From chemical and microstructural evolution of cement pastes to the development of autogenous deformations
Résumé
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 (OPC) 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 (degree of hydration of the cement a, Ca(OH)7 content, ...) and to the microstructural characteristics of the material. The effects of the curing temperature at early age (< 24 hours) in the range 10-50°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 = 7%, both defining the range where autogenous shrinkage is linearly related to a and corresponding to the precipitation of Ca(OH)2. Moreover, a W/C threshold is pointed out both at the macro-level (autogenous deformations, ...) and at the micro-level (characteristics of the hydration products, MIP porosity and pore size distribution, ...). The chemical and (micro)structural basic effects of calcium hydroxide are in particular distinguished.
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