Application of isothermal calorimetry and thermal analysis for the investigation of calcined gypsum–lime–metakaolin–water system

Abstract

The calcined gypsum–lime–metakaolin–water system and its subsystems consisting of two and three components are analyzed using the isothermal heat flow calorimetry, differential scanning calorimetry, and thermogravimetry. The hydration heat development in the initial hydration phase up to several hours is monitored in a common way, on the samples having a mass of about 1 g. In later time intervals up to 1 week when the values of the specific hydration heat power are very low, a large-volume calorimeter with better resolution is used for 300 g samples. The thermal analysis is carried out with the specimens undergoing hydration for the time periods of one to 8 days. The isothermal heat flow calorimetry shows that in the analyzed system and its subsystems, the majority of hydration heat is evolved during ~1.5 h after beginning of the hydration process. After 25 h, measurable amounts of hydration heat are produced in the gypsum–lime–metakaolin and metakaolin–lime mixes only, indicating a pozzolanic reaction in progress. In both these mixes, the differential scanning calorimetry reveals seven endothermic peaks corresponding to the thermal decomposition processes occurring during the heating up to 1000 °C. After 8 days of hydration, all Ca(OH)2 is found missing in the gypsum–lime–metakaolin and metakaolin–lime mixtures, which points to the completion of the pozzolanic reaction. The results obtained by the differential scanning calorimetry are well correlated with the mass changes measured by thermogravimetry and derivative thermogravimetry.