Slow Hydration Research Articles

Improving the strength performance of cemented phosphogypsum backfill with sulfate-resistant binders

Utilizing phosphogypsum (PG) as an aggregate for cemented backfill offers an effective waste management approach for phosphorus industry. Nevertheless, the supersulfate property of PG significantly hindered the strength development of cemented PG backfill. This study aims to enhance the strength performance of hardened backfill through the optimization of binder types. Silicate-based cements (P·O 42.5 ordinary Portland cement (OPC) and P·I 62.5 Portland cement (PC)) and (sulpho)aluminate-based cements (62.5 sulphoaluminate cement (CSA) and CA50-II calcium aluminate cement (CAC)) were selected as binders. In this study, backfill samples were prepared by PG, water and different types of binders. The strength development, porosity and hydration characteristics of hardened backfill were studied. The results show that OPC and PC exhibited susceptibility to sulfate, resulting in backfill samples with slow hydration and low strength. In contrast, backfill samples applying CSA and CAC as binders exhibited rapid hardening and significant strength enhancement, particularly in the early age. The active interaction between gypsum and the main components of CSA and CAC, such as ye’elimite (C4A3S¯) and monocalcium aluminate (CA), led to the accelerated hydration process and the generation of abundant ettringite and aluminum hydroxide (AH3). Consequently, the hydration process of CSA and CAC appeared to be exempted from the negative effects of supersulfate environment, maximizing the binding performance. This study emphasizes the favorable binding performance of (sulpho)aluminate-based cements in cemented PG backfill, contributing to the utilization of PG.

Bi-salts composites to enhance the hydration kinetics and heat storage capacity

In this study, salt hydrates were deposited on a porous carbon (PC) support to address the challenges associated with maintaining heat capacity and slow hydration kinetics in thermochemical heat storage (THS) systems. Mono-salt (MgSO4) or binary salt (MgSO4-MgCl2) composites on the PC were analyzed by thermogravimetry/differential scanning calorimetry (TG/DSC). The results show that binary salt composites exhibit faster hydration kinetics and higher water adsorption capacity than monosalt composites. The addition of MgCl2 overcomes the kinetic drawbacks caused by the hydrophobic nature of the support and the presence of salt aggregates. The PC_60binary composite achieves a water uptake of 0.75 g/g, exceeding the water uptake of PC_60MgSO4 (0.48 g/g). Furthermore, the heat released by the composites follows a similar trend to water sorption. PC_60MgSO4 produces 52 % (1356 J/g) of the heat released by pure MgSO4, while PC_60binary achieves 70 % (1840 J/g) of the heat released by pure mixed salts. These results suggest that the newly developed binary salt composites are promising for long-term heat storage applications. These experimental results highlight the improved performance of binary salt composites in terms of hydration kinetics, water adsorption and heat release. This study provides valuable pointers for the development of more efficient THS systems.

Enhanced immobilization of metal pollutants in sewage sludge ash (SSA)-cement pastes by calcium chloride and nitrate: Experimental and DFT studies

Urbanization, Portland cement production, and urban waste have caused concern about increasing environmental pollution. Sewage sludge ash (SSA) comes from the incinerating plant of sewage sludge, which is a by-product of wastewater treatment. SSA can cause metal pollution if improperly handled. Recycling SSA into cement composite such as concrete (i.e., the most widely used material after water) can obviate landfilling and mitigate CO2 and metal pollutants from cement production and SSA. Chemicals called accelerators, such as CaCl2 and Ca(NO3)2 are used to overcome the relatively slow hydration of SSA cement pastes (CPs). In this study, we found the capability of CaCl2 and Ca(NO3)2 to enhance Mo and Cr immobilization by CPs with SSA (i.e., mixed with Portland and blast furnace types of cement). The mechanistic investigation was performed by employing nitrogen gas adsorption, thermogravimetric analysis, X-ray diffraction, and scanning electron microscopy. The role of the chemical properties rather than physical characteristics of the hydrates was more significant for the immobilization. CaCl2 and Ca(NO3)2 enhanced more formation of aluminate ferrite tri-sulfate (AFt), increasing the host sites for MoO42- and CrO42-. Density functional theory (DFT) simulations showed the likelihood of SO42- in AFt substituted by MoO42- and CrO42-. The unveiled capabilities and mechanism of CaCl2 and Ca(NO3)2 to enhance the MP immobilization of SSA-CPs disclosed new knowledge for the mitigation strategy of pollution from increasingly generated SSA.

