Shrinkage cracking of concrete at an early age has notable and adverse effects on the durability and long-term performance of the material because it damages surface integrity or undermines overall uniformity of concrete. In order to mitigate early-age concrete shrinkage cracking, a new gel-type superabsorbent polymer (SAP) has been developed as an internal curing agent (ICA). This paper investigates the effects of the introduced gel-type ICA dosage and curing conditions on early-age cracking mitigation of concrete. It is found that the cracking area of concrete is greatly reduced when up to $41\mathrm{kg}/{\mathrm{m}}^3$ gel ICA is used in concrete with water:cement ratio ($w:c$ ) of 0.5, and the internal relative humidity (IRH) of concrete at 7 days can be maintained at 92% or above. The gel-type ICA also is more effective in delaying initial cracking time and lowering the decreasing rate of IRH. It is believed that the superiority of this newly introduced ICA lies not only in its uniform dispersion in concrete but also in its molecular network structures, which contribute to maintaining moisture balance in the near-surface layer of concrete. This new gel-type ICA provides a new alternative way to control early-age cracking of modern concrete and it has a promising future in concrete construction, especially in windy, drying, and high-temperature areas.

In order to increase use ratio of steel slag solid waste, the concrete containing steel slag powder and zeolite powder as admixtures was prepared by using the orthogonal test method. The effects of water-binder ratio, sand ratio, steel slag powder content and zeolite powder on working properties, mechanical strength and chloride ion permeability of the concrete was studied. It was found that the early strength of the concrete had a decrease with the mixing of steel slag and zeolite powders, but its later strength approached to pure concrete. Moreover, the physical filling and pozzolanic activity of the admixtures increased the density of the concrete, resulting in the improvement of the durability of the concrete by the migration speed of Cl− reducing. The optimum mix ratio of C40 steel slag powder-zeolite powder concrete is obtained, and which had the slump of 220 mm, the 3 d, 7 d and 28 d compressive strengths of 27.8 MPa, 37.5 MPa and 48.4 MPa, the 6 h electric flux of 950 C and the diffusion coefficient of 1.65 × 10−12 m2 /s.

In this paper, Bayer red mud and ground granulated blast furnace slag were formed into three different ratios (7:3, 5:5 and 3:7) of alkali-activated cementitious materials (AASR) using a water glass solution as the alkali activator. By studying the compressive strength of different red mud content, the effect of Bayer red mud content on the mechanical properties of AASR was analyzed. The influence mechanism of red mud content on the mechanical properties of AASR was revealed by the variation of micro-porosity, connected porosity, Isothermal calorimetry, XRD, FTIR, and BSE/EDS testing techniques. The results show that the compressive strength of the specimens at the same age decreases with the increase of red mud content. On one hand, the addition of red mud makes the connected porosity in the AASR system increase, and the densification of the slurry structure decreases; on the other hand, the inert minerals in red mud participate in the hydration reaction to a lesser extent, the overall hydration reaction process becomes lower, and the generation of hydration products providing strength decreases.

The low mechanical strength of a low-calcium fly ash (FA)–red mud (RM) geopolymer severely limits its application. Steel slag (SS) and Ca(OH)₂ can provide calcium and alkali for the hydration process of a low-calcium FA-based geopolymer. In this study, SS was used to replace part of the RM, and Ca(OH)₂ was introduced. The effects of SS and Ca(OH)₂ on the properties of the FA-RM geopolymer were investigated. The experimental results show that SS promoted the matrix to generate more C(N)-A-S-H and C-S-H gels and optimized the pore structure, thereby improving the mechanical properties of the FA-based geopolymer. The addition of 4 wt.% Ca(OH)₂ increased the hydration products of the FA-based geopolymer, the microstructure was denser, and the mechanical properties were significantly improved. The 28 d compressive strength of the FA-based ternary composite geopolymer prepared by replacing part of the RM with SS and adding Ca(OH)₂ reached 30.6 MPa, which provided an experimental basis for the resource utilization of various bulk solid wastes.

The study of the mechanisms affecting the preparation parameters of carbonated steel slag is of great significance for the development of carbon sequestration materials. In order to elucidate the mechanism of the influence of moisture on CO₂ diffusion and particle cementation in steel slag, the effects of different water–solid ratios and water contents on the mechanical properties, carbonation products, and pore structure of steel slag after carbonation were investigated. The results show that increasing the water–solid ratio of steel slag can control the larger initial porosity and improve the carbon sequestration capacity of steel slag, but it will reduce the mechanical properties. The carbonation process relies on pores for CO₂ diffusion and also requires a certain level of moisture for Ca²⁺ dissolution and diffusion. Increasing the water content enhances particle cementation and carbonation capacity in steel slag specimens; however, excessive water hinders CO₂ diffusion. Reducing the water content can increase the carbonation depth but may compromise gelling and carbon sequestration ability. Therefore, achieving a balance is crucial in controlling the water content. The compressive strength of the steel slag with suitable moisture and initial porosity can reach 118.7 MPa, and 217.2 kg CO₂ eq./t steel slag can be sequestered.

To mitigate the shrinkage properties of ultra-high performance concrete (UHPC), MgO expansion agents (MEAs) with different activity levels (R-MEA, M-MEA, and S-MEA) were prepared and incorporated into UHPC. The effect of MEA activity on the mechanical properties and volumetric stability of UHPC were evaluated by using hydration heat tests, XRD-Rietveld quantitative analysis, MIP, X-CT and SEM. The results showed that MEA addition reduces the mechanical properties of UHPC, especially at high activity levels. However, it is beneficial for compensating early shrinkage. By combining MIP and X-CT analyses, it was found that MEA effectively increases the porosity of UHPC, with R-MEA (with the strongest activity) increasing it most. The w/b ratio had a greater effect on MEA hydration than the activity level. At lower w/b ratios, R-MEA reduced autogenous shrinkage even less effectively than M-MEA. Considering both the mechanical properties and shrinkage-reducing effect, it is recommended to prepare shrinkage-reducing UHPC with a w/b ratio of 0.18 and moderately reactive M-MEA.