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Greener phosphogypsum-based all-solid-waste cementitious binder with steel slag activation: Hydration, mechanical properties and durability
... [21][22][23] However, production costs remain high because of the reliance on cement clinker, particularly when used in large quantities. 24 Therefore, replacing cement clinker with a lowcarbon, economical activator for SSC production is necessary. Calcium carbide slag (CS) is a by-product of acetylene gas manufacturing. ...
... Furthermore, the final setting time of the G-CSSC slurry decreased by approximately 26% from 558 to 413 min. The P-CSSC developed in this study achieves an initial setting time of 8.6 hours and a final setting time of 9.8 hours, which is significantly shorter than the persulphated phosphogypsum slag cement developed by Ouyang et al., 24 Figure 7(c) indicates that at 28 d, the G-CSSC, D-CSSC, and P-CSSC slurries reached their highest compressive strengths at CS/G ratios of 0.11, 0.67, and 0.11, respectively. Among these, P-CSSC exhibited the highest late compressive strength. ...
... Generally, the time for the slurry to begin hardening and the initial setting time correspond to the time of the main hydration peak and the end of the induction stage, respectively. 24 A higher pH enhances slag dissolution, thereby reducing both the induction stage and initial setting time of CSSC slurries. Figure 8(b) shows the cumulative heat of hydration for the CSSC slurries. ...
The appropriate use of industrial solid waste is an effective method for alleviating environmental pollution. In this study, the potential of using desulfurized gypsum (DG), phosphogypsum (PG), gypsum dihydrate (G), and calcium carbide slag (CS) in the production of supersulfated cement (SSC) was investigated. The effects of various types of gypsum and their dosages on the performance and hydration process of SSC were thoroughly examined. The results indicate that as the ratio of CS to gypsum (CS/G) increased, the pH of SSC increased, whereas fluidity and setting time decreased. For the same CS/G ratio, SSCs activated by DG and CS (D-CSSC) and by PG and CS (P-CSSC) exhibited longer setting times and lower pH values than those activated by G and CS (G-CSSC). Mineralogical analysis and microstructural characterization revealed that the type of gypsum influenced the hydration rate, microstructure, and hydration products (type and quantity) of the SSC, thereby affecting its compressive strength. The loose pore structure of G-CSSC significantly reduced its compressive strength. Owing to the formation of a significant amount of Calcium aluminum hydrate (C-A-H) gel, D-CSSC demonstrated greater early strength; however, hannebachite hindered its late strength development. PG continuously participated in hydration, leading to more hydration products and a denser microstructure in P-CSSC, which significantly increased its late strength. This study elucidates the effect of gypsum type on SSC performance and offers valuable insights for enhancing it.
... pH value of FG and FAB. The pH value of the medium has a great influence on hydration and structure formation in SSC [4][5][6], largely due to the activation of GBFS. Generally, under high-temperature exposure deformation of the crystal lattice of calcium sulfate dihydrate occurs due to which the pH value changes [25]. ...
... pH value of FG and FAB. The pH value of the medium has a great influence on hydration and structure formation in SSC [4][5][6], largely due to the activation of GBFS. Generally, under high-temperature exposure deformation of the crystal la ice of calcium sulfate dihydrate occurs due to which the pH value changes [25]. ...
Supersulfated cements (SSCs) are one of the promising binders characterized by low CO2 emissions. A significant advantage of SSC is the possibility of using phosphoanhydrite binders as a sulfate component, obtained by the calcination of phosphogypsum—a waste product of acid and fertilizer production. The utilization of phosphogypsum is a global problem. Differences in the properties of phosphogypsums from various industrial enterprises are determined by the difference in phosphate rock and the technological mode of production. This gives reason to believe that phosphoanhydrite binders (FABs) will also have differences in properties, which in turn will influence the process of structural formation of SSC. In the article, the effect of FAB produced at calcination temperatures of 600, 800, and 1000 °C using phosphogypsum of two different industrial enterprises was studied. It is established that the morphology and pH value of FAB particles, and the ratio of components in the binder have the greatest influence on the physical and mechanical characteristics of the SSC. The use of FAB with a high pH value (≈12) allows for obtaining free-of-cement SSC, with compressive strengths of up to 50 MPa at the age of 90 days.
... However, the main hydraulic phase in steel slag, γ-C 2 S, is thermodynamically stable and exhibits much lower reactivity than β-C 2 S in Portland cement, resulting in slow hydration and limited early-age strength contribution [23]. Therefore, physical or chemical activation is often necessary to enhance its cementitious performance [24][25][26]. Despite its low early reactivity, the slow and continuous hydration of steel slag can facilitate sustained C-S-H gel formation, leading to improved pore structure, increased matrix densification, and steady long-term strength development. ...
This study evaluates the effects of steel slag powder (SSP), fly ash (FA), and steel slag sand (SSS) on mortar compressive strength. A response surface methodology (RSM) based on central composite design (CCD) was employed to model 7-day, 28-day, and 91-day strength development, considering three quantitative variables: SSP, FA, and SSS. Statistical results confirmed the reduced cubic models were significant and predictive (R2 > 0.97), with non-significant lack of fit and adequate precision. Experimental results revealed that SSP and FA negatively affected early-age strength due to dilution effects and low initial reactivity, whereas SSS slightly improved it by enhancing particle packing. At later ages, SSP exhibited nonlinear effects, where moderate dosages enhanced strength, while excessive replacement led to strength reduction. SSS showed a continuously positive contribution across all ages, particularly at 91 days. Perturbation plots, contour maps, and gradient analyses indicated that SSS played a dominant role at later stages and that maintaining a proper balance among supplementary cementitious materials (SCMs) and aggregate replacements is crucial. The developed models and response surfaces provide practical guidance for designing slag-based mortars with improved mechanical properties and enhanced sustainability.
... Gypsum also released Ca 2+ and SO 4 2− , which formed AFt in an alkaline environment. Simultaneously, the dissolution of GGBFS released ions that facilitated the generation of C-(A)-S-H gel, thereby promoting hardening and solidification of SSC [46]. The conductivity of solution in SSC samples containing 1% SA, 2% SA, and 3% SA decreased rapidly in the initial phase before stabilizing. ...
Supersulfate cement (SSC) has received significant attention in the construction industry due to its extensive utilization of solid wastes and low carbon emissions. However, the low carbonation resistance and early strength of SSC greatly restricted its application, which was attributed to early insufficient alkalinity and slow hydration. Facilitating early hydration alkalinity is critical to promote early hydration and improve early performance for SSC. Thus, sodium aluminate (SA), an admixture with concentrations ranging from 0% to 4%, was adopted to enhance early alkalinity and investigate its impact on the initial hydration process. The results indicated that incorporating SA into SSC enhances its early performance by balancing high alkalinity and AFt stability. The addition of 2% SA accelerates hydration procession, reducing initial and final setting times by 76% and 42%, respectively, while increasing viscosity by 50% for improved structural stability. At 2% SA, 1-day and 7-day compressive strengths rose from 3.7 MPa to 8.4 MPa and from 15.1 MPa to 18.5 MPa, respectively, representing gains of 127% and 22.5%, which were facilitated by accelerated GGBFS dissolution and needle-like AFt formation, which densifies the crystal-gel network microstructure.
