Influence of paper mill sludge ash on mechanical, microstructural and durability properties of metakaolin based geocrete

This study investigates the influence of Paper Mill Sludge Ash (PMSA) on the mechanical, microstructural, and durability properties of metakaolin-based geocrete. Metakaolin (MK) was partially substituted with PMSA at varying levels (5%, 10%, 15%, and 20%), with mixes designed at alkali activator/binder (A/B) ratios of 0.40 and 0.45. The geocrete specimens were synthesized using 8 M sodium hydroxide under ambient curing (24 to 26 °C) conditions in the entire study. Mechanical properties were assessed through compressive, split tensile, and flexural strength tests, while microstructural analysis was conducted using scanning electron microscopy (SEM). Durability performance was examined through rapid chloride penetration (RCPT), water absorption, and porosity tests. The G6 mix, prepared with an A/B ratio of 0.45, consistently demonstrated higher mechanical strength than the G2 mix with an A/B ratio of 0.40, indicating that an increased A/B ratio accelerates the geopolymerization process, enhancing strength development. SEM analysis confirmed this strength improvement, which can be ascribed to the formation of a dense matrix in G6 mixes. The G2 and G6 mix exhibited superior durability, with RCPT values of 1521 and 1414 coulombs, water absorption rates of 2.8% and 2.5%, and porosity levels of 11.73% and 11.58%, respectively. The findings suggest that incorporating 10% PMSA in geocrete formulations provides a viable, low-carbon alternative to cement concrete.