The influence of nano-SiO₂ emulsion on sulfate resistance of cement-based grouts

Nano-SiO₂, with a specific surface area of 140 m²/g and an average particle size of 10 nm, was supplied by Shanghai Yuanjiang Chemical Co., Ltd. (Shanghai, P. R. China). Portland cement produced by Taizhou Yangwan Conch Cement Co., Ltd. (Taizhou, P. R. China) was utilized in these studies. The chemical composition and primary performance indicators of the cement are presented in Tables 1 and 2, respectively. The granulated blast furnace slag powder was purchased from Yangzhou Hengrun Marine Heavy Industry Co., Ltd. (Yangzhou, P. R. China). Fly ash was obtained from Jiangsu Huadian Energy Co., Ltd. (Yangzhou, P. R. China). The chemical compositions of these two mineral admixtures are also presented in Table 1. The sand was natural river sand with a fineness modulus of 2.75, a bulk density of 1425 kg/m³, and an apparent density of 2580 kg/m³. The water used for casting and curing was potable water with a pH of 7.

Polycarboxylate superplasticizer (P540) was obtained from Shanghai Chenqi Chemical Technology Company (Shanghai, P. R. China). The defoaming agent (P80) was purchased from MÜNZING Chemie Corporation (Münzingstraße, Germany). The expanding agent (YH-S) was obtained from Tianjin Weihe Technology Admixture Company (Tianjin, P. R. China). Methacryloxy propyl trimethoxyl silane (KH570) was supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, P. R. China). Sodium sulfate was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, P. R. China). All of the chemical reagents mentioned above were used directly without any further treatment, unless otherwise specified.

A nano-SiO₂ emulsion was prepared according to our previous work [27], which was reported as follows: 2 g of KH570, 10 g of P540, 5 g of nano-SiO₂, and 50 g of distilled water were added to a 100-mL glass beaker, stirring at 300 rpm for 2 h and then sonicated for 30 min to obtain the nano-SiO₂ emulsion.

To analyze the impact of nano-SiO₂ emulsion on the sulfate resistance of cement-based grouts, four samples with varying nano-SiO₂ particles or emulsions were studied, as detailed in Table 3.

The specimens were prepared with a water–cement ratio of 0.22, and a blend of Portland cement, slag powder, fly ash, and river sand, along with PCE, YH-S, and P80 was mixed at 60 rpm for 5 min using a cement mortar mixer. An emulsion of nano-SiO₂ was added to a mixer with the appropriate amount of water. Next, solid components were poured into the mixer and mechanically mixed at 60 rpm for 3.5 min. After 24 hours of casting in molds at 97% relative humidity and 20°C, all samples were then demolded and cured in a water tank for 28 days.

The sulfate resistance test was performed according to standard GB/T 50082-2019 and was reported as follows: after 28 days, the samples were submerged in separate plastic tanks containing with 5 wt% or 10 wt% sodium sulfate solutions until tested at 20°C. Also, every 15 days, the sodium sulfate solution was changed.

Before compressive strength testing, the mass of all compressed samples in a surface-dry state was tested to calculate the weight loss of each sample. The compressive strength test was performed according to standard GB/T 17671-2021 and measured using a WHY-300/10 microcomputer control universal testing machine (Shanghai Hualong Testing Instrument, Shanghai, China). X-ray diffraction (XRD) patterns were measured using a Y500 diffractometer (Dandong, China) with Cu Kα radiation at 40 kV and 140 mA from 10° to 75°. Scanning electron microscopy (SEM) images were performed using a S-4800 scanning electron microscope (Hitachi, Japan), and the fracture surface was sputter-coated with gold before being observed.

