Reinforced concrete has been widely used in infrastructure. Some defects (e.g., cracks, holes, exfoliation, etc.) occur in the structure of reinforced concrete, which may be inevitable in the service process. The long-term performance may be affected as a result of the complexity of the engineering environment in civil engineering [1–3]. The deterioration of long-term performance in reinforced concrete is one of the most prevalent and critical issues in civil engineering [2, 4]. Generally, the cement-based grout, which have high fluidity, early strength, and micro-expansion [5–9] and which directly fill the weak zone, are a key controlling factor in the long-term performance of reinforced concrete [1, 10, 11]. High durability requirements for cement-based grouts are therefore of major importance [8].
Sulfate ions are widely present in seawater, underground water, rivers, soil, and industrial wastes [4]. In a sulfate-rich environment, both concrete structures and cement grouts are susceptible to the attack of corrosive substances [1–3, 6, 10]. Once external sulfate ions
Currently, the importance of the addition of nanomaterials in cement-based materials has emerged as an attractive area of research [16–18]. Considering the extreme servicing environment for many cement-based concretes, there is a significant amount of research focused on the description of the chemical mechanisms and mechanical consequences of sulfate attack. Pastor et al. [21] conducted a study on the change in skin friction of cement pastes when the grout surface was affected by sulfate attack. Samanbar Permeh et al. [22] evaluated the transportation of sulfate ions linked to grout separation in large-scale tendon mockups. Joséarcos Ortega Álvarez et al. [23] suggested that micropiles made using fly ash and slag cement could perform well under sulfate attack due to a more refined microstructure. Rusati and Song [24] studied the impact of magnesium sulfate attack on gravel-sand-cement-inorganic binder mixtures. They proposed that increasing the inorganic binder content in the mixture could improve the grout’s performance and mitigate strength degradation caused by sulfate attack. Ortega Álvarez et al. [25] analyzed the effects of silica fume on sulfate attack in the microstructure of grouts. Grouts containing 10% silica fume exhibited a similar or even superior performance in the very long term compared to grout made with sulfate-resisting Portland cement. Chindaprasirt et al. [26] evaluated the performance of hybrid high-calcium fly ash alkali-activated grouting materials for concrete exposed to a sulfate environment. They suggested that grouts containing 10% ordinary Portland cement outperformed the materials containing basalt fiber and silica fume. These studies enriched the theory of corrosion in cement-based grouts caused by a sulfate-rich environment. Developing cementbased grouts with strong resistance to sulfate attack remains a key factor in addressing the durability of concrete structures.
In this study, a preprepared nano-SiO2 emulsion was used to disperse nano-SiO2 particles effectively in the cement matrix of cement-based grouts, aiming to enhance sulfate resistance properties. The impact of nano-SiO2 emulsion on weight loss and strength properties in cement-based grouts was thoroughly investigated. A microstructural analysis was conducted using SEM to examine the hydration products in a hardened cement matrix. The enhancement of resistance to sulfate attack may be due to the decrease in AFt and gypsum formation, as well as the directional growth of AFt crystals.
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.
Chemical composition of fly ash, slag powder and Portland cement (wt%)
Constituent | CaO | SiO2 | Al2O3 | Fe2O3 | MgO | Na2O | SO3 | Loss |
---|---|---|---|---|---|---|---|---|
Fly ash | 2.4 | 61.9 | 28.8 | 2.5 | 0.8 | 0.3 | 0.6 | 1.7 |
Slag powder | 38.1 | 30.0 | 13.6 | 0.6 | 7.6 | 0.3 | 0.3 | 4.6 |
Cement | 63.71 | 20.35 | 5.0 | 3.3 | 1.37 | 0.39 | 2.48 | 2.6 |
Performance indices of Portland cement
Indices | Density (g/cm3) | Specific surface area (m2/kg) | Setting time (min) Initial Final | Compressive strength (MPa) | |||
---|---|---|---|---|---|---|---|
1d | 3d | 28d | |||||
Cement | 3.08 | 340 | 160 | 195 | 22.1 | 37.4 | 62.6 |
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.
Mixing ratios of raw materials for cement-based grouts (g)
Sample | Cement | water | FA | Slag | sand | PCE | P80 | YH-S | NS particle | NS emulsion |
---|---|---|---|---|---|---|---|---|---|---|
700 | 220 | 200 | 100 | 1000 | 5 | 1 | 0.5 | 0 | 0 | |
700 | 170 | 200 | 100 | 1000 | 5 | 1 | 0.5 | 5 | 0 | |
700 | 170 | 200 | 100 | 1000 | 5 | 1 | 0.5 | 0 | 56.2 | |
700 | 140 | 200 | 100 | 1000 | 5 | 1 | 0.5 | 0 | 89.92 |
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 K
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.
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].
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
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
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
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.
When cement-based grouts have served in a sulfate-rich environment,
In this work, the effect of nano-SiO2 emulsion on the sulfate resistance of cement-based grouts was experimentally studied. Conclusions can be drawn as follows:
The compressive strength of all samples from the highest to the lowest was the 16NSE, 10NSE, NSP, and control samples, which may be attributed to the addition of nano-SiO2, improving the resistance against sulfate attack of cement-based grouts. Also, the effect of nano-SiO2 emulsion was better than that of nano-SiO2 particles due to the better dispersion of nano-SiO2 in cement matrix via emulsion. The weight losses of four samples determined after all exposures to both 5% and 10% sulfate solution from the highest to the lowest were the control, NSP, 10NSE, and 16NSE samples, which indicates that the incorporation of nano-SiO2 can react with CH and accelerate the hydration of cement matrix. Meanwhile, the weight losses of the samples in a 10% sodium sulfate solution were higher than that of the samples exposed to 5% sulfate solution. The mineral compositions of the control and 16NSE samples were CH, C3S, C2S, AFt, and gypsum. The formation of AFt and gypsum may be due to the reaction between the inner hydrate calcium aluminates/CH of the sample and external The microstructure of four samples consisted of hexagonal CH, C-S-H gels, needle bar AFt crystals, FA, veinlet-gypsum, and unhydrated clinkers. On the one hand, AFt crystal can fill the pores and enhance the mechanical properties of cement-based grouts at an early stage. On the other hand, the growth of AFt is restricted, resulting in generation of crystallization pressure and then crack extension. The secondary hydration of nano-SiO2, which was well predispersed in emulsion, may decrease the gypsum content, improve the density of microstructure, and then enhance the resistance against sulfate attack.
In this study, a new idea for improving the sulfate resistance of cement-based grouts in sulfate-rich environments has been suggested. However, the content of nano-SiO2 particles in the emulsion is less and limited. Also, the impact of nano-SiO2 on sulfate resistance is not very significant, so future work should be focused on the preparation and application of the emulsions with a high nanomaterial content.