# Effects of Cement Treatment on Mechanical Properties and Microstructure of a Granite Residual Soil
## 1 Introduction  
Granite residual soil (GRS), one of the most commonly encountered soils in the tropics and subtropics, is often gap-graded and characterized by bimodal grain-size distribution curves from clay to gravel. For example, the GRS widely distributed in Shenzhen, China, is usually dominated by clay and coarse-sand particles whereas the fine- and medium-sand particles are found in very small amounts [1]. Due to its poorly graded nature, the GRS is difficult to compact similar to a clayey soil and disintegrate quickly after being soaked under free conditions similar to a sandy soil. Considering the distinct heterogeneity in its soil structure, GRS is treated as special soil in geotechnical engineering that needs to be dealt with carefully. Cement treatment can improve the geotechnical performance of soil materials with small dosages of cement and allows for an immediate strength gain compared to lime and fly-ash treatment [2]. Therefore, it has been widely used in geotechnical practices such as embankment construction and ground improvement [3; 4; 5]. The mechanisms of cement treatment to improve soil properties include the occupation of large pores and a binding of the soil particles by the hydration products (i.e., C-(A)-S-H gel) of the cement component to form a denser and/or cemented soil structure [2; 6]. The efficiency of cement treatment depends on the soil and cement types, cement content, and the treating technology [7; 8; 9]. In general, to achieve the same engineering indices, higher cement contents are required for clayey soils than for sandy soils. For a given geotechnical context and soil type, a proper cement dosage is important to meet the engineering requirements. The effectiveness of cement treatment is judged primarily by the physicochemical and mechanical properties of the soil-cement mixtures. A review of past investigations showedthat cement contents higher than 10% (on a dry-weight basis) are usually required for clayey soils to achieve satisfying engineering indices; in this case, higher cement contents produced lower consistency limits and a higher strength and stiffness of the treated soils [3; 10; 11]. For sandy soils, cement contents lower than 10% (on a dry-weight basis) are often adopted; in this case, higher cement contents yielded lower optimum water contents and maximum dry densities but higher strengths and stiffnesses of the treated soils [12; 13; 14]. As GRS is usually classified as silty sand or clayey sand (which is between sand and clay), the dosages for cement treatment and the physicochemical properties of the treated soil are expected to vary with the mineralogy and grain compositions of the original soil. Nevertheless, the cement-treated GRS exhibits a higher compaction efficiency and durability than the original soil [15; 16], and cement contents of around 5% (on a dry-weight basis) are preferred or suggested in the context of highway and high-speed railway embankment construction [17; 18]. Apart from the mechanical characteristics, attention was paid to the alterations in the mineralogy and microstructure of soils following cement treatment. These efforts were mainly performed based on X-ray diffraction (XRD), scanning electron microscope (SEM), and mercury-intrusion porosimetry (MIP) tests. The presence of C-(A)-S-H can be identified from XRD patterns, Raman spectra, and thermogravimetry (TG) curves, and its morphology can be observed from SEM images. The C-(A)-S-H was observed to be fibrous at a very low cement content [19; 20]. As the cement content increased, the C-(A)-S-H fibers became shorter and eventually reticular [21; 22; 23]. In addition to the cementation from C-(A)-S-H, changes in the pore structure (especially the pore size distribution, PSD) affect the mechanical properties of the treated soils and a few investigations were performed based on MIP tests. For sandy soils with single-porosity, an overall reduction in the pore size was revealed as the water-cement ratio increased [24; 25].