To　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　degrees　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　mu　m),　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　mu　m,　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.　(C)　2022　American　Society　of　Civil　Engineers.