High early strength alkali-activated mortar from artificial slag blended with high-volume limestone powder: Reaction kinetics and thermodynamic modeling

The European Unions goal of climate neutrality by 2050 and the associated switch in pig iron production from the CO₂-intensive blast furnace process to the direct reduction process means that blast furnace slag is becoming increasingly scarce and will no longer be available in some regions in the foreseeable future. This applies to its use in classic cements as well as in alkali-activated binders. This study explores a new type of artificial slag (AS), with adapted chemical and physical properties, as an alternative to traditional BFS. Thanks to the adapted chemical and physical performance of AS, it exhibits exceptional reactivity, achieving high mechanical performance even when a significant portion is replaced with a widely available and inexpensive filler. Replacing a high volume of AS (≥70 vol%) with limestone powder (LP) in the alkali-activated system results in the formation of a hardened material with high early strength. The mortar composed of a 30–70 AS-LP volume ratio and activated with sodium silicate at a Na₂O equivalent of 5 wt% by weight of all powder exhibited the highest 2-, 28- and 90-day strengths of 67.8 MPa, 81.7 MPa, and 82.4 MPa, respectively. However, decreasing Na₂O equivalent to 3 wt% results in a comparable 90-day strength value. GEMS modeling revealed that the LP-AS content, as well as the source and dosage of Na₂O, significantly influenced the type and quantity of activation products. Furthermore, the results demonstrated that LP promotes the formation of activation products, while quartz powder had no appreciable effect.