Greenhouse gas (GHG) transport as solid natural gas (SNG) hydrate Pellets: Assessment of Self-Preservation & dissociation controls

Safe greenhouse gas (GHG) transport is an integral part of any carbon capture, utilization, and storage (CCUS) operations. This manuscript discusses the assessment of solid natural gas (SNG) hydrate technology to safely transport GHG utilizing the self-preservation capability of hydrates around 248 K. Ice shielding around hydrate grains has been previously implicated as a key mechanism for self-preservation of hydrates. However, there is a lack of thorough understanding of the hydrate grains sintering and compaction effects on self-preservation prior to the ice crust formation around the grains. In a first of its kind study, the promotional effects of hydrate grains compaction and sintering via pellet porosities, aging times, and formation times on self-preservation are presented. Furthermore, real time operational variables such as depressurization rates and system pressure build-up are studied. A unique integrated apparatus was designed to assess in-situ hydrate pellets formation, compaction, and dissociation. This method eliminated the uncertainties of possible hydrate dissociation during material transfer between the operational stages. Around 248 K, hydrate pellet stability occurs in two states: pre-self-preservation (initial fast hydrate dissociation) and extended self-preservation (slow hydrate dissociation over 200 + hours). Decreasing porosity via compaction and increasing formation and pellet-aging times via prolonged sintering increases hydrate pellet stability. The pellet stability can be further promoted by slowly depressurizing the pellet to ambient pressure and allowing pressure build-up from the dissociating hydrates. Additional transport risks are assessed through a sudden loss of temperature from 248 K, which are missing in the literature. Refrigeration failure of the pellet progresses in two stages: (1) a safe insulation stage characterized by slow dissociation and minimal gas evolution, followed by (2) a rapid dissociation stage during which complete dissociation of the pellet took place. Overall, this work elucidates the transportation factors governing self-preservation and dissociation, thus indicating the feasibility for safe GHG transport via SNG technology.

The Effect of Fly Ash Mixture on Concrete Compressive Strength

Fly ash is a waste from the steam power plant industry that can be used as added material in making concrete. This is because the chemical compounds in fly ash are almost the same as cement. The purpose of this study was to determine the compressive strength of concrete with partial replacement of cement with fly ash. The composition of partial replacement of cement with fly ash is 0%, 20%, 25%, 30%, and 35%. The planned concrete quality is 30 MPa and tested at the age of 7 days and 28 days. The test results show the average compressive strength of the concrete for all variations of both 7-day and 28-day replacement is less than 30 MPa. The greater composition of fly ash used, the smaller the compressive strength value of the concrete. This is caused by the slow hydration process because fly ash is pozzolans which delay the hydration process. In addition, the reduction of cement content will also result in a decrease in compressive strength, this is because the bonding capacity of the aggregate decreases. However, when viewed based on the density of the concrete, the fine fly ash grains can make the concrete denser.

