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Magnesium silicate hydrate (MSH) cement has the advantages of low energy consumption, minimal environmental pollution, carbon negativity, and reduced alkalinity, but excessive drying shrinkage inhibits its application. This paper analyzed the influence of steel slag (SS) dosage, carbon dioxide partial pressure, and carbonation curing time on the compressive strength, shrinkage rate, and phase composition of MSH cement. Various analysis methods, including X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP), were used to study the hydration products and microstructure. The results showed that under normal curing conditions, MSH cement mixed with different steel slag contents experienced a decline in strength at all ages. However, the greater the amount of SS incorporated, the lesser the degree of drying shrinkage. The compressive strength of all groups was improved, and the drying shrinkage was reduced by carbonation treatment. The samples with 5%, 10%, and 15% SS content exhibited shrinkage rates of 2.19%, 1.74%, and 1.60%, respectively, after 28 days of curing. The reason was that after carbonation treatment, hydrated magnesium carbonates (HMCs) were generated in the SS–MSH cement, and a Ca–Mg–C amorphous substance formed by hydration and carbonation of C2S in steel slag filled in the pores, which enhanced the density of the matrix, improved the compressive strength of the specimen, and reduced the shrinkage rate. [ABSTRACT FROM AUTHOR]

Accelerated carbonation can promote the application of steel slag in construction materials. This method can not only resolve the fatal volume expansion of steel slag, but also sequestrate CO2. The carbonation thermodynamics, carbonation degree, carbonation methods and influencing factors of steel slag were reviewed. The theoretical CO2 uptake of steel slag is between 25% to 50%. However, the actual CO2 uptake distribution curve obeys normal distribution with a median value of 15%, which affected by carbonation methods, different influencing factors such as curing temperature, CO2 concentration and pressure, particle size of steel slag, liquid to solid ratio, and extraction agents. The improvement mechanism of accelerated carbonation on mechanical properties and stability of steel slag-based building materials was analyzed. Besides, different accelerated carbonated steel slag-based products with enhanced properties were summarized. Finally, some valuable suggestions concerning accelerated carbonation of steel slag were presented for further research and industrial application. [ABSTRACT FROM AUTHOR]

The use of tricyclic copolymer latex (AMPS) can effectively improve the carbonation resistance of sulphoaluminate cement. This paper investigated polymer AMPS and polycarboxylic acid to modify sulphoaluminate cement materials by exploring the carbonation level of sulphoaluminate cement paste and mortar and the strength before and after carbonation. Then, the optimal dosage of polymer and polycarboxylic acid was obtained so that the carbonation resistance of sulphoaluminate cement reached the best state. The compressive strength was significantly improved by adding AMPS for sulphoaluminate cement paste and mortar. After carbonation, the strength decreased and combined with the carbonation level; it was concluded that the carbonation resistance of sulphoaluminate cement materials was the best when the optimal dosage of AMPS and polycarboxylic acid was 5% and 1.8%, respectively. Due to the addition of AMPS, the hydrated calcium aluminosilicate (C-A-S-H) and hydrated calcium silicate (C-S-H) gels, generated by the hydration of sulphoaluminate cement and the surface of unreacted cement particles, are wrapped by AMPS particles. The water is discharged through cement hydration. The polymer particles on the surface of the hydration product merge into a continuous film, which binds the cement hydration product together to form an overall network structure, penetrating the entire cement hydration phase and forming a polymer cement mortar with excellent structural sealing performance. To prevent the entry of CO2 and achieve the effect of anti-carbonation, adding polycarboxylic acid mainly improves the sample's internal density to achieve the anti-carbonation purpose. [ABSTRACT FROM AUTHOR]

