As a major steel producer, China is now eager to develop feasible solutions to recycle and reuse steel slag. However, due to the relatively poor hydration activity of steel slag, the quantity of steel slag used as a supplemental binder material is limited. In order to improve the cementitious properties of steel slag, the strength and carbonation degree of the high-content steel slag powder–cement–metakaolin composite cementitious material system under CO₂ curing conditions were investigated. The compressive strengths of the mortar specimens were tested and compared. The carbonation areas were identified and evaluated. A microscopic analysis was conducted using X-ray diffraction (XRD), thermogravimetry analysis (TG), and scanning electron microscopy (SEM) to reveal the chemical mechanisms. The results showed that CO₂ curing significantly increased the early strength as the 3D compressive strength of the specimens increased by 47.2% after CO₂ curing. The strength of the specimens increased with increasing amounts of metakaolin in a low water-to-binder ratio mixture. The 3D compressive strength of the specimens prepared with 15% metakaolin at a 0.2 water-to-binder ratio achieved 44.2 MPa after CO₂ curing. Increasing the water-to-binder ratio from 0.2 to 0.5 and the metakaolin incorporation from 0% to 15% resulted in a 25.33% and 19.9% increase in the carbonation area, respectively. The calcium carbonate crystals that formed during carbonation filled the pores and reduced the porosity, thereby enhancing the strength of the mortar specimens. The soundness of the specimens after CO₂ curing was qualified. The results obtained in the present study provide new insight for the improvement of the hydration reactivity and cementitious properties of steel slag powder.

The application of sludge in the manufacture of ceramic material provides an outlet for waste disposal. In this study, we aimed to produce a new lightweight aggregate applications in concrete. The influence of burning temperature on the ignition loss rate, cylinder compressive strength, and the water absorption rate of ceramsite mixed with sludge and excavated soil was investigated. The slump flow, apparent density, and mechanical strength (flexural and compressive strengths) of cement concrete with ceramsite were determined. Moreover, the chloride ion permeability coefficient and the thermal conductivity were tested. Finally, scanning electron microscopy, X-ray diffraction, and thermal analysis were applied to analyze the mechanisms of the properties of ceramsite. Results show that the ignition loss rate and the burning temperature are in a quadratic relationship. The cylinder compressive strength shows a positive quadratic relationship with the burning temperature. However, the water absorption rate negatively correlates with the burning temperature. The addition of sludge can increase the ignition loss rate and cylinder compressive strength of ceramsite. Meanwhile, the effect of sludge on the water absorption rate is the opposite. Ceramsite decreases the slump flow and the apparent density of cement concrete. Cement concrete with 10% ceramsite shows the highest mechanical strength and the lowest chloride ion migration coefficient. Correction of the chloride ion migration coefficient and the content of ceramsite was performed as an exponential equation. Ceramsite exerts a negative effect on the thermal conductivity of cement concrete. Concrete with sludge ceramsite shows higher slump flow, apparent density, mechanical strength, and resistance to chloride ion penetration and thermal conductivity than concrete sludge with clay ceramsite. The mullite content of sludge ceramsite is higher than that of clay ceramsite. Additionally, sludge ceramsite exhibits a denser structure than that of clay ceramsite.

Purpose: This paper aims to study the influence of NaNO₂ on the chemical composition of passivation film. Design/methodology/approach: X-ray photoelectron spectroscopy and X-ray diffraction were selected to determine the composition of passivation film of steel bars in mortar. The specimens were exposed to the chloride solution, carbonation environment and the coupling effects of chloride solution and carbonation. The chemical composition and micro structures at 0 and 5 nm from the outer surface of the passivation film of steel bars were analyzed. Findings: Results showed that the nitrite inhibitor improved the forming rate of the passivation film and increased the mass ratio of Fe₃O₄ to FeOOH on the surface of steel bars. The component of Fe₃O₄ at 5 nm of the steel passivation film was more than that at 0 nm. Sodium ferrite in the pore solution was easily hydrolyzed and then FeOOH was formed. Therefore, due to the nitrite inhibitor, a “double layer structure” of the passivation film was formed to prevent steels bars from corrosion. Originality/value: This is original work and may help the researchers further understand the mechanism of rust resistance by nitrite inhibitor.

Water absorption is closely related to the durability of concrete. To determine the effect of steam curing on water absorption of concrete at early ages, this paper investigated the water absorption of concrete which had different compositions of cementitious materials under three types of technological measures condition (technology A, B and C), and performed water absorption comparisons of various types of concrete by cutting the specimens into three slices: top-level, internal, and bottom. Results indicate that the water absorption of all specimens appears to be gradient distribution from top-level to bottom, and top-level specimens have the highest water absorption. However, in terms of water absorption, there is no significant difference among the other layers. The highest water absorption value is achieved by the top-level of concrete with technology A, which is 42% higher than that of its mid-level. However, the water absorption of top-level, mid-level, and bottom layer of various concretes with technology B and C are all decreased, especially the concretes incorporating admixture A with technology C.The differential value of water absorption between the top-level and mid-level is 3%∼18% commonly. By use of suitable technological measures and admixtures, the harm of steam curing on the surface structure of concrete will be alleviated, and the inhomogeneity of the internal concrete can be improved.

In precast concrete elements manufacturing, steam-cured concrete incorporating 30% fly ash encountered the problem of a too low demoulding compressive strength. To resolve it, this paper developed a new steam-cured concrete (AFSC) incorporating fly ash and a chemical activator. Experiments were conducted to investigate the mechanical properties of AFSC. The corresponding mechanism was also discussed by testing the microstructure of concrete. Results indicate that the demoulding compressive strength of AFSC can meet production requirements, and compressive and flexure strength of AFSC at later ages increase well. Compared with that of ordinary steam-cured concrete, AFSC has a higher tensile strength, and the capability of AFSC to resist cracks is enhanced remarkably. At an early age, addition of the chemical activator can distinctly accelerate the extent of hydration of the fly ash cement systems, and thus the microstructure of concrete becomes denser.

This paper developed a new high performance steam-cured concrete (HPSC) incorporating grade I fly ash (FA I ) and a chemical activator, which can be used for high speed railway precast elements. Experiments were carried out to investigate the mechanical properties and durability of HPSC. The corresponding mechanism was also discussed by testing microstructure of concrete. Results indicate that the demoulding compressive strength of HPSC is more than 50MPa, and compressive and flexure strength of HPSC at later age increase well. The partial replacement of cement by FA I can significantly improve the adaptability of concrete to steam curing. Compared with that of ordinary steam-cured concrete (OSC), HPSC has lower brittleness, higher plastic deformation and less dry shrinkage, and the capability of HPSC to resist chloride attack is enhanced remarkably. Addition of FA I can improve compositions of hydration products and microstructure, and thus the durability of concrete under steam curing condition.