Preparation of Hydrophobic Low-Clinker Binder Using Stearic Acid and Multicomponent Solid Waste

Abstract The rapid development of industry has generated a large amount of solid waste, which poses significant environmental pressure. How to efficiently utilize these solid wastes is an urgent problem to be solved. At the same time, traditional cement-based materials exposed to the natural environment for a long time are subject to erosion by rainwater and destruction by freeze–thaw cycles, making the durability of cement-based materials an increasing concern. The hydrophobic modification of cement-based materials has gradually attracted attention. This study prepared hydrophobic cement by mixing stearic acid with different raw materials in a multicomponent solid waste system. The results showed that stearic acid significantly enhanced the hydrophobicity of the raw materials and cement. Among them, the best hydrophobic angle was achieved with stearic acid–modified sintered sludge ash (SSA) at 147.7°. For cement, the best result was also obtained with SSA as the carrier, reaching a hydrophobic angle of 95.8°. However, the hydrophobicity also hindered the hydration reaction of cement, leading to a decrease in mechanical properties. The most significant decline was observed when limestone powder was used as the carrier, with a 38% and 31% reduction in compressive and flexural strength after 28 days, respectively. Isothermal calorimetry and microstructural analysis further revealed the retarding effect of stearic acid on cement hydration and its impact on the internal structure of the material. The main reasons were the hydrophobicity of stearic acid preventing sufficient contact between cement and water, and the formation of calcium stearate from the reaction between limestone powder and stearic acid, which led to the loss of its ability to provide attachment sites.