Low Carbon Cementitious Materials Research Articles

Modification mechanism of flue gas desulfurization gypsum on fly ash and ground granulated blast-furnace slag alkali-activated materials: Promoting green cementitious material

Alkali activation and the combination of fly ash (FA) and ground granulated blast-furnace slag (GGBS) has been used for cementitious material, which is seen as an effective method to dispose of industrial solid waste. The GGBS-FA AAMs have the characteristic of drying shrinkage and a large number of tiny pores distributed in the matrix, resulting in strength deterioration. The paper is aimed to optimize the strength property by incorporating another solid waste of flue gas desulfurization gypsum (DG), which could be regarded as a ternary alkali-activated materials (AAMs). The unconfined compression tests are carried out to study the effects of DG content on strength with different curing times, and the effects from the modulus of the alkali activator are also analyzed. Then X-ray diffraction (XRD) and Scanning Electron Microscope (SEM) are performed to reveal the microscopic mechanism of the effects of DG on the mechanical property. Results show that the incorporation of DG can significantly improve the early strength due to the increase of calcium. The SO42- in DG continuously reacts with Ca2+ and active silicate to form ettringite during the curing period, which can fill micro-pores and tiny cracks to form a denser matrix structure. In addition, the presence of SO42- has the effect of secondary enhanced activation, further promoting the polymerization reaction and enhancing the strength, especially for the low alkaline reaction system. These all contribute to the strength improvement. It should be noted that the DG content must be controlled to prevent the volume-filling effect that leads the AAMs matrix to be loose and porous, and the generation of geopolymer gel would also be impeded in a high-calcium reaction system, further influencing the final strength. The results will promote the application of high-performance, cost-saving, and green low-carbon cementitious material based on the synergistic utilization of GGBS, FA and DG.

Preparation and Mechanical Properties of Low Carbon Cementitious Materials with Superfine Cement Reverse Filling High-Volume Mineral Admixtures.

In order to increase the content of mineral admixtures in cement, this study proposes a method for preparing a high-volume mineral admixture cementitious material (HMAC) using superfine cement as a reverse filling material. Firstly, superfine cement is prepared through mechanical grinding. Then, the activity of mineral admixtures (such as slag and fly ash) is enhanced by mechanical grinding, sulfate activation, and alkali activation methods. Meanwhile, the evolution of HMCM from microstructure to macroscopic mechanical behavior is studied by combining a laser particle size analyzer and a scanning electron microscope. Furthermore, the reverse filling mechanism of superfine cement on mineral admixtures under different activation conditions is proposed. Results show that superfine cement can largely improve the utilization rate of cement clinker and the compressive strength of cementitious materials. In the condition that the compressive strength is not lower than that of the control group (without mineral admixture), the content of mineral admixture can be increased to 50%, 70%, and 90% after mechanical grinding, sulfate activation, and alkali activation, respectively. Analysis indicates that the reverse filling effect of superfine cement is the main reason for improving the density of the HMCM.

Interpretable machine learning model for autogenous shrinkage prediction of low-carbon cementitious materials

Supplementary cementitious materials (SCM) are key components of low-carbon cementitious materials. However, their effects, especially for the emerging high cement replacement ratio formulations, are sometimes very challenging to predict, leading to restrictions in most standards. Through advanced Machine Learning techniques, this study provides novel insights into the autogenous shrinkage properties of concrete containing various kinds of supplementary cementitious materials such as slag, fly ash, silica fume, calcined clay, and filler. Four machine learning models are optimized and their predictions are compared on a dedicated database, including ternary and quaternary cement blends. The Extreme Gradient Boosting (XGBoost) model exhibited the highest accuracy among these models. Interpretative tools such as SHAP analysis are then used to obtain novel information about the relative influence of the SCM on the autogenous shrinkage depending on their dosage. The model is finally compared to the recently modified versions of the B4 and the CEB analytical models to predict the autogenous shrinkage of unknown concrete formulations. The model showed significantly better predictions than the analytical models, and a web application is proposed, paving the way for precise and interpretable autogenous shrinkage predictions of cementitious materials incorporating SCM.

