The Enhancement Mechanism of Calcium Sulfate Whiskers on the Mechanical Properties and Microstructure of Coral Aggregate Concrete

To upgrade the mechanical properties and reduce the high brittleness of coral aggregate concrete (CAC), calcium sulfate whisker (CSW) has been innovatively used as a reinforcing material in this study. Five incorporation levels (0%–4%) were designed to systematically investigate the evolution mechanism of CAC mechanical, microstructure, and pore characteristics at different curing ages. The results showed that CSW incorporation can significantly improve the mechanical properties of CAC; 1% CSW can bring 36.7% enhancement to 14-day compressive strength, and 11.9% improvement to 28-day splitting tensile strength with 2% CSW. Mechanism analysis revealed that appropriate CSW content effectively suppressed microcrack propagation through whisker bridging effects and remarkably enhanced the cement paste–coral aggregate interfacial bond strength by 71%, promoting a transition in failure mode from interfacial failure to aggregate fracture. At the same time, CSW improved the pore structure by reducing the proportion of macropores and increasing the micropore proportion to 76% with 1% CSW content. However, the performance deterioration of CAC caused by CSW excess (4%) was mainly due to the defect formation resulting from whisker agglomeration. The proposed strength prediction models (R2 > 0.93) based on experimental data can reliably describe the enhancement effect of CSW.