Effect of Water-Cement Ratio of Cement Pastes on Microstructural Characteristics Related to Carbonation Process

The carbonation of concrete depends both on the carbon dioxide penetration through the pore network and on the accessibility of the calcium-containing compounds such as calcium hydroxide or C-S-H gel. The aim of this work was to determine the influence of the water-cement ratio (W/C) on the microstructural characteristics of cement pastes. Investigations were performed during the hydration process at early age and on 28-day and 2-year old hydrated cement pastes made with a Type I normal portland cement. The W/C were 0.25, 0.35, 0.45, and 0.60. The main results are as follows: Porosity and autogenous strain measurements, completed with SEM observations, show that the microstructural characteristics are very different between pastes having W/C above and below 0.35-0.40. Above this value the porosity strongly increases; the autogenous deformations exhibit a swelling phase before self-desiccation shrinkage; and the calcium hydroxide appears in large crystals. SEM examinations and EDX analysis show that, for an equivalent degree of hydration, the calcium hydroxide amount increases and that the CaO/SiO2 ratio of the C-S-H decreases as the W/C increases. Explanations of the results are proposed, taking into account the ability of the water to diffuse through the structure and to react with unhydrated cement. The implications of these results in carbonation process are also discussed.