Performance enhancement of coral aggregate concrete through pre-coating coral aggregate with cement-metakaolin-coral powder ternary slurry

It is challenging to produce high-strength coral aggregate concrete (CAC) due to the high porosity and low strength of coral aggregate (CA). To enhance the properties of CAC, this study proposes a novel pre-treatment protocol for CA, using the cement-metakaolin-coral powder (CMC) ternary slurry or the pure cement (PC) slurry to pre-coat CA. The physical properties of CA and the workability, mechanical properties and capillary water absorption of CAC were tested before and after pre-treatment of both slurries. Additionally, the interface transition zone (ITZ) between untreated/treated CA and the matrix were analyzed using scanning electron microscopy coupled with energy dispersive spectroscopy and microhardness testing. Results show that the recommended component content of the CMC ternary slurry (70 % cement, 20 % metakaolin, and 10 % coral powder) is determined using the simplex centroid design method. Pre-coating CA with the PC slurry or the CMC ternary slurry can improve the workability, mechanical properties, and water permeability resistance of CAC by enhancing physical properties of CA and optimizing the ITZ. Compared to the PC slurry, the CMC ternary slurry exhibits a superior improvement effect, which is attributed to its higher compressive strength and achieving a denser ITZ. Specifically, at the curing age of 120 d, the CMC ternary slurry can increase the compressive strength, splitting tensile strength, and fracture energy of CAC by 39.2 % (reaching 67.5 MPa), 43.9 %, and 70.5 % respectively, and decrease the open porosity of CAC by 39.4 %. Based on these findings, it is recommended to prioritize the use of the CMC ternary slurry for pre-treating CA in marine engineering projects, particularly for island construction, to meet the demand for high-strength CAC.