Compression Casting of Alkali-Activated Mortars: Forming Mechanisms, Alkali Regulation, and Impact on Mechanical Performance

Abstract The compression casting process improves pore structure and mechanical performance in cementitious systems. However, in alkali-activated materials (AAMs), the effects of activator loss during compression casting on reaction behavior remain poorly understood. This study investigates the mechanical performance of AAM via compression casting. Response surface methodology was used to analyze the effects of precompression rest time, casting pressure, alkali equivalent, and liquid-to-solid (L/S) ratio on compressive strength in compression casting AAM. A method was proposed to evaluate retained alkali equivalent during compression casting and its role. The experimental results demonstrate that compression casting reduces AAM porosity and optimizes pore size distribution by increasing gel and transition pores, enhancing strength through physical densification. Microstructural analyses revealed elements enrichment and increased Ca/Si, Al/Si, and Na/Si ratios in compression casting AAMs, accompanied by enhanced gel polymerization and reduced bound-water loss, collectively contributing to the improvement of compressive strength. At 5% alkali equivalent, compression casting effectiveness improves with increasing L/S ratio and precompression rest time. At 7% alkali equivalent, strength enhancement reaches saturation. Casting pressure effects depend on initial alkali content and precursor development, showing both positive and negative impacts during activation. Compression casting improves alkali utilization efficiency, and despite pressure reducing alkali equivalents, higher retained alkali levels produce superior enhancement effects.