Influence of Raw Material Composition on the Early Mechanical Strength of Calcium Magnesium Silicate Composite Cementitious Materials

In order to develop alkali activated cementitious materials with high safety, low cost, and environmental friendliness, slag and fly ash were used as the main raw materials, Ca(OH)2 and Mg(OH)2 were used as weak alkali activators in this paper. The effects of raw material composition on the early mechanical strength and workability of composite cementitious materials were systematically studied. The mechanism of Ca/Mg ratio, fly ash and slag ratio on the hydration products was explored through tests of flowability, compressive strength, XRD and SEM analysis. The results show that the slag can effectively generate C-S–H gel under the excitation of Ca(OH)2, which significantly improves the early strength; Moderate introduction of Mg(OH)2 (Ca:Mg = 6:4 ~ 5:5) can partially form C-M-S–H gel and improve the mechanical properties, but excessive Mg(OH)2 will inhibit the hydration reaction, resulting in a decrease in fluidity and strength. The addition of fly ash helps to improve the fluidity of the slurry, while slag significantly enhances the early strength and development potential of the material. In the fly ash: slag = 7:3 system, the optimal 7-day compressive strength is achieved when Ca:Mg = 5:5. Research has confirmed that reasonable regulation of raw material composition can optimize early mechanical properties while ensuring construction performance, providing theoretical basis and technical support for the application of weak alkali activation systems in ecological building materials.