Pore structures and interfacial properties between hydrated magnesia carbonates-modified recycled aggregate and reactive magnesia paste

Abstract

To further increase the sustainability of reactive magnesia cement (RMC) concrete while not compromising their overall strength, this study used the carbonation technique to pretreat recycled aggregates during the preparation of RMC-based samples, and their strength, pore structures and carbonation degrees were assessed by the compression test, XRD, TG-DTG, FTIR and X-CT. Greater carbonation degrees are considered to contribute to the strength improvement due to the formation of interconnected and well-developed carbonates and reduced porosity in RMC-based samples. However, this study shows that lower w/c ratios, which greatly refined their pore structures, favor the improved mechanical performance of RMC-based samples, albeit their carbonation degrees are lower. In addition, the much weaker correlation between the carbonation degree and strength of RMC-based samples could also be explained by the formation of different types of hydrated magnesium carbonates (HMCs), where samples containing nesquehonite (∼needle-like HMCs), albeit their carbonation degrees could be lower, reveal better strength than samples with hydromagnesite (∼rosset-like HMCs). Therefore, using pretreated recycled aggregates in RMC-based samples delayed their inside CO2 diffusion when they were cured in CO2, which slowed down their strength development, but when cured in air, RMC-based samples containing pretreated recycled aggregates reveal better strength owing to their more compacted pore structures. This study could also prove that CO2 diffusion among RMC-based samples could be through the pores within ITZs instead of capillary pores resulting from water.