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Porous magnesium oxysulfate (MOS) cement pastes were successfully fabricated by injecting presaturated bentonite into modified MOS cement paste. Their pore structure and hardened performance were investigated. The results indicated that the 20MgO-MgSO4·7H2O-18H2O system modified by citric acid (C6H8O7⋅H2O) and ethylene diamine tetraacetic acid was suitable to fabricate porous MOS cement paste. Bentonite slurry led to significant refinement of pores, generating nanosized pores in MOS cement pastes. When volume replacement of bentonite slurry in MOS cement paste rose between 0 and 60%, pore size corresponding to the peak in the pore size distribution curve of MOS cement-based materials decreased from 180.0 nm to 22.8 nm and then increased to 163.0 nm, and the porosity linearly increased from 21.1% to 58.1%. These small pores caused the successful preparation of porous MOS cement paste with dry bulk density of 760–1650 kg/m3, compressive strength of 7.8–69.8 MPa, and thermal conductivity of 0.25–0.85 W/(m·K).

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