ABSTRACT
The expansive behavior of cement pastes containing different admixtures due to delayed ettringite formation was investigated. Specimens consisted of high-early-strength Portland cement, substituted with fly ash, silica fume, metakaolin, aluminum hydroxide (Al2O3) or ground granulated blast furnace slag. Expansion was suppressed by additives in most cases, although low amounts instead functioned to accelerate its onset, and effectively so when they contained aluminum present as Al2O3. 2 7Al and 29Si solid-state NMR analysis revealed that heat curing transforms aluminum in additives to its third aluminum hydrate (TAH) phase, likely serving as a source for the monosulfate and ettringite phases despite its consumption by calcium aluminosilicate hydrate (C-A-S-H). Cement additives with high aluminum content appear to reduce the SO3/Al2O3 ratio to suppress the conversion of monosulfate into ettringite.