Permeability damage and hydrate dissociation barrier caused by invaded fracturing fluid during hydrate reservoir stimulation

Hydraulic fracturing is a promising stimulation technology for enhancing the gas productivity and energy efficiency of low-permeability hydrate reservoirs. Presently, most studies focus on the fracturing feasibility of hydrate-bearing sediments and recovery efficiency after fracturing. Although the fracturing fluid inevitably invades the hydrate reservoirs during fracturing, the effects of invaded fracturing fluid on sediment permeability and hydrate dissociation have not been investigated yet. In this study, the invasion of water-based fracturing fluid into hydrate-bearing sediments was experimentally studied to clarify the dynamic characteristics of the fracturing fluid invasion, including its effects on sediment permeability and hydrate dissociation. The results revealed that the fracturing fluid loss rate was initially high but abruptly became extremely small, potentially because of the filter cake deposition on the fracture surface and the secondary hydrate formation in the invaded zone. However, the invaded fracturing fluid increased the dissociated gas flow resistance and inhibited the hydrate dissociation and gas production that yielded a two-stage gas production process, including slow hydrate dissociation in the invaded zone followed by relatively fast hydrate dissociation in the uninvaded zone. Furthermore, the degree of permeability damage in the invaded zone gradually decreased with the increasing invasion distance that was caused by the variations in the microscopic filtrate–sediment reactions. Interestingly, the water saturation measured by NMR gradually decreased from about 90% to 60% (initial water saturation) along the invasion distance. This phenomenon indicates that water sensitivity and water lock acted as the primary sources of permeability damage to the clayey–silty sediments after fracturing fluid invasion, suggesting that inhibiting clay swelling and increasing the flowback rate of the invaded fluid were essential for reducing permeability damage. Noted that using the fracturing fluid with breaker and KCl exhibited the lowest degree (25%) and shortest distance (<15 cm) of sediment permeability damage. The present results are vital for applying hydraulic fracturing in the field tests of hydrate reservoirs to optimize the fracturing fluid and improve the fracturing stimulation effect.

Water hydration of polyethylene glycol dimethyl ether.

In this work, GHz and THz complex dielectric spectra of a polyethylene glycol dimethyl ether (2000 g/mol) aqueous solution were studied. The reorientation relaxation of water in this kind of macro-amphiphilic molecule solution can be well described by three Debye models, including under-coordinated water, bulk-like water [water molecules in a tetrahedral hydrogen bond network (bulk water) and bulk water molecules affected by hydrophobic groups], and slow hydrating water (water molecules donating hydrogen bonds to hydrophilic ether groups). The reorientation relaxation timescales of bulk-like water and slow hydration water both show increases with concentration from 9.8 to 26.7ps and from 46.9 to 100.1ps, respectively. By estimating the ratios of the dipole moment of slow hydration water to the dipole moment of bulk-like water, we calculated the experimental Kirkwood factors of bulk-like and slow hydrating water. The experimental Kirkwood factor of bulk-like water increased from 3.17 to 3.44 with concentrations, while the experimental Kirkwood factor of slow hydrating water roughly remained constant at 4.13 from concentrations of 15%-60%. The estimated water molecule numbers of three water components around monomers also confirm our sorting for water components.

Study of the Mechanical Properties and Microstructure of Alkali-Activated Fly Ash-Slag Composite Cementitious Materials.

Composites that use fly ash and slag as alkali-activated materials instead of cement can overcome the defects and negative effects of alkali-activated cementitious materials prepared with the use of an alkali-activated material. In this study, fly ash and slag were used as raw materials to prepare alkali-activated composite cementitious materials. Experimental studies were carried out on the effects of the slag content, activator concentration and curing age on the compressive strength of the composite cementitious materials. The microstructure was characterized using hydration heat, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM), and its intrinsic influence mechanism was revealed. The results show that increasing the curing age improves the degree of polymerization reaction and the composite reaches 77~86% of its 7-day compressive strength after 3 days. Except for the composites with 10% and 30% slag content, which reach 33% and 64%, respectively, of their 28-day compressive strength at 7 days, the remaining composites reach more than 95%. This result indicates that the alkali-activated fly ash-slag composite cementitious material has a rapid hydration reaction in the early stage and a slow hydration reaction in the later stage. The amount of slag is the main influencing factor of the compressive strength of alkali-activated cementitious materials. The compressive strength shows a trend of continuous increase when increasing slag content from 10% to 90%, and the maximum compressive strength reaches 80.26 MPa. The increase in the slag content introduces more Ca2+ into the system, which increases the hydration reaction rate, promotes the formation of more hydration products, refines the pore size distribution of the structure, reduces the porosity, and forms a denser microstructure. Therefore, it improves the mechanical properties of the cementitious material. The compressive strength shows a trend of first increasing and then decreasing when the activator concentration increases from 0.20 to 0.40, and the maximum compressive strength is 61.68 MPa (obtained at 0.30). The increase in the activator concentration improves the alkaline environment of the solution, optimizes the level of the hydration reaction, promotes the formation of more hydration products, and makes the microstructure denser. However, an activator concentration that is too large or too small hinders the hydration reaction and affects the strength development of the cementitious material.