In this study, ternesite was proposed as a low-lime CO 2 sequestration binder, and the ternesite slurry and compacts were carbonated in different periods. The evolution of ternesite carbonation products and properties was studied by qualitative and quantitative analysis, and the kinetics equation of ternesite carbonation was established. According to the microstructure, crystal size and phase composition of carbonated products, a conceptual model of the carbonation hardening process of ternesite was constructed. The mechanism of ternesite carbonation process was analyzed. The results showed that ternesite CO 2 sequestration capacity and compressive strength of ternesite reached 15.1 % and 91.95 MPa after carbonation for 120 min. Ternesite has been shown to quickly obtain a hardened product with good properties through carbonation curing, opening up the field of CO 2 sequestration binder. • Ternesite has been proven to be a significantly competitive CO 2 sequestration binder. • The carbonation products and reaction kinetics of ternesite were analyzed. • The compressive strength of carbonated ternesite for 120 min is 91.95 MPa. • A conceptual model of ternesite carbonation reaction process was established. • Anhydrite and its hydrates act as mediators to promote the carbonation of ternesite. [ABSTRACT FROM AUTHOR]

• Poorly-crystalline calcium carbonates were produced during carbonation. • Well-crystalline calcite with a single crystal structure was formed. • Calcite crystal growth was investigated in terms of crystallite size, particle size, etc. • A compact stack and a strong mechanical bond between well-crystalline particles were observed. • β-C 2 S carbonation products reached 127 MPa after 168 h of carbonation. Larnite (β-C 2 S) is a low-calcium mineral with a low rate of early hydration which has a high CO 2 sequestration potential. Synthetic β-C 2 S was exposed to pure CO 2 for gas–solid carbonation for varied time periods to investigate changes in mineral composition, calcite crystal growth, and the development of physico-chemical properties. Results show that the ultimate degree of carbonation reached 61.5% and the ultimate CO 2 sequestration capacity was 314.7 g/kg raw β-C 2 S. High-resolution transmission electron microscopy, 13C magic angle spinning nuclear magnetic resonance spectroscopy, the Rietveld method, and thermogravimetric analysis confirmed that amorphous and/or poorly-crystalline calcium carbonates were produced during carbonation. Well-crystalline calcite with a single crystal structure was formed, having a crystallite size of 107 nm and an average particle size of 1.8 μm after 168 h of carbonation. The compact stack and the strong mechanical bond between well-crystalline calcite particles were observed in scanning electron microscopy images, and they are thought to contribute to a strong micro level force, thereby leading to the good macro level performance of carbonation products which reached 127 MPa after 168 h of carbonation. This study contributes to a more insightful understanding of the vital role of calcite in promoting the mechanical and physico-chemical properties of β-C 2 S-containing wastes and cements produced by carbonation. A further investigation into the micro level mechanical properties of calcite is required to understand the relation between carbonation and increased strength. [ABSTRACT FROM AUTHOR]

[Display omitted] • Carbonation treatment of semidry high calcium content light-burned magnesium. • A carbonated-517 phase was found to have better stability. • The carbonated products exhibited better mechanical properties. With the large amount of high-quality magnesite mining, low-grade magnesite is discarded, which not only pollutes the environment but also wastes resources. Therefore, to utilize high-calcium magnesite tailings, carbonated light-burned magnesium with a high calcium content was used to prepare magnesium oxysulfate (MOS) cement. Characterization was performed by FT-IR, SEM, TEM, TG-DTG, TG-MS, MIP, XRD, and zeta potential measurements. The following conclusions are drawn: a porous shell layer was formed on the surface of the carbonated powder particles; thus, the hydration reactivity was reduced, but a better dispersibility was obtained. The setting time of the samples increased with increasing carbonation treatment time, and the early strength was lower than that of the control, but it increased significantly with the curing period. The 28 d compressive strength of the samples treated with carbonation for 2 min was 12.1 % higher than that of the control, and the volume density was 3.7 % lower than that of the control. In addition, a carbon-containing 517 phase (C- 517) structure was found. This C-517 phase exhibited better stability and resistance to sodium chloride erosion. After immersion in sodium chloride solution for 28 d, the softening coefficient was 0.84, which was 385 % of the control, so the carbonated samples had better durability than the ordinary MOS cement. The semidry method is used to carbonate light-burned magnesium with high calcium content, which avoids the negative effect of calcium oxide on the hydration process of MOS cement and provides a method to solve the discarding of high-calcium magnesite tailings. [ABSTRACT FROM AUTHOR]

