Characterization of mechanical properties and microstructure of silica mortar and coral mortar based on nuclear magnetic resonance technology

It is crucial to use coral sand to replace traditional sand and gravel aggregates to produce coral aggregate concrete for island development and construction to minimize environmental damage to offshore islands and reefs. Coral sand particles are irregular in shape, porous, and break easily, resulting in substantial differences in engineering mechanical properties and the microstructure compared to land-based sand. Uniaxial compressive strength tests were carried out on coral mortar and silica mortar to determine the influence of the sand-cement ratio on the mortars' compressive strength. Low-field nuclear magnetic resonance (NMR) technology was used to analyze the compressive strength of the two mortars at the microscopic scale. The optimum sand-cement ratio of coral mortar was determined. At the same mixture ratio, the compressive strength of coral mortar was lower than that of silica mortar. In the early hydration reaction, only one main peak occurred in the transverse relaxation time distribution of the silica mortar, but a secondary peak was observed in the coral mortar, and a transition peak appeared 84 h later. The pore size range of the internal pores of the coral mortar was large, and the interval was continuous. The significantly higher proportion of harmful pores and very harmful pores resulted in the lower strength of the coral mortar than the silica mortar. The research results can provide a reference for using NMR to evaluate coral mortar.