Strength Enhancement and Mechanism of Alkali-activated Sodium Silicate in Cement-stabilized Soil

[Objective] In recent years, the increasing number of buildings constructed on soft ground has made the treatment of soft soil foundations particularly important. Investigating the strength variation characteristics of silt foundations under different types of stabilizing agents and curing ages,as well as exploring a preliminary method for identifying the strength level of non-standard soil samples obtained in the field,is of great practical significance for engineering applications. [Methods] Representative silty soil layers from the Zhongshan area were selected. Cement-only mixing tests were first conducted to optimize cement content (ratio of cement mass to the mass of treated wet soil). Subsequently,silt was stabilized using lignosulfonate acid (LA),triethanolamine (TEA),and alkali-activated sodium silicate (AS) as stabilizing agents,respectively. Scanning electron microscopy (SEM) tests were then carried out to analyze the microstructures and stabilization mechanisms of untreated and stabilized soils. Finally,ArcGIS was used to construct independent elevation models from the SEM images of untreated and stabilized soils and to process them into three-dimensional images. Scatter plots were plotted in double-logarithmic coordinates,and the soil porosity and particle fractal dimension were further calculated and analyzed. [Results] 1) Using the unconfined compressive strength of specimens as the evaluation index,under the same curing age, the cement content was positively correlated with the unconfined compressive strength. Before a curing age of 7 days, the unconfined compressive strength increased rapidly; during 7-14 days, the growth rate slowed down; and from 14 to 28 days, the unconfined compressive strength continued to increase. Considering economic cost and code requirements, the optimal cement content was 18% of the wet soil mass. 2) At certain mixing ratios, single incorporation of LA, TEA, and AS all enhanced the strength of cement-stabilized soil and could be used as stabilizing agents. Based on the optimal contents of the three stabilizers, ternary mixing stabilization tests were conducted. The results showed that single incorporation of AS exhibited better stabilization performance than the other single-additive groups and the ternary mixing group. When the cement content was 18% and the AS content was 0.9%, the unconfined compressive strength of the stabilized soil reached a maximum value of 2.39 MPa. 3) SEM tests indicated that the specimens of the 18S blank group failed to generate sufficient gel-like hydration products (C-S-H) and needle-like Aft crystals. As a result, limited cementitious material existed between soil particles, and numerous pores were observed. After stabilization with 18S-0.9AS, a large amount of gel-like C-S-H hydration products and needle-like Aft crystals were rapidly generated, which interwove to form a large-area spatial network structure and initially formed a skeleton. This process led to particle bonding and aggregation and filled the interparticle pores. Overall, the 18S-0.9AS group exhibited the best stabilization effect. 4) Three-dimensional SEM images of untreated and stabilized soils were constructed and processed using ArcGIS. Data calculation and analysis showed that the untreated soil had 97 663 226.34 pore pixels, accounting for 53.22% of the total image pixels (porosity), with a fractal dimension of 1.399 8. The 18S-0.9AS stabilized soil had 50 153 642.75 pore pixels, accounting for 27.27% of the total image pixels (porosity), decreased by 25.95 percentage points compared with the untreated soil. The fractal dimension of this group was 1.853 5, and the unconfined compressive strength reached the maximum value of 2.39 MPa. [Conclusion] 1) The larger the particle fractal dimension of a specimen, the more complex the particle structure, the higher the surface roughness, the lower the porosity, and the higher the compressive strength. 2) The porosity of specimens and the compressive strength exhibit a nonlinear decreasing relationship, whereas the particle fractal dimension and the compressive strength exhibit a nonlinear increasing relationship. 3) When standard specimens cannot be obtained at construction sites, the compressive strength of specimens can be preliminarily inferred by using the porosity and particle fractal dimension of non-standard specimens, based on the physical significance and correlations of fractal dimensions among different specimens.