28d Compressive Strength Research Articles

Upcycling of molybdenum tailings, coal gangue and slag into sustainable repaired composites by geopolymerization technology: Workability, embodied carbon and mechanical-microstructural development

The major disadvantage of reinforced concrete structure was multi cracks under loading, seriously affecting its normal performance, but using traditional repaired materials to strengthen reinforced concrete structures can bring some side-effects, such as higher carbon oxide and cost. Herein, this study successfully designed an eco-friendly geopolymer repaired composites developed by molybdenum tailings, ground granulated blast-furnace slag and coal gangue. Firstly, the fresh and harden properties of geopolymer repaired composites was systematically studied including water bleeding rate, fluidity, setting time, unconfined compressive strength and bonding strength. The micro performance of geopolymer repaired composites was analyzed via scanning electron microscopy, thermogravimetry-derivative thermogravimetry, X-ray diffraction and fourier transform infrared spectroscopy to give some new viewpoints for revealing the coupled geopolymerization mechanism of molybdenum tailings/coal gangue/ground granulated blast-furnace slag in geopolymer repaired composites. Meanwhile, the gray level of concrete contact surface is calculated. The results show that the properties of geopolymer repaired composites with 5% NaOH designed by 1:3 binder/sand, 0.5 w/c and 1:3 molybdenum tailings/ground granulated blast-furnace slag were optimal properties with 1.73% water bleeding rate, 192 mm fluidity, 58 min and 178 min the initial and final setting time, respectively, 21.443 MPa 28-d unconfined compressive strength and 2.642 MPa bonding strength. Excessive NaOH amount would inhibit the unconfined compressive strength and bonding strength of geopolymer repaired composites, while the increase of ground granulated blast-furnace slag would improve unconfined compressive strength and bonding strength. The primary hydration products of geopolymer repaired composites were C–S–H and C-A-S-H gels. Molybdenum tailings, coal gangue, and ground granulated blast-furnace slag showed an important synergistic effect, improving its geopolymerization reaction and strength. Besides, bonding strength of geopolymer repaired composites was highly correlated with unconfined compressive strength and fractal dimension of surface roughness. The relation coefficients are 0.92005 and 0.94006, respectively. With the higher the unconfined compressive strength of geopolymer repaired composites and the larger fractal dimension of the concrete contact surface, the bonding strength of geopolymer repaired composites is higher and the bond between geopolymer repaired composites and concrete is denser. The increase in the amount of NaOH and the increase in ground granulated blast-furnace slag have an increasing and decreasing trend on the carbon emissions of geopolymer repaired composites, respectively. The most obvious increasing and decreasing trends are that the carbon emissions of GRC increase from 66.41 kg CO2-e/m3 to 96.28 kg CO2-e/m3 and decrease from 79.81 kg CO2-e/m3 to 76.37 kg CO2-e/m3. The increase in the amount of NaOH is the main reason for the increase in the carbon emissions of geopolymer repaired composites, while the increase in ground granulated blast-furnace slag helps to reduce the carbon emissions of geopolymer repaired composites. The carbon emission of geopolymer repaired composites was reduced by 75.08%–83.4% compared with traditional mortar. The 28-day per unit unconfined compressive strength carbon emission of the geopolymer repaired composites with the optimal mix is 46.63% lower than that of the traditional mortar. Overall, this study can provide new viewpoints and novel channels for converting solid waste, including molybdenum tailings and coal gangue, into green recycled construction materials, provide highly promising insight for designing green construction products and new ideas in the development of low-carbon remediation materials for environmental sustainability, contribute an important force to environmental protection, and promote the growth of economic benefits in the process.