Mechanism of regulating the mechanical properties and paste structure of supersulfated cement through ultrafine iron tailings powder

To improve the defects of supersulfate cement (SSC), such as slow hydration rate and low strength, this research investigated the influence of ultra-fine iron tailings powder (UITP) on the strength of SSC. The mechanism of UITP's influence on the mechanical properties of SSC was studied by scanning electron microscope (SEM), X-ray diffraction (XRD), thermogravimetric–differential scanning calorimetry analysis (TG–DSC), and chemical titration analysis of ettringite (AFt). Results show that the compressive strength of SSC paste can be significantly improved when the UITP content was 30%–60%. When the UITP content was 40%, the compressive strength of the SSC paste was increased by 20.4% for 3 d and 44.1% for 28 d compared with that of the control sample. The hydration products of UITP and slag powder were consistent, which were calcium silicate aluminate hydrate (C-A-S-H) and ettringite (AFt); The addition of UITP can increase the amount of hydration products in SSC paste and regulate the ratio of AFt to C-A-S-H, thereby affecting the density of paste microstructure. Moreover, the pore structure of the SSC hardened paste can be improved by adding UITP, and the pore size distribution was optimal when the content of UITP was 40%. This study confirms that UITP and slag powder exhibit a synergistic hydration effect in the SSC system. This synergistic effect significantly improves mechanical properties and volume stability of the SSC hardened paste.

Durability of Concrete with Partial Replacement of Portland Cement by Incorporating Reactive Magnesium Oxide and Fly Ash

In this research, the durability performance of sustainable concrete with the incorporation of reactive magnesium oxide (MgO) and fly ash (FA) was evaluated. The partial replacement of cement with these two materials is an appealing solution for the construction sector due to sustainability benefits and shrinkage reduction. The incorporation of FA by partial replacement of cement was carried out at 0%, 15% and 30%. The incorporation of MgO in concrete was carried out at 0%, 5%, 10% and 20%. Two types of MgO were used, one from Australia and another of Spanish origin. These two materials were evaluated in terms of their individual incorporation, and then an evaluation was carried out when the two were simultaneously used. In terms of durability, performance losses between 3% and 95% were obtained in all tests (water absorption by capillarity and immersion, carbonation depth and resistance to chloride penetration). However, over time, the difference in performance relative to the reference concrete tends to decrease due to the slow hydration that characterizes these two alternative materials. It was found that, in most of the tests, no overlapping of the negative effects occurred. In other words, the simultaneous incorporation of MgO and FA caused performance losses lower than the sum of the losses of their individual incorporation.

Development of Cemented Paste Backfill with Superfine Tailings: Fluidity, Mechanical Properties, and Microstructure Characteristics.