Highlights • Aerated steel slag blocks are prepared by accelerated carbonation. • C 4 AČH 11 formed during the hydration process of steel slag-aluminum system. • C 4 AČH 11 converted to aragonite, vaterite, calcite, and amorphous aluminum hydroxide by carbonation. • The formation and carbonation reactions of C 4 AČH 11 can be produced thermodynamically. Abstract The present paper discusses investigated the foaming behaviors and microstructure of three aerated steel slag block (ASSB) samples prepared with three different foaming agents (namely animal protein, hydrogen peroxide, aluminum powder) and by accelerated carbonation. The composition and microstructure of the ASSB samples before and after accelerated carbonation were characterized using X-ray diffraction (XRD), Rietveld method, thermogravimetric analysis (TGA), fourier transform infrared spectroscopy (FT-IR) and field emission scanning electron microscopy (FE-SEM). It is an advisable method for foamed steel slag products to use the aluminum powder and accelerated carbonation technology. The aluminum powder favored a good foaming behavior for steel slag owing to (i) a high porosity and low dry density, (ii) a high hydration degree of minerals in steel slag, including brownmillerite, larnite, and lime, which was confirmed by the analysis of mineral compositions using Rietveld method, and (iii) the interlaced lamellas of monocarboaluminate and calcium silicate hydrate (C-S-H) gel strengthen the bubbles wall, which was confirmed by morphology analysis using FE-SEM. After accelerated carbonation, the monocarboaluminate shifted to aragonite, vaterite, calcite and amorphous aluminum hydroxide (Al(OH) 3), which was confirmed by XRD, TGA and FT-IR. The network skeleton formed by aragonite crystals improved the compressive strength of ASSB prepared by aluminum powder. In addition, the reactions of the formation and carbonation of monocarboaluminate phase were proved to be thermodynamically possible in this work. [ABSTRACT FROM AUTHOR]

Highlights • The carbonation rates of minerals were ordered (descending) CH, β-C 2 S and C 4 AF. • CH was more readily carbonated than other phases due to its high solubility. • Carbonation was an effective method for reducing the pH of BOFS. • Full carbonation was not reached due to the lack of CO 3 2− with a final pH <10. Abstract Basic oxygen furnace slag (BOFS) contains different alkaline Ca-bearing minerals that lead to high CO 2 sequestration and high pH. The high alkalinity of BOFS is an environmental issue. In this study, we investigated the relationship between accelerated carbonation, pH, and leaching by analyzing changes in mineral composition during carbonation process. Results indicate that carbonation of Ca-bearing minerals in BOFS was mainly from, from highest to lowest, Ca(OH) 2 , β-C 2 S and C 4 AF. Ca(OH) 2 was most likely to be carbonated because of its high solubility. Carbonation greatly reduced the alkalinity of BOFS, from an initial pH of 12.73 (non-carbonated BOFS) to a final pH of 9.36. When the pH value was <10, carbonation was hindered by lack of CO 3 2−. The increase in the degree of carbonation greatly reduced Ca leaching but significantly increased Si leaching. The pH value and Ca leaching from non-carbonated BOFS were determined by Ca(OH) 2. The pH value and Ca leaching from carbonated BOFS tended to equilibrate with a saturated CaCO 3 solution, and Si leaching was determined by amorphous SiO 2 generated during carbonation. [ABSTRACT FROM AUTHOR]

This paper presents a study on rapid hardening behaviors of β-CS by accelerated carbonation curing. β-CS cubes compacted at various molding pressures were subjected to different CO concentration for accelerated carbonation curing. The CO uptake and microstructure changes were analyzed by thermogravimetric analysis (TG), QXRD, FT-IR and MAS-NMR. The results indicated that CO uptake was affected by molding pressure and CO concentration seriously. TG analysis indicated that the carbonation reaction was rapid in the first hour. The carbonation degree reached 21.6% and giving a compressive strength of 85.7 MPa after 6 h carbonation in 99.9% CO concentration. And it showed a much less carbonation degree in 20.0% CO. Calcite, vaterite and amorphous silica-rich phase formed in the carbonation progress. The FT-IR and NMR analysis indicated β-CS was decalcified to C-S-H gel and further decalcified to formation of an amorphous silica gel composed of Q and Q silicate tetrahedral. The chain length of C-S-H gel increased from to 2.67 to 6.36 with prolonged carbonation time, showing a lower C/S ratio and higher polymerization and also resulting in a lower C-S-H content. [ABSTRACT FROM AUTHOR]

