Effect of superplasticizers on properties of one-part Ca(OH)₂/Na₂SO₄ activated geopolymer pastes

This paper investigates the effect of three different superplasticizers (SPs), Naphthalene (N), Melamine (M) and Polycarboxylate (PC), on the properties of one-part fly ash/slag geopolymer pastes activated by Ca(OH)₂/Na₂SO₄ powder combination. The flowability, setting time, and compressive strength of the achieved geopolymer pastes were assessed. It was found that the superplasticizers significantly improved the flowability, retarded the setting time, and increased the compressive strength of the one-part Ca(OH)₂/Na₂SO₄ geopolymer pastes. The most recommended superplasticizer was found to be polycarboxylate. The use of polycarboxylate SP for decreasing the water content (i.e. w/b) in Ca(OH)₂/Na₂SO₄ geopolymer pastes has resulted in reducing the porosity and enhancing the compactness of the aluminosilicate gel, therefore, improved the compressive strength. It was found that the excessive use of polycarboxylate (i.e. 3%) for additional water reduction in Ca(OH)₂/Na₂SO₄ geopolymer pastes had an adverse effect; it extended the induction period and delayed the participation of reaction products, thus significantly prolonged the setting time. Furthermore, a comparison between the achieved Ca(OH)₂/Na₂SO₄ geopolymer pastes and the conventional Na₂SiO₃-anhydrous one-part geopolymer pastes was carried out. It was found that Ca(OH)₂/Na₂SO₄ geopolymer exhibited significantly lower compressive strength, higher flowability and considerably longer setting time compared to Na₂SiO₃ geopolymer. Hence, such binder can be implemented as a green non-structural material.