Effect of carbonation treatment on fracture behavior of low-carbon mortar with recycled sand and recycled powder

This paper presents a systematic experimental study on the fracture behavior of low-carbon mortars with various contents of carbonated recycled sand and recycled powder (CRS and CRP). Three-point bending tests on pre-notched beams were performed to explore the load-induced fracture processes. Based on the strain and displacement fields measured using the digital image correlation technique, the evolution of the fracture process zone (FPZ) was studied qualitatively and quantitatively. Results indicated that the initial cracking toughness (KIcini), unstable fracture toughness (KIcun) and fracture energy (Gf) of mortar reduced gradually with the increasing recycled sand (RS) content but kept relatively steady with the rising recycled powder (RP) dosage except for KIcini. Carbonation treatment could effectively improve the KIcini of mortar with low RP content and Gf with a high proportion of RS but had limited effect on KIcun. A combination of the FPZ evolution characteristics and crack growth resistance curves suggested that the use of RS would increase microcracking in the stable crack growth stage and thus facilitate the connectivity of microcracks in the unstable fracture stage, while the use of RP mainly affected the crack initiation stage of cement paste in the mortar. The improvement of mortar fracture by CO2 treatment of RS or RP also corresponded to these stages. Regarding fracture characteristics and carbon footprint, mortars with CRS and CRP could achieve a synergy of safety and low carbon and thus can be considered as low-carbon cementitious materials compared to normal mortar.

Study on the composition of calcium alumina silicate hydrate (CASH) in lime-activated low carbon cementitious materials: The influence of alkaline additives

The composition of CASH in lime-activated low carbon cementitious materials (LCM) was investigated in this study, and the influence of alkaline additives on the composition of CASH in LCM was analyzed, particularly for the hydration rim of slag. Compared to LCM-control mixture, alkali additives effectively improved the hydration of GGBFS and significantly increased the thickness of the hydration rim as the inner hydration product. The enlargement of the hydration rim zone led to a lower Ca/(Si + Al) ratio of the hydrated gel mainly due to the interference of Al from intermixing phases in the rim. For LCM with alkaline additives, the hydration rim of slag contained C(N)ASH, ettringite, U-phase, and a minor amount of amorphous or poorly crystalline hydrotalcite-like phase, and the hydrated gel in paste was mainly composed of C(N)ASH due to the uptake of Na+ ions in CASH.

Strength prediction and optimization for ultrahigh-performance concrete with low-carbon cementitious materials – XG boost model and experimental validation

In recent years, supplementary cementitious materials (SCMs), as well as other unconventional materials [such as recycled glass powder (GP)], have gained much attention for the formulation of ultra-high-performance concrete (UHPC). Machine learning (ML) techniques have become more prevalent in everyday life in the current era of cutting-edge technology. Their applicability is readily evident in a wide range of areas, including civil engineering applications. This research aimed to develop, validate, and apply the novel XG Boost model to predict the characteristic compressive strength of UHPC containing recycled GP under normal moist curing conditions. Based on 309 datasets, the study developed and trained an ML model using different components, including four supplementary cementitious materials (silica fume (SF), fly ash (FA), quartz powder (QP), and glass powder (GP). The feature importance of the developed ML model was assessed using two methods (the Gini index and the SHAP values). Based on the validation datasets, the predicted-target data was within a 95% accuracy range. The optimized model was helpful tool for making accurate predictions over a wide range of data sets. It was observed from the results obtained that the most significant features were SF, water–binder ratio, water–powder ratio, and virtual packing density. The developed ML model and the tested mixtures showed a mean ratio of tested-to-predicted values of 0.956, with most data lying within a margin of error of 15%. As a result, an intuitive user interface was developed for optimizing UHPC systems, including SF, FA, QP, and GP.

