Effect of Hydrophobic Treatment with Potassium Methylsilicate on Mechanical Properties of Ceramsite Sand Engineered Cementitious Composites

To mitigate the adverse effects of the high-water absorption characteristic of ceramsite sand on the properties of ceramsite sand engineering cementitious composites (CS-ECC) and clarify the mechanism of hydrophobic treatment, this study used solid potassium methylsilicate powder as a hydrophobic agent to modify the surface of ceramsite sand at four concentrations: 0%, 0.5%, 1%, and 3%. Systematic tests were conducted on the crushing value and water absorption rate of the ceramsite sand, as well as the compressive strength, flexural strength, tensile strength, and ultimate elongation of CS-ECC. Additionally, the scanning electron microscopy (SEM) was employed to observe the microstructure. The results indicate that hydrophobic treatment has no significant impact on the strength of the ceramsite sand, with the fluctuation of the crushing value being less than 2%. The 0.5% concentration of the hydrophobic agent can reduce the water absorption rate of the ceramsite sand from 18.32% to 9.02%, while concentrations of 1% and 3% lead to a rebound in water absorption. This concentration (0.5%) can also improve the early mechanical properties of CS-ECC: the 7-day compressive, flexural, and tensile strengths increase by 26.9% and 14.2%, respectively. In contrast, high concentrations (1% and 3%) of the hydrophobic agent result in a decrease in the mechanical properties of CS-ECC. Microstructural analysis reveals that 0.5% hydrophobic treatment optimizes the ceramsite sand–matrix interface bonding and alleviates fiber pull-out and fracture. However, high concentrations of the hydrophobic agent inhibit the hydration of cementitious materials, causing interface voids and weakened bonding.