Mechanism of Impurity Effects on Performance of α‐Calcium Sulfate Hemihydrate and Its Application Optimization

ABSTRACT As a high‐performance cementitious material, the stability of α‐calcium sulfate hemihydrate (α‐HH) is significantly constrained by impurities. This study systematically investigates the effects of four typical impurities (calcite [CC], quartz [SI], calcium sulfate dihydrate [DH], and type III anhydrite [AH]) on the setting performance and mechanical strength evolution of α‑HH at 2 h, 3 d, and 7 d. A Random Forest model was employed to predict the safe dosage ranges of impurities. The results show that the influences of these impurities are phase‐dependent and age‐dependent. Inert impurities CC and SI primarily act as physical fillers, with minor effects on setting time. At low dosages, they enhance 2 h and 3 d strengths, but excessive incorporation or prolonged curing to 7 d leads to strength deterioration. DH acts as seed crystals, accelerating the hydration reaction and refining the crystal grains; however, its incorporation into the crystalline network disrupts structural continuity, leading to strength retrogression of the hardened body. AH exhibits a dual‐action mechanism with relatively minor impacts. In multi‐impurity systems, coupled effects of grain‐boundary deterioration and internal defects lead to complex performance evolution. Feature importance analysis from the Random Forest model reveals a dynamic shift in dominant factors: early strength is governed by CC and SI, while later strength is more sensitive to DH. Based on the model, safe dosage ranges for multiple impurities to achieve the α40 strength grade were successfully predicted. This study elucidates the influence mechanisms of multiple impurities on α‐HH performance and provides a data‐driven approach for product design and optimization. The outcomes support the sustainable management of industrial by‐product gypsum by enabling its high‐value utilization, which aligns with the principles of circular economy and cleaner production.