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.

As awareness of the impact of anthropogenic activities on climate change increases, the concepts of durability, resilience, and sustainability in concrete tend to be adopted more seriously in the concrete construction industry. In this sense, one of the concrete technologies that began in the 1980s and that significantly contributes to maximize the beneficial effect on all these concepts are the ultra-high-performance concretes, a very attractive technology because it presents ultra-high strength and durability performances far superior to those of conventional concretes, a performance that is leading to a permanent increased demand. However, the development of these concretes has been widely criticized due to their high ecological impact, which is mainly attributable to the high cement dosages required for their production (800–1000 kg/m³). To address this criticism in a comprehensive manner and thereby reduce the embodied carbon attributable exclusively to the material, this research was oriented to determine the effect of an industrial by-product of vitrified clay, as a partial or total substitution for cement, silica fume, and limestone aggregate, on the compressive strength, flexural toughness, and embodied CO₂. For the UHPC’s evaluated in this work with a dosage of 2% by volume of steel micro-fibers, the results evidence the feasibility that the following substitutions by mass: 30% of the Portland cement, 100% of the silica fume, and 30% of the limestone aggregate and powder, do not detract the fresh stage, the compressive strength, the static modulus of elasticity, and the flexural strength, leading to significant reductions of the embodied CO₂.

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.

The development of cementitious materials with multifunctional performance is increasingly important to address environmental demands and durability requirements in modern infrastructure. This study investigates calcium sulfoaluminate (CSA) cement partially substituted with blast furnace slag (BFS), fly ash (FA), and TiO₂ nanoparticles, aiming to combine sustainability with photocatalytic self-cleaning functionality. Phase analysis by X-ray diffraction confirmed the formation of characteristic CSA hydration products, including ettringite, ye’elimite, anhydrite, and calcite, indicating that partial substitution did not disrupt the primary hydration mechanisms. Microstructural observations revealed that the incorporation of BFS, FA, and TiO₂ induced noticeable morphological changes, with increased porosity and microstructural heterogeneity at higher replacement levels. Mechanical testing showed that moderate BFS contents of 5 to 10 wt% enhanced compressive strength in reference mixtures, while systems containing TiO₂ exhibited slightly lower strength values and increased dispersion, particularly at elevated slag contents. The photocatalytic performance, evaluated through Rhodamine B degradation under solar irradiation, demonstrated a marked improvement for TiO₂-containing samples, reaching degradation efficiencies of up to 80%, in contrast to negligible activity in unmodified systems. These results confirm that the combined use of industrial by-products and photocatalytic nanoparticles in CSA-based matrices represents a viable strategy for producing sustainable cementitious materials with added environmental functionality, without compromising fundamental structural performance.

En esta investigación se ensayaron especímenes bajo carga sostenida a compresión axial para obtener el comportamiento por flujo plástico a corto plazo, esto es, duración de una hora. Los especímenes fueron sometidos a diversas cargas (20%, 50% y 80% de su capacidad) a diversas edades (7, 28 y 90 días), registrándose la deformación longitudinal y transversal en el tiempo. Posteriormente, los especímenes fueron ensayados a la falla, obteniéndose la gráfica esfuerzo-deformación, la capacidad a compresión y módulo de elasticidad. Se encontró que los especímenes sometidos al 20%, la capacidad y el módulo de elasticidad se incrementan ligeramente, mientras que en especímenes sometidos al 50% y 80%, su capacidad y módulo de elasticidad decrecen, para todas las edades.