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.

The associated effect of sodium chloride and dihydrate gypsum on the mechanical performance of a slag-based geopolymer activated by quicklime was investigated by compressive strength, shrinkage, and square circle anti-cracking tests of mortar with a 0.5 water–binder ratio and a 1:3 binder–sand ratio, as well as paste soundness, powder X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), and mercury intrusion porosimetry (MIP) of the paste. The results indicate that (1) when dihydrate gypsum is used alone, it combines with calcium aluminate hydrate (C-A-H) to form calcium sulfoaluminate hydrate (AFt), which encourages the hydration process of slag. A 7.5% addition can result in an increase of 97.33% and 36.92% in 3-day and 28-day compressive strengths, respectively. When NaCl is used by itself, it facilitates the condensation of the aluminum silicate tetrahedron unit and generates zeolite. A 2% dosage can lead to a 66.67% increase in the 3-day compressive strength, while causing a 15.89% reduction in the 28-day compressive strength. (2) The combined effect of 2% NaCl and 7.5% gypsum results in the formation of needle-like and rod-shaped AFt, Friedel’s salt, and plate-like Kuzel’s salt in the geopolymer. This leads to an increase in 3-day and 28-day compressive strengths by 148% and 37.85%, respectively. Furthermore, it reduces the porosity by 18.7%. (3) Both NaCl and gypsum enhance the paste soundness of the slag-based geopolymer, and they do no harm to the crack resistance of the geopolymer. The drying shrinkage of the geopolymer at 28 days is just 0.48 × 10⁻³, which is only 66.7% of OPC. This slag-based geopolymer has a simple preparation process, good volume stability, low raw material cost, low energy consumption, and low carbon emissions. It can be used instead of 32.5 slag Portland cement in plain concrete applications, and has high engineering, economic, and environmental values.