Physio-chemical reactions in recycle aggregate concrete

Concrete waste constitutes the major proportion of construction waste at about 50% of the total waste generated. An effective way to reduce concrete waste is to reuse it as recycled aggregate (RA) for the production of recycled aggregate concrete (RAC). This paper studies the physio-chemical reactions of cement paste around aggregate for normal aggregate concrete (NAC) and RAC mixed with normal mixing approach (NMA) and two-stage mixing approach (TSMA) by differential scanning calorimetry (DSC) and scanning electron microscopy (SEM). Four kinds of physio-chemical reactions have been recorded from the concrete samples, including the dehydration of C₃S₂H₃, iron-substituted ettringite, dehydroxylation of CH and development of C₆S₃H at about 90 °C, 135 °C, 441 °C and 570 °C, respectively. From the DSC results, it is confirmed that the concrete samples with RA substitution have generated less amount of strength enhancement chemical products when compared to those without RA substitution. However, the results from the TSMA are found improving the RAC quality. The pre-mix procedure of the TSMA can effectively develop some strength enhancing chemical products including, C₃S₂H₃, ettringite, CH and C₆S₃H, which shows that RAC made from the TSMA can improve the hydration processes. © 2008 Elsevier B.V. All rights reserved.