Properties of Alkali-Activated Slag Cement Activated by Weakly Alkaline Activator

Sodium sulfate (Na₂SO₄) and sodium carbonate (Na₂CO₃) are weakly alkaline activators. Alkali-activated slag (AAS) cement prepared with them shows the special advantages of long setting time and low shrinkage, but it shows slow development of mechanical properties. In the paper, Na₂SO₄ and Na₂CO₃ were used as activators and compounded with reactive magnesium oxide (MgO) and calcium hydroxide (Ca(OH)₂) to optimize the setting time and mechanical properties. The hydration products and microscopic morphology were also studied using XRD, SEM, and EDS. Furthermore, the production cost and environmental benefits were compared and analyzed. The results show that Ca(OH)₂ is the main influencing factor for setting time. It reacts preferentially with Na₂CO₃ to form CaCO₃, which makes AAS paste lose plasticity rapidly and shortens the setting time, and then produces strength. Na₂SO₄ and Na₂CO₃ are the main influencing factors for flexural and compressive strength, respectively. Suitably high content is beneficial to promote the development of mechanical strength. The interaction of Na₂CO₃ and Ca(OH)₂ shows a great effect on the initial setting time. High content of reactive MgO can shorten the setting time and increase the mechanical strength at 28 days. There are more crystal phases in hydration products. Considering the setting time and mechanical properties, the composition of activators are: 7% Na₂SO₄, 4% Na₂CO₃, 3-5% Ca(OH)₂, and 2-4% reactive MgO. Compared with ordinary Portland cement (OPC) and AAS cement activated by sodium hydroxide (NaOH, NH) and water glass (WG) with the same alkali equivalent, the production cost and energy consumption are greatly reduced. Compared with P·O 42.5 of OPC, CO₂ emission is reduced by 78.1%. AAS cement activated by weakly alkaline activators shows excellent environmental and economic benefits and good mechanical properties.