Synthesis and characterisation of supersulfated cements from kaolin, quartz and sodium hydroxide

Supersulfated cements (SSCs), as potential green alternatives to Portland cements, are in demand for developing new kinds of precursors instead of blast-furnace slag. A mixture of kaolin, quartz and sodium hydroxide was calcined to prepare SSC by grinding clinkers with lime and gypsum. Preparation factors were investigated and multiple characterisation methods were adopted to explore the calcination and hydration mechanisms. The sodium hydroxide additive was found to reduce the activation temperature of the kaolin and a moderate addition of sodium hydroxide improved the compressive strength of SSC made from calcined kaolin. Quartz added into the kaolin calcined with a moderate amount of sodium hydroxide also promoted the compressive strength. Comprehensive characterisations indicated that the alkali-added calcination of the kaolin generated pozzolanic zeolite and sodium aluminosilicates along with inert nepheline at a calcination temperature ≤950°C. The hydration of SSC pastes tended to produce C(N)–A–S–H/N(C)–A–S–H gels and crystalline ettringite. During sodium hydroxide-added calcination, quartz added into the kaolin hindered the generation of inert nepheline and produced cementitious N–S gels. The research facilitates the utilisation of the massive discharged coal gangues rich in kaolinite and quartz by synthesising SSCs economically.