Enhancement in compressive strength of carbide slag activated ground granulated blast furnace slag by introducing CaCl2 and NaCl

Alkali-activated materials was the green and low carbon cementitious materials. In this study, the calcium chloride (CC) and sodium chloride (SC) were used as accelerator to promote the compressive strength of carbide slag (CS) activated ground granulated blast-furnace slag (GGBFS). The compressive strength of samples was investigated and the related mechanism was comprehensively characterized with hydration heat, XRD, TG-DTG, SEM, 29Si NMR and MIP in details. These results showed that 5% SC and CC significantly improved early strength with an increase of 58.8% and 127.1% at 3 days, respectively. Interestingly, the presence of CC also increased the 28-day and 90-day strength from 16.6 MPa and 30.4 MPa to 50.2 MPa and 70.5 MPa, respectively. These results citified that the addition of CC and SC accelerated the consumption of CS at early age. At late age, the enhancing effect of SC was lower than CC, reflecting the Ca2+ made a greater contribution to strength development than Na+. The addition of CC still accelerated the rapidly precipitation of aluminium phase in the form of hydrocalumite, which could improve further dissolution and hydration of GGBFS at late age. Therefore, the addition of CC produced more hydration products that would refine microstructure and enhance strength at late age. These findings suggested a novel way to improve the mechanical properties of alkali-activated GGBFS.

Enhancement of sintered sludge ash-modified cement paste with CaSO4 and CaCl2

For the combination of ecological environment governance with cement industry upgrading, this paper seeks innovation by proposing a low-carbon cementitious material, in which CaSO4 and CaCl2 are used in combination with cement to solidify sintered sludge ash (SSA). Additionally, XRD, FTIR, TGA and SEM are used to explored systematically the mechanism of CaSO4 and CaCl2 enhancing SSA-modified cement paste from three perspectives: hydrate assemblage, hydration degree and micromorphology. The sludge activation system of “drying-grinding-calcining at 850 ℃-grinding” is adopted. High-temperature calcination makes little difference to the layered structure of the sludge, but the compactness of its surface improves and the content of amorphous Al2O3 increases significantly. Compared with the cement paste containing 50 % SSA, the addition of 1 % CaCl2 and 5 % CaSO4 shortens the initial setting time by 39.4 % and the final setting time by 31.5 %. Besides, the 28d-compressive strength increases by 32.6 % and the holes clearly observed are repaired effectively. Due to the competition between SO42- and Cl- for reaction with Al2O3, the number of Ettringite, Kuzel's salt and Friedel's salt is redistributed and Hydrotalcite-SO4 is generated in large amounts. The incorporation of CaSO4 and CaCl2 performs better in improving the early hydration degree of SSA-modified cement paste, with an increase of 30.1 % and 16.0 % in 7 and 28 days, respectively. The microstructure is transformed from porous matrix with microcracks to the dense grid formed by overlapping CSH, resulting from pozzolanic reaction of SSA, Ettringite expansion and aluminate filling in combination. Additionally, the enhanced SSA-modified cement paste demonstrates a lower embodied CO2 index and a lower integrated cost index than ordinary cement.

Preparation of epoxy resin-geopolymer (ERG) for repairing and the microstructures of the new-to-old interface

Geopolymer with great durability and bonding properties, as a kind of low-carbon cementitious material, has become a potential repairing material for concrete repair. In this work, epoxy resin-geopolymer (ERG) were prepared, and the new-to-old interface was characterized. Results show the incorporation of epoxy resin (ER) would increase the mechanical properties and bonding performance, while it would also increase the porosity of ERG. Geopolymer with 4% ER (mass ratio) exhibited both great compressive strength and flexural strength. The interaction between ER and geopolymer was clarified by X-Ray Diffraction (XRD), Thermogravimetric-Differential Thermal Analysis (TG/DTA) and Fourier Transform infrared spectroscopy (FTIR), indicating that there might be formed hydrogen bonds between ER and geopolymer while ER would not affect the main types of geopolymerization products. For repaired composites using geopolymers, the results of the newly defined interfacial water absorption coefficient were consistent with the interfacial flexural-tensile strength results, probably providing an easy and non-destructive method for evaluating the bonding performance of repairing materials. From the interfacial water absorption results, the incorporation of ER would reduce the porosity of the new-to-old interface, which was also verified through Back Scattered Electron (BSE) images analysis, explaining the improved bonding performance of geopolymer with ER.

