Heat cured cement mortars may result in linear expansion due to later age ettringite precipitation, popularly termed as delayed ettringite formation (DEF). However, later age ettringite precipitation may not result in expansion always. To differentiate the expansive and non-expansive nature of heat-cured cementitious systems, in this research pore solution chemistry of various cementitious systems cured at 90°C for 12 hours were studied. Linear expansion of mortar bars was measured periodically. Besides, ettringite precipitation was studied using X-ray diffraction (XRD) and thermal gravimetric analysis (TGA). The XRD, TGA results show the absence of early age ettringite and its delayed precipitation in all the heat-cured cementitious systems irrespective of expansive or non-expansive mortars. However, the pore solution ionic concentration reveals that sulphate ion concentration at the end of heat curing is vital to predicting whether the mortar will expand or not in the future. The curing temperature increased the solubility of sulphates and total alkalis present in the pore solution. The expanded mortars had a relatively higher amount of sulphates in the pore solution at the age of 30 hours, compared to the non-expansive mortars. Further, at the age of 7 and 30 days, a drop in sulphate, sodium, potassium ions concentration in pore solution was noticed, suggesting the leaching of alkalis and consumption of Sulphate for the precipitation of ettringite.