Effect of Alkali Content and Water Glass Modulus on the Mechanical Properties and Microstructure of Slag-Based Geopolymer Mortar

Geopolymer materials represent a novel green cementitious material characterized by excellent mechanical properties and unique microstructural features. This study developed geopolymer mortar using slag as the primary raw material by adjusting alkali content and water glass modulus. Characterization methods, including nanoindentation testing, mercury intrusion porosimetry (MIP), and X-ray diffraction (XRD), were employed to systematically analyze the influence mechanisms of alkali content and water glass modulus on the mechanical properties and microstructure of slag-based geopolymer mortar. Results demonstrated that compressive strength exhibited an initial increase followed by a decline with rising alkali content and water glass modulus, while flowability first increased and then decreased. When the water glass modulus was 1.4, and the alkali content reached 8%, the geopolymer mortar achieved a 28-day compressive strength of 86.5 MPa and flexural strength of 10.2 MPa. At 10% alkali content, flowability reached 240 mm. Compressive strength showed a trend of initial increase followed by a decrease with increasing alkali content, reaching a maximum value of 86.4 MPa at 8% alkali content after 28 days. Nanoindentation analysis revealed that the primary strength-forming phase in geopolymer mortar was C-A-S-H gel. Variations in alkali content and water glass modulus primarily affected the volume fractions of C-A-S-H gel, porous phases, and unreacted slag particles, with limited impact on micromechanical parameters of individual phases. These findings not only provide a theoretical basis for optimizing the mix design of slag-based geopolymer mortar but also offer practical guidance for its application in high-strength and workable construction materials.