Development of low-carbon cement: Carbonation of compounded C2S by β-C2S and γ-C2S

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 C2S. Both β-C2S and γ-C2S can rapidly develop compressive strength and sequestrate CO2 after carbonation. To explore the role of β-C2S and γ-C2S played on compressive strength and carbonation degree in C2S paste, several groups of C2S samples composed of pure β-C2S and pure γ-C2S 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-C2S content, and the carbonation degree positively correlated with the proportion of γ-C2S content. Different ratios of β-C2S and γ-C2S mainly affected the growth rates of compressive strength and carbonation degree. When β-C2S was 60 wt% and γ-C2S 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-C2S content, while the crystallite size of calcite decreased with the γ-C2S content. Through the established carbonation kinetics equation and the measured carbonation heat curves of compounded C2S, the possible synery effect of β-C2S and γ-C2S was proposed. Combined β-C2S and γ-C2S in cement clinker can reduce CO2 emissions. Carbonating high-belite cement composed of a certain proportion of β-C2S and γ-C2S will contribute to developing early compressive strength and improving CO2 sequestration of this low-carbon cementitious material.

Preparation and Characterization of Low-Carbon Cementitious Materials Based on Soda-Residue-Activated Ground Granulated Blast-Furnace Slag: A Case Study on Cemented Paste Backfills

To investigate the issues related to significant environmental damage and poor resource utilization of soda residue (SR), the composition and microstructure of hydration products of the GGBS (ground granulated blast-furnace slag) synergistically activated by NaOH-SR are characterized by an X-ray diffraction (XRD), Fourier-transform infrared spectrometry (FTIR), thermogravimetric–differential thermogravimetric (TG-DTG) analysis, and scanning electron microscope X-ray energy dispersive spectrometry (SEM-EDS). The effect of SR proportion, activator dosage, and water-to-binder ratio on the hydration process is studied. Results indicate that the hydration products mainly include hydrated calcium chloroaluminate (3CaO·Al2O3·CaCl2·10H2O, FS), hydrated calcium aluminosilicate (Ca2Al3(SiO4)3OH, C-A-S-H), halite (NaCl), calcite (CaCO3), and C-S-H gel. With the increase in SR proportion (especially from 80% to 90%), the C-S-H gel yield decreases significantly, while the FS yield rapidly increases, and the T-O-Si (T = Al or Si) peak shifts to a higher wavenumber range (955 cm−1 to 975 cm−1). With the decrease in activator dosage (40% to 15%), the hydration reaction gradually weakens, and the FTIR band of the T-O-Si (T = Al or Si) shifts to the lower wavenumber range (968 cm−1 to 955 cm−1). Then, cemented paste backfills (CPBs) are prepared with iron tailings as the aggregate. At mass content of 75%, SR proportion of 80%, and activator dosage of 30%, the fluidity of the CPB reaches 267.5 mm with a 28-day unconfined compressive strength (UCS) of 2.4 MPa, confirming that SR- and NaOH-synergistically-activated GGBS has great application prospects in backfill mining.

Study of 3D printed concrete with low-carbon cementitious materials based on its rheological properties and mechanical performances

Using high-volume cement in 3D printed concrete (3DPC) can lead to later crack formation. Accordingly, mineral admixtures can be used as cement replacements to mitigate this disadvantage. In this study, fly ash, slag, and a type of silica fume (with a high water consumption) are selected as supplementary materials in 3DPC under different substitution rates. A method for the printability evaluation of each test group in the orthogonal experiment is proposed based on a comprehensive analysis of the rheological behavior and mechanical performance of the test samples. The results indicate that a cement replacement ratio of 50% can be achieved in 3DPC while retaining its rheological behavior and mechanical performance. According to the actual printing test, the selected silica fume improves the buildability of 3DPC and can serve as an economical and effective cement substitute. The findings in this study also reveal the potential application of poor-quality mineral admixtures in 3DPC, which can increase economy and reduce CO2 emissions.