Expanding the availability scope of sulfate activators in supersulfated cement (SSC) systems will further the development of low–carbon cementitious materials techniques. The use of industrial solid wastes with comparable properties in SSC systems shows significant potential. However, the types of potential sulfate activators and their impacts on SSC performance remain inadequately defined. To address this gap, this study explored the feasibility of substituting gypsum with desulfurization gypsum (DG) and sodium−based desulfurization ash (SDA) as alternative sulfate activators in SSC. The workability, setting time, thermodynamic processes, dissolution characteristics, and mechanical properties of SSC systems prepared with various sulfate activators were characterized comprehensively, and the phase compositions and reaction mechanisms were elucidated using multiple microscopic techniques. The results indicated that fluidity and consistency were primarily dependent on the physical characteristics of the sulfate activators, while setting time was more strongly influenced by chemical dissolution and hydration processes. DG–SSC exhibited thermodynamic, phase composition, and mechanical properties similar to those of G–SSC. The incorporation of SDA increased the system’s pH value and facilitated the dissolution of slags, shifting the product system from ettringite−driven to gel−driven. The formation of gel products optimized matrix densification and enhanced compressive strength. This study not only provides a scientific basis for the application of desulfurization by−products in SSC systems but also offers additional insights into the resource utilization of desulfurization by−products, contributing to the sustainable development of the construction materials industry.
Metals extraction and processing generate a substantial amount of industrial waste, posing significant environmental hazards. To reduce the accumulation of such waste, this paper incorporates steel slag (SS) into a lime (CH)–sodium sulfate (SN) composite-activated cementitious system. The mechanical properties, hydration products, and microstructure of the cementitious system with varying SS contents were characterized. The results indicate that the addition of SS not only enhances the mechanical properties of the cementitious system but also significantly reduces its cost and energy consumption. When the content of SS is lower than 30%, it does not reduce the generation of C-(A)-S-H and ettringite in the cementitious system, and can be used as a micro-filling to make the matrix dense, thus improving the mechanical properties of the cementitious system. However, as the SS content continues to increase, the formation of C-(A)-S-H and ettringite in the cementitious system decreases, leading to an increase in the number of harmful pores in the matrix and a subsequent reduction in the mechanical properties of the system. This study provides important insights into the sustainable development of metal extraction and processing.
Currently, research aimed at developing alternative binders that can partially or completely replace Portland cement is relevant. At the same time, the priorities are minimal impact on the environment (including dust and CO2 emissions), energy saving, and rational use of natural resources through the disposal of man-made waste. Extremely promising from these positions are supersulfated cements (SSC), which consist of 80–85 % of the aluminosilicate component; 10–20 % from the sulfate component and 3–5 % alkaline activator. The traditional aluminosilicate component for SSC is granulated blast furnace slag (BFS), however, due to the removal of requirements for the content of aluminum oxide in its composition (not lower than 14 %), it has become possible to partially or completely replace it with other technogenic or natural aluminosilicate products. Instead of natural gypsum-containing raw materials, technogenic ones (phosphogypsum, desulfurization gypsum) are increasingly being considered as a sulfate component, both in the form of dihydrate and in the form of anhydrite and hemihydrate. Due to this composition and the possibility of using waste as the main components, this type of cement is characterized by significantly lower CO2 emissions into the environment and energy costs for production compared to Portland cement. The article describes the features of the hydration mechanism of SSC, which determine its main properties: low heat generation, high resistance in sulfate and chlorine environments, acid resistance. Disadvantages are considered - long hardening times, reduced frost resistance, high carbonization rates. Ways to combat the shortcomings of SSC and prospects for further research are described
Artificial aggregate is an effective method to recycle for waste phosphogypsum (PG) in large quantity. This article proposes PG-muck artificial aggregates (PMAs) that combine PG and muck using compaction granulation to achieve high-volume utilization of PG. Varied PG and muck contents are applied to prepare PMAs, and the density, water absorption, strength, leaching stability, microstructure, and porosity of PMAs are tested. It is found that addition of muck can improve the PMA properties significantly, especially the leaching stability. Muck showed excellent immobilization capacity on contaminates in PMAs. Effects of muck addition are discussed comprehensively, and an optimized recipe for preparing PMAs is recommended.
Traditional Portland cement is widely used in the preparation of various hydraulic concrete. However, the high alkalinity produced by cement hydration threatens the survival of aquatic animals and plants. In this paper, a new eco-friendly, ultra-low alkalinity, cementitious material was prepared with industrial waste phosphogypsum, granulated ground blast slag (GGBS) and sulphoaluminate cement. When appropriate proportions are used, the pH value of the test blocks’ pore solutions at different ages were all less than 9, showing the remarkable characteristic of ultra-low alkalinity. The XRD and SEM analyses showed that the 56 d hydration products were mainly ettringite and hydrated calcium silicate, and the content of Ca(OH)2 was not detected. The new cementitious material also has the advantages of short setting time, low heat of hydration, high strength of cement mortar and the ability to fix harmful substances in phosphogypsum, such as phosphate, fluoride and Cr and Ba elements. It has a broad application prospect in the construction of island and reef construction, river restoration and so on.
The urge to reduce greenhouse gas emissions, in particular carbon dioxide, is a global problem, not only in spatial terms but also in terms of the scope of activities and sectors involved. Nevertheless, some sectors/industries are more critical due to their overall contribution to the problem, which is the case of the Portland cement industry. The present research estimates the energy consumption and carbon emissions associated with a novel process for producing cement by recycling used concrete and mortars. The novel process assessed resorts to the magnetic separation of the cement paste from the aggregates, followed by the thermal reactivation of the cement paste. Comparing the recycled cement production with the clinker production, higher energy consumption (over 9000 MJ/t compared with roughly 4000 MJ/t for Portland cement) and lower carbon dioxide emissions (average 730 kg CO2/t compared with more than 800 kg CO2/t for Portland cement) were estimated. However, the potential benefits in an industrial application are potentially much higher with the optimization of the production process. In particular, improvements in the washing and drying of the material prior to the magnetic separation will be critical since most of the energy is consumed in the process of drying.
The global disposal of urban waste glass has been facing a great challenge while the glass products are still continuously produced annually. Discarding waste glass, a nonbiodegradable material, to landfills is not offering a sustainable management of this municipal solid waste. In recent years, against the backdrop of prevailing OPC-less or even OPC-free cementing materials, increasing attention has been paid to the application of waste glass powder (GP) in sustainable cement and concrete, since GP is rich in amorphous silica (∼70%). This paper provides a comprehensive review of GP used as the eco-supplementary cementitious material in ordinary Portland cement (OPC) and the precursor of alkali-activated materials (AAMs). The effect of GP on mechanical and durability properties of cementitious materials is systematically discussed. The review also includes the recent progress on measuring the reaction degree of GP and the prediction of hydration products by thermodynamic modelling. However, the related research on the durability of GP-based AAMs is limited, especially on the aspect of ASR expansion. In addition, the majority of published studies were based on laboratory-scale experiments. More case studies are required on the performances of real concrete structure containing GP.
Dicalcium silicate is one of the main mineral phases of steel slag. Ascribed to the characteristics of hydration and carbonation, the application of slag in cement production and carbon dioxide sequestration has been confirmed as feasible. In the current study, the precipitation process of the dicalcium silicate phase in steel slag was discussed. Meanwhile, the study put emphasis on the influence of different crystal forms of dicalcium silicate on the hydration activity and carbonation characteristics of steel slag. It indicates that most of the dicalcium silicate phase in steel slag is the γ phase with the weakest hydration activity. The hydration activity of γ-C2S is improved to a certain extent by means of mechanical, high temperature, and chemical activation. However, the carbonation activity of γ-C2S is about two times higher than that of β-C2S. Direct and indirect carbonation can effectively capture carbon dioxide. This paper also summarizes the research status of the application of steel slag in cement production and carbon dioxide sequestration. Further development of the potential of dicalcium silicate hydration activity and simplifying the carbonation process are important focuses for the future.