Previous studies have shown that the effectiveness of superfine tailings cemented paste backfill (SCPB) is influenced by multiple factors. To optimize the filling effect of superfine tailings, the effects of different factors on the fluidity, mechanical properties, and microstructure of SCPB were investigated. Before configuring the SCPB, the effect of cyclone operating parameters on the concentration and yield of superfine tailings was first investigated and the optimal cyclone operating parameters were obtained. The settling characteristics of superfine tailings under the optimum cyclone parameters were further analyzed, and the effect of the flocculant on its settling characteristics was shown in the block selection. Then the SCPB was prepared using cement and superfine tailings, and a series of experiments were carried out to investigate its working characteristics. The flow test results showed that the slump and slump flow of SCPB slurry decreased with increasing mass concentration, which was mainly because the higher the mass concentration, the higher the viscosity and yield stress of the slurry, and thus the worse its fluidity. The strength test results showed that the strength of SCPB was mainly affected by the curing temperature, curing time, mass concentration, and cement-sand ratio, among which the curing temperature had the most significant effect on the strength. The microscopic analysis of the block selection showed the mechanism of the effect of the curing temperature on the strength of SCPB, i.e., the curing temperature mainly affected the strength of SCPB by affecting the hydration reaction rate of SCPB. The slow hydration process of SCPB in a low temperature environment leads to fewer hydration products and a loose structure, which is the fundamental reason for the strength reduction of SCPB. The results of the study have some guiding significance for the efficient application of SCPB in alpine mines.

Toward performance improvement of supersulfated cement by nano silica: Asynchronous regulation on the hydration kinetics of silicate and aluminate

Supersulfated cement (SSC) is a traditional low-carbon cement, but its slow hydration and strength development has limited its practical applications. Nano silica (NS) was used to activate the hydration of SSC by taking advantage of its ability to regulate silicate and aluminate reactions. The mechanical performance of various mixes was determined, as a function of sulfation degree and NS addition, as pore structure, phase assemblage, hydration degree, and microstructure. Results showed that NS improves the hydration degree of slag, densifies the microstructure, and significantly increases both early- and late-age compressive strength. The enhancement was attributed to its effects on the hydration of slag in SSC: delaying ettringite formation, but promoting C-(A)-S-H precipitation, reducing microporosity. This study reveals the critical role of the regulation of hydration kinetics of silicate and aluminate in controlling the performance of SSC as NS does.

Thermodynamics, Kinetics, Morphology, and Raman studies for sH Hydrate of Methane and Cyclooctane

Natural gas is expected to be the major energy source in the near future, and storing it in the form of gas hydrate is a safe, clean, and economical approach. However, required thermodynamic conditions and slow kinetics are the key challenges that need to address for process viability. This study involves an experimental investigation of methane and cyclooctane sH hydrate formation for possible applications in gas storage using thermodynamics, kinetics, morphology, and Raman analysis. The hydrate formation is carried out at such thermodynamic conditions where only sH hydrate would form. The four-phase (Lw-LHC-H-V) sH hydrate equilibrium is studied for the methane and cyclooctane system via dissociation along the phase boundary method which is a robust method as it delivers a greater number of equilibrium data points in a single experimental run compared to other available methods. The sH hydrate formation helps in lowering the equilibrium conditions compared with sI hydrate formation. The slow sH hydrate formation kinetics can be improved by using low tryptophan concentrations. In this work, 0.1 wt % is the optimum tryptophan concentration as the gas uptake, and the hydrate formation rate is found to be the highest compared to 0.01, 0.05, and 1 wt % tryptophan concentrations. Here, we also visually investigate the sH hydrate formation and observed that the hydrate formation occurs below the interface for the system with no tryptophan; however, hydrate formation occurrence above the interface increases with an increase in the tryptophan concentration. The increase in the hydrate formation could be dedicated to the increased gas uptake due to the increasingly porous nature of hydrate formation. The Raman analysis confirmed the presence of methane and cyclooctane in sH hydrate cages. The higher intensity of the peaks using tryptophan additionally confirms the higher hydrate formation compared to the system with no tryptophan.

