Effects of mineral admixtures on the thermal expansion properties of hardened cement paste

Concrete is a heterogeneous composite material with multiple components that have different thermal expansion properties. Internal stresses are generated if the concrete is rapidly heated or cooled, causing cracking in the micro- or macro-scale. If the coefficient of thermal expansion (CTE) of hardened cement pastes (HCP) is adapted, the thermal stress caused by the mismatch of thermal expansion properties of the components can be mitigated. The effects of the commonly used mineral admixtures (fly ash, ground granulated blast furnace slag and silica fume) on the thermal expansion properties of HCP are investigated. The thermal dilation rates (TDR) of HCP in which Portland cement is partially replaced with mineral admixtures are tested with a dilatometer and the CTE are derived. Replacing Portland cement with mineral admixtures is found to lower the CTE of HCP, which is beneficial for mitigating the thermal mismatch between the HCP and the aggregates. The extent of this lowering effect increases with the increasing replacement proportions of mineral admixtures in the pastes. Through quantitative determination of the porosity and the amount of portlandite in the paste and microscopic observations, it is revealed that the effects of the mineral admixtures are mainly due to the change of the porosity and the amounts of hydration products in the pastes.