Synergistic utilization of waste paper sludge ash and superabsorbent polymer in lightweight metakaolin-based geopolymer concrete: Mechanical properties, microstructure, and shrinkage evaluation

This research presents a comprehensive investigation of the synergistic interaction between waste paper sludge ash (WPSA) and superabsorbent polymers (SAP) on the mechanical and durability performance of metakaolin-based lightweight geopolymer concrete (LWGC). Although WPSA has demonstrated promising performance in Portland cement–based systems, its potential in geopolymers remains largely unexplored. The CaO-rich ash may compensate for the low CaO content in MK and could facilitate the formation of hybrid C–S–H and N–A–S–H gels, potentially enhancing bonding and structural integrity. Additionally, SAP, with its superior ability to absorb and retain water, facilitates internal curing, mitigates drying shrinkage, and helps prevent cracking. To experimentally verify this hypothesis, WPSA was calcined at 650–800 °C for 2–5 h and characterized using TGA, XRD, XRF, SEM, BET, and density tests. Calcination at 750 °C for 2 h produced the highest amorphous phase and surface area, enhancing reactivity. The effects of Na₂SiO₃/NaOH ratios (1, 1.5, 2 wt%), WPSA replacement (0, 10, 20 wt%), and SAP contents (0, 1, 2 wt%) on slump, density, compressive and splitting tensile strengths, water absorption, electrical resistivity, shrinkage, and microstructure were investigated. Furthermore, the evolution of binding phases and matrix structure in selected mixes was examined using FTIR and XRD analyses, complemented by SEM observations. Incorporating 10 wt% WPSA and 1 wt% SAP at Na₂SiO₃/NaOH=1.5 increased 28-day compressive strength by 10% and tensile strength by 5%, reduced water absorption by 15%, increased resistivity by 28%, and decreased drying shrinkage by 40%. SEM observations confirmed a denser gel structure and lower porosity. Overall, the combination of WPSA and SAP resulted in LWGC with improved mechanical performance, durability, and volumetric stability, offering an environmentally friendly and sustainable approach for construction applications. • WPSA was utilized as a reactive precursor in LWGC. • Optimal calcination (750 °C–2 h) yielded high amorphous content and BET surface area. • SAP improved internal curing and reduced drying shrinkage. • Combined WPSA–SAP use enhanced strength and microstructural densification of LWGC. • SEM, XRD, and FTIR results confirmed reduced porosity and improved geopolymeric gel formation • Electrical resistivity and shrinkage tests indicated improved durability of LWGC mixes.