An alternative approach to reducing the clinker factor, i.e., worldwide CO₂ emissions resulting from the production of composite cement, is to replace these materials with supplementary aluminosilicate-based materials that promote the formation of alkali-activated cements, whose elevated temperature resistance, limited permeability, strong binding properties, excellent durability, high chemical corrosion resistance, confinement of toxic waste, and environmentally low impact have attracted a lot of attention in the cement industry. The principal aluminosilicate-based supplementary materials (SCMs) used in the cement industry are fly ash and blast-furnace slag. Recently, limestone has been proposed for use in alkali-activated cement to improve mechanical resistance and promote nucleation sources for the hydration of hybrid gels. In the current research work, the effect of 5 and 10 wt% limestone additions to slag and fly ash/slag alkali-activated cements with NaOH-4M was studied to evaluate the mechano-chemical and microstructural properties of alkali-activated cement. The effect of limestone was studied using mechanical resistance, XRD, FTIR, SEM-EDS, and calorimetry methods. The XRD, FTIR, and SEM-EDS results demonstrated the formation of portlandite Ca(OH)₂ after the activator solution’s reaction with limestone. The limestone’s dissolution in Ca²⁺ contributes to hybrid gel formation ((N, C)-A-S-H, N-A-S-H, and C-A-S-H), resulting in compressive strength higher than 20 MPa, the recommended resistance for commercial cement.

The partial replacement effect of Portland cement by geothermal nano-SiO₂ waste (GNSW) for sustainable Portland-cement-based concrete was investigated to improve the properties of concrete exposed at high critical temperatures. Portland cement was partially replaced by 20 and 30 wt.% of GNSW. The partial replacement effect on Portland-cement-based concrete subjected to 350, 550, and 750 °C was evaluated by measuring the weight changes, ultrasonic pulse velocity, thermogravimetric and differential thermal analysis, X-ray diffraction, surface inspection, and scanning electron microscopy under residual conditions. The ultrasonic pulse velocity results showed that the GNSW specimens maintained suitable stability after being heated to 350 °C. The SEM analysis revealed a denser microstructure for the 20 wt.% of partial replacement of Portland cement by GNSW specimen compared to the reference concrete when exposed to temperatures up to 400 °C, maintaining stability in its microstructure. The weight losses were higher for the specimens with partial replacements of GNSW than the reference concrete at 550 °C, which can be attributed to the pozzolanic activity presented by the GNSW, which increases the amounts of CSH gel, leading to a much denser cementitious matrix, causing a higher weight loss compared to the reference concrete. GNSW is a viable supplementary cementitious material, enhancing thermal properties up to 400 °C due to its high pozzolanic activity and filler effect while offering environmental benefits by reducing industrial waste.

Nowadays, the study of the improvement of mechanical properties is of great importance in the ceramic industry. The strenuous demand to which industrial processes are subjected leads to the search for new raw materials in the production lines that transfe

This study addresses a key unresolved question in alkali-activated materials: how externally supplied calcium from limestone influences calcium-induced hybrid gel formation under NaOH-only activation in precursor systems with different initial calcium availability. Unlike previous studies focusing mainly on strength development or using combined alkaline activators, this work establishes a composition-dependent mechanistic framework linking calcium availability, reaction kinetics, microstructural evolution, and engineering performance. Limestone additions (5–10 wt.%) were incorporated into fly ash (FA0) and slag–fly ash (FAS0) binders activated with 8 M NaOH and cured at 65 °C. In the low-calcium fly ash system, compressive strength increased from 29 to 38 MPa (≈30%), with nearly 90% of the ultimate strength achieved within the first 24 h. In contrast, slag-containing binders exhibited strength variations below 5%, indicating limited additional benefit once sufficient calcium was intrinsically available. Integrated characterization (SEM/EDS, XRD, FTIR, isothermal calorimetry, and TGA–DSC) consistently supports the progressive transformation from predominantly N–A–S–H gel toward hybrid calcium-containing binding environments. The combined evidence suggests that localized calcium availability associated with limestone accelerates hybrid gel formation and microstructural densification rather than acting solely as an inert carbonate filler. These findings provide practical guidelines for designing low-carbon hybrid alkali-activated binders with tailored reaction kinetics, optimized engineering performance, and improved material efficiency.