Key factors controlling the kinetics of secondary hydrate formation in the porous media

Secondary hydrate formation (SHF) usually occurs in hydrate formation and decomposition experiments, and its manifestations behave differently in promoting gas consumption and water-hydrate conversion. In this work, in order to investigate the kinetic processes and controlling factors of the SHF in the porous media, we conduct a series of formation and dissociation experiments in a three-dimensional cubic hydrate simulator (CHS). Hydrate dissociation is accomplished by the depressurization method after the formation experiments. The experimental results reveal that a rapid growth rate of equal magnitude in the SHF stage follows a relatively slow hydrate growth in the process of hydrate formation. The occurrence of the SHF is accompanied by a sudden pressure drop and temperature rise, and a heterogeneous temperature distribution shows significant effects on hydrate formation behaviors in the vessel. It has been verified that the SHF is controlled by hydrate thermodynamic stability at a subcooling degree of −0.8 °C during the hydrate formation. The SHF is initiated by the collapse and crack of the local thin hydrate film, which results in an accelerated triggering of gas diffusion for further hydrate formation. In the dissociation experiments, the slight temperature increase caused by the SHF can promote the hydrate dissociation rate. The subcooling degree in the hydrate formation can also be suitable for predicting SHF in the dissociation experiment. It is found that a rapid depressurization rate can effectively shorten the duration of SHF. These insights can be beneficial for the energy transition and natural gas hydrate extraction in the future.

Нуклеация гидрата метана в присутствии нанофибриллярной целлюлозы

Is it possible to slow down gas hydrate nucleation by introducing additional surfaces into the system? This process usually occurs at phase boundaries, i.e., the appearance of new surfaces (solid particles) in the system should rather increase the probabilityof hydrate formation. In this work, the effect of carboxylated cellulose nanofibrils (CNF) dispersed in water on the formationof methane hydrate under intensive stirring was investigated. It was found that, under identical conditions, 0.5 mass% CNF allowed reducing the hydrate nucleation rate and suppressing the "memory" effect characteristic for hydrate formation from distilled water (DW). The analysis of the "survival" functions revealed that the mechanism of the CNF inhibitory action is to reduce the number of hydrate nucleation centers. This can be explained by the participation of CNF in the disturbance of the hydrogen bond network:for example, mechanical "destruction" of pre-critical nuclei by inhibitor particles under intensive stirring. The data obtained suggest the development of a new class of kinetic hydrate inhibitors based on dispersions.

Self-assembled micro-nano flower-like/spherical magnesium hydroxide for heat-energy storage

In response to global energy problems, industrial waste heat storage systems are a useful strategy as important as clean energy. Slow magnesium oxide hydration rate and incomplete hydration are the main obstacles to the application of MgO/Mg(OH)2 to heat storage systems. In this study, porous structures are introduced into pure magnesium oxide materials to increase their specific surface area and enrich their pores, thereby increasing their hydration rate and conversion. The effective exothermic densities of the self-assembled micro-nano flower-like and spherical samples with high specific surface area are 955.6 kJ/kgMg(OH)2 and 1034.8 kJ/kgMg(OH)2, respectively, about 2 to 3 times that of commercially purchased magnesium hydroxide. Their hydration conversion rates are 51.0 % and 57.7 % at 20 min, respectively. They have potential for use in heat storage systems.

Effect of multilayer graphene oxide on the hydration and early mechanical strength of cement mortar in low temperature

The mechanical strength of cement mortar is normally decreased in a low-temperature environment, which is mainly due to the slow cement hydration reaction rate and an increase in the total pore volume, especially the harmful pore volume. With the modification of nano-materials, the early mechanical strength of cement-based materials can be effectively compensated. In this study, the effect of multilayer graphene oxide (MGO) on the early mechanical properties and microstructure of cement mortar at low temperatures of 0 °C, 5 °C, and 10 °C is studied by measuring the fluidity, setting time, mechanical property, scanning electron microscope (SEM), thermogravimetric (TG), and nitrogen adsorption/desorption test. Compared with the mechanical strength of control mortar with curing temperature of 20 °C, the mortars with the addition of MGO curing at low temperature present higher mechanical strength and denser microstructure. And the mechanism of the interaction between cement particles and MGO nano particles at low temperatures (0 °C, 5 °C, and 10 °C) is further discussed. The initial hydration reaction of MGO mortar is accelerated, which is mainly due to the nucleation effect of MGO and chemical reactions between –COOH on the edge of MGO nanosheets and the Ca2+ of Ca(OH)2. The promotion of the initial hydration refined the pore diameter of samples by reducing the pore size and total porosity, thus making the microstructure of samples denser and enhancing the strength of specimens.

