During the construction of a roller-compacted concrete dam, there is a problem that some areas cannot be compacted, and the application of metamorphic concrete paste injection technology can solve this problem [ 1 ]. The stability, fluidity, rheology, and time-varying properties of metamorphic concrete paste are all important properties that need to be considered [ 2 3 ]. The stability characterizes its ability to maintain its original properties, and the bleeding rate is usually used to characterize the stability of the paste [ 4 ]. The fluidity and rheology of the paste directly influence the effect of grouting or infiltration, which in turn affects the construction quality of metamorphic concrete [ 5 6 ]. The time-varying characteristics can significantly affect the diffusion effect of paste [ 7 8 ]. However, traditional metamorphic pastes often have problems such as poor rheology, insufficient strength, and poor stability. In addition, as the main component of the paste, the extensive use of cement in grouting leads to the rapid consumption of resources and energy and the generation of harmful gases [ 9 10 ]. Therefore, considering the grouting performance, resources, environment, and cost of the metamorphic paste, it is a challenging task to improve grouting with new additives.
12,13,14,Through recent research and engineering applications, some green mineral admixtures such as fly ash, basalt powder, zeolite powder, etc. have also been used in the grouting field and showed good performance [ 11 15 ]. However, compared with the actual demand of engineering construction, the supply of green mineral admixtures in the grouting field is still very limited. Therefore, the selection of available materials must be carefully studied to meet the requirements of environmental and economic costs while considering the good properties of metamorphic paste [ 16 17 ]. The silica fume (SF) is processed from natural quartz and fused quartz, and the production process is non-toxic, non-polluting, and low in cost. The storage of quartz is large in nature, which can meet the large demand in the short term, so it can be used as an alternative material.
Many scholars have studied the role of SF in the performance of cement paste, mortar, and concrete and drawn corresponding conclusions. Smita Sahoo [ 18 ] conducted a comprehensive experimental study on the strength, durability, and microstructure properties of SF-containing concrete, and believed that 15% was the optimal amount of SF used in concrete. With the addition of 15% SF, the concrete’s compressive strength, sulfuric acid corrosion resistance, chloride corrosion resistance, and carbonation resistance were all improved to varying degrees. Mahdi Shariati [ 19 ] explored the effect of SF on the durability and microstructure characteristics of alkali-activated slag geopolymers and found that SF significantly increased the resistivity and reduced the water absorption rate of the paste by generating additional CSH gel. Xiaodi Dai [ 20 ] found that SF can improve the drying shrinkage and mechanical properties of alkaline activated slag cement (AASC) and has a slower structure accumulation and better workability maintenance in the early stage. Guangwei Liang [ 21 ] reported that proper SF can significantly refine the porosity of geopolymer slurry and reduce the porosity, thereby improving the mechanical properties. G. Appa Rao [ 22 ] found that the addition of silica powder increased the initial setting time of cement slurry, reduced the gas content and workability of mortar, and enhanced the early hydration reaction of C3A and C3S in paste.
Some researchers also achieved good results by using SF in combination with other mineral admixtures or chemical additives. Aneel Kumar [ 23 ] reported that the effect of SF on the compressive strength of burnt clay bricks (BCB) was better than that of fly ash. The 4% SF increased the compressive strength of BCB by 27.55%, while the 4% FA increased the compressive strength by 17.36%. Sherong Zhang [ 24 ] combined SF with waste marble powder (WMP) and found that cellular concrete containing 10% SF and 5–20% WMP showed the best mechanical and durability properties. Mojtaba Nili [ 25 ] used concrete mixed with air and silica fume to show the best performance in salt scale tests, the addition of SF reduced the total pore volume and refined the pore distribution. The mechanism of SF in cement-based materials and concrete has also been explored. Jeong proved [ 26 ] that silica fume provides seed sites for cement hydration nucleation and subsequent acceleration and provides active silica for the pozzolanic reaction. Chenxin Ni [ 27 ] focused on the influence of silica fume fineness on the hydration process of cement paste. The hydration of cement changed with the SF particle size. Coarse DSF particles prolonged the acceleration period of cement hydration and reduced the rate of heat release of cement hydration. Coarse SF was conducive to the formation of AFm crystals. Fine SF accelerated the hydration of C3S, leading to an increase in the amount of CH. Xinming Wang [ 28 ] found that under low sulfate conditions, SF was adsorbed on the surface of hydration products as nucleation seeds, which promoted the precipitation and chemical shrinkage of hydration products.
It can be seen from the above that SF has been widely used in the field of cement-based and concrete materials, and its influence on durability, mechanical properties, and microstructure has also been explored. However, there are few studies on the rheology and stability of silica fume in cement-based and concrete materials. These properties are important in some engineering fields. For example, in the field of grouting, rheology is a prerequisite for determining whether the grouting material is pourable. If the grouting material does not have good rheology, it cannot enter the cracks or voids to play an effective filling role. Stability determines whether the grouting material exhibits bleeding and sedimentation. Slurry plugging and poor diffusion uniformity result in poor construction quality and severe consequences. Time-varying characteristics is the scale that determines the rheological properties and stability of grouting materials with time. In addition, the current explanation of the mechanism of the influence of silica fume on the rheology and stability of cement paste is relatively simple, and new theories need to be developed to explain the influence mechanism of silica fume, which is convenient for the application of silica fume to provide theoretical guidance in the field of grouting.
Therefore, based on the above discussion, an experimental study on the characteristics of the cement paste containing silica powder (SF) was carried out. The main purpose of this research is twofold: (i) The influence of SF on the rheology, fluidity, stability, and time-varying properties of cement paste were evaluated, which provided a theoretical basis for the application of SF in the field of grouting. (ii) The water film thickness theory, bleeding sedimentation model, microstructure observation, and hydration heat analysis methods were used to explain the mechanism by which SF changes the fresh and time-varying properties of cement paste.