The purpose of this study is to solve the problem of industrial waste storage. Waste from the production of mineral fertilizers is considered in this study (as an example—phosphogypsum LLC «Industrial group «Phosphorit»). Waste storages on a landfill have a significant negative impact on the environment. This fact has been confirmed by studies of wastewater from the dump (drainage water). Phosphogypsum utilization as a mineral component of an organic-mineral mixture without preliminary purification is the most promising approach. In this case, the joint utilization of lignin sludge as an organic component is required. The absence of excess content of toxic elements and the presence of plant nutrients in the required quantities was established as a result of laboratory studies of soil additive. In addition, the authors noted an increased content of stable strontium in phosphogypsum. Localization of strontium in the soil mixture is ensured by fractional application. Strontium is replaced by covalent calcium when the components are absorbed by the plants. The developed soil mixture should be considered as a single-use organomineral additive with prolonged action. Forestry activities, reclamation of disturbed lands, slopes of highways and landfills of solid municipal waste are promising areas of implementation.
Magnesium phosphate cement (MPC) is an excellent repair material for civil and road engineering, but its volume stability under various environmental conditions significantly influences these applications. In this study, the volume stability of MPC under different curing conditions (e.g., air, standard, and water curing) is investigated. Moreover, the phases, weight loss, microstructure, and pore structure of the samples have been determined by X-ray diffraction, thermogravimetry, scanning electron microscopy, and Brunnauer–Emmet–Teller method. The results show that MPC will shrink by 8 × 10⁻⁴ under air-curing conditions. At the same time, MPC will expand by 9 × 10⁻⁴ under water-curing and standard curing conditions, which means that curing conditions influence the volume stability of MPC. Not only that, compared with air-curing conditions, the compressive strength of MPC under standard curing and water-curing conditions will decrease by 30% and 60%, respectively, which implies that greater humidity will reduce the mechanical properties of the repair material. Therefore, air curing is the best curing condition for MPC. To get a better repair effect, the environment should be avoided as much as possible in a humid state. The microscopic analysis results show that the volume expansion of MPC is related to hydration products, and the volume shrinkage occurs owing to drying shrinkage caused by internal moisture evaporation.
Illustrating the influence of Fe dosage and temperature on the formation of sulfoaluminate hydrates is of great significance to understand the early hydration process of high ferrite cement under steam curing. Herein, the impact of Fe dosage on the ettringite formation at different temperatures was revealed through the combination of experiments and computational simulations. A Fe dosage no more than 20% conspicuously accelerated the crystal growth of ettringite by increasing the surface energy in the (0 0 1) direction, whereas a higher dosage suppressed the formation of ettringite as the high incorporation of Fe ions into the ettringite crystal was energetically unstable. The chemical environment analysis of Fe in products shows that Fe(OH)6³⁻, compared with Al(OH)6³⁻, prefers to participate in the formation of Al2O3‐Fe2O3‐mono than ettringite, which is also confirmed by the calculated thermodynamic properties’ results. The understanding of the impact and mechanism of Fe on the formation of ettringite under steam curing conditions plays a pivotal role in the utilization of high ferrite cement.
Natural adsorptive materials are mainly based on physical adsorption and have limited adsorption capacity. Artificial adsorptive aggregates lead to great potential in improving water quality for water treatment. This study aims to develop a porous and sustainable adsorptive aggregate combining both chemical and physical adsorption capacities. Industrial by-product steel slag (SS) powder is used in conjunction with porous expanded silicate (ES) powder applying a non-sintered pelletizing method for producing sustainable high adsorptive aggregates, and bio-based miscanthus (M) is used to further increase its permeability and porosity. The results show that the bulk density of the resulting adsorptive aggregates varies from 570 kg/m³ to 882 kg/m³, with a bulk crushing strength of up to 5.1 MPa. Moreover, all adsorptive aggregates have outstanding resistance to salt and freeze-thaw cycles. Phosphorus (P) adsorption tests show that the adsorptive aggregates remove the P in aqueous solution (168 mg/L), with an adsorption capacity of 4.2 mg/g. The research demonstrates that sustainable high adsorptive aggregates with good mechanical properties can be produced applying the facile pelletizing method, suitable for the application in the field of water treatment such as adsorptive concrete, constructed wetlands and rainwater gardens.
Cement-based materials (CBMs), such as paste, mortar and concrete, are highly alkaline with an initial high pH of approximately 12.0 to 13.8. CBMs have a high pH due to the existing oxide mineral portlandite and alkali metal contents in Portland cement. The high pH of concrete provides excellent protection and reinforces the steel bars against corrosion. The pH of concrete does not remain constant due to ageing and other defect-causing factors, such as chloride ingress, alkali leaching, carbonation, corrosion, acid attack, moisture and biodegradation process. Reducing the concrete pH has negative impact on the strength, durability and service life of concrete buildings. However, the high pH of concrete may also cause concrete structure deterioration, such as alkali silica reaction, porosity and moisture related damages in concrete structures. The pH of CBMs can be influenced by high temperatures. For instance, the extremely high volume (85%–100%) of slag-blended cement pastes shows considerable pH reduction from 12.80 to 11.34 at 800 °C. As many concrete structure deterioration are related to concrete pH, using an accurate and reliable method to measure pH and analyse the durability of reinforced concrete structure based on pH values is extremely important. This study is a comprehensive review of the pH of CBM in terms of measurement, limitations and varying values for different CBM types.
This study presents an analytical investigation based on thermodynamic simulation aimed at achieving a holistic management of the phase assemblages of alkali-activated materials (AAMs) and gaining insights into the designing of precursors. Gibbs free energy minimization method was conducted on AAMs spanning the compositional envelopes at (metastable) thermodynamic equilibrium. The stability regions and quantities of simulated phases were identified in the SiO2-CaO-Al2O3 ternary contour diagrams, yielding the overall relationships between the chemical components of precursors, phase assemblages and pH in pore solution. The analytical results are in good agreement with the available experimental observations that the main precipitation regions of C-(N-)A-S-H appear at CaO/SiO2 = ~1.0 and relatively low Al regions while N-A-S-H phases generally dominate the Ca-deficient regions of the contour diagrams. Strätlingite mainly occurs at intermediate levels of Si, Al and Ca. Katoite and AFm phases form at Al2O3/CaO = ~3.0 under Si-deficient conditions. The findings also suggest the precipitation regions of a product may span a range of pH of aqueous solution, making it possible to simultaneously control pH and maintain the precipitation amount of the phase. The application of this work in designing precursors can achieve a more precise control of the phase assemblages for AAMs.
For clarifying the effect of dolomite on the volume stability of Portland dolomite cement (PDC) at high reaction degree, the length change of PDC pastes with the replacement levels of 10–30 wt% dolomite and cured in different conditions was examined, in contrast with Portland limestone cement (PLC). The hydration products and microstructure were investigated using XRD, TGA, and SEM. Results indicate that at 40 and 60 °C the PDC and PLC show similar deformation patterns, with a small expansion. At 80 °C, however, the PDC are characterized by a higher expansion than the PLC, especially at high replacement levels. The reaction process of dolomite in PDC is dependent on the availability of aluminate phases. In the presence of free alumina, dolomite would react preferentially to form carboaluminates, hydrotalcite, and calcite. When the alumina is exhausted, dedolomitization reaction takes place producing brucite and calcite. The high expansion in PDC is mainly associated with the dedolomitization, which may result in the reinforcing frame volume and crystallization pressure due to the formation of brucite and calcite in confined space. However, the expansion of PDC is too small to cause damage to hardened pastes. Therefore, the incorporation of dolomite in PDC has no adverse effect on the volume stability of cement-based materials.
