Effects of heating and drying on the strength and stiffness of high-early-strength Portland cement pastes

Changes in the bending strength and Young's modulus after drying and heating up to 90 degrees C of high-early-strength Portland cement paste with water to cement ratios (W/C) of 0.30 and 0.55 were measured. The bending strength decreased until 40% RH and then increased below 40% RH in the case where the W/C equaled 0.55. For a W/C of 0.30, the bending strength only changed minorly for relative humidity above 70%, while the strength increased significantly below 70% RH. Based on microstructural characterizations, for a W/C of 0.30 the strength change of the paste can be evaluated by a combination of changes in the strength of a solid skeleton composed of C-S-H agglomerations and the macropores, which is related to the strength characteristics of porous materials. When the W/C equals 0.55, changes in the solid strength and macropores or mesopores influence the strength variation, but the mesopores have more impact on the strength than macropores. Since the increase in the solid strength becomes predominant from higher RHs for lower W/C paste, the starting point of strength increase (SPSI) of lower W/C paste appears at higher RH values than in higher W/C cases. Despite the relatively large scatter in the experimental results the bending strength and Young's modulus standard deviation of HCP were 16% (W/C = 0.55) and 10% (W/C = 0.30) - consistent and reliable tendencies were found.