Performance evaluation of pigmented fair-faced concrete under various environments: Mechanical properties, apparent properties, and color stability

Fair-faced concrete is green concrete that is directly decorated by its texture. For a more stunning visual impact, pigmented fair-faced concrete with more colors on the surface is prepared. However, the effects of different pigments and service environments on pigmented fair-faced concrete need to be explored. Therefore, four iron oxide inorganic pigments were employed to prepare pigmented fair-faced concrete, and the changes in its fresh properties and mechanical properties were studied through slump, slump flow, and compressive strength tests. Meanwhile, camera analysis and colorimetry were utilized to evaluate the changes in its apparent properties and color stability under four different distinct service environments in this experiment. Results indicate that incorporating pigments reduces both the slump and slump flow of the fresh mixture, and its fresh properties gradually decrease as the pigment content increases. The slump and slump flow of concrete with 6 % iron oxide yellow (IOY) were 30.00 % and 44.00 % lower than the control. Adding IOY and iron oxide red (IOR) can improve the 7-d and 28-d compressive strength of concretes. The 28-d compressive strength of concrete with 2 %-6 % IOY increased by 7.72 %, 11.78 % and 15.09 % over the control. Meanwhile, the surface pore area of concrete with IOY is the lowest and that of concrete containing 2 % IOY is 3.97 cm2/m2. Additionally, the color stability of concrete performs best in water immersion environments and worst in natural environments. It is noteworthy that the concrete added with IOY exhibits excellent color stability under all four service environments. This study aims to evaluate the effects of different pigments on concrete properties and investigate the color stability of four types of concrete under diverse service conditions. The findings will promote the application of pigmented fair-faced concrete in construction and provide technical insights for future research.

Study on optimization of mixing ratio and shrinkage property of alkali-activated ultrafine fly ash-slag mortar

Alkali-activated cementitious materials have received widespread attention due to their favorable mechanical properties and environmental benefits. However, there are fewer studies on the optimal mixing ratio of alkali-activated ultrafine fly ash-slag (AAUS) mortar, and shrinkage cracking is a critical issue that hinders its further application. Based on this, the study used the response surface method (RSM) to optimize the mixing ratio of AAUS mortar, explored the effect of basalt fibers (BF) on the compressive strength and drying shrinkage of AAUS mortar, and finally analyzed the energy consumption and carbon emission of fiber-reinforced AAUS mortar. The experiment used X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques for microstructural characterization to research the microstructure and mineral phase changes of AAUS mortar. The results demonstrated that the 28d compressive strength of AAUS mortar reached its maximum when the substitution ratio of ultrafine fly ash (UFA) was 14.5 %, the alkali equivalent was 10.34 %, and the modulus of the activator was 1.6, based on the RSM; BF of appropriate length and volume fraction can productively improve the compressive strength of AAUS mortar and limit drying shrinkage; AAUS mortar with optimal mixing ratio can be sufficiently activated by alkali activator to construct a considerable number of C-(A)-S-H gels to establish a high-density microstructure; the carbon emission of the B124 test group was reduced by 61.1 % compared with ordinary Portland cement (OPC) mortar.

Valorization of phosphate mine waste rock as alternative aggregates for high-performance concrete

Natural aggregate quarrying significantly impacts the environment while phosphate mine waste rock (PMWR) offers alternative aggregates for use in high-added value construction materials. This study was the first to investigate the application of three aggregate types from PMWR as a complete substitute to natural aggregate to produce high-performance concrete (HPC). For this aim, three coarse aggregate (CA) types: Flint (F), Phosflint (PF), and Dolomite (D) from PMWR in Morocco were selected and compared with a natural Reference CA (R). Two mixtures were tested: HPC-0.40 and HPC-0.36 with a w/c ratio equal to 0.40 and 0.36. D-HPC and PF-HPC achieved a slump exceeding 20 cm and a 28-d compressive strength (CS) equal to 55 MPa ± 5 MPa on average, similar to the R-HPC, while F-HPC did not meet the requirement for HPC (i.e. a 28-d CS superior to 55 MPa). Scanning electron microscope observations showed that D-HPC exhibited a very dense and indistinguishable interfacial transition zone (ITZ), while F-HPC showed a debonding of F-CA and a larger ITZ size which explain its lower CS. Life ycle assessment revealed a major positive environmental impact on ecosystem quality that reduced land occupation and land transformation by up to 32 %. A secondary evaluation of the valorization of PMWR as alternative fine aggregate (FA) in high-strength mortar (HSM) was investigated and showed that the 28-d CS is equal to 93 MPa for PF-HSM and equal to 81 MPa for D-HSM, superior to RS-HSM which is equal to 61 MPa. Dolomite and Phosflint FA offered a better alternative to natural River sand to overcome the ceiling effect. These findings encourage the construction sector to adopt a sustainable consumption of aggregates by using PMWR as alternative CAs and FAs to produce high-strength construction materials.