A large amount of solid waste is produced due to the simultaneous production and consumption activities of an increasing population in the world. Solid wastes-based geopolymer as a green alternative has the potential to replace traditional cement concrete to lower CO2 emissions. In this study, the sample size effects of waste glass powder (WGP) and ground blast furnace slag (GBFS)-based geopolymer pastes cured at different conditions and ages on its physical properties and micro-characteristics were systematically investigated, including the mass loss, compressive strength, microstructures, and reaction products at different geopolymerization stages. The results show that the sample size effect was significant in the geopolymer pastes, and the compressive strength decreased with the increasing sample size. A higher reaction degree was observed in the smaller-sized geopolymer specimens based on the microstructural investigation. The higher curing temperature and higher relative humidity (RH) could contribute to the generation of C–S-Hs and lead to better mechanical performance. The analysis of coring and cutting samples reveals that the alkaline concentration gradient was formed during the reaction process, and the precipitation of alkali reagents was more obvious in the larger-sized geopolymer sample. In addition, the XRD, TGA/DSC, mass loss, and carbonation test results indicate that the moisture distribution was vital and had different functions during the geopolymerization process at different reaction stages.
This paper presents an experimental study on the effects of calcined gangue and wet co-milling on the properties of phosphogypsum-based excess-sulphate cementitious materials (PESCMs), including setting time, rheology, heavy metal solidification and strength development. For PESCMs containing 0%-50% calcined gangue, with and without wet co-milling, we examined hydration and microstructural evolution. Results indicate that the addition of calcined gangue shortens the initial and final setting time of fresh PESCM pastes, and impairs stability, fluidity and strength development. After wet co-milling, the rheological and mechanical properties of PESCM pastes are improved, where the binders with 10–20% calcined gangue show a 40% increase in 28-d compressive strength. Hydration of calcined gangue is rapid and weakens the retarding effect of impurities, providing more active aluminates and silicates for further hydration. Wet co-milling promotes physical dispersion, chemical dissolution and thus hydration, which helps refine microstructure for a better development of strength and heavy metal solidification capability.
Titanium gypsum (TG), fly ash (FA) and calcium carbide residue (CCR) are harmful industrial by-products that need to be addressed. In this study, TG, FA and CCR were selected to prepare the eco-friendly FA-TG-CCR (FTC) cementitious materials. Respond surface methodology (RSM) was undertaken to optimize and understand the influence of design factors of m(TG/FA), m(CCR/FA) and W/B on compressive strength, and the mechanical properties, microstructure, hydration mechanism and environmental performance of FTC cementitious materials were investigated. The morphology and structure were characterized by X-ray diffraction (XRD), thermogravimetry- differential scanning calorimetry (TG-DSC) and Scanning electron microscopy energy dispersive X-ray (SEM-EDX), the concentration of hazardous elements was determined via inductively coupled plasma optical emission spectrometer (ICP-OES). The results show that the optimal mixture with RSM methodology (m(TG/FA) = 30.80 %, m(CCR/FA) = 27.85 %, W/B = 0.62) reached 18.93 MPa at 28 days of curing, which confirmed via experimental verification. The FTC cementitious materials are mainly consist with C-(A)-S-H, ettringite; the existence of Fe³⁺ was disadvantageous to early age strength, however partial Fe³⁺ participated in the formation of Fe-ettringite as the curing time increased, which is beneficial to later age strength. The leaching results indicate that the FTC materials can effectively consolidate the heavy metal ions in raw materials, and the leaching concentration of ions all meet the Chinese Standard for Pollution Control on the Landfill Site of Municipal Solid Waste. Therefore, the synergistic of FTC material offers a novel approach to the efficient utilization of original TG and other industrial by-product wastes.
Phosphogypsum (PG) is a residue from the fertilizer industry used by Portland cement companies as a source of calcium sulfate to control the setting time during the hydration (or hardening process). However, certain impurities can alter the cement hydration kinetics. Thus, this paper aims to analyze the soluble phosphate, fluoride, and pH effects of Brazilian phosphogypsum used as a setting retarder on the hydration of Portland cement. Different sources of calcium sulfate were analyzed: natural gypsum (NG), alkaline phosphogypsum (PGA), and acid phosphogypsum (PGB). The materials were characterized physically and chemically, and Portland cement pastes were assessed by isothermal calorimetry, DRX, and TGA to identify the main hydrated products. Complementary compressive strength was evaluated in mortars. Furthermore, chemically soluble fluoride and phosphate contents were added to cement with NG, whose hydration was monitored by isothermal calorimetry. Portland cement with acid PG, soluble phosphates and fluorides presented hydration delays. However, soluble phosphates showed a more significant delay effect on cement than soluble fluorides. Cement setting times were extended with P2O5 soluble from 0.33% to 1.32%. Also, there is no relevant difference between the hydration products and compressive strength of alkaline PG cement and NG cement. Thus, the results suggest that low phosphate – fluoride and alkaline PGs may substitute NG in Portland cement.
Phosphogypsum-based excess-sulphate slag cement (PESSC) shows great resistance to sulphate and chloride ion erosion, and is expected to be an effective sink of waste phosphogypsum, to alleviate environmental pressure. Understanding the effect of magnesium ions is essential for the engineering application of PESSC in marine environment. In this paper, heat evolution, pore solution composition, phases assemblage, and mechanical properties of PESSC are characterized and analysed to reveal the effect of magnesium ions on early hydration and strength development. Results demonstrate that magnesium ions prolong the induction period and lead to a longer setting time. Increasing magnesium ions concentration from 0 mol/L to 0.2 mol/L makes the acceleration period appear earlier, and affects content of hydration products such as ettringite and C-(A)-S-H gel at early stage. Compressive and flexural strength can be significantly improved with magnesium ions addition at various degrees. It turns out that seawater could be favourable for PESSC if applied in marine environment.
This study investigated the effects of curing conditions such as dry air curing, moist air curing, water curing, and water-bath curing (30°C, 50°C) on hydration performance and mechanical properties of magnesium potassium phosphate cement (MKPC). The hydration temperature, compressive strength, and volume stability were investigated. Furthermore, microstructural analyses, including X-ray diffraction (XRD), scanning electron microscope (SEM), thermogravimetric analysis (TG/DTG), and mercury intrusion porosimetry (MIP) were adopted. Experimental results showed that the compressive strength and volume stability of MKPC paste improved under early moist air curing and 30 °C water-bath curing, whereas they worsened under water immersion and 50 °C water-bath curing. The XRD and SEM examinations also indicated the instability and partial dissolution of K-struvite in water, with the formation of new precipitates. Early moist air curing and elevating-temperature curing below 50℃ both produced faster early strength development and good volume stability for the MKPC paste. However, further increase in humidity and curing temperature resulted in porous structures, with detrimental effects on the dimensional stability and strength.
