Chemical Bond Characteristic and Strength Activity of Geopolymer Mortar Containing Fly Ash and Chemically Pretreated Paper Mill Sludge Ash

Geopolymer is a synthesized inorganic composite through alkaline activation of aluminosilicate materials, which commonly originated from industrial byproducts such as fly ash. The combination of fly ash with other source materials promotes excellent mechanical and durability properties and improves some specifications of geopolymer concrete. Thus, this study introduces paper mill sludge ash (PMSA) as an alternative partial substitute for fly ash in geopolymer mortar production, tackling the environmental concerns of PMSA disposal. The high calcium (Ca) content in PMSA, contrary to its low alumina and silica levels, poses challenges in the geopolymerisation process, especially during setting, hardening and later age strength. To address this challenge, a pretreatment method involving acid leaching with varying concentrations of hydrochloric acid (HCl) was adopted. This aims to decrease the Ca content and simultaneously enrich the silica and alumina levels in PMSA. Using molarities of 0.5 M, 1.0 M, and 1.5 M HCl, PMSA replaced 5.00 % to 15.00 % of fly ash, and the samples were cured at 30 and 90 °C. A series of tests were conducted to determine the strength and polymeric bonds of geopolymer mortar containing PMSA. Notably, 1.5 M HCl pretreatment effectively reduced calcium from 52.60 % to 22.00 % while elevating silica and alumina levels to 44.00 % and 19.60 %, respectively. The high proportion of silica and alumina accelerated Si-O-Al-O bond formation. The geopolymer mortar formulated with the pretreated PMSA showcased superior strength and a more compact microstructure, attributed to the abundant formation of N-A-S-H gels.