Based on economic and environmental considerations, the recycling economy of mineral waste has been found to have great potential and economic benefits worldwide, in which alkali-activated cementitious materials are one of the main developing directions. The alkali activators commonly used in alkali-activated cementitious materials are the composite activators of sodium silicate solution and solid sodium hydroxide, which not only need to deal with high viscosity and corrosive chemicals, but also need to be prepared in advance and properly stored. In this paper, ground granulated blast furnace slag (GGBS) and fly ash were used as precursors, while solid sodium silicate powder was applied as the alkali activator. In addition, the precursors were mixed with the activator in advance and activated by adding water to prepare alkali-activated GGBS/fly ash cement. The influence of precursor components, the dosage of the alkali activator and the liquid–solid ratio on the working performance, mechanical strength and hydration process of alkali-activated cement was studied. The results showed that the further incorporation of GGBS accelerated the alkali activation reaction rate and improved the strength of the specimen. However, in the specimen with GGBS as the main component of the precursor, the main hydration product was C-A-S-H gel, which was different in the structural order and quantity. The compressive strength indicated that there was the best amount of activator to match it in terms of the precursor with certain components. A too high or too low amount of activator will hinder the alkali activation reaction. This study can provide some significant reference material for the use of solid alkali activators in alkali-activated cementitious materials.

Solar light with high-energy ultraviolet (UV) radiation acting on the surface of cement-based materials easily changes the properties of cement-based materials by affecting their carbonation reaction. In order to elucidate the difference in the carbonation process under UV radiation in cement-based materials with different supplementary cementitious materials (SCMs), the carbonation depth (apparent pH values), chemical composition (XRD, FTIR, and TG analysis), and mechanical properties (compressive strength and microhardness) of cement-based materials were evaluated. The results revealed that UV radiation acting on the surface of cement-based materials accelerated the carbonation reaction, which enhanced the decrease rate of pH and formation of stable calcite, thereby improving the macromechanical and micromechanical properties of cement-based materials. In addition, the carbonation process under UV radiation differs according to the added SCM. In particular, silica fume substantially increased the carbonation of cement-based materials under UV radiation, resulting in a 53.3% increase in calcium carbonate coverage, a 10.0% increase in compressive strength, and a 20.9% increase in mean microhardness, whereas the incorporation of blast furnace slag resulted in a smaller effect on UV irradiation-induced carbonation. In addition, UV radiation facilitates the crystallographic transformation process of cement-based materials containing metakaolin, resulting in more stable crystals of carbonation products. This study provides a theoretical framework and serves as an important reference for the design of cement-based materials under strong UV radiation for practical engineering applications.

This study compared the physical properties and mechanical strength development of PCBAs with water, sealed, standard, and open ambient air curing over 28 days to find a suitable curing method for the production of phosphogypsum-based cold-bonded aggregates. The types and relative amounts of hydration products, microstructural morphology and pore structure parameters were characterized utilizing XRD, TGA, FTIR, SEM and nitrogen adsorption methods. According to the results, water curing leads to rapid increases in single aggregate strength, reaching 5.26 MPa at 7 d. The standard curing condition improved the 28 d mechanical strength of the aggregates by 19.3% over others by promoting the generation of hydration products and the transformation of the C-S-H gel to a higher degree of polymerization and by optimizing the pore structure. Further, PCBAs achieved an excellent solidification of phosphorus impurities under all four curing conditions. This work provides significant guidance for selecting an optimized PCBA curing method for industrial production.