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Coal fly ashes (COFA) are readily available and reactive materials suitable for CO[sub.2] sequestration due to their substantial alkali components. Therefore, the onsite collaborative technology of COFA disposal and CO[sub.2] sequestration in coal-fired power plants appears to have potential. This work provides an overview of the state-of-the-art research studies in the literature on CO[sub.2] sequestration via the mineralization of COFA. The various CO[sub.2] sequestration routes of COFA are summarized, mainly including direct and indirect wet carbonation, the synthesis of porous CO[sub.2] adsorbents derived from COFA, and the development of COFA-derived inert supports for gas-solid adsorbents. The direct and indirect wet carbonation of COFA is the most concerned research technology route, which can obtain valued Ca-based by-products while achieving CO[sub.2] sequestration. Moreover, the Al and Si components rich in fly ash can be adapted to produce zeolite, hierarchical porous nano-silica, and nano-silicon/aluminum aerogels for producing highly efficient CO[sub.2] adsorbents. The prospects of CO[sub.2] sequestration technologies using COFA are also discussed. The objective of this work is to help researchers from academia and industry keep abreast of the latest progress in the study of CO[sub.2] sequestration by COFA.

The desulfurization capacity of top slag in the process of pre-desulfurization of hot metal containing vanadium and titanium was researched. The top slag system of CaO-SiO[sub.2]-MgO-Al[sub.2]O[sub.3]-TiO[sub.2]-VOx that was formed by blast furnace slag and a CaO desulfurization agent reduced the sulfur in hot metal from 0.08 wt.% to 0.02 wt.%. It was found that the resulfurization of the slag happened in the later periods of the desulfurization process. The vanadium–titanium oxides were both acidic in the desulfurization slag. TiO[sub.2] and VOx reacted with the basic oxides to form CaTiO[sub.3] and MgV[sub.2]O[sub.4] at 1623 K, which reduced free CaO and was not conducive to top slag desulfurization. The results of calculation showed that the top slag desulfurization accounted for 15% of the total desulfurization. Using the ionic and molecule coexistence theory of slag structure, it is shown that the desulfurization efficiency could be enhanced by adjusting both the amount of desulfurization agent and the composition of the blast furnace slag before pre-desulfurization.

Gamma dicalcium silicate (γ-2CaO∙SiO[sub.2], abbreviated as γ-C[sub.2]S) is considered a potential candidate as a construction material owing to its high carbonation reactivity and consequent CO[sub.2] absorption. This study investigates the diffusion of CO[sub.2], a physical process, into hardened cement paste and the resulting carbonation, a chemical process. CO[sub.2] diffuses from a region of high concentration to one of a lower concentration, which is the inner core of the hardened cement. This study aimed to examine whether the diffusion of CO[sub.2] into the ordinary Portland cement (OPC)/γ-C[sub.2]S composite paste followed the conventional laws of diffusion. We also studied the diffusion of CaCO[sub.3] to determine if carbonation products were formed in the pores and examined the capture of CO[sub.2]. The paste specimens were prepared and subjected to CO[sub.2] in the carbonation chambers for varying periods. The results showed that the CaCO[sub.3] deposited in the pores affected the rate of diffusion of CO[sub.2] in the mortars and pastes, resulting in the densification of such bodies and a decreased rate of diffusion, leading to the shutdown of diffusion. The diffusion of CO[sub.2] in hardened cement pastes made from OPC and γ-C[sub.2]S follows Fick’s second law, wherein there is a change in the concentration of CO[sub.2] diffusing at a particular distance with time.

Slag compositions are significant for the viscosity of blast furnace slag. An improved Urbain model (IUM) was proposed by introducing R[sub.5] ((X(CaO) + X(MgO) + 2X(TiO[sub.2]))/(2X(SiO[sub.2]) + 3X(Al[sub.2]O[sub.3]))) and N (X(MgO)/3X(Al[sub.2]O[sub.3])) as the model parameters. By comparing IUM with other models, the model parameters of R[sub.5] and N are more reasonable and suitable for TiO[sub.2]-bearing blast furnace slag, and IUM for predicting viscosity has a higher precision, and its relative error is only 10%. The viscosity isolines of the CaO–SiO[sub.2]–15%Al[sub.2]O[sub.3]–MgO–2.5% TiO[sub.2] system were plotted, and the results show that the viscosity center of the slag is between Rw[sub.2] (w(CaO)/w(SiO[sub.2])) = 0.77–1.39 and Nw (w(MgO)/w(Al[sub.2]O[sub.3])) = 0–1.37, the value of the viscosity center is 0.3 Pa·s, the viscosity increases gradually from the center to the outside, and the viscosity of the slag gradually decreases with the increase in Nw and Rw[sub.2]. Furthermore, FTIR (Fourier Transform Infrared Spectroscopy) analysis was carried out in order to understand the mechanism between the slag structure and viscosity. With the increase in Nw and Rw[sub.2], the peak values of the symmetrical stretching vibration of non-bridging oxygen in the Si–O tetrahedral structure of slag decrease, and the slag structures depolymerize, which leads to the decrease in the viscosity of the slag.

