Synthesis and characterization of supersulfated cements from kaolin, quartz and NaOH

Supersulfated cements (SSC), being possible green alternatives to Portland cements, have demand to develop new kinds of precursors instead of blast furnace slag. The mixture of kaolin, quartz and NaOH was calcined to prepare SSC by grinding the clinkers with lime and gypsum. The preparation factors were investigated and multiple characterization methods were adopted to explore the calcination and hydration mechanisms. It is found that NaOH additive reduced the activation temperature of the kaolin and moderate addition of NaOH improved the compressive strength of SSC made from calcined kaolin. Quartz added into the kaolin calcined with moderate amount of NaOH promoted the compressive strength as well. Comprehensive characterizations indicated that the alkali-added calcination of the kaolin generated pozzolanic zeolite and sodium aluminosilicates along with inert nepheline at 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. Too much NaOH added into the kaolin calcined at temperature ≤ 950 °C led to generating calcium-absent geopolymer hydrates that restrained the compressive strength development. During NaOH-added calcination, quartz added into the kaolin hindered generating inert nepheline and produced cementitious N-S gels. The research facilitates the utilization of the massive discharged coal gangues rich in kaolinite and quartz by synthesizing SSCs uncostly.