This study investigates mechanisms of carbonation on the microstructure of phosphogypsum-based supersulfated cement (PSSC) paste at different CO2 concentrations (atmospheric, 5% and 20%). The products was characterized using XRD, TGA, FTIR and ²⁷Al MAS NMR. It is observed that the main hydration products of PSSC are ettringite, C-A-S-H gels and third aluminate hydrate (TAH), and the carbonation products are calcium carbonate (mainly vaterite and calcite), secondary gypsum, and C-A-S-H gels and separate TAH with varying degrees of decalcification. The quantitative analysis of BSEM images shows that the volume change of products occurring on carbonation significantly increases porosity and pore size of the PSSC paste. Results indicate that the carbonation of PSSC at different CO2 concentrations results in a progressive carbonation degree, 5% CO2 supports the accelerated carbonation experiment in PSSC to simulate the natural carbonation process due to the same phase composition and similar microstructure with atmospheric CO2 and 20% CO2 further loosens the microstructure of PSSC for the complete decomposition of ettringite and the severe decalcification of gel products.
Landfilled phosphogypsum would cause severe environmental issues, but the waste can be recycled for preparing excess-sulphate cement, an eco-friendly alternative to conventional cement. This paper investigates the effect of metakaolin (0–50%) on early hydration, phase assemblages and mechanical properties of the excess-sulphate phosphogypsum cementitious materials (ESPCMs). Results indicate that metakaolin is related to a new exothermic peak and significantly shortens the induction period. Setting time of ESPCM pastes is reduced by 13%–38% with 0%–50% dosage of metakaolin. More ettringite and highly disordered C-(A)-S-H gel are characterized when metakaolin dosage is below 20%, leading to 70% increase in 28-d compressive strength. With above 20% metakaolin dosage, portlandite consumption at early stage is promoted and hydration degree at late state is reduced. It turns out that 20% metakaolin dosage is efficient to optimise setting time and strength development of ESPCMs, where slag and metakaolin synergistically promote the formation of ettringite and C-(A)-S-H gel to bind the unhydrated cement particles effectively.
While traditional disposal of phosphogypsum either occupies excessive amounts of land or causes serious pollution, novel recycling of phosphogypsum is desirable. Making phosphogypsum-based cold-bonded aggregates (PCBAs) through granulation technology is a potential recycling strategy. In this study, the quality of the recycled PCBAs is assessed, considering physical properties, mechanical strength, impurity stabilization ability, and microstructure. With the increase of phosphogypsum content from 60% to 90%, 28-day bulk density of the PCBAs decreased from 1080 to 950 kg/m³, cylinder compressive strength in over dry (OD) condition decreased from 16.5 MPa to 3.9 MPa, and water absorption increased from 5.9% to 13.6%. SEM, XRD, and MIP analyses showed that as the phosphogypsum content increased, hydration products in the aggregates diminished and proportion of harmful pores (greater than200 nm) increased. Leaching test on 28-day PCBAs showed a reduced concentration of phosphorus and heavy metals in the leachate compared to the leaching of the original phosphogypsum. The study concluded that the PCBAs with the phosphogypsum content up to 80 % are qualified lightweight aggregates (LWAs) for concrete.
Reducing or replacing cement clinker is one of the most important ways for the sustainable development of the cement industry. In the present work, three types of industrial solid wastes, wet ground granulated blast-furnace slag (WGGBS), phosphogypsum (PG) and carbide slag (CS), were designed as clinker-free, high-performance ternary binders for applications such as construction and underground filling. The results show that the wet grinding process with PG and CS can synergistically excite the GGBS and make the compressive strength of the sample surpass that of the cement, reaching 45.6 MPa. However, its cost and carbon emissions are only 51% and 12% of cement, which is expected to completely replace cement as a low carbon cementitious material. In addition, with the increase of PG content and the decrease of CS content, the ettringite content increased significantly and its microstructure changed from long and thin to short and thick, and the porosity gradually decreased, which was beneficial to the development of compressive strength. The type and morphology of hydration products and the amount of crystalline hydrates are closely related to the ratio of PG/CS, and explain the development of compressive strength and microstructure of the clinker-free ternary binder.
Supplementary cementitious materials (SCMs) have attracted increasing research interests due to the energy-intensive process and high greenhouse gases emissions of conventional cement. Meanwhile, large amounts of domestic wastes such as waste glass are causing serious environmental issues due to their nonbiodegradable characteristic. Thus, recycling waste glass is a sustainable way to mitigate the environmental problem, conserve natural materials, minimize landfill spaces, and save energy during the recycling production process. This paper experimentally studied the effect of using waste glass powder (WGP) as an ordinary Portland cement (OPC) supplementary material on the paste specimens at ambient temperature and after exposure to high temperatures (800 °C, 1000 °C, and 1200 °C). The physical-mechanical properties such as workability, setting time, compressive strength, and bonding strength of the OPC paste samples containing different mass contents of WGP (0%, 10%, 20%, and 30%) were investigated and discussed. The potential alkaline aggregate reaction (ASR) effect of WGP on OPC was investigated by the accelerated mortar-bar method. The results show that the addition of WGP contributed to the workability but led to prolonged setting time of the OPC paste. On the other hand, the introduction of WGP in OPC mitigated the degradation of the OPC matrix under high temperatures and could reduce the ASR effect. Therefore, the WGP could be regarded as a feasible SCM to OPC to improve the safety of OPC-based structures both at ambient and under high temperatures.
Innovations in the cement industry strive towards achieving an eco-friendly alternative to traditional cement. A low-carbon cement, calcium sulfoaluminate-activated supersulfated cement (CSA-SSC), has been recently developed. This cement consists of 80% granulated blastfurnace slag (GBFS), 15% anhydrite, and 5% high-belite calcium sulfoaluminate cement (HB-CSA) clinker. The hydration mechanism of CSA-SSC was experimentally investigated using isothermal calorimetry, X-ray diffraction, thermogravimetric analysis, and scanning electron microscopy, and was numerically studied using thermodynamic modeling. CSA-SSC shows a moderate compressive strength at the early stage, which is mainly attributed to the rapid formation of ettringite from the hydration of C4A3 S¯ with CaSO4 in HB-CSA clinker. Meanwhile, the hydration of f-CaO in HB-CSA clinker supplies an alkaline environment for the dissolution of GBFS and the formation of more ettringite. In the late stage, apart from ettringite, the hydration of GBFS forms C–S–H, leading to the continuous increase in late strength. A statistical analysis reveals that the high volume of GBFS in CSA-SSC results in a very low direct CO2 emission, which is only 8.7% of that of Portland cement.
In this paper, the mechanical properties of excess-sulfate phosphogypsum slag cement (PPSC) were studied by experimental data, microscopic tests and molecular dynamics (MD) simulations. Specifically, the properties of PPSC in terms of setting time, compressive and flexural strengths were investigated by experiment. The hydration products of PPSC were tested and observed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Furtherly, at the atomic scale, the relationship between nanoscale and macroscopic mechanical properties of PPSC was established. The mechanism of the effect of chemical composition on the mechanical properties of PPSC was studied at multiple scales, and the compound synergistic effect between chemical compositions was further studied. The results show that the mechanical properties of PPSC increase with the increase of CaO/SO3, and decrease with the increase of SiO2/Al2O3. CaO and Al2O3 can improve the mechanical properties of PPSC. The high mobility and structural instability lead to CaO/SO3 has greater effect on the mechanical properties of PPSC than that of SiO2/Al2O3. When CaO/SO3 is 1.8 ∼ 2.0 and SiO2/Al2O3 is 3.5 ∼ 3.7, the mechanical properties of PPSC are better. Moreover, when CaO/SO3 is 2.0 and SiO2/Al2O3 is 3.5, the mechanical properties of PPSC are the best. At this time, the compound synergistic effect of alkaline activator and sulphate activation is the best. This paper provides an important reference for the composition design and application of PPSC.
