Since the introduction of Portland limestone cement (PLC) to the CSA A3000 2008 edition many efforts were made to evaluate the effects of Limestone Filler (LF) on the early and later age strengths of PLC concrete cured under ambient temperature. However, little attention has been given to evaluate the effect of steam curing. This paper aims to investigate the effects of LF content and size on the early age properties of steam cured high early strength cement mortars. CSA High Early Strength Cement "Type HE" was used with water to cement ratio of 0.37. The limestone filler contents were 0, 5, 10, and 15% with particle size of 3, 12 and 17 microns. Mortar and paste specimens were steam cured at 55°C for 12 and 16 hours. Compressive strength was evaluated at 12 and 16 hrs; 3 and 7 days. Heat of hydration was evaluated using Isothermal Calorimetry. Calcium hydroxide Ca(OH)_2 and calcium carbonate CaCO_3 contents were measured using Thermal Gravimetric Analysis and Differential Thermal Analysis (TG/DTA). The results showed increase in the heat of hydration and Ca(OH)_2 content with the increase in LF content and decrease in LF size. The early age compressive strength was increased with the increase in LF content and decrease in size. In general, mix designs containing LF showed better early age hydration and strength compared to control mix without LF.
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