Property and Microstructure of Waterborne Self-Setting Geopolymer Coating: Optimization Effect of SiO2/Na2O Molar Ratio

As a kind of coating material, the inorganic coating of alkali-activated metakaolin geopolymer cured at high temperature has been studied a lot for special applications. To our best knowledge, however, not much attention has been given to investigate the influence of SiO2/Na2O molar ratio on property of the geopolymer coating. This paper is, thus, dedicated to investigate the role of SiO2/Na2O molar ratio on property and microstructure of metakaolin-based geopolymer coating at ambient temperature. The effects on setting behavior, adhesive strength, shrinkage deformation and permeability are discussed. Multiple experiments were used to reveal microstructure changes of the geopolymer coating with different ratios of SiO2/Na2O, including Mercury Intrusion Porosimetry (MIP), Scanning Electron Microscope (SEM) and Fourier Transform Infrared Spectroscopy (FTIR). The results indicated that the optimal ratio of SiO2/Na2O was 1.0 for good properties of adhesive strength, shrinkage and impermeability. In addition, it has been found that the setting time of geopolymer coating increased with SiO2/Na2O ratio which increased from 0.8 to 1.5. That agrees well with the other property and results of exothermal rate of alkali-activated metakaolin. As for the microstructural changes, the SiO2/Na2O ratio of 1.0 reduced pore size and porosity of the geopolymer coating and particularly increased volume percentage of pores with a size lower than 20 nm. Besides, FTIR results suggested that geopolymer prepared by the ratio of 1.0 was likely to produce more heterogeneous geopolymer due to a greater silicate structural reorganization