Both sodium carbonate (NC) and sodium phosphate (NP) could immediately precipitate in the presence of calcium ions (Ca²⁺), and these precipitates would deposit on the surface of cement as one layer in cement paste. However, these precipitates have different effects on the hydration of cement. In this study, effect of NC and NP on cement hydration was discussed, and the different effect between these two was investigated. Setting time, hydration heat, conductivity of cement suspension, and compressive strength of cement paste were examined, and the hydrates were analyzed with XRD and TG-DTG. The precipitates synthesized in saturated calcium hydroxide solution were characterized with XRD, TEM, and SEM. Then one model was proposed to illustrate the mechanism of the effect of NC and NP on cement hydration. The results indicated that NC accelerated cement hydration, but NP retarded it. And the results of TEM and SEM showed that calcium carbonate (the precipitate formed by NC and Ca²⁺)) could well crystalize with directional growth, there is a large amount of interspace in the calcium carbonate-based layer, which would benefit the transports of water molecules and dissolved ions. However, the calcium phosphate precipitate (the precipitate formed by NP and Ca²⁺)) seemed to have a large amount of amorphous structure with a much more compact layer structure than calcium carbonate. The difference in composition and structure of the layer was responsible for the effect of NC and NP on cement hydration. The mechanism behind accelerating effect of NC was due to the interspace in the calcium carbonate based layer which facilitated the ion dissolution of system, while that for retarding effect of NP was because the calcium phosphate-based layer formed on the surface of cement grains seemed compact enough to restrict the transports of water molecules and dissolved ions. Such consequences were expected to give deeper insight into the effect of the surficial structure of cement grains on early hydration of cement paste.
This study focuses on the influence of liquid-solid ratio, carbonation time and curing method on the carbonation reaction, and then mainly discusses the carbonation effects of four curing methods and reaction mechanisms to improve the carbonation efficiency of steel slag (SS)-desulphurisation gypsum (DG) and the strength of carbonated products. The results showed that the compressive strength of the SS–DG samples could be increased to 57.56 MPa by optimising the liquid–solid ratio to 1:5, carbonation time to 12 h, and curing method to 1dW (hydration for 1 d) + 2 dC (carbonation for 2 d). Furthermore, the compressive strength of samples cured for 1dW+2dC could increase by 33.40% compared to that cured for 2dC. The predominant mineral phases that react with CO2 in steel slag were dicalcium and tricalcium silicates, according to microanalysis (including XRD, TG–DTA, SEM-EDS and FTIR). Approximately 8% ettringite and 5% Ca(OH)2 could be generated during the hydration process and they would be then further carbonated to afford CaCO3. In the curing process, the method of hydration followed by carbonation considerably promoted the carbonation efficiency and formation of CaCO3 by approximately 5% compared with the method of direct carbonation. The overall time of hydration and carbonation was the same while the sequencing was different; the reactants CaCO3 obtained from the curing method of 1 dW + 2 dC was increased by 2% compared with the curing method of 1dC + 1dW + 1dC, which further indicated that the curing method of hydration and then carbonation could better improve the carbonation reaction efficiency.
In this study, refining slag containing high hydration active component C12A7 was added in GBFS−SS−DG−based clinker−free cementitious material. The optimum was determined by an orthogonal test and the influence of heat treatment was studied. The participation of refined slag makes the compressive strength of 3 days reached 27.4MPa, 60.6% of that at 56 days, which overcomes the shortcoming of low initial strength of GBFS−SS−DG−based clinker−free cementitious material. Microscopic analysis showed that quantities of ettringite occurred in 3 days. During the hydration, C3AH6 was formed by the hydration reaction of C3A and C12A7 and then reacted with the CS¯H2 to form ettringite which constituted the basic framework. Under the alkaline condition provided by steel slag, SiO4 tetrahedra and AlO4 tetrahedra were constantly dissociated from GBFS, then C–S–H gels and ettringite were further developed, which was corroborated by results of IR and XPS. In middle and late period, amounts of C–S–H gels wrapped AFt crystals and filled the pores, and finally formed a unified composite structure, which was responsible for the high strength of 43.0 MPa at 28 days.
Urban waste glass powder (GP) has been identified as the Si-rich (>70%) but Ca-and Al-deficient precursor material for use in alkali-activated materials (AAMs). To facilitate the recycling and utilization of GP, this study explored the possibility of using calcium sulfoaluminate cement (CSA) as both reactive alumina source and shrinkage-reducing agent to improve the strength and durability properties of alkali-activated waste GP. The experimental and thermodynamic simulation showed that 0%-50% of CSA substitution could contribute to the formation of calcium (sodium) aluminosilicate hydrate [C─(N─)A─S─H] and N─A─S─H gels due to the release of Ca and Al from CSA. Nonetheless, more than 50% of CSA resulted in the change of reaction paths and the precipitation of AFt and AFm as the major phases, which reduced the porosity significantly. A higher CSA replacement ratio generally increased the early strength development rate, while the delayed strength gain could be observed due to the slower but progressive dissolution of GP. More importantly, CSA effectively reduced the overall shrinkage of hardened AAM pastes, and the mixtures with up to 50% of CSA had lower shrinkage than the reference ordinary portland cement sample. However, the GP-rich mixtures showed viscoelastic/viscoplastic response to the capillary pressure-induced stress upon drying, possibly due to the structural rearrangement of C─(N─)A─S─H and N─A─S─H. It was also found that the moderate amount of CSA in mixtures could considerably reduce the leaching of free alkali and therefore lower the potential environmental impacts. This study can improve the limited understanding on the hydrate assemblages and properties of AAMs based on GP and CSA, and expand the toolkit of cementitious materials based on recycled urban waste glass.
Cement is widely produced and used. Cement production consumes a significant amount of energy and natural resources, and it accounts for approximately 7% of man-made greenhouse gas (GHG) emissions in CO2-equivalents globally. The cement industry is continually conducting research and development into energy conserving and low-carbon technologies to reduce CO2 emissions. Furthermore, the industry will be at the heart of a carbon recycling system along cement and concrete value chain using newly developed mineral carbon capture and utilization (MCC&U) technology to reduce CO2 emissions substantially during clinker and/or cement production. This paper provides fundamental calculations for the determination of CO2 emissions from the MCC&U technology in the cement industry, which will contribute to the calculation of GHG inventories and life cycle assessments of the system. Demonstration experiments were conducted to develop an accounting formula and clearly determine CO2 emissions for recarbonate utilization as a cement raw material.
The cement industry is presently facing the demanding challenge of reducing its large amount of carbon emissions in order to meet the targets set to fight climate changes. One recent, and very promising, approach to reduce the carbon footprint is the production of more eco-efficient recycled cement from cement-based waste materials. This study aims at comparing the difference in terms of energy consumption and carbon dioxide emissions between recycled cement and conventional clinker production. In a conservative scenario, the estimated carbon dioxide emissions from recycling cement was as low as 58%–74% of the clinker production. From the sensibility analysis, it was found that the most influencing factors for the carbon emissions from the recycled cement production are: i) the waste cement water content; ii) the fraction of cement paste on waste material; and iii) the dryer energy intensity. The main drawback of the recycled cement production process is the pretreatment stage related with washing and drying of waste materials. The carbon dioxide emissions from recycled cement production can be potentially reduced to only 13% of the emissions from clinker production, if these pretreatment stages are avoided by developing a dry process.
