AbstractTo clarify the role of limestone powder (LP) in cement mortars under a magnesium sulfate solution at room temperature, the hydration products, microstructure, and mechanical properties of cement-based materials with different amounts and particle sizes of LP were investigated by Fourier transform infrared (FTIR), thermogravimetric analysis (TGA), X-ray diffraction (XRD), scanning electronic microscopy (SEM), energy dispersive spectrometer (EDS), appearance change, and compressive strength. The specimens were immersed in 5% MgSO4 solution at 20°C for up to 9 months. LP content had little effect on the content of ettringite, but it significantly influenced the content of gypsum and Ca(OH)2. When the LP content increased from 0% to 30% (average particle size: 32.42 μm), the ettringite content in mortars ranged from 4.1% to 5.3%, the gypsum content increased from 7.7% to 57.0%, and the Ca(OH)2 content decreased from 9.4% to 0%. The particle size of LP content had little effect on the content of ettringite and Ca(OH)2 when its content was 30%, but it influenced the gypsum content. When the average particle size of LP decreased from 32.42 to 8.07 μm, the ettringite content ranged from 4.6% to 5.1%, the Ca(OH)2 content was always 0%, and the gypsum content decreased from 57.0% to 51.8%. The compressive strength variation coefficients of mortars with different LP content ranged from 7.7% to 57.0%, and it ranged from 82.3% to 86.7% for mortars with different LP particle sizes.