Research Progress of Low-Carbon Cementitious Materials Based on Synergistic Industrial Wastes

Cementitious material based on synergistic industrial wastes can be used as a new product for low-carbon transformation. It can aid in resource recycling and suitable consumption and utilisation of various industrial wastes. The proposed material can reduce a large amount of CO2 emitted during calcination in cement production and the decomposition of raw limestone. In addition, the material exhibits high durability and high resistance to corrosion in the marine environment that can further reduce CO2 emissions over the lifecycle of the carbon footprint of the building. Currently, many similar chemical kinetic processes and mineralogical reaction processes of particle migration and rebinding exist in the hydration and hardening reactions, service processes and durability evolution of different industrial waste cementitious systems for low-carbon production. The theoretical basis of preparing various low-carbon cementitious materials (LCCMs) with industrial waste systems is discussed herein, including the two theories of ‘complex salt effect’ and ‘isomorphic effect from tetrahedral coordination of silicon-oxygen’. Further research on LCCM is based on the theoretical foundation of ‘passive hydration kinetics’. Furthermore, this study presents the CO2 reduction potential of LCCM prepared using industrial wastes and provides future research directions in this regard.

Understanding the strength evolution of alkali-activated slag pastes cured at subzero temperature

As a kind of low-carbon cementitious materials, alkali-activated slag (AAS) can make contributions to the construction in cold areas because of its low freezing point of alkaline activator solution. This paper investigates the evolution mechanism of the strength and microstructure of AAS binders cured at −10 °C. Subzero-cured AAS pastes activated with different molarities of NaOH solution were made comparisons with the specimens cured at 20 °C in terms of various properties including setting time, compressive strength, pore structure, and microstructure. The results indicated that subzero temperature delayed the hydration process of AAS pastes, leading to prolonged setting times. The negative effects of subzero temperature were highlighted via the significantly decreased compressive strength, deteriorated pore structure, and porous microstructure. The phenomenon that subzero-cured AAS pastes exhibited lower compressive strength than ambient-cured specimens was attributed to the reduced reaction rate caused by subzero temperature, which influenced the formation position and compactness of hydration products. Interestingly, the compressive strength of subzero-cured AAS pastes still increased with the increase of NaOH molarity, which is believed to be associated with the fact that the increase in NaOH molarity caused the increase of hydration degree.

Fly Ash and Slag as Partial Replacement of Cement for the Synthesis of Low Carbon Cementitious Materials

Tailings known as solid waste are generated by the mining industry. The development of tailings as wet shotcrete aggregates has significant economic and environmental benefits. The fine particle size of the tailings results in a large consumption of traditional cement as a cementitious material and insignificant improvement in strength. Therefore, a composite cementitious system of cement and solid waste resources (fly ash and slag powder) is explored for this study. In this paper, the response surface methodology (RSM) is used to optimize the experimental design and a multivariate nonlinear response model with cement, fly ash and slag powder contents as variables are constructed, which can investigate the effect of the composite cementitious system on the strength of tailing wet shotcrete (TWSC). In addition, the information entropy (IE) is introduced and combined with the RSM to evaluate the composite cementitious system. Finally, the desirability function (DF) combined with RSM is used to optimize the composite cementitious system. The results show that the response model constructed in this paper has R2 = 0.96 and P-value < 0.01 (the test result of the model is P-value < 0.01), which indicates that the model has high reliability. The higher the content of slag powder and cement in the composite cementitious system, the higher the strength and comprehensive score of the TWSC. There is a critical value of fly ash content, which makes the maximum cementation of the composite cementing system. The optimal mix proportion of the composite cementitious system is obtained based on RSM-DF, which leads to the strength of TWSC at different curing time to achieve the expected index.