The relationship between slag structure and viscosity is studied, employing Raman spectroscopy for the five-component slag system of MnO-SiO[sub.2]-CaO-Al[sub.2]O[sub.3]-MgO and its subsystems. This study aims to investigate the influence of variations in slag composition on viscosity, which is crucial for optimizing industrial processes. Based on industrial slag compositions produced in a silicomanganese submerged arc furnace, 17 slags with a fixed content of MnO of 10 wt% are synthesized with varying contents of SiO[sub.2] of 33 to 65 wt%; CaO within the range of 14 to 40 wt%; and fixed contents of Al[sub.2]O[sub.3] and MgO of 17 and 6 wt%, respectively. The slag compositions are divided into four groups, ranging from low basicity (0.38) to high basicity (0.80), with each group containing the four slag systems of MnO-SiO[sub.2]-CaO, MnO-SiO[sub.2]-CaO-Al[sub.2]O[sub.3], MnO-SiO[sub.2]-CaO-MgO, and MnO-SiO[sub.2]-CaO-Al[sub.2]O[sub.3]-MgO, with fixed basicity. Additionally, a five-component composition with the lowest basicity of 0.28 is considered. Raman spectroscopy measurements are performed in the wavenumber range of 200 to 1200 cm[sup.−1] using a green source laser with a 532 nm wavelength. The high-wavenumber region of the Raman spectra (800 to 1200 cm[sup.−1]) is deconvoluted to quantitatively investigate the effect of each oxide on the slag structure and the degree of polymerization (DOP) of the silicate network. Results indicate that measured NBO/T increases with increasing basicity, demonstrating a reduction in DOP of the silicate structure. This depolymerization effect is more pronounced in slags containing Al[sub.2]O[sub.3] compared to those without it. In a group of slags with similar basicity, the substitution of SiO[sub.2] with Al[sub.2]O[sub.3] leads to further depolymerization. In contrast, substituting CaO with MgO has little effect on the silicate structure in slags without Al[sub.2]O[sub.3] but causes depolymerization in slags containing Al[sub.2]O[sub.3]. To study the relationship between structure and viscosity, viscosity data obtained from FactSage are used as reference values. The predictions of slag viscosity using the Raman-structure model and the NBO/T viscosity model are then compared to the FactSage results. The adjustable parameters of the Raman-structure model are re-determined using the FactSage data for the studied slag compositions. The NBO/T viscosity model employs both calculated NBO/T values from the slag compositions and measured NBO/T values from the deconvolution results. The findings of this study reveal good agreement between the predictions of the Raman-structure model and the FactSage viscosity data.

About 120 million tons of steel slag are produced annually in China, making it one of the largest sources of industrial solid waste; however, its utilization rate remains only around 30%. The presence of f-CaO is the main factor in its widespread application. Currently, the carbonation of steel slag is mainly through indirect wet mineralization, which is difficult to implement on an industrial scale. Direct dry carbonation, on the other hand, consumes more energy due to its slow kinetics. In this study, steam coupled with CO[sub.2] was used to directly mineralize steel slag, a process fully compatible with existing iron and steel industry treatment processes. The required temperature can be achieved using the waste heat from hot steel slag, eliminating the need for additional heat supply. With 15% steam injection, the CaCO[sub.3] content increased to 12.02 g/100 g (52.8 kg CO[sub.2] t[sup.−1] slag utilization), representing a 16.7% improvement. After mineralization, the f-CaO decreased to 0.61%, with 91.73% of f-CaO in steel slag mineralized. The mineralization efficiency of f-CaO increased by 20.24%. This enhancement was attributed to steam entering the interior pores of steel slag, generating intermediate Ca(OH)[sub.2], causing steel slag particle breakage and fully exposing the previously enclosed f-CaO for complete carbonation. To further utilize flue gas, the effects of different CO[sub.2] concentrations on carbon fixation were investigated. At a concentration of 20% CO[sub.2], the carbon fixation reached 69.90% of that achieved at 100% CO[sub.2]. This research not only addresses the stability issues of steel slag but also reduces CO[sub.2] emissions and effectively utilizes waste heat, making the process suitable for large-scale industrial application.