High concentrations of corrosion ions are foundin sulfate-rich environments. These ions can react with hydration minerals of hardened grout matrix, which changes the type and quantity of minerals and degrades long-term stability [8]. The compressive strength of cement-based grouts determined after 28, 56, and 90 days immersed in 5% and 10% sulfate solution is presented in Figure 1. It is clearly shown that the compressive strength of all samples determined under nonsulfate solution exposure (C0) increases with an increase in the term of exposure (Fig. 1a). For example, the compressive strength of the 16NSE sample after 28, 56, and 90 days is 121.3, 124.3, and 129.5 MPa, respectively. On the one hand, the nano-SiO₂ can react with Ca(OH)2 produced in the cement hydration process and generate additional hydrated calcium silicate (C-S-H). On the other hand, the incorporation of nano-SiO₂ can also fill the voids between cement grains, resulting in the formation of a denser matrix, and then improve the compressive strength of the cement-based material. The compressive strength of all samples determined in 5% sulfate solution (C5) increases with the increase in exposure term, reducing the trend of increase after exposure. For instance, the compressive strength of the 10NSE sample determined after 28, 56, and 90 days is 115.9, 117.2, and 114.1 MPa, respectively. While the compressive strength exposed in 10% sulfate solution (C10) decreases with an increase in exposure term. At an early stage, the formation of AFt may fill the pores of the cement matrix; at a later stage, the excess AFt may lead to cracking in the cement matrix and thus a decrease the mechanical properties [13, 28], i.e., compressive strength, which will be discussed in section 3.4 (Fig. 5). Furthermore, at a later stage, the compressive strength decreases along with an increase in the sulfate concentration.

Fig. 1a.

Fig. 1b.

Fig. 1c.

The compressive strength of the 16NSE sample determined after 56 days in 5% and 10% sulfate solution is 119.8 and 116.8 MPa, respectively (Fig. 1a), while the strength determined after 90 days is 117.3 and 110.4 MPa, respectively (Fig. 1b). A high concentration of sulfate solution, high AFt content, and high gypsum formation, may result in the formation of cracks and exfoliation [13, 15]. It should be noticed that the compressive strength of these four samples from the highest strength to the lowest is 16NSE, 10NSE, NSP, and control samples. This result indicates that the addition of nano-SiO₂ can improve resistance of cement-based grouts against sulfate attack. Meanwhile, the effect of s nano-SiO₂ emulsion on resistance to sulfate attack is better than that of nano-SiO₂ particles as a result of better dispersion of nano-SiO₂ in the emulsion. The effect of nano-SiO₂emulsion increases with an increase in emulsion content.

Several researchers have reported a loss in the weight of cement-based materials when immersed in a sulfate-rich environment [29]. When sulfate attacks cement-based materials, it reacts with CH and C-S-H gels to form gypsum, which causes the dissolution and swelling of hardened cement pastes [4, 8, 16, 29]. Figure 2 shows the weight loss of cement-based grouts determined after 28, 56, and 90 days’ exposure to 5% sodium sulfate solution. It is seen that the weight losses determined after 28 days are all negative, which may be attributed to the continuous hydration of the cement matrix [30]. The weight loss listed in order from the highest to the lowest reduction is the control, NSP, 10NSE, and 16NSE samples, indicating that the degree of hydration of nano-SiO₂-modified samples is higher than that of the control sample. The incorporation of nano-SiO₂ can react with CH and accelerate the hydration of the cement matrix [31].

Fig. 2.

The weight losses determined after 56 and 90 days are higher than that of 28 days, showing that sulfate has penetrated the inner core of the grout matrix. The weight losses of the 16NSE sample determined after 28, 56, and 90 days, are all the lower than those of other samples over the same timeframes. This result indicates that cement-based grouts modified with nano-SiO₂ emulsion, which was preprepared and in which the nano-SiO₂ particles have been well dispersed, show excellent resistance to sulfate attack. The enhancement may be attributed to the secondary hydration of nano-SiO₂, improving the density of the microstructure and reducing the migration channel of SO42−\[\text{SO}_{4}^{2-}\] [25]. For exposure over 90 days, the formation of gypsum, which is likely due to the reaction between SO42−\[\text{SO}_{4}^{2-}\] and CH, may lead to exfoliations and increased weight loss.

The weight loss of cement-based grouts determined after 28, 56, and 90 days of 10% sodium sulfate solution exposure is shown in Figure 3. It is clearly seen that the weight losses of samples immersed in 10% sulfate solution are higher than those of the samples exposed to 5% sulfate solution. For instance, the weight loss after 90 days of 5% sulfate solution exposure is 0.97, 0.71, 0.36, and 0.28% for the control, NSP, 10NSE and 16 NSE samples, respectively, whereas these weight losses are 1.1, 1.02, 0.7, and 0.52% for these samples receiving 10% sulfate solution exposure. The reduction of weight may be attributed to the exfoliations, which in turn is owing to gypsumtype corrosion [28, 29]. Additionally, the formation and growth of AFt may lead to crystallization pressure, which may be due to the restraint in a certain direction and increase with an increase in the concentration of sulfate solution [13]. The cement matrix may crack and provide a penetration channel for SO42−\[\text{SO}_{4}^{2-}\] penetration.

