Compound Effects of Sodium Chloride and Gypsum on the Compressive Strength and Sulfate Resistance of Slag-Based Geopolymer Concrete

Based on compressive strength, sulfate resistance, mass change, and relative dynamic elastic modulus tests, and XRD and SEM analysis, the effects of sodium chloride (NaCl) and gypsum on the mechanical properties and resistance to sulfate attack of slag-based geopolymer concrete activated by quicklime as well as the mechanism of action were studied. The results indicate that: (1) with appropriate dosages of NaCl or gypsum, the compressive strength of geopolymer concrete can be increased by 55.8% or 245.3% at 3 days and 23.9% or 82.3% at 28 days, respectively. When NaCl and gypsum are combined, Friedel’s salt, Kuzel’s salt, and NaOH are generated, and the strength is increased by 90.8% at 3 days, and 180.3% at 28 days. (2) With 2% NaCl alone, the mass loss is reduced from 5.29% to 2.44%, and the relative dynamic elastic modulus is increased from 0.37 to 0.41. When compounded with 7.5% gypsum, the mass is increased by 0.26%, and the relative dynamic elastic modulus is increased to 1.04. With a further increase of NaCl to 4%, the mass is increased by 0.27%, and the relative dynamic elastic modulus is increased to 1.09. The sulfate corrosion resistance coefficient of geopolymer concrete is increased from 0.64 to 1.02 when it is immersed with 7.5% gypsum alone for 90 days, and it can be further increased to 1.11 when compounded with 4% NaCl. (3) The geopolymer prepared with sodium chloride: gypsum: quicklime: slag = 4:7.5:13.5:75 can be used to replace 32.5 slag Portland cement in plain concrete. The cost and carbon emissions are reduced by 25% and 48%, respectively, and the sulfate corrosion resistance coefficient is higher by 38.8% than with slag Portland cement.