Iron mineral admixtures improve the sulfuric acid resistance of low-calcium alkali-activated cements

We investigated the sulfuric acid resistance of low-calcium alkali-activated materials (i.e., geopolymers) supplemented with an iron mineral admixture (i.e., hematite). Geopolymers without and with 5% hematite were produced at two alkali contents (Na:Al = 0.86 and 1.39). Acid degradation reactions were comprehensibly studied through three replenishes of acid. Results demonstrate that hematite is chemically active upon acid exposure yielding a short-term increase in acid neutralization capacity. Prolonged acid resistance was enhanced in high alkali content formulations with hematite. Acid exposure revealed minimal changes to mineralogy, molecular structure, and micro-scale porosity in these samples, resulting in less dealumination and silicon leaching. Thus, results indicate that the acid buffering capacity of geopolymers, specifically at higher alkali content formulations, increases due to the addition of hematite. The increased buffering capacity leads to lower degrees of dealumination of the N-A-S-H cementitious binder. These results are important as they may help explain the increased acid durability of alkali-activated materials synthesized from industrial aluminosilicate precursors (e.g., slag, fly ash, lateritic clays) that may contain iron minerals.