Geopolymers are typically cured either at ambient temperature (~25 °C) or subjected to short-term heat curing before being stored under ambient conditions until testing. However, in hot-arid regions, the daily ambient temperature may exceed 45 °C during summer months. Therefore, such conditions should also be considered as high ambient curing, and their influence on low-calcium geopolymer performance needs to be investigated. In this study, pumice- and fly ash-based geopolymer mortars were produced to evaluate the effects of different curing regimes. In the pumice-based mixtures, 10 wt% of pumice was replaced with metakaolin to enrich the alumina content. Three curing conditions were applied: ambient curing, high ambient curing, and heat curing. Setting times of geopolymers were determined based on each curing regime. Physical properties, including density, water absorption, and sorption coefficient, were assessed. Compressive strength development was evaluated over 90 days. In addition, durability performance was assessed through water resistance, freeze–thaw durability, and resistance against sulphuric and hydrochloric acid. Fourier transform infrared spectroscopy and X-ray diffraction confirmed that geopolymerisation reactions continued significantly up to 90 days under high ambient curing, while mercury intrusion porosimetry showed a reduction in porosity. These findings explain the continuous increase in compressive strength. Pumice-based geopolymers cured under this condition exhibited significantly better long-term strength than those cured under other regimes. High ambient-cured fly ash-based geopolymers, a 3-day strength of 40.3 MPa was achieved, eliminating the need for heat curing. Thus, high ambient curing enables the in situ use of these geopolymers and offers a cost-effective and eco-friendly alternative.

The effect of sodium hydroxide (SH) concentration and sodium silicate (SS) content in alkali activator on mechanical properties and wet-dry resistance of fly ash added pumice based geopolymer paste were investigated. In the study, the concentration of SH was used as 8, 10, 12 and 14 molar, while the weight ratio of SS to SH was used as 0, 1, 2 and 3. While the density of geopolymer paste samples increased significantly with the increase of SH molarity in mixtures with low SS content, it was less affected by SH molarity in mixtures with high SS content. For all SH molarity values, the compressive strength of the geopolymer paste samples increased with the increase of the SS/SH ratio, while a slight decrease was observed with the SS/SH ratio increasing from 2 to 3. However, with the increase of the SH molarity, the compressive strength of the samples was less affected by the SS content. As large cracks or splits occur in paste samples activated only with SH under the effect of wet-dry, their compressive strength could not be measured. The wet-dry performance of the paste samples activated with SS + SH was not affected much by the SH molarity and the SS/SH ratio and were close to each other. When looking at the microstructure studies, FTIR analyzes show that SS content provides a better geopolymerization, and SEM images show that micro-cracks relatively decrease with SS content.

Alkali-activated cements have gained attention as sustainable alternatives to Portland cement-based concretes. This study aims to enhance the early strength development of alkali-activated slag (AAS) mortars by preheating the alkali activator, ground granulated blast furnace slag, and aggregates to temperatures reaching up to 65°C before their utilization. Preheating at 50°C enhances both early and ultimate strength of AAS, compared to specimens that were not subjected to preheating. Similarly, preheating at 65°C results in a substantial increase in early strength but leads to some strength reduction after three days. SEM and FTIR analyses reveal that preheating at 50°C improves the interfacial transition zone between paste and aggregate, reduces microcracks, and promotes alkali activation. Furthermore, FTIR analysis confirms the occurrence of hydration reactions in AAS upon contact with water.

This study investigates the feasibility of using calcined pumice as a sustainable precursor for geopolymer production. Natural pumice was calcined at different temperatures (600, 750, and 900 °C) and durations (1, 2, and 4 h). The effects of calcination were evaluated through color change, particle size distribution, scanning electron microscopy, energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, and X-ray diffraction. The results showed that calcination induced structural and mineralogical modifications in pumice, including increased disorder in the aluminosilicate network and partial recrystallization, which enhanced its reactivity. Consequently, geopolymer mortars produced with calcined pumice exhibited significantly improved compressive strength, with the highest strength of 53.5 MPa obtained for the sample calcined at 750 °C for 1 h, corresponding to an 84.5% increase compared to the mortar produced with raw pumice. In addition, calcination at 600 °C and 900 °C significantly improved water resistance. Considering mechanical performance, durability-related properties, and energy efficiency together, the calcination condition of 600 °C for 2 h was identified as the optimum treatment. These findings demonstrate that calcined pumice is a promising and sustainable precursor for geopolymer production.