Granulated phosphorus slag (GPS) is an industrial byproduct generated from the yellow phosphorus manufacturing process through the electric furnace method, which not only occupies enormous fields but also causes serious environmental issues. This study investigated the effect of GPS on physical, mechanical and microstructural characteristics of Class F fly ash (FFA) based geopolymer. Samples synthesized from FFA with different GPS content (0%, 10%, 20%, 30%, 40% and 50%) were produced by reacting with a mixture of NaOH and Na2SiO3 solution. Meanwhile, other variables including different NaOH concentrations (2.5 M, 5 M, 7.5 M and 10 M), various curing temperatures (ambient 20℃ and elevated 60℃) and curing times (3, 7, 14 and 28 days) were also considered. The setting time and compressive strength of geopolymer paste were measured, respectively. The results indicated that the GPS addition decreased the setting time but did not result in the flash setting of FFA based geopolymer paste. Besides, the incorporation of GPS increased the compressive strength of geopolymer samples. Increasing NaOH concentration and curing temperature were beneficial for the strength improvement. Scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) analysis were utilized to identify the microstructure and phase composition of geopolymer samples. The test results showed that after the hydration reaction, the relatively high amount of CaO in the GPS generated calcium silicate hydrate (C-S-H) gel which coexisted with sodium aluminate silicate hydrate (N-A-S-H) gel and thus resulted in a denser and more homogeneous microstructure. This study confirmed the feasibility of utilizing GPS to partially replace FFA, which could not only improve the mechanical property of the derived geopolymer but also realize the synergy between granulated phosphorous slag recycling and geopolymer industry.
Geopolymer is a potential solution to the high carbon emissions caused by cement. Life cycle assessment (LCA) methodology was applied in this study to evaluate the environmental impacts of pavement road bases with reuse and recycling strategies. We considered four types of stabilized road base materials to evaluate environmental impacts: waste glass-fly ash based geopolymer stabilized macadam (WFAG), fly ash based geopolymer stabilized macadam (FAG), cement stabilized macadam (CS) and cement-fly ash stabilized macadam (CFAS). Two alkaline activators were used to synthesize geopolymer road base materials. The results showed that the application of geopolymer road bases reduced global warming (GWP) significantly. Comparing with CS, the GWP of WFAG and FAG declined by 17.9% CO2, eq per function unit. The combined alkaline activator consisting of sodium hydroxide (NaOH) and water glass had lower environmental impacts compared to the one with pure NaOH alkaline activator. The ozone layer depletion (ODP) of geopolymer stabilized road bases using pure NaOH solution was an order of magnitude higher than the ODP using the combined alkaline activator consisting of NaOH and water glass. For other indicators except GWP, the environmental impacts of conventional road bases were lower than geopolymer stabilized road bases, indicating a pollution transfer during application of geopolymer road bases.
Desulphurisation gypsum (DG), desulphurisation ash (DA) and fluor gypsum (FG) are three types of industrial by-product gypsum. The production and stockpiling of these types of gypsum may cause waste of resources and environmental problems. In this study, we focus on the activation mechanisms of these materials on steel slag (SS)–granulated blast furnace slag (GBFS)-based binders and their hydration mechanisms. Results showed that the binder with DA had a lower early strength and higher long-term strength than the other two binders. DG and FG provided Ca²⁺ and SO4²⁻ to generate C–S–H gels with SiO4⁴⁻ and ettringite with AlO4⁵⁻. The attached spherical C–S–H gel particles and aciculate ettringite formed a reticular structure to achieve improvements in strength. The portlandite in DA promoted the dissolution of AlO4⁵⁻ and SiO4⁴⁻ in GBFS during the production C–S–H gels. However, the generation of ettringite was resisted by the less gypsum content in DA, thereby providing space for C–S–H gels to generate the network structure. The C–S–H gels in the binders containing DA exhibit a low Ca/Si atom ratio and enhanced mechanical properties. Further, the SO2⁻ could be observed after 28 days, and hannebachite was consumed and gypsum was observed to increase. The conversion of SO3²⁻ to SO4²⁻ in binder with DA may occur at 20 ℃. DA and FG have a similar activation effect to that of DG on SS–GBFS-based binders, offering an alternative utilisation for solid wastes.
As a low-carbon cementitious material, the supersulfated cement (SSC) is composed of a thimbleful of cement clinkers and massive industry by-products, i. e: gypsum and slag. However, the concrete made from SSC suffers low strength, poor carbonation resistance, inferior frost resistance, etc. In such a scenario, two types of alkali activators (lactates and sodium hydroxyl) were employed to improve the microstructure and mitigate the performance of supersulfated cement concrete. The results indicate that the incorporation of lactates effectively improves mechanical performance, water impermeability, and carbonation resistance of supersulfated cement concrete. However, the addition of sodium hydroxide failed to result in a performance enhancement of SSC concrete. Furthermore, the mercury intrusion porosimetry (MIP), X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR) and scanning electron microscope (SEM) were comprehensively utilized to illustrate the microstructure changes of supersulfated cement concrete. It indicates the hydration degree of supersulfated cement was promoted by the addition of lactates, which refined the pore structure of concrete. A more compact matrix conduces to overall performance enhancement. This work may shed new lights on the microstructure optimization of solid-waste-based binding materials and further promote the application of SSC concrete.
The search for cements with lower CO2 emissions and lower consumption of non-renewable materials has been a challenge for the cement industry. The aim of this study was to investigate the resistance of SSC concrete to carbonation and chloride ingress. Portland cements (CEM I and CEM III/B) were used as a comparison. The results showed that the carbonation was very advanced in the SSC concretes. This is related to the exclusive formation of C-S-H and ettringite as hydration products, which present a higher rate of carbonation than portlandite. The SSC concrete has a good performance in terms of chloride penetration, mainly due to the increased ability to combine chloride ions both physically and chemically. The service life estimation showed that SSC concretes could be suitable for rural and urban environments when low w/c ratios (0.40) are used. All SSC mixtures studied were suitable for chloride environments.
In this study, effects of steel slag on the hydration and strength development of calcium sulfoaluminate (CSA) cement were investigated. Pastes with 0%, 10%, 20%, and 40% (by mass) of steel slag replacements for CSA cement were tested for setting time, chemical shrinkage, electrical resistivity, and compressive strength. The hydration products, bound water content, pore structure, and pH value of the pore solutions of these pastes were also analyzed. The results indicate that at early ages (from 2 h to 28 d), steel slag worked like a filler and had a dilution effect on CSA cement hydration. Therefore, setting time increased and chemical shrinkage, electrical resistivity, chemically bound water content, and compressive strength of the pastes reduced with increase of steel slag replacements. But, at late ages (≥90 days), these properties in pastes with steel slag replacements surpassed those of the plain CSA cement paste due to hydration of steel slag, which increased pH values of pore solutions and subsequently stabilized ettringite in the pastes. Steel slag replacement for CSA cement also accelerated strätlingite formation. Both ettringite stabilization and strätlingite formation appeared to be the primary mechanisms for strength compensation of CSA cement paste at late ages.
Supersulfated cements (SSCs) are composed primarily of blast furnace slag, calcium sulfate, and a small content of alkaline activator. SSC has been discussed in the literature because it has a low content of clinker and consequently low CO2 emissions. However, the current absence of durability studies is the main challenge for the large-scale implementation of SSC as a building material. The objective of this study was to investigate the behavior of phosphogypsum-based SSC mortar under sulfate attack. SSC mortars were submitted to MgSO4 and Na2SO4 solutions. Portland cements were also used as a comparison. SSC mortar showed good durability against sodium sulfate attack but presented an expansive behavior when exposed to magnesium sulfate. This behavior in the MgSO4 solution was attributed to the low portlandite reserve, which anticipated the attack of C-S-H. Besides, the non-formation of the brucite layer accelerated the entry of this type of sulfate.
