# Effects of Cement Treatment on Mechanical Properties and Microstructure of a Granite Residual Soil
## 5 Conclusions  
Mechanical and microscopic tests were conducted on the GRS treated with Portland fly-ash cement, and the effects of cement treatment on the uniaxial and oedometer compression behaviors and the soil structure were analyzed. The following conclusions were drawn based on the findings of this study:  
(1) The UCS and secant modulus increased linearly with the volumetric cement content in the range of \(C_{\rm{vi}}\)\(\leq\) 3% and with the curing time in \(t\)\(\leq\) 7-14 d. As the volumetric cement content exceeded 3% or the curing time exceeded 14 d, the increase of the UCS and secant modulus with the cement content and curing time diminished. A unified formula using \(\eta C_{\rm{vi}}\) as the key parameter can estimate the UCS and secant modulus of the cement-treated specimens based on the untreated states. (2) Compressibility decreased markedly as the volumetric cement content increased to 2% and then varied slightly as the cement content further increased. Vertical yielding pressure decreased with the cement content in the range of \(C_{\rm{vi}}\) < 2-3%, followed by a continuous increase as the cement content further increased. The diminished increase of the compressibility with the vertical pressure indicated the breakage of the interparticle cementation and rearrangement of the soil particles upon compression. (3) The C-S-H is fibrous (C-S-H of Type I) in the case of \(C_{\rm{vi}}\) = 1-2% and becomes reticular (C-S-H of Type II) in the case of \(C_{\rm{vi}}\) = 3-5%. The soil structure remained of single-porosity in the case of \(C_{\rm{vi}}\) < 2% and a dual-porosity structure formed as the volumetric cement content exceeded 3%. (4) The mechanisms of soil structure to be modified through cement treatment were supposed in three aspects: (i) the formation of C-S-H and non-detective micro-pores within C-S-H; (ii) the intrusion of C-S-H into macro- and micro-pores; and (iii) the emergence of aggregates along with inter- and intra-aggregate pores. The macro- and micro-void ratios decreased with the cement content in the case of \(C_{\rm{vi}}\)\(\leq\) 2%, indicating that the role of the second aspect surpassed that of the first aspect (the third aspect is invalid) in this case. Figure 22: Macro-, micro-, and total void ratios of as-compacted and cured cement-treated GRS depending on cement content (Hollow symbol indicates a curing time of 28 d). (a) Initial state and after curing to 28 d; (b) Difference between initial and cured states. The macro- and micro-void ratios started to increase due to the third aspect in the case of \(C_{\text{vi}}\geq 3\%\) and exceeded their initial values as the third aspect began to dominate in the case of \(C_{\text{vi}}>4\%\). However, the coupling effects of the second and third aspects on the macro- and micro-void ratios need to be quantified in future investigations. (5) A volumetric cement content of 2-3% is suggested for treating the GRS (a silty sand) based on mechanical and microstructural characteristics of the saturated treated specimens. Volumetric cement contents lower than 2% provide inadequate interparticle cementation, whereas volumetric cement contents higher than 3% are inefficient as the inter-aggregate pores emerge along with the aggregates. In view of economic efficiency and environmental protection, combined treatment of cement and green binders is needed to reduce the cement content to its lower level and obtain more favorable physicochemical properties in a given geotechnical context. Writing--original draft: X.D. ; Writing--review & editing: X.B. ; Validation: H.C.; Supervision: C.X. ; Conceptualization: X.C. All authors have read and agreed to the published version of the manuscript. This research was supported by the National Natural Science Foundation of China (Grant Nos. 52022060, 4220298), Postdoctoral Science Foundation of China (Grant No. 2021M702234), and Joint Fund of National Natural Science Foundation of China-Railway Corporation for basic research of high-speed railway (Grant No. U1934208). Not applicable.