# Effects of Cement Treatment on Mechanical Properties and Microstructure of a Granite Residual Soil
## 4 Discussion
### Effect of Cement Treatment on Oedometric Compression Behaviour  
The cementation induced by C-S-H would deteriorate to varying degrees during the loading of a cured specimen. The complete destruction of the cementation at high stress was supposed to yield a consistent intrinsic compression line (ICL) or paralleled ICLs for weakly cemented soils [45; 46; 47], however, this may not be the case for strongly cemented soils. Figure 19 shows the integrated plots of the \(e-\)log\(\sigma_{v}\) relationship during successive loading of the saturated untreated and cement-treated specimens. The ICL of the saturated untreated specimen was presented as a reference, and the oedometric compression curves of the saturated treated specimens were extrapolated to judge the potential convergence or parallelism of ICLs at high stress. The oedometric compression curve of the untreated specimen intersects those of the treated specimens due to its highest initial void ratio and compressibility. This is also the case at \(C_{vi}=1\%\) due to its secondary initial void ratio and much higher compressibility when compared to the cases of \(C_{vi}=2\)-5% (see Figure 11a). In  
Figure 18: Modeling of \(E_{50}\) of saturated cement-treated GRS (Hollow symbol with cross indicates data excluded in the fitting). (**a**) Fitting of \(E_{50}\) at four curing times; (**b**) \(\alpha_{E50}\) dependant on curing time.  
the case of \(C_{\rm vi}\) = 1%, the ICL of the specimen cured to 28 d is likely to parallel with that of the untreated specimen at high stress. In the case of \(C_{\rm vi}\) = 2-5%, the ICLs of the specimens cured to 7 d and 28 d tend to parallel each other at high stress despite the initial void ratio decreasing with the cement content. This resulted in an overall increase of the vertical yielding pressure with the cement content in the range of \(C_{\rm vi}\) = 2-5% (see Figure 11c).  
The deterioration of the soil cementation can also be judged via the evolution of the compression index upon compression. The compression index (\(C_{\rm c}\)) at a vertical pressure of \(\sigma_{\rm v}\) is calculated by:  
\[C_{\rm c}(\sigma_{\rm vi})\ =-\left(e_{\rm i+1}-e_{\rm i}\right)/\left(\log \sigma_{\rm v,i+1}-\log\sigma_{\rm v,i}\right) \tag{7}\]  
where \(e_{\rm i}\) is the void ratio of the specimen after being loaded to \(\sigma_{\rm v,i}\).  
The evolution of the compression index during successive loading of the specimens is shown in Figure 20. In all the cases, a diminishing increase of the compression index with the vertical pressure was revealed, indicating a gradual approach of the specimens into the normal consolidation state; this is a consequence of the breakage of the interparticle cementation and rearrangement of the discrete soil particles upon compression. In addition, for the specimens cured to 7 d, the difference in the compression index is not distinct in the case of \(C_{\rm vi}\) \(\geq\) 2% compared to the case of \(C_{\rm vi}\) \(\leq\) 1%; this may be because the interparticle cementation was not fully established after curing to 7 d and the differences in the compressibility depend mainly on the presence and types of C-S-H (see Figure 13). The interparticle cementation may have been fully established after the specimens were cured to 28 d and, in this case, the compression index at a given vertical pressure depends largely on the cement content. Nevertheless, the compression index tends to converge at high vertical pressures in the case of \(C_{\rm vi}\) \(\geq\) 2%.  
Figure 19: Integrated plot of the \(e-\)log\(\sigma_{\rm v}\) relationship of saturated untreated and cement-treated GRSs. (**a**) Curing time = 7 d; (**b**) Curing time = 28 d.