# Effects of Cement Treatment on Mechanical Properties and Microstructure of a Granite Residual Soil
## 2 Experimental Procedures  
### Materials  
The GRS used in this study is obtained from Shenzhen, Guangdong Province, China. The bulk GRS is air-dried and pulverized carefully using a rubber hammer to avoid breakage of the sand particles. The grain size distribution (GSD) of the air-dried GRS is measured by dry-sieving (after washing) in combination with the Hydrometer method [27]. The overall GSD curve of the GRS is shown in Figure 1; it is observed to be a gap-graded soil dominated by 40% fine particles (\(d\) < 0.075 mm) and 25% coarse particles (\(d\) > 2 mm) on a dry-weight basis. Sand particles coarser than 5 mm account for no more than 1%; for convenience, they are eliminated from the original soil in the following test procedures. The specific gravity of the fractions finer than 5 mm is measured using the Pycnometric method [28]. The consistency limits of the fractions finer than 0.5 mm are determined using a liquid-plastic limit combined tester [29]. A bimodal compaction curve is revealed by the standard Proctor test [30] (seeFigure 2); this may be due to the GRS's gap-graded nature. The optimum water content and maximum dry density are identified at the second peak of the compaction curve. The basic physical properties of the GRS are summarized in Table 1. According to the Unified Soil Classification System [31], the GRS is classified as silty sand. The XRD profile of the GRS with a grain size finer than 0.5 mm is shown in Figure 3, and the basic minerals are identified as Kaolinite (90%) and Quartz (10%) on a dry-weight basis. A SEM image of the GRS with a grain size finer than 0.5 mm is shown in Figure 4. The clay lamella of Kaolinite and its stacks (i.e., quasicrystal) are vivid in the SEM image, and the irregular quartz is wrapped in the clay lamellae. Despite the high clay content, clay aggregates (i.e., stacks of quasicrystals) are not recognized from SEM image, indicating a single-porosity structure of GRS compared to the dual-porosity structure of clays [32; 33]. \begin{table}
\begin{tabular}{c c c c c c} \hline \(\mathbf{G_{\mathrm{s}}}\) (-) & \(\mathbf{\omega_{\mathrm{L}}}\) (\%) & \(\mathbf{\omega_{\mathrm{P}}}\) (\%) & \(\mathbf{I_{\mathrm{P}}}\) (-) & \(\mathbf{\omega_{\mathrm{opt}}}\) (\%) & \(\mathbf{\rho_{\mathrm{dmax}}}\) (Mg/m\({}^{3}\)) \\ \hline
2.64 & 53.5 & 30.7 & 22.8 & 18.8 & 1.58 \\ \hline \end{tabular} Note: \(\mathbf{G_{\mathrm{s}}}\) = specific gravity; \(\omega_{\mathrm{L}}\) = liquid limit; \(\omega_{\mathrm{P}}\) = plastic limit; \(I_{\mathrm{P}}\) = plastic index; \(\omega_{\mathrm{opt}}\) = optimum water content; and \(\mathbf{\rho_{\mathrm{dmax}}}\) = maximum dry density.  
\end{table} 
Table 1: Basic physical properties of granite residual soil. Figure 1: Grain-size distribution curves of GRS and Portland fly-ash cement. Figure 2: Compaction curve of GRS from standard Proctor test. The Portland fly-ash cement used in this study is of a commercial type (PF32.5 [34]) containing 25% of active fly-ash on a dry-weight basis; this cement type is preferred in the context of embankment construction due to its economy and low hydration heat. The grain-size distribution of the powder cement is obtained by laser particle-size analysis with dry dispersion and the result is added to Figure 1. The specific gravity of the powder cement is estimated to be 3.10 via the Pycnometric method [28] with Kerosene (instead of water) used as the dispersion medium. The XRD profile of the oven-dried powder cement is shown in Figure 5, and the basic mineral compositions are identified as Tric calcium silicate (C\({}_{3}\)S, 71%), Quartz (12%), Gypsum (8%), Calcite (6%), and Dicalcium silicate (C\({}_{2}\)S, 2%) on a dry-weight basis. The XRD results do not reveal the presence of Tric calcium aluminate (C\({}_{3}\)A) and Tetra-calcium aluminoferrite (C\({}_{4}\)AF), possibly because of their low fractions. Figure 4: A SEM image of GRS finer than 0.5 mm and at a dry density of 1.44 Mg/m\({}^{3}\).