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Abstract A coupled CO 2 -water curing regime was employed on Ordinary Portland cement (OPC) paste samples immediately after casting, which allowed carbonation and hydration of OPC to proceed simultaneously. The strength development and microstructural evolution was evaluated by using multiple-techniques. The results indicated that, compared to the normal hydrated counterpart, a lower porosity, higher amorphous phase content and overall reaction degree can be achieved in the coupled CO 2 -water cured OPC sample. By combining with the morphological observations, a new mechanism was proposed for the rapid hardening of OPC. It is shown that the carbonation reactions led to the formation of calcite particles, which provided more nucleating sites for C S H gel growth; and thus, an increase in the overall reaction degree of the cement paste can be achieved within the first 24 h compared to the conventional water curing process. Graphical abstract Image 1 Highlights • Chemically bond water in coupled CO 2- water cured paste was less than that in normally hydrated paste. • Strength gain from subsequent water curing varied inversely with the initial CO 2 uptake. • Apparent porosities of the pastes were similar after 28 days of curing, regardless of any curing regime. • Different morphologies of C S H were observed under different curing regimes. • A new mechanism was proposed for the rapid hardening of OPC under coupled CO 2 -water curing. [ABSTRACT FROM AUTHOR]

Strengthening the attached old cement mortar of recycled concrete aggregate (RCA) is a common approach to enhance the RCA properties. Accelerated CO 2 curing has been regarded as an alternative way to enhance the properties of RA. However, the improvement of the properties of RCA was limited by the shortage of reactive components in the old cement mortar available for the carbonation reactions. In this study, a CO 2 curing process associated with a limewater saturation method was performed cyclically on cement mortar samples, aiming to enhance the properties of cement mortars via artificially introducing additional calcium into the pores of the cement mortars. The results indicated that the adopted treatment method promoted the level of carbonation which was demonstrated by higher CO 2 uptake by the limewater saturated cement mortar when compared to that without limewater treatment. After 3-cycles of limewater-CO 2 treatment, the density of the cement mortar slightly increased by 5.7%, while the water absorption decreased by over a half. For mechanical properties, the compressive and flexural strength were increased by 22.8% and 42.4%, respectively. Compared to the untreated cement mortar samples, the total porosity of cement mortar was reduced by approximately 33% and the densified microstructure therefore resulted in a higher microhardness. [ABSTRACT FROM AUTHOR]

Carbonation treatment is a promising option to enhance recycled concrete aggregate (RCA) quality and eventually improve concrete performance. This study investigated the multi-scale mechanical properties of calcium silicate hydrates (C-S-H) in a mature alite paste subjected to accelerated carbonation treatment by testing macro compressive strength, microhardness and nanoindentation. The results show that, carbonation treatment resulted in a heterogeneous distribution of carbonation products in the alite sample. Reinforcement of C-S-H by the randomly precipitated calcite and vaterite crystals prevailed over the nano-structural and compositional alteration of C-S-H induced by carbonation, leading to an increase of nano-indentation modulus at the edge of the sample. Also, the microhardness at the edge zone was markedly higher than that at the core of the carbonated sample. The carbonated peripheral zone with improved nano-scale modulus and microhardness was likely serving as a "hard shell", contributing to the improved macro compressive strength. The new insights into the multi-scale mechanical properties of carbonated C-S-H contribute to further understanding of the enhanced properties of RCA after carbonation treatment. [ABSTRACT FROM AUTHOR]