Physical and Mechanical Properties of Novel Gypsum-Based Self-Leveling Mortars: The Effect of Sand Gradation and Type

Gypsum-based self-leveling mortar (GLSMs) has good workability and economic benefits, but poor mechanical properties limit its application. In this study, the sand gradation of different sand types was designed based on Fuller–Thompson theory and the effect on the physical and mechanical properties of GLSMs was investigated. The mathematical model developed for the fluidity loss of GLSMs, the analysis showed that the addition of sand had a positive effect on the fluidity loss, and reduced the initial and 30 min fluidity of GLSMs. Moreover, river sand particles contained mud, resulting that the smaller the particle diameter, the greater the loss of fluidity to GLSMs. Manufactured sand achieved an optimal connection and interlocking between the gypsum paste and sand, which reduced the porosity and improved the mechanical properties of the GLSMs. The flexural and compressive strengths of the GLSMs using the manufactured sands increased by 65.5% and 57.8%, respectively. Further, the GSLMs with irregular sand could absorb a considerable amount of energy during fracture and exhibit superior mechanical properties. The excellent mechanical properties allow GLSMs to be used in factories, car parks and warehouses, broadening the application scenarios for GLSMs.

The crack path of the GSLMs with sand containing sharp aggregates was more complex than those with sand containing rounded shapes, resulting in higher energy consumption during crack propagation in the GSLMs with sharp aggregates. The crack formation in the material required debonding between the aggregate particles and gypsum slurry, with the cracks progressively expanding as the bending stresses increase. The crack propagated continuously from the surface of the particles during debonding to deflection or branching, which further elongated the crack expansion path and consumes more fracture energy. As a result, the mechanical properties of the GSLMs with sand containing sharp aggregates were improved.