An experimental study of mitigating coastal dune erosion by using bio-carbonization of reactive magnesia cement

Abstract

Due to coastal storm surge and wave or man-made activities, widespread coastal erosion events are emerging. In this study, the feasibility of bio-carbonation of reactive magnesium cement (RMC) to mitigate coastal erosion was investigated by conducting laboratory-scale wave erosion tests. The accumulative erosion volume (V), erosion resistance duration (Dre), and volume erosion rate (VIS) were calculated to evaluate the erosion resistance of sand dunes with the different cement thickness (T = 3 mm, 4 mm, and 5 mm) and RMC content (R = 1%, 1.5%, and 2%). The surface penetration resistance and the cementation product content (brucite/hydrated magnesia carbonates (HMCs)) were also measured to assess the effectiveness of bio-carbonation in sand dunes. Results show that in comparison with RMC hydration, the VIS decreased by 77%, Dre increased by 333%, and peak penetration resistance increased by 42% after the bio-carbonation treatment of sand dunes. Further improvements in these results were obtained with increasing T or R. Given varying T or R, we identify two types of erosion process responses under wave action. For bio-carbonation treatment with higher T or R, the erosion process response follows four phases. In contrast, for hydration treatment, or bio-carbonation treatment with lower T or R, Phase II does not exist because of small crust thickness or weak bond. The presentation of Phase II effectively delays the slope failure depending on larger erosion volume (V ≥ 93 cm3) and turns sudden into flexible failure. Scanned electron microscopy observations further revealed that HMCs formed by RMC bio-carbonation provided better filling and cementation effects than brucite formed by RMC hydration, however, massive amounts of HMCs concentrated in the shallow surface layer, causing hindered penetration and reducing the number of cementation products at higher depths.