Effect of magnesium formate as compared with magnesium oxide on the strength enhancement and microstructures of CaO-activated Class F fly ash system

Abstract

This study investigated the influences of magnesium formate (Mg(HCOO)2) and magnesium oxide (MgO) as potential additives on strength development and reaction products in the CaO-activated Class F fly ash system. Originally, the use of Mg supplies (MgO and Mg(HCOO)2) was expected to increase strength through the formation of magnesium silicate hydrate (M-S-H) gel, but no M-S-H gel actually formed, although both Mg supplies increased the strength of the system in different ways. The use of Mg(HCOO)2 is clearly effective in boosting strength by significantly increasing the dissolution degree of the main activator (CaO); although the formate ions did not form any reaction products contributing to strength enhancement, they functioned as a booster of Ca concentration, leading to considerably more formation of calcium silicate hydrate (C-S-H) and strength. However, the use of MgO was much less efficient because it rapidly precipitated to Mg(OH)2, which is insoluble in an aqueous system; thus, the addition of MgO had little effect on the formation of C-S-H, but largely enhanced the formation of Mg(OH)2 (brucite). As brucite is known to have relatively high strength, the strength improvement after adding MgO was likely simply attribute to the formation of Mg(OH)2.