As the demand for alkali-activated cementitious materials (AACM) increases, reliance on blast furnace slag (BFS) raises concerns about potential shortages. This study evaluates the substitution of BFS with diatomaceous earth (DE) in AACM, using three DE types-raw (RDE), calcined (CDE), and spent (SDE)-at substitution rates of 0-30 %. The impact of these substitutions on fluidity, setting time, and compressive/flexural strength of the prepared mortar was assessed, along with an analysis of the mechanisms through hydration heat, product composition, and pore structure. Results indicate that DE, characterized by the porous structure and high content of amorphous silica, enhances early hydration, reduces fluidity, accelerates setting times, and refines pore structure, thereby improving strength. CDE, with its low organic content and large surface area, exhibited the best performance, while SDE was effective despite impurities, and RDE performed the poorest. Increasing DE substitution raised water absorption, impacting fluidity and setting times, with 20% identified as the optimal substitution rate, balancing strength and workability. The entropy method-analytic hierarchy process (EM-AHP) confirmed this rate, with CDE as the optimal variant. Life cycle assessment (LCA) showed that while SDE reduces environmental impacts significantly, CDE achieves a greater reduction when considering both environmental and mechanical performance. In conclusion, CDE and SDE are viable BFS substitutes in AACM, offering enhanced performance and reduced environmental impacts.