From the perspective of effective utilization of resources and environmental preservation, accelerated carbonation of waste cement paste is a promising and environmentally beneficial application. Greenhouse gas CO 2 can be captured, waste hydrated cement can be reused, and building materials with excellent performance can be produced. In this study, samples were exposed to high CO 2 concentration (99.9 wt%, 0.2 MPa) for accelerated carbonation. The compacts with water/solid ratio of 0.15 absorbed 19.8 wt% CO 2 after carbonated for 2 h, and the compressive strength was 28.6 MPa. The total pore volume decreased from 0.41 cm 3 /g to 0.26 cm 3 /g due to the precipitation of calcium carbonate and silica gel. Rietveld refinement quantitative phase analysis and TG analysis showed calcium carbonate are mainly from Ca(OH) 2 and C–S–H, accounting for about 70–80% of the total calcium carbonate. Ca(OH) 2 has superiority in the early carbonation period, and the carbonated compacts has a relative higher “compressive strength-to-CO 2 uptake” value, corresponding. When the carbonation degree of C–S–H and other carbonation phase exceed Ca(OH) 2 in the later stage, the compressive strength has a slower gain tendency. [ABSTRACT FROM AUTHOR]

In this study, calcium silicate hydrate(C-S-H) with CaO/SiO ratio of 1.5 target mixture is synthesized and CSH with 93.4 % purity is obtained. The dried C-S-H was compacted at 8 MPa to 20 mm × 20 mm × 20 mm for accelerated carbonation. The carbonation products and carbonation degree of synthetic calcium silicate hydrate are characterized and quantitatively evaluated by QXRD (Rietveld refinement), mass gain method, and TG/MS analysis. After accelerated carbonation at 0.2 MPa CO pressure for 2 h, the carbonation degree from mass gain method is 71.5 %, and from TG/MS analysis is 78.0 %. Calcite, aragonite, vaterite, and amorphous phase exist simultaneously in the accelerated carbonated samples, accounting for 33.98, 17.13, 18.74, and 30.15 %, respectively. Two mass-loss stages were observed for calcium carbonate decomposition. The first mass-loss stage (300-660 °C range) was mainly caused by the decomposition of aragonite and vaterite, and calcite formed in the accelerated carbonation process has a relatively high decomposition temperature which is mainly centered in the 660-800 °C range. [ABSTRACT FROM AUTHOR]

Highlights: [•] Magnesium and calcium are widespread in nature. [•] Mineral carbonation is becoming a effective method in reusing waste mineral. [•] Calcium to magnesium mole ratio is a key parameter in this study. [•] Thermogravimetric analysis has been used in this study. [•] Thermogravimetric analysis is an useful method on the calculation of carbonation efficiency. [Copyright &y& Elsevier]

[Display omitted] • Proposes and evaluates a novel solution to address the carbonation issue of CSA. • Modified CSA with fine CSA cement-sand mix is submitted to accelerated carbonation. • Mechanical, mineralogical, and microstructural evaluation of carbonated MCSA. • MCSA having 20 wt% fine CSA cement-sand mix could be regarded as optimum MCSA. • OMCSA results in negligible loss of ettringite due to fewer pores. Calcium Sulfoaluminate (CSA) cement is regarded as a plausible alternative to Ordinary Portland Cement (OPC) to reduce environmental burdens caused by the production of the latter; however, susceptibility to carbonation of the former restricts its utilization in the construction sector. This study proposes and evaluates a novel solution to address the carbonation issue of CSA by partial replacement with a fine CSA (<2.4 μm) cement-sand (1:3) mix. For this purpose, the modified CSA (MCSA) samples were prepared by substituting fine CSA cement-sand paste in conventional CSA mortar by 0 to 30 wt% and then submitted to accelerated carbonation. The performance of different MCSA mixes, OPC, and conventional CSA was compared after carbonation from the mechanical, mineralogical, and microstructural perspectives. The investigation reveals that the MCSA having 20 wt% fine CSA cement-sand mix could be regarded as optimum MCSA (OMCSA) in terms of carbonation degree, mechanical and microstructural characteristics after carbonation. The OMCSA shows a decrease of 8% and 23% in carbonation degrees and an increase of 12% and 25% in compressive strength upon carbonation as compared to OPC and CSA. Mercury intrusion porosimetric analyses (MIP) show that the porosity of OMCSA is reduced upon carbonation compared to conventional CSA. In addition, mineralogical analyses reveal that OMCSA contains the highest amount of ettringite and less carbonated contents upon carbonation than other subjected samples. OMCSA results in negligible loss of ettringite due to fewer pores as carbon dioxide is unable to penetrate the system, making it resistant to carbonation. Thus, the current solution reduces the carbonation impacts of CSA cement and is a step forward to make it more durable and practical. [ABSTRACT FROM AUTHOR]

