, indicating the presence of an amide ionogen.

Thermal and XRD analysis

In Fig. 6, the weight loss of the developed mortars is plotted against the temperature. The variation in PAM content influences the decomposition of bound water and C-S-H. The introduction of PAM also influences the decarbonation peaks, wherein the weight loss curves shift to the bottom with an increase in PAM content. As the content of PAM increases, the temperature of the decomposition and dehydration of calcium hydroxide shifts to the left, implying that the thermal properties of heat capacity deteriorate after the incorporation of PAM. As PAM increases, the Ca(OH)₂ (CH) content (mass loss from 400°C to 450°C) demonstrates a descending trend, where specimens with 1% PAM exhibit a CH content equivalent to that of the control mixture, indicating that adding a small amount of PAM has a limited impact on cement hydration. Samples with 3% and 5% PAM exhibit considerably lower CH content in comparison with that of the reference sample. The decreasing trend in the concentration of portlandite can also be qualitatively verified in Fig. 7, where the peak intensity of CH decreases with increasing PAM. This can be ascribed to the fact that PAM can react with Ca²⁺ produced by cement hydration and hence reduces the CH concentration. The decrease in CH also suppresses the hydration of cement, resulting in a low mechanical strength of mortar at an early age.

Pore size distribution of PAM-modified mortars

Capillary pores (10–10000 nm) are critical for material strength and harmful ion (such as chloride and sulfate ions) transport in cementitious materials. The variations in the cumulative pore volume and BET surface area vs. the content of PAM are plotted in Fig. 8. The control mortar exhibits the largest cumulative pore volume and the highest pore volume (0.024 cm³/g) among the mixtures. The cumulative pore volume of samples with PAM is lower than that of the reference mixture owing to the filling effect of the polymer and the formation of network structures between the polymer and cement hydrates. However, when 3% and 5% PAM are incorporated into the mortar, the cumulative pore volume is greater than that of the specimens with 1% PAM. This can be ascribed to an increased entrained air volume as the PAM content increases. Furthermore, the number of pores occupied by the polymer for mortars with 3% and 5% PAM is greater than that of mortars with 1% PAM. The samples were dried before the BET tests, which caused the polymer inset in the cement matrix to lose water and shrink acutely, thereby augmenting the pore volume of the cement mortar modified with 3% and 5% PAM. The polymer-modified specimens have a lower surface area in comparison with that of the control sample, further demonstrating that the existence of PAM fills the large pores in mortars and compacts the inner structure.

Water absorption

The ability of water absorption can be used to characterize the internal compactness and durability of cementitious materials because most of the aggressive substances are transported into cement-based materials with water. The amount of water uptake vs. time and the capillary absorption coefficient vs. the PAM content are depicted in Fig. 9, where the average values are obtained for three samples for each mix. Notably, the mass of water absorption exhibits a sharp increase due to the contact with water for 24 h, as seen in Fig. 9(a). The mass increase in water absorption grew gradually from 24 h to 14 d and reached a constant value at 14 d. Furthermore, the reference mixture presented the highest water absorption value in comparison with those of samples containing PAM within 24 h of water exposure, whereas specimens with 5% PAM exhibited the lowest water absorption value in the same time range. The moisture absorption value of the control group was 6.99 kg/m², which was approximately 28% higher than that of mixtures with 5% PAM at 24 h. The PAM-modified mortar has better resistance to water penetration, which can be related to the filling effect of the polymer. After the dried sample came in contact with water, the PAM in the cement matrix absorbed water immediately, leading to the expansion of the polymer, which blocked the pores, increased the tortuosity of the pores, and reduced the connectivity between pores. Consequently, it is difficult for water to be transported into the mortars, resulting in a reduction in water penetration.

The profile of the water absorption coefficient vs. PAM content for cement mortars is depicted in Fig. 9(b). The water absorption coefficient was determined by calculating the slope of the water uptake curves (0 to 24 h). It is apparent that the water absorption coefficient decreases significantly with the increment in PAM, which has a linearly dependent coefficient of 0.94. With the addition of 5% PAM, the capillary water absorption coefficient of the samples was approximately 24% lower than that of the blank mixture, indicating that the introduction of PAM strengthens the ability of the samples to resist water penetration.

Chloride diffusion

The chloride penetration depth of samples exposed to 3% sodium chloride solution at 7, 14, and 28 d is depicted in Fig. 10. As expected, a clear decline in the chloride penetration depth was observed for all mortars as the PAM content increased from 0 to 5%. Incorporations of 1% and 3% of PAM reduced the 28 d penetration depths to 2.2 cm and 1.8 cm, respectively. Incorporation of 5% PAM further reduced the depth to 1.5 cm at 28 d. Samples with 5% PAM exhibited a chloride penetration depth of 1.5 cm at 28 d, which was approximately 37% lower than that of plain mortar. The reduction in chloride penetration depth indicates that the introduction of PAM significantly contributes to the chloride resistance capacity. The enhancement in the resistance to chloride penetration by adding PAM may be attributed to the refinement in the porosity that results from the filling effect of polymers in mortars. Moreover, the polymer expands rapidly when exposed to water, plugging the pores and restraining the diffusion of chloride ions. The polymer also compacts the ITZ between the cement and fine aggregate [²⁰], impeding the migration of chloride ions.

Conclusions

Based on the obtained results, the following conclusions can be drawn.

1) In situ polymerization of AM monomers produced an organic-inorganic network in the cement matrix, limiting the advancement of microcracks. The PAM and hydrated products of cement can react with each other to produce crosslinks, resulting in an improvement in flexural strength. A decreasing trend in compressive strength was observed with an increase in PAM.

2) The interaction between Ca²⁺ and PAM reduced the content of calcium hydroxide, influencing the hydration of cement.

3) The porosity decreased because PAM refined the pore structure of mortars. The existence of PAM retarded the early hydration of cement, which can be reflected in the growth rate of the mechanical strength.

4) The capillary water absorption coefficient decreased sharply as the PAM content increased from 0 to 5%, where samples with 5% PAM exhibited the lowest water absorption coefficient of 1.15. Similarly, the chloride penetration depth exhibited a descending trend with an increase in PAM concentration.

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