Revisiting the effects of carbon nanotube agglomerates in cement

Agglomerates of carbon nanotube (CNT) are typically considered as defects in cement matrix, yet without a clear physiochemical picture regarding why and how. By representing CNT agglomerates with CNT sponge powders (CNTSPPs), we discovered that hydrates can precipitate in the pores of CNTSPPs to form a CNT/hydrates nanocomposite shell, the thickness and compactness of which are dependent on the curing temperature. In contrast to the detrimental effects of CNTSPPs for the samples cured under standard condition, the addition of 0.05 wt % CNTSPPs increased the 28d compressive strength by ∼16 % compared to the reference sample under steam curing condition. This is because steam curing condition significantly improves the hydrophilicity of CNTSPPs, resulting in its absorption of pore solution and calcium ions, which eventually increase the CNT/hydrates nanocomposites shell thickness and microhardness. Such nanocomposites shell with high mechanical properties can screen the negative effects of the porous inner CNTSPPs.

Effect of particle size distribution and content of limestone powder on compressive response of high-early-strength cement mortars

This study examined the effect of particle size distribution and content of limestone powder on the compressive response of high-early-strength portland cement mortars cured either in water or under ambient conditions. Nineteen mortar mixtures were classified into three groups based on the particle size distribution of the pulverized limestone powders: L (blaine = 3467 cm2/g), M (blaine = 4710 cm2/g), and H (blaine = 5764 cm2/g). The design equations for the compressive strength gain rates and stress–strain relationship of ordinary portland cement (OPC) concrete were adapted for the present mortars. The test results revealed that the 28-d compressive strength (fc′) of the mortars tended to decrease with an increase in limestone powder content, even for mortars containing 5 % limestone powder with small particle sizes. This contrasts with the common trend of the positive effect of limestone powder on the compressive strength of OPC concrete up to a certain substitution level. Furthermore, mortars containing finer limestone powders exhibited a greater rate of decrease in fc′, regardless of the curing conditions. The modified equations show promise for reliably assessing both the compressive strength gain rates at different ages and the stress–strain behavior of high-early-strength portland cement mortars containing limestone powders.

Evaluation on Preparation and Performance of a Low-Carbon Alkali-Activated Recycled Concrete under Different Cementitious Material Systems.

A green, low-carbon concrete is a top way to recycle waste in construction. This study uses industrial solid waste slag powder (S95) and fly ash (FA) as binders to completely replace cement. This study used recycled coarse aggregate (RCA) instead of natural coarse aggregate (NCA). This is to prepare alkali-activated recycled concrete (AARC) with different cementitious material systems. Alkali-activated concrete (AAC) mixtures are modified for strength and performance based on the mechanical qualities and durability of AARC. Also, the time-varying effects of the environment on AARC properties are explored. The results show that with the performance enhancement of RCA, the mechanical performance of AARC is significantly improved. As RCA's quality improves, so does AARC's compressive strength. At a cementitious material content of 550 kg/m3, AARC's 28d compressive strengths using I-, II-, and III-class RCA were reduced by 2.2%, 12.7%, and 21.8%, respectively. I-class AARC has characteristics similar to natural aggregate concrete (NAC) in terms of shrinkage, resistance to chloride penetration, carbonization, and frost resistance. AARC is a new type of green building material that uses industrial solid waste to prepare alkali-activated cementitious materials. It can effectively reduce the amount of cement and alleviate energy consumption. This is conducive to the reuse of resources, environmental protection, and sustainable development.

Revealing the hydration mechanisms in iron-rich calcium sulfoaluminate cements with elevated bauxite residue content

The goal of the study was to investigate the reactivity at different Blaine, hydration behavior, and performance of calcium sulfoaluminate-ferrite cement (CSA-F). The raw meal incorporated bauxite residue (38 wt%) fired at 1260 °C and 1300 °C for 30 and 60 minutes. Based on multi-objective optimization, an optimum dosage of citric acid ranges between 0.4<x<0.6 wt% with a w/b ratio of 0.45 was chosen for the mortar evaluation and hydration studies. At an early age, monosulfates-AFm only started appearing for 1300_30. In the later age, an increase in ettringite formation was only reported at 1300°C for 60 min due to the dissolution of the Ternesite phase after 7d of hydration. Later-age studies after 28d reported ettringite, monosulfates, hydrogarnets (except for 1300_60), and strätlingite had formed. The 28d highest compressive strength was reported for 1300_60 corresponding to the highest bound water and amorphous content from the pore filling ettringite and amorphous stratlingite.

