Hydration behavior of calcium aluminate cement mortars with mineral admixtures at different curing temperatures

Abstract

Cement-based materials that are produced with calcium aluminate cement (CAC) exhibit severe strength losses with time and temperature due to the conversion of metastable hydrates (CAH10 and C2AH8) into the stable hydrates (C3AH6 and AH3). On the other hand, using mineral admixtures at different ratios during the production of cement-based materials change the hydration behavior and, thus, mechanical properties of these materials. Therefore, this research focuses on the effects of mineral admixtures on the properties of CAC mortars at two different curing temperatures (20 °C and 38 °C). Mortars were prepared by using constant CAC content, and the sand was replaced by class F fly ash (FA) as 10%, 30%, 60%, granulated ground blast furnace slag (GGBFS) as 10%, 30%, 60% and silica fume (SF) as 5%, 10%, 15%. In addition to fresh mortar tests, other tests, including isothermal calorimetry tests up to 72 h, the flexural and compressive strength tests at 3, 7, 28, and 90 days and the capillary absorption tests, were also conducted. The mineral admixtures, when used as sand replacement materials, accelerated the hydration reactions of CAC mortars. 10% replacement of sand by FA and especially GGBFS resulted in better mechanical strengths at 20 °C (7% and 16% increases in compressive strengths for 10% FA and 10% GGBFS replacements at 90 days). In addition, they were least affected with the increase in temperature to 38 °C as compared to other GGBFS and FA used mixes. On the other hand, silica fume used mixtures had relatively lower strengths, but they showed some improvements with the increase in temperature at later ages (10% and 25% increases at 90 days for 5% and 15% SF, respectively). GGBFS and FA additions in different amounts decreased the heat of hydration values at 38 °C as compared to 20 °C (except 10% GGBFS addition) while silica fume increased the values.