Alkali-treated porous palm fibre as internal curing agent for metakaolin-based alkali-activated materials

High drying shrinkage remains a major challenge in alkali-activated materials (AAM). In this study, porous palm fiber (PF) was modified with NaOH solution, saturated with water via vacuum impregnation, and incorporated as an internal curing agent into metakaolin-based AAM. The influence of PF on hydration behavior and microstructure was examined using scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Vicat setting tests, and low-field nuclear magnetic resonance (LF NMR). The results showed that PF incorporation promoted the formation of additional hydration products and enhanced microstructural densification. Flexural strength was significantly improved, with the sample containing 1.5 % of 6 mm PF achieving the highest enhancement—an increase of 66.7 % compared with the control. After 28 days, a small amount of PF slightly increased the compressive strength. Vertical mortar expansion tests confirmed that PF effectively reduced drying shrinkage, with the most pronounced reductions observed in mixtures containing 1 % of 3 mm PF and 1.5 % of 6 mm PF, which decreased shrinkage by 42.9 % and 41.1 %, respectively, compared with the MK-AAM control sample (P0, 0 % PF). Overall, PF markedly mitigates drying shrinkage, improves flexural performance, and shows strong potential as a sustainable and environmentally friendly internal curing agent for AAM.