Substitution of blast furnace slag by melting furnace slag as active component in green concrete application

Blast furnace slag (BFS), a commonly used substitute for cement clinker, is in large demand and gradually increasing in price. Melting furnace slag (MFS), a by-product of the co-disposal of waste incineration fly ash and metallurgical dust sludge, has potential hydration properties, and its large accumulation also poses a serious threat to the environment. In this study, we focus on the effect of MFS replacing BFS on concrete properties and its hydration mechanism. Results showed that MFS significantly enhanced the compressive strength at the later stage of hydration, and the 28-d compressive strength of MSD concrete was 15.15 % higher than BSD concrete. MFS enriched with a large amount of amorphous silica and alumina played a key role in the hydration process. This is mainly shown as the increase in the fluctuation of Si-O and Al-O bond strengths and the decrease in the energy separation between Ca 2p and Si 2p levels. In addition, the increased substitution of aluminum for silicon in MSD results in the formation of C-A-S-H gels, which are tightly bonded with ettringite to form a composite structure. Subsequent durability assessments indicate that the incorporation of MFS diminishes the permeability to chlorides and sulfates, while also bolstering the concrete’s resistance to carbonation. These findings imply a positive impact on the concrete’s enduring service life.

The strength, reaction mechanism, sustainable potential of full solid waste alkali-activated cementitious materials using red mud and carbide slag

Alkali-activated cementitious material (AACM) is a green building material which effectively utilizes industrial solid wastes, such as red mud (RM), ground granulated blast furnace slag (GGBS). However, there are problems with using conventional alkali activators (NaOH, Na2SiO3), including strong corrosivity and high cost. In order to solve this problem, this study prepares full solid waste cementitious materials (FSWCM) by activating RM and GGBS using carbide slag (CS) instead of traditional alkali activator. Based on the response surface method (RSM), the effects of RM content, CS content and water-binder ratio on the performance of FSWCM were studied. The microstructure changes of FSWCM with different RM contents were analyzed by XRD, TG, FTIR and SEM-EDS. The results showed that the optimized content of RM and CS was 40 % and 20 % respectively, and the 28-d compressive strength reached 14.11 MPa, which met the requirements of road base materials. The regression model established by RSM had high accuracy and could accurately predict the performance response values. The alkaline environment created by CS-RM facilitated the decomposition of active components in GGBS, resulting in the formation of a considerable amount of C-(A)-S-H gel via polycondensation. With the increase of age, Na+ dissolved in RM replaced Ca2+ in C-A-S-H to form N-A-S-H gel. The hydration products N-A-S-H, C-(A)-S-H and CaCO3 were intertwined to form a stable and dense three-dimensional network structure, which provided excellent mechanical properties. Sustainability analysis shows that FSWCM has significant advantages in cost control and environmental friendliness compared with AACM.

Synergistic hydration mechanism of steel slag-metakaolin based on ionic dissolution properties combined with industrial CT analysis

Replacing cement with solid waste materials is a key approach to preserving natural resources and mitigating carbon dioxide emissions. In this research, we explored elemental complementation as an eco-friendly and low-carbon approach to develop composite cementitious materials (CCMs) by partially substituting cement with steel slag (SS) and metakaolin (MK), thereby overcoming the low reactivity of SS and facilitating its broader application. We scrutinized the synergistic effects of SS-MK CCMs on parameters such as solubility, water demand, setting time, compressive strength, the formation of hydration products, and microstructure. Employing a suite of analytical techniques, including Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES), X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FT-IR), Scanning Electron Microscopy (SEM), measurement of hydration heat, and X-ray computed tomography. The findings revealed that the alkaline environment produced by SS-cement powder promoted the breakdown of Si-O and Al-O bonds within MK, facilitating both the early hydration process and the development of strength.MK compensates for the lack of strength caused by the slow hydration of SS in the early stage, while the SS accelerated a pozzolanic reaction of MK, leading to a 28d compressive strength of 54.77 MPa for the composite, while the 28d strength of SS20 was 39.74 MPa; MK effectively utilized portlandite, enhancing the hydration of SS and increasing the production of calcium silicate hydrate, which optimized the pore structure and improved sphericity. Furthermore, the SS-MK CCMs addressed the issues of f-CaO excess and prolonged setting times in the SS-cement system, while also ameliorating the high calcium demand and reduced fluidity in the MK-cement system. Additionally, its lower cumulative heat release from hydration enhances the material’s volumetric stability. This study presents a strategy for the large-scale application of SS and contributes to the value-added, eco-friendly use of industrial solid wastes.

