Preparation and properties of geopolymer containing phosphoric acid-activated fly ash and mechanically-milled kaolinite: Experiments and density function theory

The environmental pollution resulted from the stacking of solid wastes, and the 250 kg CO2 emission caused by calcined a ton of kaolinite have become an urgent problem to be solved. In this study, a novel phosphoric acid-activated geopolymer was developed, which was composed of fly ash (FA), mechanically activated-kaolinite (MAK). The influences of phosphoric acid concentration, mechanical grinding duration, and curing temperature were investigated. Microstructural properties of FAMAK geopolymers were examined through a serious of tests. Furthermore, the Gibbs free energy and nano-scale structure characteristics of acid-activated hydration products were calculated for the first time by using density functional theory. The results show that the optimal mass ratio of FA to MAK is 7:3 with 8 mol/L phosphoric acid, which led to the maximum compressive strength of 36.6 MPa at 56-day. Besides, the optimal grinding time is 3 h and temperature is 40 °C. Moreover, the content of silicon promotes the polymerization reaction. The valence and bonding of aluminum are primarily affected by its coordination number. The key outcomes in this study provides a low-carbon cementitious materials, which can achieve the double benefits of carbon reduction and the utilization of solid wastes.