Development of kaolin based geopolymer coating for lumber wood applications

Abstract

The use of ordinary Portland cement is evident for centuries now especially in civil industries. Lacking of greener option with equal or enhanced properties forced consumers to ignore the shortcomings of ordinary Portland cement. Investigation of kaolin based geopolymer coating was aimed to not only increase the productivity and applications of geopolymer but also to potentially replace the use of ordinary Portland cement in terms of coating technology. Initial aim for this study was to prepare an optimum geopolymer coating paste made up of kaolin, as the aluminosilicate source and alkaline activator solution. The optimum mix design was mainly judged by its mechanical strength, followed by physical, phase analysis and scanning electron microscopy micrographs as crucial parameters of sodium chloride concentration, solids-to-liquid (S/L) ratio and alkaline activator ratio was studied. Kaolin, the geopolymer source material was characterized by using particle size analysis, phase, chemical composition, and scanning electron microscopy testing. Kaolin geopolymer paste with 8 M sodium hydroxide molarity, solids-to-liquid (S/L) ratio of 0.9 and alkaline activator ratio of 0.40 cured at 70 °C for 24 hours, gave highest strength values of 2.4 MPa (7 days), 2.98 MPa (28 days) and 4.56 MPa (90 days). This mix design also proven to have good workability, density, semi crystalline phase, and homogeneous compacted geopolymer matrix through morphology micrographs, in agreement to strength values. Continues improvement of kaolin geopolymer paste over time showed promising potentials towards preparation of kaolin based geopolymer coating. To the best of our knowledge, no attempts have been made previously to produce kaolin based geopolymer coating for lumber wood application, thus making it a novel work. The geopolymer paste were then coated on most unlikely substrate to investigate the extent of its bonding capabilities. This was evidently studied through bonding, physical, mechanical and morphological results. Optimum kaolin geopolymer coating successfully adhered to lumber wood substrate and provided high strength value of 4.3 MPa (7 days), 4.9 MPa (28 days) and 5.96 MPa (90 days). Early mechanical strength of kaolin geopolymer coated lumber wood differs from mechanical strength exhibited by kaolin geopolymer paste due to moisture absorption into substrate as supported by interfacial layer micro images. Hardness value was in agreement with flexural strength of kaolin geopolymer coated lumber wood and phase analysis of kaolin geopolymer paste. Zeolite peak contributes to high early strength development but disrupts geopolymer structure over time that leads to drop in strength upon sample age. Investigation of sample over different time period is proven to be important for long term usage of geopolymer coating. Ultimately, this study clearly demonstrated the processing and feasibility of kaolin geopolymer coating material.