Investigation on a sustainable magnesium phosphate cement (MPC) with recycled waste MPC powders

Magnesium phosphate cement (MPC) has been used for the structure rapid-repair of damaged buildings. To reduce the cost and resource consumption during MPC preparation, recycled MPC powders were used to prepare an environmental-friendly MPC by replacing the dead burned MgO powders. The workability and early hydration temperature development of MPC paste with recycled powders were investigated. To characterize the effect of recycled powders on the performance of the MPC paste, the mechanical properties, volume stability, phase developments and microstructures were determined. The results showed that due to the small particle size and the large specific surface area, the recycled powders absorbed part of the water during the mixing and reduced the fluidity of the MPC paste. The chemical reaction activity of recycled powders was lower than that of MgO powders, resulting in the reduction of the hydration reaction rate and hydration products of MPC. With the recycled powder content increasing, the mechanical strength and volume stability of MPC decreased. The decreased strengths could be attributed to the poor MPC paste structure with increased pores and microcrack content caused by recycled powders. Nevertheless, when the recycled powder content was 40 %, MPC paste obtained a workability of 20.8 cm, setting time of 24.75 min, 3-h compressive strength of 30.9 and 28-d compressive strength of 50.3 MPa. Therefore, under this replacement rate, the MPC paste still had acceptable properties, which could still meet the rapid-repair requirements.

Strategies for reusing multiple recycled powders: Evaluating the mechanical performance and sustainability of Engineered Cementitious Composites (ECC)

Engineering Cementitious Composites (ECC) are fibre-reinforced cementitious materials with excellent tensile strain capacity. Recycling powder from building and demolition waste can improve the ecological efficiency of concrete and reduce the cost of raw materials. In this study, recycled glass powder (RGP) was used to partially replace traditional cement. Based on using a 1:1 replacement of recycled brick powder (RBP) and recycled concrete powder (RCP) for fine aggregates, two sets of variables were set: the RGP substitution rate (10 %, 15 %, 20 %, and 25 %) and RGP particle-size range (38–75 μm, 75–150 μm, and 150–300 μm). Comprehensive material testing and mechanical experiments were conducted, including X-ray diffraction analysis, thermogravimetric analysis, shrinkage tests, uniaxial compressive tests, uniaxial tensile tests, and scanning electron microscopy. The results indicated that fully substituting quartz sand with RBP and RCP enhanced the ECC performance. Incorporating RGP significantly improved the ultimate tensile strain and crack-propagation ability of the ECC while reducing the 90-d shrinkage rate. The 28-d compressive strength and tensile strength have some loss, but can meet the strength requirements. SEM showed that incorporating RGP weakened the bonding strength of the fibre matrix, making it easier for the fibres to be pulled out. By evaluating the experimental results, the mix ratio of a 20 % substitution rate and 75–150-μm size was selected. A material-sustainability indicator (MSI) analysis of the raw materials and production process of RGP-ECC was performed. The embodied energy, carbon emissions, and economic indices were reduced to a certain extent, thereby improving the ecological efficiency of concrete.