Fig. 3.

Analysis of the mineral changes of cementbased grout matrix after sulphate attack process is of great interest and importance, as it will contribute to understanding the sulfate attack degradation mechanism [4]. The XRD patterns of the control and 16 emulsion samples determined after 56 days of 5% sodium sulfate solution exposure are illustrated in Figure 4. Compared with the XRD patterns of other reported cement-based grouts [1], it can be observed that the mineral compositions of these two samples are CH, dicalcium silicate (C₂S), tricalcium silicate (C₃S), AFt, gypsum, and quartz (SiO₂). The additional mineral compositions, AFt and gypsum, can be attributed to the reaction between inner hydrate calcium aluminates/CH of the sample and external SO42−\[\text{SO}_{4}^{2-}\] [13, 31]. The secondary AFt may cause internal stress and result in cracking and reduction of compressive strength [26, 32]. The formation of gypsum may induce the consumption of CH and decalcification of C-S-H gels, which may cause a strength loss in cement-based grouts [4]. It should be noted that the peak intensity of CH and gypsum in the XRD pattern of the 16NSP is lower than that of the control sample, which indicates that the incorporation of nano-SiO₂ can react with CH, consume additional CH, and then restrain the formation of gypsum [31]. Moreover, the intensity of AFt peaks also decreases, demonstrating that the introduction of the nano-SiO₂ emulsion may also restrain the formation of AFt.

Fig. 4.

It is well known that the deterioration of cement-based grouts is closely related to microstructural changes. Thus, studies of the microstructure development of cement matrix may be beneficial to the understanding of the sulfate attack process [4]. The SEM images of all samples determined after 90 days of 10% chloride solution exposure are presented in Figure 5. As shown in Figure 5a, the microstructure of the control sample consists of hexagonal CH, C-S-H gels, fly ash (FA), needle bar AFt crystals, veinlet-gypsum, and unhydrated clinkers. The morphologies of the control (Fig. 5b), 10NSE (Fig. 5c) and 16NSE (Fig. 5d) samples show that the hydration products are CH, C-S-H gels, FA, and AFt crystals.

Fig. 5a.

Fig. 5b.

Fig. 5c.

Fig. 5d.

When cement-based grouts have served in a sulfate-rich environment, SO42−\[\text{SO}_{4}^{2-}\] can enter into the inner core of the cement matrix and react with the hydrate calcium aluminates and CH of hydrate products to form AFt and gypsum [13, 28]. At an early stage, AFt crystal can fill the pores and enhance the mechanical properties of cementbased grouts [33]. As hydration time is prolonged, the concentration of AFt in cement paste solutions may become oversaturated. The growth of AFt is restricted by limited space and, along a certain direction, lead to generation of crystallization pressure, which may result in an expansion of cementbased grouts and crack extension (Fig. 5a) [19, 34]. The formation of gypsum can consume additional CH and decrease the alkalinity of the pore solution [35]. To achieve ionic equilibrium, Ca²⁺ may be precipitated from CH and C-S-H gels, which may lead to the destruction of the gel microstructure [28]. While C-S-H gels play a cementitious role in a cement matrix, the destruction may result in exfoliation and softening of cement-based grouts [1]. Moreover, the formation of a large number of AFt crystals may lead the matrix to crack, which can provide an entrance channel for the migration of SO42−\[\text{SO}_{4}^{2-}\] and decrease the mechanical properties over time. In Figure 5b, there is a small amount of nano-SiO₂ particle agglomeration, which may result from the high surface energy and charge of nanoparticles [36–38]. The secondary hydration between nano-SiO₂ particles and CH may be affected by this agglomeration, which leads the cement matrix to crack due to the formation and growth of gypsum crystals via the reaction of CH and sulfate [36–38]. No obvious crack appears in the matrix of the 10NSE and 16NSE samples (Figs. 5c and 5d), which may be attributed to the consumption of CH and the formation of additional C-S-H gels via the reaction between well-dispersed nano-SiO₂ particles and CH [30, 39, 40]. For the 16NSE sample, the secondary hydration of nano-SiO₂, which was well predispersed in emulsion, may decrease the gypsum content, improve the density of the microstructure, reduce the migration channel of SO42−\[\text{SO}_{4}^{2-}\], and then improve the resistance to sulfate attack.