Lime mortars are considered the most compatible material for monuments and historic buildings, and they are widely used in restoration works. A key factor determining the mechanical and physical properties of lime mortars is carbonation, which provides strength and hardness. This paper indicates the properties gained in lime mortars produced by Ca(OH)2 and CaO reinforced with different bio-fibers (hemp and lavender) when exposed to the natural environment and in accelerated carbonation. At 90 and 180 days of manufacture, the mechanical and physical properties of the produced composites have been tested. The results show that the carbonation reaction works faster in the case of hot lime mortars, increasing their compressive strength by up to 3.5 times. Hemp-reinforced mortars led to an enhancement in strength by up to 30%, highlighting the significance of bio-fibers in facilitating CO2 diffusion. This was also verified by the thermogravimetric analysis and the determination of the carbon content of the samples. Optimal mechanical properties were observed in mixtures containing quicklime and hemp fibers when conditioned with 3% CO2 at the tested ages.

Hempcrete is a sustainable biocomposite that can reduce buildings’ embodied energy while improving energy performance and indoor environmental quality. This research aims to develop novel insulating hemp-lime composites using innovative binder mixes made of recycled and low-embodied energy pozzolans. The characterization of composites’ mechanical and hygrothermal properties includes measuring compressive strength, splitting tensile strength, thermal conductivity, specific heat capacity, and moisture buffer capacities. This study also investigates the impact of sample densities and water content on compressive strength at different ages. The findings suggest that mixes with a 1:1 binder to hemp ratio and 300−400 kg/m3 density have hygrothermal and mechanical properties suitable for insulating infill wall applications. Hence, compressive strengths, thermal conductivity, and specific heat capacity values range from 0.09 to 0.57 MPa, 0.087 to 0.10 W/m K, and 1250 to 1557 J/kg K, respectively. The average moisture buffer value for all hempcrete samples of 2.78 (gm/m2 RH%) indicates excellent moisture buffering capacity. Recycled crushed brick pozzolan can enhance the hygrothermal performance of the hemp-lime composites. Thus, samples with 10% crushed brick have the lowest thermal conductivity considering their density and the highest moisture buffer capacity. The new formulas of hydrated lime and crushed brick have mechanical properties comparable to metakaolin and hydraulic lime formulas.

Calcium-based binders, such as ordinary Portland cement (OPC) and lime (CaO), are the most common artificial cementitious materials used worldwide for concrete and soil improvement. However, using cement and lime has become one of the main concerns for engineers because they negatively affect the environment and economy, prompting research into alternative materials. The energy consumption involved in producing cementitious materials is high, and the subsequent CO2 emissions account for 8% of the total CO2 emissions. In recent years, an investigation into cement concrete’s sustainable and low-carbon characteristics has become the industry’s focus, achieved by using supplementary cementitious materials. This paper aims to review the problems and challenges encountered when using cement and lime. Calcined clay (natural pozzolana) has been used as a possible supplement or partial substitute to produce low-carbon cement or lime from 2012–2022. These materials can improve the concrete mixture’s performance, durability, and sustainability. Calcined clay has been utilized widely in concrete mixtures because it produces a low-carbon cement-based material. Owing to the large amount of calcined clay used, the clinker content of cement can be lowered by as much as 50% compared with traditional OPC. It helps conserve the limestone resources used in cement manufacture and helps reduce the carbon footprint associated with the cement industry. Its application is gradually growing in places such as Latin America and South Asia.

We investigated the phase transitions, mechanical properties, and chemical durability of a composition of 9 mol% CaO-stabilized zirconia (9CSZ) doped with 2–4 mol% CeO[sub.2] under thermal stress against molten slag. The monoclinic phase fraction of 9CSZ was 7.14% at room temperature, and CSZ doped with 2–4 mol% CeO[sub.2] showed a slightly lower value of 5.55–3.72%, with only a minor difference between them. The microstructure of 9CSZ doped with 2–3 mol% CeO[sub.2] was similar to that of undoped 9CSZ, whereas the microstructure of 9CSZ doped with 4 mol% CeO[sub.2] exhibited noticeable grain refinement. The mechanical properties of CSZ at room temperature tended to improve as the CeO[sub.2] doping concentration increased. The Vickers hardness increased from 1088.4 HV to 1497.6 HV when the CeO[sub.2] doping amount was 4 mol%, and the specific wear amount decreased from 1.5941 to 1.1320 × 10[sup.5] mm[sup.3]/Nm. This tendency remained similar even after applying thermal stress. The monoclinic phase fraction of 9CSZ increased from 7.14% to 67.71% after the erosion experiment with the CaF₂-based slag. CeO[sub.2]-doped CSZ had a lower monoclinic phase fraction than CSZ after the erosion experiment, but as CeO[sub.2] content increased from 2 to 4 mol%, the fraction rose to 4.07%, 30.85%, and 77.11%.