Microstructure of cement paste subject to early carbonation curing

Microstructure of Ordinary Portland Cement paste subjected to early age carbonation curing was studied to examine the effect of early carbonation on performance of paste at different ages. The study was intended to understand the mechanism of concrete carbonation at early age through the microstructure development of its cement paste. Early carbonation was carried out after 18-hour initial controlled air curing. The microstructure characterized by XRD. TGA, (29)Si NMR and SEM was correlated to strength gain, CO(2) uptake and pH change. It was found that early carbonation could accelerate early strength while allowing subsequent hydration. The short term carbonation created a microstructure with more strength-contributing solids than conventional hydration. Calcium hydroxide was converted to calcium carbonates, and calcium-silicate-hydrate became intermingled with carbonates, generating an amorphous calcium-silicate-hydrocarbonate binding phase. Carbonation modified C-S-H retained its original gel structure. The re-hydration procedure applied after carbonation was essential in increasing late strength and durability. (C) 2011 Elsevier Ltd. All rights reserved.