[Display omitted] • Carbonation properties of different groups of synthetic C 2 S were studied. • The CO 2 sequestration capacity of C 2 S was increased with the content of γ-C 2 S. • S2-C group showed the capacity of 321.9 g/kg CO 2 sequestration. • S2-C group had the densest microstructure and the highest compressive strength. • Different content and microstructure of CaCO 3 was formed in carbonated sample groups. In order to deal with the current global climate crisis, it is urgent to reduce greenhouse gas emissions and store CO 2 in the atmosphere through Carbon Capture and Storage (CCS) technology. Both C 3 S and C 2 S are the main mineral components of Ordinary Portland Cement; through accelerated carbonation reaction, C 2 S could sequestrate more CO 2 and gain a rapid development of early strength. β-C 2 S and γ-C 2 S, as two stable crystallines of C 2 S, can both capture CO 2. Compared to β-C 2 S, the γ-C 2 S essentially shows no hydration activity, but γ-C 2 S shows higher reactivity to CO 2 during the carbonation process. The conversion of β-C 2 S to γ-C 2 S would cause volume expansion, saving electricity during the grinding process and reducing CO 2 emissions. To synthetic different proportions of β and γ polymorphic of C 2 S, the different weight content of B 2 O 3 (B 2 O 3 wt%) was mixed with the analytical pure chemical reagent of CaCO 3 and SiO 2 , and different cooling rates were used. The synthetic C 2 S (F1, S1, F2, S2) were exposed to pure CO 2 gas with 2 bar pressure for 2 h. After carbonation, compressive strength was tested, carbonation degree was calculated. The synthetic C 2 S (S2) containing 57.8 wt% β-C 2 S and 42.2 wt% γ-C 2 S showed the highest compressive strength and the higher CO 2 sequestration capacity, with the compressive strength of 46.3 MPa and the 14.14% CO 2 uptake. The XRD results and TG-DTG analysis have proved that different calcium carbonates, including calcite, aragonite, vaterite, and ACC (amorphous calcium carbonate), were formed after carbonation. FT-IR observed the characteristic vibration of calcium carbonate, the microstructure of different calcium carbonate crystals was observed by FE-SEM. The calcite in the S2-C group showed more thermodynamic stability, confirmed by TG analysis at the temperature stage of 680℃～820℃. The mechanical bond force of rhombic calcite contributed to the high compressive strength. And S2-C group had the lowest porosity, which contained the highest volume of capillary pores and the lowest volume of macropores among the 4 groups. The combination of β-C 2 S with γ-C 2 S exhibited rapid strength development and high sequestration of CO 2 after carbonation. The carbonation of combination of β-C 2 S and γ-C 2 S was able to devote to the rapid development of compressive strength and rather a high capacity of capturing CO 2 , and the proper proportions of β-C 2 S and γ-C 2 S were beneficial to develop belite-rich cement, as well as speed up the absorption and storage of CO 2 in the atmosphere. [ABSTRACT FROM AUTHOR]