Thermal co-treatment of aluminum dross and municipal solid waste incineration fly ash: Mineral transformation, crusting prevention, detoxification, and low-carbon cementitious material preparation

Harmless disposal and resource utilization of hazardous industrial wastes has become an important issue with the green development of human society. However, resource utilization of hazardous solid wastes, such as the production of cementitious materials, is usually accompanied by a pretreatment process to remove adverse impurities that contaminate the final product. In this study, aluminum dross (AD) was thermally co-treated with another hazardous waste, municipal solid incineration fly ash (MSWI-FA), to synergistically solidify F and Na, control leaching of heavy metals, and remove chloride impurities. Significant crusting was observed when AD was thermally treated by itself, but not when AD and MSWI-FA were thermally co-treated. In the process of co-thermal treatment, the remaining Cl, Na, and K contents were reduced to as low as 0.3%, 1.8%, and 0.6%, respectively. CaO and SiO2 in MSWI-FA reacted with Na3AlF6 and Al2O3 in AD, and formed CaF2 and Na6(AlSiO4)6, which contributed to the prevention of crusting and limited the leaching concentrations of F and Na to below detection thresholds and 270.6 mg/L, respectively. In addition, heavy metals were well solidified, and dioxins were fully decomposed during thermal treatment. Finally, a sulfoaluminate cementitious material (SACM) with high early- and later-age strengths was successfully created via synergetic complementarity using thermally co-treated AD and MSWI-FA together with other solid wastes. Collectively, this study outlines a promising method for the efficient and sustainable utilization of AD and MSWI-FA.

Enhancement of ultra-fine slag on compressive strength of solid waste-based cementitious materials: Towards low carbon emissions

Utilization of solid wastes in building materials can reduce the amount of cement and thus reduce carbon emissions. However, low compressive strength largely restrained the development of solid waste-based cementitious materials (SWBCM). In this study, steel slag (SS), phosphogypsum (PG), and ground granulated blast-furnace slag (GGBS) were used to prepare SWBCM. Ultra-fine slag (UFS) was used to partially replace raw GGBS in SWBCM, with intent to enhance the compressive strength of SS-PG-GGBS binders. Results showed that the strength of SS-PG-GGBS mortars was 17.0 MPa and 24.0 MPa at 7 d and 28 d, while 20% UFS mortars were 33.2 MPa and 44.9 MPa. The reason was that the high activity of UFS and nucleation-inducing effect could accelerate early hydration of SS-PG-GGBS, and the higher hydration degree of GGBS and the refined pore structure enhanced the later strength. Additionally, the promotion mechanism of UFS on SS-PG-GGBS system was proposed. These results showed an effective method to promote the strength of SWBCM. This type of binders showed greatly potential in low carbon cementitious materials.

Mechanical properties and hydration mechanism of high-volume ultra-fine iron ore tailings cementitious materials

• A blended low-carbon cementitious materials containing high-volume mine solid waste activated ultra-fine IOT is designed. • IOT2 has higher pozzolanic activity and better dense filling effect. • IOT2 can consume more portlandite produced from cement hydration to form more rigid hydration products. • The increase of later compressive strength owes to the synergistic action of the improvement of hydrates and filler effect. Iron ore tailings (IOT) are a type of mining solid waste and cause many environmental and safety problems. The aim of this paper was to investigate the effect of high-volume ultra-fine IOT on the mechanical properties and hydration mechanism of cementitious materials. To obtain ultra-fine particles, IOT was processed via grinding in water, and the particle characteristics were characterized using a laser particle-size analyzer, scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FT-IR) and inductively coupled plasma (ICP). The influence of the IOT particle size (D50 = 47.63 μm, 4.32 μm, 1.09 μm), substitution percentage (30 %, 40 %, 50 %), and hydration age (1 d, 7 d, 28 d) on the compressive strength of mortar specimens were also studied. The results show that ultra-fine treatment enhances the hydration activity of IOT particles and has a positive effect on the mechanical properties of cementitious materials. The addition of IOT increases the setting time and reduces the hydration heat at early hydration stage; however, the ultra-fine treatment can minimize this problem and meet the requirements of practical engineering applications. Compared with raw IOT, the ultra-fine IOT exhibits more remarkable filling effect and hydration activity. They can consume a large amount of portlandite, which is produced by cement hydration to form densified calcium silicate hydrate gel and accelerate the development of strength. This study provides a design method and experimental data support for the preparation of a kind of low-carbon cementitious materials.

