Hybrid cement has become one of the most viable options in the reduction of CO₂ emissions to the environment that are generated by the cement industry. This could be explained by the reduction of the content of clinker in the final mixture and substitution of the remaining percentage with supplementary cementitious materials with the help of an alkaline activation. Following that, properties that are provided by an Ordinary Portland Cement and of a geopolymer are mixed in this type of hybrid material and could be achieved at room temperature. Thereafter, the main objective of this research was to synthesize hybrid cements reducing the clinker content of Portland Cement up to 20% and use metakaolin and fly ash as supplementary cementitious materials in different proportions. The mixtures were alkaline activated with a mixture of sodium silicate and sodium hydroxide, calculating the amounts according to the percentage of Na₂O that is present in each of the activators. The samples were then characterized using Compressive strength, X-ray diffraction, Fourier Transform Infrared Spectroscopy, and Scanning Electron Microscopy with energy-dispersive X-ray spectroscopy. The results indicated that the hybrid cements have similar mechanical properties than an Ordinary Portland Cement, and they resulted in a dense matrix of hydration products similar to those that are generated by cements and geopolymers.

In recent decades, alkali activated and blended cements have attracted great interest worldwide due to their advantages of low energy cost, high strength, and good durability. This study evaluated the effects of replacing 50% of Portland cement with a mixture of three waste materials: ground granulated blast furnace slag (GGBFS), fly ash (FA), and geothermal waste (GS), with and without external alkaline activation, and activated with different alkali agents: 4 and 7% Na₂O equivalent of sodium hydroxide, sodium silicate (water glass), and sodium sulfate. After 90 days of curing, samples were characterized using compressive strength tests, scanning electron microscopy, X-ray diffraction, and thermogravimetric analyses. The results showed that sodium hydroxide caused an alkali–silica reaction and reduced the strength, while sodium silicate and sodium sulfate improved the strength and hydration products formation. Moreover, the addition of fly ash decreased the compressive strength but increased the workability, while the addition of slag and geothermal waste increased strength and densified the matrix with the formation of additional hydration products. The blended cements without activation also showed better performance than pure cement and a more compact matrix of hydration products. The study demonstrated the feasibility of using waste materials to produce blended cements with low energy costs and high durability.

Supplementary cementitious materials are considered a viable and affordable way to reduce CO₂ emissions from the cement industry’s perspective since they can partially or nearly entirely replace ordinary Portland cement (OPC). This study compared the impact of adding spent coffee grounds (SCGs), fly ash (FA), and volcanic ash (VA) to two types of cement: OPC and calcium sulfoaluminate cement (CSA). Cement samples were characterized using compressive strength measurements (up to 210 days of curing), scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), attenuated total reflection infrared spectroscopy, and hydration temperature measurements. In all the studied systems, the presence of SCGs reduced compressive strength and delayed the hydration process. CSA composite cement containing 3.5% SCGs, 30% FA, and 30% VA showed compressive strength values of 20.4 MPa and 20.3 MPa, respectively, meeting the minimum requirement for non-structural applications. Additionally, the results indicate a formation of cementitious gel, calcium silicate hydrate (C-S-H) in the OPC-based composite cements, and calcium alumino-silicate hydrate (C-A-S-H) as well as ettringite in the CSA-based composite cements.

The effective utilization of steel slag waste offers a sustainable approach to advancing construction materials, particularly in cold regions. This study investigates the influence of alkaline activators and low-temperature curing (10°C) on the performance of steel slag cement mortar. The setting time of mixed slurry with varying steel slag powder content was analyzed, along with the effects of different activators and water glass modulus on compressive strength. Advanced characterization techniques, including thermogravimetric analysis (TGA), X-ray diffraction (XRD), and scanning electron microscopy (SEM), were employed to examine the hydration products and microstructural evolution of optimally activated samples. The results indicate that a Na₂O content of 4% significantly enhances the compressive strength of steel slag cement mortar. The composite activator (containing water glass and NaOH) exhibited superior performance compared to neat NaOH, increasing the strength. Activated samples demonstrated increased early hydration products, as evidenced by intensified Ca(OH)₂ diffraction peaks. The activators facilitated calcium ion dissolution, reduced the activation energy of cementitious materials, and accelerated hydration kinetics, leading to the enhanced formation of hydration products. Microstructural analysis revealed that the activators disrupted the vitreous network structure of the steel slag powder, promoting more complete hydration and the formation of dense C-S-H gels and Ca(OH)₂. This densification reduced porosity and significantly improved the mechanical properties of the cement matrix. These findings highlight the potential of alkaline-activated steel slag cement mortar as a sustainable and high-performance material for construction applications in cold climates.