• A novel method, zeolite partially replacing steel slag, was found to further improve the compressive strength and carbonation degree of steel slag compacts by accelerated carbonation curing. • In terms of compressive strength and carbonation degree, the optimum content of steel slag substituted by zeolite was 5 wt.% and 15 wt.%, respectively. • XRD, QXRD, TG, FT-IR, ICP-OES, MIP and SEM had been done to analyse the mechanism of zeolite improving carbonation degree and compressive strength of steel slag compacts. In this paper, a novel method, zeolite partially replacing steel slag, was proposed to futher improve the properties (compressive strength and carbonation degree) of steel slag compacts futher after CO 2 curing. Inductively coupled plasma (ICP) results illustrated that the Ca, and, Fe ion concentrations in the steel slag matrix substituted by 5 wt% zeolite (water-to-solid ratio of 10) were 78.19 mg/L and 4.09 mg/L, 19.3% and 99.0% higher than that of pure steel slag after 1 d of hydration curing respectively. The decreased crystallite size of C 4 AF and increase in Fe ion concentration illustrates that zeolite with pozzolanic properties could react with CaO-FeO X matrix components such as brownmillerite (C 4 AF), to produce C-S-H gel, which possesses higher solubility than the CaO-FeO X matrix, to improve the Ca ion concentration. In terms of the compressive strength and carbonation degree results, the optimum zeolite substitution ratios were 5 wt% and 15 wt%, respectively. After 1 d hydration and 2 h of carbonation curing, the compressive strength of the steel slag compacts with 5 wt% zeolite (CSZ5) was 14% higher than that of pure steel slag compacts (CSZ0). With respect to carbonation degree, the steel slag substituted with 15 wt% zeolite (CSZ15) had a 10.2% higher carbonation level than that of CSZ0. [ABSTRACT FROM AUTHOR]

Magnesium oxysulfate (MOS) cement has the advantages of lightweightedness, high strength, and low thermal conductivity, but the utilization of MOS cement is limited due to low water resistance. This paper studied the influence of steel slag and CO2 treatment on the compressive strength and water resistance of MOS cement. The hydration products and microstructures were characterized by X-ray diffraction (XRD), thermogravimetric analysis–differential scanning calorimetry (TG–DSC), scanning electron spectroscopy (SEM), and Fourier transform infrared spectroscopy (FTIR). The results showed that the strength of MOS cement reached 89.7 MPa with steel slag and CO2 treatment; the water-resistance coefficients of the control and samples containing 10%, 20%, and 30% reached 0.91, 0.81, 1.01, and 1.08 MPa, respectively. The improvement in the strength and water resistance coefficients was because of carbonation that accelerated the hydration of C2S in the steel slag and formed a Ca–Mg–C amorphous substance. The carbonation products contributed to better water stability and denser matrix denser while inhibiting the hydration of MgO, which led to improving the water resistance of the sample. [ABSTRACT FROM AUTHOR]

Widespread utilization of Portland cement (PC) with a large-scale carbon footprint has caused a vital climate crisis. The application of low-carbon materials rich in belite is hindered by the low hydraulic activity of C 2 S. Both β-C 2 S and γ-C 2 S can rapidly develop compressive strength and sequestrate CO 2 after carbonation. To explore the role of β-C 2 S and γ-C 2 S played on compressive strength and carbonation degree in C 2 S paste, several groups of C 2 S samples composed of pure β-C 2 S and pure γ-C 2 S in different weight contents (wt.%) were homogenized and carbonated. Compressive strength was tested, and XRD, TGA, FT-IR, and SEM analysis methods were applied to observe the carbonation properties. Experimental results showed that compressive strength positively correlated with the proportion of β-C 2 S content, and the carbonation degree positively correlated with the proportion of γ-C 2 S content. Different ratios of β-C 2 S and γ-C 2 S mainly affected the growth rates of compressive strength and carbonation degree. When β-C 2 S was 60 wt% and γ-C 2 S was 40 wt%, both compressive strength (54.4 MPa) and carbonation degree (36.7%) of carbonated samples can reach an optimum value. The calcite content increased with γ-C 2 S content, while the crystallite size of calcite decreased with the γ-C 2 S content. Through the established carbonation kinetics equation and the measured carbonation heat curves of compounded C 2 S, the possible synery effect of β-C 2 S and γ-C 2 S was proposed. Combined β-C 2 S and γ-C 2 S in cement clinker can reduce CO 2 emissions. Carbonating high-belite cement composed of a certain proportion of β-C 2 S and γ-C 2 S will contribute to developing early compressive strength and improving CO 2 sequestration of this low-carbon cementitious material. [ABSTRACT FROM AUTHOR]