Influence of water-to-binder ratios on the performance of limestone calcined clay cement-based paste for mining applications

This study aims to investigate the impact of various water-to-binder (w/b) ratios by mass (0.3–2.0) on the performance of limestone calcined clay cement (LC3) pastes. The flowability, setting time, density, unconfined compressive strength, hydration, and microstructure of LC3 pastes under different w/b ratios were thoroughly investigated. The results show that increasing the w/b ratio extends the flowability and setting time while significantly reducing the unconfined compressive strength of LC3 pastes. LC3 pastes with w/b ratios above 0.6 exhibited a final flow diameter surpassing 167 mm, indicating good flowability. The 28-d unconfined compressive strength decreased from 82.3 to 1.3 MPa as the w/b ratio increased from 0.3 to 2.0. This study confirmed that the relationship between unconfined compressive strength and the w/b ratio of LC3 follows Abram’s law. Furthermore, a modified gel/space ratio was proposed to reflect the concentration of solid products, effectively explaining the influence of the w/b ratio on strength, with an R2 of 0.96.

Carbonation curing of cement-based materials by using potassium glycine solution: Uniformly CO2 sequestration and secondary strengthening

In this research, an internal carbonation curing method was proposed to achieve inside-outside synergistic carbon capture and performance improvement of cement-based materials. Potassium glycine (KG) solution with various concentrations was used as the CO2 carrier. The results indicate that the addition of CO2-rich KG solution increases 7-d and 28-d compressive strength by 29.8% and 16.2%, as well as achieving an overall CO2 uptake of 4∼11% in cement paste. However, at higher KG concentrations, the rapidly generated amorphous calcium carbonate covers the surface of cement particles, which shortens the setting time and inhibits early-age hydration. The presence of carbamate in CO2-rich KG solution improves the stability of metastable CaCO3 polymorphs and prolongs the period of dissolution-recrystallization. Calcite generated by the slowed recrystallization within 14 days having a filling and binding ability in the cement matrix, significantly reducing porosity and providing a secondary strengthening effect.

Comparative study on preparation and hydration mechanism of composite cementitious materials containing copper slag

Copper slag (CS) is a kind of non-ferrous solid waste with low content of aluminosilicate and relatively high content of iron oxide. In this paper, the feasibility and mechanism of CS as the mineral admixture and precursor of alkali activated cementitious material were studied. The effects of CS on mechanical properties were studied and the hydration products were determined by several characterization methods. Scanning Electron Microscopy (SEM) and Mercury Injection Porosimetry (MIP) were performed to characterize the microstructure. The results show that CS composite fly ash (FA) has more advantages than CS alone, but its application as a mineral admixture is limited by the low activity. The alkali activated cementitious material prepared by CS and FA as precursor has excellent mechanical properties. The compressive strength of specimen at 28 days age can reach the maximum of 81.3 MPa when the dosage of CS is 30 %. The 3d and 28d compressive strength of the specimen contained 60 % CS obtained 57.7 MPa and 70.7 MPa, respectively. The research exhibits great potential for the future utilization of CS.

A methodology to enhance ye'elimite in calcium sulfo-aluminate ferrite clinker from bauxite residue

Calcium sulfoaluminate-ferrite (CSA-F) clinkers were prepared using the alumina production waste bauxite residue (BR) as one of the raw materials, next to limestone, kaolin, and gypsum. A complementary approach was employed, aiming to maximize the desired ye’elimite phase. This consisted of thermodynamic modelling and computational statistics via response surface modelling, which enabled the identification of the best raw meal by varying the proportions of the chosen raw materials and the optimal clinkering conditions by unraveling the interaction between clinkering temperature and time. The optimal parameters were for a raw meal consisting of 38 wt% BR, 42 wt% limestone, 5 wt% kaolin, and 15 wt% of gypsum, fired for 60 min at 1300 °C. The accuracy of the model was assessed experimentally on a clinker synthesized at a temperature range of 1250–1300 °C for 30 and 60 minutes, followed by rapid cooling. Based on the characterization results, firing at 1300 °C for 60 minutes was the optimal set of conditions to maximize ye'elimite formation, which was in accordance with the response surface model. In addition, clinker 1300_60 exhibited the highest 28d compressive strength of 33 MPa due to the presence of Ternesite phase. In conclusion, the combined methodology endorsed and presented herein is a pragmatic and relatively accurate methodology for producing low-CO2 clinkers from bauxite residue and can likely be transferred to other unexploited secondary raw materials.

