FROM CHEMICAL AND MICROSTRUCTURAL EVOLUTION OF CEMENT PASTES TO THE DEVELOPMENT OF AUTOGENOUS DEFORMATIONS. IN: AUTOGENOUS DEFORMATION OF CONCRETE

This paper seeks to contribute to the understanding of physical and chemical mechanisms that are at the origin of the autogenous volume changes and cracking of cementitious materials, focusing on the first 24 hours. A micro-macro experimental study has been performed, from the end of mixing up to several months, on a set of plain cement pastes prepared with the same type I ordinary Portland cement and various water-to-cement ratios (W/C), and cured at various constant temperatures. Chemical shrinkage, volumetric and one-dimensional autogenous deformations have been measured and analyzed in relation to the hydration process and to the microstructural characteristics of the material. The effect of the curing temperature at early age (<24 hours) in the range 10-50 deg C, and of W/C in the range 0.25-0.60, have been investigated. The temperature-induced changes recorded on both the magnitude and the kinetics of volumetric autogenous shrinkage clearly show the irrelevance of using the usual maturity concept to describe such phenomena within the whole early-age period. In addition, a threshold is pointed out at about a degree of hydration of the cement = 7%, both defining the range where autogenous shrinkage is linearly related to degree of hydration of the cement and corresponding to the precipitation of calcium hydroxide. A W/C threshold also is pointed out both at the macro-level (autogenous deformations, etc.) and at the micro-level (characteristics of the hydration products, pore size distribution, etc.). The findings indicate the critical two-fold effect of calcium hydroxide: a chemical effect, in which the carbon hydroxide content of the cement past plays the role of a chemical indicator of the hydration process, and a microstructural effect, generated by large-sized carbon hydroxide crystals.