Continental Mid‐Lithosphere Discontinuity: A Water Collector During Craton Evolution

AbstractContinental mid‐lithosphere discontinuity (MLD) seems to be ubiquitous beneath cratons around the world with the dominant depth of 70–100 km, and is characterized by a shear‐wave velocity drop of 2%–7%, according to geophysical observations. The MLD is often considered to be related to a rheologically weak layer; however, the mechanism of MLD formation is widely debated. In this study, we have conducted systematic numerical modeling with a new and simplified deep hydration method to study the dynamics of craton evolution and MLD formation. The results indicate that the MLD may be induced by slow hydration processes within the mantle lithosphere during craton evolution. The top boundary of this hydrated layer is characterized by high water content and low shear‐wave velocities, and is consistent with the depth and properties of natural MLD observations. Thus, we propose that the MLD may act as a water collector during craton evolution.

Eco-friendly Super Sulphated Cement Concrete Using Vietnam Phosphogypsum and Sodium Carbonate Na2CO3

Sustainable development is one of the critical topics in the construction industry today, especially in reducing CO2 emissions and production energy costs. There have been many studies worldwide on using ground granulated blast furnace slag combined with phosphogypsum (PG) to replace binder (B) in making concrete. However, this topic in Vietnam has not received much attention despite the large backlog of phosphogypsum waste. One of the main disadvantages limiting the feasibility of super-sulphated binders in concrete is the relatively slow hydration and hardening processes, which affect the rate of strength development of mortar and concrete, especially at an early age. In this study, the use of Na2CO3 salt as a quick, solid additive can overcome the disadvantages of this type of binder. Research results show that using 15 to 25% phosphorus gypsum waste (PG) and a combination of 60 to 80% finely granulated blast furnace slag (GGBFS) with a small amount of cement and an activator like Na2CO3can replace cement in making concrete. The concrete mix has good workability, and the maximum compressive strength after 28 days can reach over 50 MPa. Using industrial wastes as the main ingredients to make binders will improve sustainable development, reducing environmental pollution and the cost of mortar and concrete products in construction. Doi: 10.28991/CEJ-2022-08-11-06 Full Text: PDF

Influence of wet grinded slag on the hydration of phosphogypsum-slag based cement and its application in backfill tailings

Phosphogypsum-slag based cement (PSBC) has a great potential in tailings backfill because of low cost and CO2 emission in comparison with Portland cement. However, slow early hydration was often observed, and this problem was generally solved by adding cement clinker, which could provide alkalinity to hasten the hydration of ground granulated blast furnace slag (GGBS). In this study, GGBS by wet grinding (i.e., WGGBS) was used to partially replace raw GGBS in PSBC, with intent to promote the early hydration and cementitious performance in backfill tailings. Results showed that 60 % replacement ratio of GGBS by WGGBS enhanced the 3 d compressive strength by 507 %, and that for a 28 d was 45 MPa, which meet the strength requirement of 42.5 grade cement; besides, its carbon emission was only 5 % of that of cement. Due to the high reactivity of WGGBS at the early age, pH of PSBC system was promoted, and this hastened dissolution of GGBS and formation of hydrates. This was the reason for the significant increase in early compressive strength. Moreover, with the continuous hydration of PSBC, the hydrates were produced, and the microstructure was densified over time, leading to the increase in later compressive strength. It seemed that the PSBC with WGGBS showed an excellent cementitious performance in iron tailings filling. Under the condition of 1:10 wt ratio of cementitious material to iron tailings, 28 d compressive strength of PSBC with WGGBS was up to 3.1 MPa, while that for cement was only 0.8 MPa.