The effects of SiO2/Na2O molar ratio on the characteristics of alkali-activated waste catalyst–metakaolin based geopolymers

• Samples with higher S/N appeared to have denser structures, high strength and low porosity.
• The S/N mole ratio of the geopolymer has a significant effect on its properties.
• Waste catalyst can be used as a source material for producing geopolymer.
• Recycling waste catalyst can not only save the costs significantly and lead to environmental benefits.

Geopolymers, which are a new form of aluminosilicates, are environmentally friendly materials that exhibit high mechanical strength. Waste catalysts that contain a large amount of amorphous SiO2 and Al2O3 are highly soluble in alkaline solutions. This study investigated the effects of the SiO2/Na2O ratio (S/N) and various levels of waste-catalyst replacement on the properties of waste-catalyst metakaolin-based geopolymers (WCMBGs). The mechanical and microstructural properties of the materials were studied by performing compressive strength testing, porosity analysis, thermogravimetric analysis, 29Si magic-angle spinning nuclear magnetic resonance (29Si MAX NMR) spectroscopy, and scanning electron microscopy. When S/N was 1.93, WCMBGs containing 10% of the waste catalyst exhibited high compressive strength and low porosity. Thermogravimetric analysis indicated that the weight loss of the materials was related to the structural composition. 29Si MAX NMR results showed that the major structure of the elements Si and Al was Q4(3Al). The experimental results indicated that waste catalyst can potentially serve as a partial replacement for metakaolin and exhibits favorable mechanic characteristics.