A novel alkali-slag cemented tailings backfill: Recycling of soda residue and calcium carbide slag

In order to address the safety and environmental concerns associated with high disposal cost and low disposal rate associated with mine tailings, low-cost cemented paste backfills (CPBs) were prepared from solid wastes such as calcium carbide slag (CS), soda residue (SR), ground granulated blast-furnace slag (GGBS) and iron tailings (TA) in this study. The effects of mass concentration of filling slurry, GGBS content, activator dosage, and SR proportion on the mechanical properties of CPBs were studied. The results showed that a flow of 235.8 mm and 28-d compressive strength of 2.3 MPa for the freshly mixed slurry could be attained with 75 % mass concentration, 14 % GGBS content, an activator-to-binder ratio of 0.4, and a CS-to-SR ratio of 3:7. The strength of CPBs first increased and then decreased with the increasing of activator dosage or SR proportion contents, and reached the optimal value for the activator-to-binder ratio of 0.4 and SR of 70 % – 75 %. Microstructural analyses by TG-DTG, FTIR, SEM-EDS, and XRD revealed that mainly C-(A)-S-H gel, hydrotalcite (HT), and Friedel's salt (FS) were formed as the hydration products in CS-SR synergistically-activated GGBS paste. The Ca2+ dissolved from CS facilitated GGBS hydration to generate the C-(A)-S-H gels and further reacted with Cl− in SR and [Al(OH)6]3− dissolved from GGBS to generate FS. The combined effect of C-(A)-S-H gel and FS enhances the strength of CPBs. When the percentage of SR is in the range of 70–75 %, the synergistic excitation of slag by alkali slag and calcium carbide slag is superior, and the matrix structure is denser.

Experimental design, mechanical performance and mechanism analysis of C30 phosphogypsum-based concrete

The present study aimed to increase the utilization of phosphogypsum and reduce cement consumption. Single-factor experiments were designed to the effects of water-cement ratio (0.45–0.30), silica fume (1 %∼7 %), cement dosage (0 %∼20 %), and the RPG/HPG (20/80–0/100) on mechanical properties of phosphogypsum-based concretes (PGBCs). A new phosphogypsum-based concrete was prepared to meet the compressive strength requirement of C30 grade. The maximum 28d compressive strength of this PGBC reached 45.1 MPa. Simultaneously, the effect mechanisms of above four factors on the mechanical properties of PGBCs were discussed by combining XRD and SEM results. Moreover, the volume stability, elastic parameters, and compressive constitutive model of this PGBC with compressive strength above 30 MPa were investigated. This PGBC used more than 80 % phosphogypsum and 10∼20 % cement, exhibiting well economy and environmental friendliness. Moreover, the constitutive model was established to well predict the compressive behavior of PGBC with different RPG/HPG relative ratios. This PGBC had better volume stability and weaker deformation resistance than cement-based concrete. The present results can provide more theoretical and technical supports for the large utilization of PG.

Design and evaluation of alkali-activated slag-calcined coal gangue cement and the shrinkage control by calcium carbonate whisker

The rapid solidification and significant shrinkage of alkali-activated slag cement are key issues that restrict its practical engineering application. This paper explores the composition design of alkali-activated cements (AAC) with slow coagulation and shrinkage reduction. This paper adjusts the proportion of slag (SG) and calcined coal gangue powder (CCGP), regulates the content of Na2O·2.4SiO2-NaOH-Na2CO3 ternary activator components, and uses calcium carbonate whiskers (CW) instead of SG to prepare AAC with slow coagulation and shrinkage reduction. When the activator is composed of (66 %) Na2O·2.4 SiO2-(2 %) NaOH-(32 %) Na2CO3 mixed composition, the CO32- and Ca2+ in the system react preferentially to produce CaCO3. The decrease of Ca2+ concentration delays the formation of C-(A)-S-H, resulting in the initial and final setting time of AAC reaching 334 min and 346 min, respectively. When the CW substitution rate is 3 %, the 28d compressive strength of AAC reaches 102.2 MPa. The 49d drying shrinkage of the AAC is 7020μm/m and the 49d autogenous shrinkage is 2387μm/m. This is attributed to the combined effect of enhancement mechanisms such as whisker bridging, crack deflection, whisker pullout, whisker–matrix coalition pullout and whisker breakage. This study establishes the relationship between the composition and performance of high performance AAC, which is significance for the resource utilization of coal gangue and the practical engineering application of AAC.