Using solid wastes to prepare cementitious materials that can replace ordinary Portland cement is an effective way to recycle solid wastes. This present paper investigated the initial setting time and compressive strength of the quaternary binder which consists of ground granulated blast furnace slag, red mud, steel slag and flue gas desulfurization gypsum based on the synergy theory, and the synergetic effect was systematically analyzed by XRD, FTIR, and SEM-EDS. And the artificial neural network was used to predict the initial setting time and compressive strength of the quaternary binder with different raw material proportions. The results showed that the quaternary binder has the highest mechanical strength than that of the ternary binder and the binary binder, which proved the presence of synergetic effect between the raw materials. Sodium silicate, steel slag and flue gas desulfurization gypsum all could accelerate the hydration reaction and increase the compressive strength to some content. The artificial neural network models proved that they are efficient models to predict the initial setting time and compressive strength, and the usage of established ANN prediction models for grouting engineering was provided, and the optimal content for the compressive strength of the quaternary binder was determined by genetic algorithm.
Increasing the gypsum and slag contents in the blended cements containing slag is conducive to promote its green degree. The blended cement was prepared by mixing slag, gypsum and Portland clinker. Contour maps were designed to determine the optimal proportion of slag, gypsum and clinker by comprehensive considerations of strength, expansion, hydration, microstructure and crystallization stress. The strength contour map shows that there are two separate regions with high cement strength in the ternary slag-gypsum-clinker system; and the expansion deformation of these two regions are relatively low. The Rietveld XRD and pore solution analysis reveal that the occurrence of the destructive expansion results from the high crystallization stress of ettringite. It is illustrated that the gypsum content could be increased up to 25% when the clinker content is less than 5%, while the gypsum content should be limited below about 10% when the clinker content is more than 20%. The [CaO][SO3]/[Al2O3] molar ratio is suggested to direct the optimization of the slag, gypsum and clinker in blended cement. As long as the [CaO][SO3]/[Al2O3] ratio is not up to 1.5, more gypsum and slag could be added into the blended cement to obtain relatively excellent performance.
In order to explore the difference of the activation effect on the ground granulated blast furnace slag (GGBFS) subjected to variant activators as well as to further clarify the depolymerization mechanism of GGBFS, investigation on the dissolved mechanism, hydrated characteristics and microstructural evolution were carried out based on slag immersed in solutions with variant pH, and FTIR, NMR, XRD, SEM and EDS were used to analyze structure of GGBFS, the change of valence and bond, structural coordination, hydrated products, micromorphology and elements distribution. The results show that the H⁺ destroyed the structure of GGBFS, and a small amount of akermanite and gehlenite particles were formed on the surface of slag, Ca²⁺, Mg²⁺ and Al(OH)²⁺ ions were released as 3 ≤ pH < 7. For the pH < 3, cauliflower-like chabazite particles were accumulated, and the dissolved Si and Al gradually formed monomeric siloxane and hydrated aluminium complex ion [Al(H2O)6]³⁺, respectively. The H⁺ ionized by the water eroded the slag surface and corrugated melilite particles, together with a layer of Si(OH)4 and Al(OH)3 gel attached on the slag surface for pH = 7. As 7 < pH ≤ 12, the hydrated magnesium aluminate and calcite particles were generated on the slag surface, and Ca²⁺, Mg²⁺ and Al(OH)4⁻ ions were released under the influence of OH⁻. The Si-O-Al bond was destructed and the monomers of Al(OH)4⁻ and Si(OH)4 were released for pH > 12. The Si(OH)4 and Al(OH)4⁻ performed polycondensation in the presence of the cations such as Ca²⁺, and that finally led to formation of large number C-(A)-S-H with a corrugated morphology.
This study investigates the effect of NaOH content on alkali/sulfate-activated binders from 90 wt% ground granulated blast furnace slag (GGBFS) and 10 wt% phosphogypsum (PG). Alkali activators were prepared with a NaOH molarity ranging from 0 M to 4 M. The hydration was monitored using in-situ X-ray diffraction (XRD) and isothermal calorimetry. The hydration product assemblage was investigated using XRD, thermogravimetric analysis (TGA), Fourier-transformed infrared (FTIR) spectroscopy, nitrogen adsorption/desorption and compressive strength tests. A molarity of 0 M NaOH gave rise to the highest porosity and highest strength, although setting occurred only after 7 days. From a molarity of 2 M NaOH and higher, ettringite disappeared and got replaced by a monosulfate phase (i.e., NaCa4Al2O6(SO4)1.5.15H2O) and amorphous aluminum-hydroxide. This study shows the potential of using GGBFS and PG for the development of novel by-product based cementitious binders.
As a major mineral of Portland cement, the hydration of tricalcium aluminate (C3A) has a significant impact on various properties of cement-based materials. However, there is little consensus on its mechanism, especially the interaction between C3A and gypsum which usually co-exist in cement systems. Recent researches have shown that sulfate ions play an important role in the kinetics of C3A hydration, which may provide more promising alternatives on controlling cement hydration besides the present solution on adjusting gypsum contents. In this study, effects of different kinds of solid sulfates (gypsum, Na2SO4, MgSO4) on the heat released during the early age of C3A hydration as well as hydration products were investigated first by isothermal calorimetry, SEM, XRD and thermal analysis. Retardation was observed in all the three systems (C3A-gypsum, C3A-Na2SO4 and C3A-MgSO4). But the duration of induction period varied with cations. Then barium nitrate (Ba(NO3)2) aiming to remove the sulfate ions was innovatively employed to highlight the role of sulfate ions. Interestingly, the renewed hydration of C3A occurred immediately after Ba(NO3)2 solution was added into the three systems with different amount of AFt remained. According to the characterization of hydration products, little monosulphoaluminate (s-AFm) was detected in the induction period of C3A-gypsum hydration and a large amount of h-AFm was found to cover the surface of C3A particles after adding Ba(NO3)2 solution. It is concluded that the coexistence of calcium and sulfate ions absorbed at C3A surface is more likely to dominate the retardation of C3A hydration in the presence of gypsum, rather than the alleged physical barrier mechanism caused by the precipitation of hydration products.
A pervious concrete with carbonated ground steel slag powder as binder and crushed steel slag as aggregate was prepared, which provided the key material for construction of sponge city and explored a new approach to massively utilize steel slag. This paper presented series of experimental studies on the properties of steel slag carbonized consolidation matrix and carbonated steel slag pervious concrete. Results indicated that the phosphogypsum promoted the carbonization ratio of steel slag. When the content of phosphogypsum was 2.5%, strength and carbonization ratio of carbonated steel slag from pressing formation reached 82.6 MPa and 52.1% respectively. The main product of the carbonated steel slag was calcite and C–S–H. The strength and porosity of carbonated steel slag pervious concrete reached 9.4 MPa and higher than 20% respectively. And it also had a good plantability when it is used in sponge city construction. Compared with ordinary pervious concrete, carbonated steel slag pervious concrete saved 75.8% cost of materials, was 100% made of solid waste and absorbed around 100 kg/m³ of CO2, which turns out to be a green and high-efficiency approach to utilize the solid waste steel slag, saving the natural resource and reducing the carbon footprint of the construction of sponge city.