Effect of accelerated carbonation on fine cement paste aggregates

Accelerated carbonation is suggested as a potential alternative for carbon dioxide sequestration, which also improves the microstructure of recycled concrete aggregates. In this study, the carbonation parameters of model aggregates derived from cement pastes with two different water/cement ratios (i.e., 0.5 and 0.7) were evaluated in order to maximize the benefits of the carbonation process on their density and absorption. For this purpose, in a first phase, the initial moisture content of the model aggregates and the reaction time were evaluated. Furthermore, in a second phase of mortar production, the fraction of fine natural aggregates was replaced, considering each type of unprocessed and carbonated model aggregate at volumetric replacement percentages of 50% and 100%, varying the amount of water in the mortar mix to maintain a similar workability in all the series. In this way, the properties of the eight series of mortars are evaluated in comparison with the control series made only with natural aggregates. The results indicate that the accelerated carbonation process positively influences density and absorption in the model aggregate. Regarding the parameters of the accelerated carbonation environment analyzed, for a higher w/c ratio in the original model aggregate, higher initial water content was necessary for the carbonation process to be efficient. As for the mechanical performance of the mortars with model aggregate, the series that incorporated model aggregate aggregates with a w/c ratio of 0.5 achieved better mechanical behavior compared to the series replacing model aggregate with a w/c ratio of 0.7; this was related to the formation of a more compact matrix in the mortar with model aggregate of cementitious pastes.