Physical and microstructural characterization of wood ash–based geopolymer mortars with ceramic powder under different curing regimes

In this study, geopolymer mortars were produced by activating wood ash with NaOH at three different dosages. In the mortar mixtures, wood ash was partially replaced by ceramic powder (CP) at three weight fractions (10, 20 and 30 wt%). The mixtures were cured at three different temperatures (20, 70 and 100 °C). The resulting geopolymer mortars were characterized in terms of unit weight (bulk density), water absorption and apparent porosity; compressive strength and ultrasonic pulse velocity tests were also performed. Microstructural and mineralogical analyses were carried out using SEM/EDS, XRD and FTIR analyses. Unit weights of the mortars were generally below 2.00 g·cm⁻³ . Compressive strength improved with increasing curing temperature and with higher Na and CP contents. SEM and XRD observations revealed the formation of geopolymeric gels resembling C(N)ASH and the presence of needle-like crystals similar to ettringite. The results indicate that the produced geopolymer mortars have potential for use in cement-free lightweight composite applications, and that wood ash and ceramic powder can be jointly employed as suitable precursors for geopolymer synthesis. The best overall physical and mechanical performance was obtained for the mixture coded N14CP10–100 (produced with 14 wt% Na and 10 wt% CP, cured at 100 °C). • Wood ash activated with NaOH and partly replaced by 10–30 wt.% ceramic powder produced lightweight geopolymer mortars. • Higher NaOH dosage, ceramic powder content and curing temperature markedly reduced porosity and water absorption. • Compressive strength increased with Na and CP contents and 100 °C curing; optimum mix was N14CP10-100. • SEM/EDS and XRD analyses revealed C(N)ASH/CSH-type geopolymer gels and needle-like ettringite crystals. • FTIR and mineralogical data confirmed effective geopolymerization of wood ash–CP blends for cement-free composites.