Influence of cement fineness and water-to-cement ratio on mortar early-age heat of hydration and set times

• Studied the influence of cement fineness on heat of hydration.
• Studied the influence of water-to-cement ratio on heat of hydration.
• Developed heat indexes to quantify heat of hydration process.
• Developed an innovative procedure to predict set times from calorimetry test.
• Compared set time predicted from the calorimetry method to the ASTM set time test.

In this study, hydration of mortars containing portland cement of three different finenesses and prepared at four different water-to-cement ratios (w/c) were investigated using isothermal calorimetry tests. Results showed that the hydration heat generated from cement with higher fineness was larger and faster compared to coarser cements in early ages. The lower w/c resulted in a higher heat of hydration rate at earlier hours but reduced after that. While the maximum heat of hydration rate was lower with higher w/c, total heats of hydration within the first 24 h were found to be approximately the same, regardless of the different w/c used. Initial and final set times determined from isothermal calorimetry were found to relate to set times determined from ASTM C403 penetration tests. The set times increased with the increasing of the w/c and decreasing of the cement fineness. However, due to the very different mechanisms and test setups in determining setting times, the relationship between these two methods may vary among different cements. Longer setting times were generally obtained from the calorimetry method compared to the ASTM set time test.