• The tetrahedral framework structure of clinoptilolite-type zeolite was stable after undergoing low-temperature calcination curing (below 400 ℃), degradation was initiated at a temperature of 400℃ and complete structural collapse occurred at 800 ℃. • Comparing the compressive strength, porosity, and carbonation degree in specimens CSZ5-25 to CSZ5-800, the optimum calcination temperature of zeolite was 800 ℃. • CHZ matrix confirmed that the pozzolanic reaction products were C-S-H gel and AFmc by XRD, FT-IR, and MAS NMR techniques. Steel slag partially substituted with zeolite (SZ) is beneficial for further improving the compressive strength and carbonation degree, after a combined curing (hydration and carbonation) process, owing to the pozzolanic reaction. In this study, the effect of clinoptilolite-type zeolite calcinated at different calcination temperatures (200–800 ℃) to modify the degree of the pozzolanic reaction is investigated. Moreover, the characteristics of SZ specimens after combined curing are also investigated. Additionally, the degree of the pozzolanic reaction and its mechanism in CHZ (Ca(OH) 2 mixed with calcinated zeolite after 1 day (1d) of hydration) specimens are clarified and compared by various microscopy techniques. Compressive strength results revealed that the optimum calcination temperature was 800 ℃. The compressive strength and carbonation degree of CSZ5-800 (steel slag specimen containing calcined zeolite (800 ℃) after 1d of hydration followed by 2 h of carbonation curing) improved by 23.2 and 13.8 %, respectively, compared with that of the reference CSZ5-25. The porosity of CSZ5-800 was 30.8 and 21.9 % higher than that of CSZ0-25and CSZ5-25, respectively. New minerals AFmc and C-S-H gel were detected by X-ray diffraction (XRD) and mass angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy in CHZ specimens. Notably, the C-S-H gel and AFmc content in CHZ-800 were 7 and 57.6 % higher than that of CHZ-25, respectively. [ABSTRACT FROM AUTHOR]

Steel slag (SS) exhibits low hydration activities and expansive phases, which should be pretreated prior to using as binding materials. Thus, these properties of SS were improved through a combined treatment of alkali and then wet carbonation (A_C) in this study. Effect of sodium silicate modulus (M = 1.0–2.5) and carbonation temperature (50–200 °C) on cementitious property of SS was investigated. Results showed that the optimum modulus and temperature were 2.0 and 150 °C under single alkali and single carbonation treatment of SS, respectively. In case of combined treatment, the optimal modulus was 2.0 with an optimum temperature of 100 °C (A2.0_C100°C-SS). The 28 d compressive strength increased by 27.1% and expansion rate decreased by 17.7% of A2.0_C100°C-SS-PC compared to untreated SS-PC. This was attribute to the fact that the newly formed CaCO 3 with micro-nano size and high activity existed in A_C-SS can react with Al-containing minerals in OPC to form monocarboaluminate, and Si-gels produced during carbonation can react with CH to form C-S-H phases. The early hydration activity of OPC was more obviously improved by adding A_C-SS compared to adding A_SS or C_SS. This was due to the micro-nano size effect of CaCO 3 and Si-gels can provide nucleation sites for C-S-H formation. Alkali pretreatment can improve the SS carbonation, reaching the maximum carbonation degree of 53% at 100 °C compared to carbonation alone (47% at 150 °C). The embodied CO 2 index of A2.0_C100-SS-OPC system decreased to 9.843 compared to OPC (10.511), indicating that the A_C treatment for SS was a more sustainable and environmentally friendly method. [ABSTRACT FROM AUTHOR]