Quantification of CO2 emission from the preparation and utilization of solid waste-based sulphoaluminate cementitious materials

The cement industry as a high-emission and an energy-intensive sector, has always been a key concern for the sustainable development. In 2020, China produced over half of the world's cement and the relevant carbon emission is enormous. It's an urgent need to development low-carbon cementitious materials, such as the sulphoaluminate cement (SAC), to achieve a low-carbon society. The prerequisite of low-carbon development is to properly quantify the amount of CO2 emissions during the preparation and utilization of cement. However, there are currently only studies on the calculation of the carbon emission factor of Portland cement (PC), and the carbon emission factor of SAC, especially the calculation of SAC produced using solid wastes is lacking. This study focuses on the quantification of CO2 emissions of solid waste-based sulphoaluminate cement (SW-SAC) from the preparation (raw material transportation, fuel combustion and transportation, electricity consumption, calcination and product transportation) to the utilization processes (carbonation process), and the influence of supplementary cementitious materials (SCMs) on carbon emissions of cementitious materials is also considered. The results show that the carbon emission factor of SW-SAC clinker produced using steel slag, red mud, aluminum ash, FGD gypsum and limestone tailing is 361.021–401.641 kgCO2/tcl, which is 28.14%–35.40% lower than that of PC. And the net carbon emission of SW-SAC is 448.221–488.841 kgCO2/tcl, which is significantly lower than that of PC (693.780 kgCO2/tcl). The carbon emission factor of SW-SAC incorporating SCMs is only 111.395–135.767 kgCO2/tcl, which accounts for 30.9–33.8% of SW-SAC's, and 36.95–45.0% of that of PC incorporating SCMs. By comparing different sectors, it is found that the calcination process is the main carbon reduction sector of SW-SAC. This study quantitatively affirms that the SW-SAC can benefit the low-carbon development of cement industry, but multiple efforts, such as utilization of renewable energy and reduction of clinker proportion, are also required to achieve carbon neutrality.

Preparation and properties of alkali and sulfate coactivated ternary cementitious matrix and its application in high-strength cold bonded lightweight aggregates

In this study, a novel alkali and sulfate coactivated ternary cementitious matrix (ASAM) was produced using sulfur-aluminum low-carbon cementitious material (LCM), red mud (RM), and blast furnace slag (SG). The mechanical properties, mineral components, hydration characteristics, microstructures and pore size distributions of ASAM were systematically investigated, and the results indicated that the compressive strength of ASAM was apparently higher than that of the LCM–RM binary matrix. Based on the alkali and sulfate coactivation effect, the ettringite formed by sulfate-activated SG and the C(N)–A–S–H generated by alkali-activated SG, as well as the U-phase products were the main mineral components of ASAM, which jointly contributed to the compressive strength of ASAM. The maximum compressive strength of ASAM was 34.5 MPa at a curing age of 56 d, when the mass ratio of SCM:RM:SG was 3:5:2. Furthermore, the leaching behaviors and solidification mechanism of Na+ in ASAM were analyzed. The results illustrated that the Na+ could be effectively solidified by C(N)–A–S–H and U-phase, through physical encapsulation and chemical adsorption. The maximum solidification ratio of Na+ was up to 44.3%, when the mass ratio of LCM:RM:SG was 3:5:2. Finally, the optimal ASAM was employed to prepare cold bonded lightweight aggregates (CBLA), and high-strength CBLA with a crushing strength of 11.6 MPa, loose bulk density of 1180 kg/m3 and water absorption of 5% were obtained. Hence, this work provides a new approach for the preparation of a novel ASAM using 100% solid waste, and also verified its potential application in CBLA production.