Reaction of Hardened Portland Cement Paste With Carbon Dioxide

The effect of age, water-cement ratio, and evaporable-water content on the reaction of hardened cement paste with carbon dioxide was investigated, using small cylinders of cement paste. The rate and extent of the reaction are subject to manipulation by varying the evaporable water content of the paste. Pastes of different age and water-cement ratio tend to dry at different rates, and this difference exerts an important indirect effect on carbonation. Analysis of the specimens showed that less than one molecule of nonevaporable water was released for every molecule of carbon dioxide gained, except in paste of low water-cement ratio or at low levels of carbonation in other pastes. If calcium hydroxide were the only phase attacked, equimolecular CO2–H2O stoichiometry would be expected. The observed behavior does not rule out the possibility of some preferential attack on calcium hydroxide, but other phases can react while calcium hydroxide is still present. Water-cement ratio, particularly in very dense paste, may exert some effect on the comparative accessibility of different phases.