β-C 2 S is the main active mineral component in Basic Oxygen Furnace slag (BOFS), while the content of β-C 2 S in BOFS is usually limited and fluctuated, which hindered the improvement of CO 2 capture during carbonation. To enhance the carbonation reactivity of BOFS products, different ratio of γ-C 2 S was blended with BOFS powder and carbonated, followed by hydration curing. XRD and TGA analysis were used to determine the mineral composition of each blending group after carbonation. SEM analysis was applied to observe the microstructure, and the MIP method was used to analyze the pore size structure. The results show that the carbonation degree and compressive strength of BOFS cubes were increased by the addition of γ-C 2 S. With 20 wt% blending of γ-C 2 S, the carbonated BOFS obtained the optimum increasement of carbonation degree and compressive strength, reaching 39.3% and 42.4 MPa, respectively. However, the subsequent hydration reactivity of BOFS was inhibited when blended with 20 wt% of γ-C 2 S. Despite this, the increased poorly crystalline CaCO 3 filled the system and densified the matrix, contributing to a 17.9% increase in compressive strength of SH20 after 56 days of hydration curing. • For SS20 group, the optimum results of carbonation degree (39.3%) and compressive strength (42.4 MPa) can be obtained. • As excessively blending of γ-C 2 S, the increased generation of poorly crystalline CaCO 3 may reduce the compressive strength. • Later hydration of SS20 was inhibited, while the formation of the denser microstructure enhanced its compressive strength. [ABSTRACT FROM AUTHOR]

Poor volume stability of steel slag (SS) is due to the existence of free-CaO, free-MgO, and RO phases. Combined treatments of alkali and wet carbonation (AWC) were conducted on the RO phases to stimulate hydration activity. The hydration degree of AWC_RO phases with MgO molar ratios of 0.84 increased from 10.6 % to 57.6 %. The effects of FeO-MgO-MnO proportions on the hydration activity of RO phases were investigated electronically based on the density functional theory. Results showed that Fe imparted higher inhibition effects on MgO hydration than Mn. The compressive strength of mortar with added AWC_SS was 34.4 % higher than with added untreated SS at 28 days, with an expansion rate decreased from 0.0447 % to 0.0298 %. The main products of AWC_RO phases were Mg(OH) 2 and Mg 5 (CO 3) 4 (OH) 2 ·4H 2 O, which exhibited seed crystal effects, accelerating the early hydration of cement. This method provides guidance towards a meaningful solution to poor SS volume stability. • The effects of FeO-MgO-MnO proportions on the hydration activity of RO phases were investigated by DFT and experiment. • Substituting cement with AWC_SS achieved a 34% increase in strength and 50% decrease in the expansion rate. • The RO phases with various FeO-MgO-MnO proportions were synthesized using analyses of extracted RO phases in SS. [ABSTRACT FROM AUTHOR]

• Alkali treatment improve the hydration activity of RO phases with a K m > 0.89. • Carbonation promotes the RO phases hydration due to the increase in Mg-leaching induced by CO 2 dissolution. • Substituting cement by ADC_SS achieves 20% increases in strength and 26% decreases in expansion rate. • The alkali excitation reaction of silica gels and monocarboaluminate contribute to the strength gain. RO phases account for 8.5–16.9 wt% of steel slags and exhibit delayed hydration activity, which may result in poor volume stability of steel slags (SS). Treatment of alkali activator (sodium silicate) followed by dry carbonation technology (ADC) was conducted on synthetic RO phases with various molar ratios of FeO, MgO, and MnO to promote its early hydration activity. Results show that 28 d compressive strength of mortar added by ADC_SS is 19.67% higher than that of added untreated SS and 26% decrease in expansion rate. Hydration degree of RO phases increases obviously during alkali treatment from 10.6% to 21.7% (RO-6), and is further stimulated by subsequently carbonation (reaching 44.1%), both resulting in much consumption of expansive component (i.e., MgO) in RO phases. The early hydration activity of cement promoted by adding ADC_RO phase/SS, which mainly attributes to the nucleation effect of hydration and carbonation products (i.e., Mg(OH) 2 , pyoaurite, and MgCO 3 ·3H 2 O). The alkali excitation reaction of silica gels produced during carbonation as well as the formation of monocarboaluminate further contribute to the strength gain. ADC_SS used as admixture for cement makes pastes have advantage of higher strength, higher hydration rate, lower heat release, and less expansion rate compared to the system contained untreated SS. ADC may be a potential method to treat SS and makes SS to be used as admixture for Portland cement. [ABSTRACT FROM AUTHOR]