Investigating correlations between microstructural characteristics and mechanical properties of coral sand mortar with different silica fume contents

Abstract

The use of coral sand in concrete and mortar for ocean engineering has led to significant reductions in construction costs and improved construction efficiency. However, this approach poses challenges such as low strength, high brittleness, and high porosity. To address these challenges, this study incorporated silica fume to enhance the mechanical properties of the mortar and investigated the effects of different silica fume contents on the hydration process, pore structure, and strength of coral sand mortar. Additionally, this study introduced a novel iterative approach combining low-field nuclear magnetic resonance and scanning electron microscopy image analysis to determine transverse relaxivity (ρ2). During the early hydration process, the intensity spectrum of coral mortar exhibited three peaks at 298, 1841, and 299 au, corresponding to transverse relaxation times (T2) of 0.64, 6.83, and 83.10 ms, respectively. Over a 6 h hydration period, the spectrum of the specimen prepared with white cement exhibited an additional peak that gradually merged with the main peak. As the curing period extended from 7 to 28 days, the porosity of coral sand mortar decreased by 3.77–4.55 %, while the strength increased by 7.43–14.35 MPa across silica fume contents ranging from 6 % to 24 %. With increasing silica fume content, the calcium–silicon ratio of the samples gradually decreased, promoting the formation of more floccus C–S–H gels. Consequently, the sample porosity first decreased slightly. After 28 days of curing, the coral sand mortar containing 12 % silica fume exhibited the highest strength owing to the increased amount of floccus C–S–H gels. These gels gradually filled the pores between the fibrous C–S–H gels, forming a compact spatial reticular structure. This structure mitigated the adverse impact of high porosity on mortar strength.