Hydration Of Slag Research Articles

A case study for Improving the performance of CaO-activated slag by the reaction behavior of CaO and NaHCO3

In Alkali-Activated Slag, a contradiction exists among pH, setting speed, and strength development. In this study, the reaction behavior between CaO and NaHCO3 is harnessed to regulate the setting speed while optimizing the material properties. The results demonstrated that the pore solution pH of Alkali-Activated Slag rose to over 12.4. At C5S100, the workability and setting time of the material were comparable to or even surpassed those of ordinary Portland cement. It exhibited a compressive strength of 53.9 MPa at 28 days, which was an 85.2 % increase compared to C5S0, and over 70Mpa at 90 days. Additionally, in the microscopic characterization tests, the composite activator facilitates the dissolution hydration of slag, generating more hydration products. This leads to a more compact structure and lower porosity, thereby enhancing the overall performance. This composite activator strategy effectively reconciled the conflict between the practicality of Alkali-Activated Slag and the setting speed and strength development.

Design and microstructural analysis of the mixture proportion of alkali-activated fly ash-slag composite cementitious material

Abstract To explore the resource utilization of fly ash, slag, and coal gangue, the composition of hydration products and strength characteristics of fly ash-slag composite cementitious material (FSGF) were studied with NaOH as an alkali activator. First, response surface analysis was used to determine factors influencing the optimal NaOH content, basalt fiber dosage, and length to obtain the complete mix ratio of the composite cementitious material. Microscopic techniques such as XRD, FTIR, TG-DSC, and SEM were employed to analyze the crystal structure, thermal properties, and micro-morphology of the composite cementitious material, and to investigate the mechanism of NaOH-activated fly ash-slag cementitious material. The results indicated that the sensitivity of each factor affecting the mechanical properties of the composite cementitious material followed this sequence: NaOH content > basalt fiber length > basalt fiber dosage, with varying degrees of interaction among them. When the mass ratio of fly ash, slag, and coal gangue was 5:1:4, with 3% NaOH by weight, 2% basalt fiber dosage, and a fiber length of 3 mm, the optimal mix was achieved. The composite material achieved a compressive strength of 8.97 MPa after 28 days of standard curing at room temperature. NaOH, as an alkali activator, provided the strong alkaline environment required for the initial hydration of fly ash-slag composite cementitious materials, promoting the hydration of slag and fly ash. The hydration products in the fly ash-slag composite system were unevenly distributed, primarily consisting of gels like C-S-H, C-A-H, and C-A-S-H. NaOH was highly effective as an alkali activator in the fly ash-slag system.

Solubility-controlled early age hydration of carbonate-activated slag: Underlying mechanisms and acceleration via seeding

The low environmental impacts and good performance of carbonate-activated binders have attracted substantial interest, while their intrinsically delayed reaction and prolonged setting times constrain widespread implementation. This study aims to investigate the early-age reaction kinetics of carbonate-activated binders, elucidate the underlying mechanisms and propose controlling methods. Results reveal that hydrotalcite and C-A-S-H gel precursors rapidly reach supersaturation within hours, yet precipitation of these phases remains absent. This phenomenon, coupled with high Si/Ca ratios in the pore solution, exhibits characteristics that may contribute to the observed reaction delays. A prolonged induction period and lethargic strength development are observed owing to the lack of strength-giving phases. The hydrotalcite and C-A-S-H concentrations surpass their theoretical solubility thresholds due to rising ionic strength and salt-in effects. The addition of a pH-neutral layered double hydroxide salt accelerates the reaction, confirming seeding impacts control pore solution saturation limits, constituting the underlying accelerator mechanism.

Performances enhancing of supersulfated cement (SSC) using waste alkaline activators: Red mud and carbide slag

Supersulfated cement (SSC), since its invention, showed lower early-age strength gain, which has limited its practical applications. The urgent issue is how to regulate and enhance its early-age performance. In this context, solid waste materials such as bauxite residue-red mud (RM) and carbide slag residue (CS) from acetylene production by calcium carbide hydrolysis were used to improve the hydration and performance of SSC. By utilizing a range of analytical techniques such as isothermal calorimetry (IC), X-ray diffraction (XRD), backscatter scanning electron images (BSE), and thermogravimetric analysis (TGA), the hydration process, phase assemblages and microstructure of CS/RM-modified SSC were systematically investigated. It’s found that CS, as a calcareous alkaline activator, act as a regulator for adjusting the two main hydration reactions of C-A-S-H deposition and ettringite formation in SSC. The alkaline activator induces the hydration of slag and promotes the formation of hydration products. However, an excessively high dosage of carbide slag delays the primary ettringite formation reaction because it inhibits the dissolution of slag and promotes the formation of outer products. It leads to a looser and more porous paste matrix, resulting in higher expansion strain and poor strength gain at full curing ages. In contrast, red mud improves the hydration rate and enhances the hydration degree of SSC within the studied content (up to 30 wt%). With addition of red mud, SSC hydration is significantly advanced and deepened, resulting in more hydrates, particularly inner products. Red mud facilitates the depolymerization of the slag glass network and promotes the forming of nuclei and growth of hydration products. However, an excessively high red mud content would lead to a looser and more porous paste matrix due to the characteristics of red mud particle itself. An optimal red mud content has been identified as 20 wt%.

Insights into the role of slag fineness on the hydration of slag-sulfoaluminate cement

Slag-sulphoaluminate cement (G·SAC), an effective alternative to Portland cement, has garnered significant attention for its low hydration heat, good mechanical properties and durability. However, the role of slag fineness on the hydration of G·SAC remains unclear. To address this, the study employed three levels of slag fineness to evaluate its effects on G·SAC in terms of hydration heat, hydrates, mechanical properties and pore structure. Results indicate that increasing slag fineness from 400 m²/kg to 800 m²/kg shortens the induction period by 2 hours but does not significantly alter the hydration heat ∼ 110 J/g or 3-day strength (around 30 MPa). In contrast, slag with a specific surface area of 1300 m²/kg significantly enhances the hydration of G·SAC, evidenced by an increased hydration heat of 180 J/g and a higher degree of slag hydration from 24 % to 79 %. This leads to greater formation of hydrates and a higher polymerization degree of C-S-H, which in turn improves pore structure density and early strength, reaching 38.8 MPa at 3 days. To sum up, a slag fineness ∼ 400 m²/kg is optimal for engineering applications, offering a balance between performance and economic considerations.

Substantially enhanced reaction of steel slag binder stimulated by the addition of sodium sulfate and thermal treatment

An industrial by-product of steel slag has a huge potential as an alternative binder and new supplementary cementitious material (SCM). In both pure steel slag and composite systems, the amount of sulfate can be a critical factor to control early-age performance. This study aims to improve the hydration of steel slag stimulated by using a small amount of Na2SO4. The results showed that 1 % Na2SO4 enhanced both C2(A,F) and C2S hydration, leading to improved strength and denser microstructures with the formation of ettringite, AFm, and siliceous hydrogarnet. Initial thermal treatment at 40 °C with 3 % Na2SO4 provided a fast development of compressive strength at 3 days over 15 MPa. Meanwhile, an excessive amount of alkali supplied by Na2SO4 affects the relative stability between SO4-rich AFm and ettringite and has a detrimental effect on mechanical performance. The result implies that Na2SO4 content should be precisely controlled when used as a stimulator in a composite system with steel slag.

Influence of calcium oxide and hydroxide on non-ferrous metallurgical slag hydration in alkaline environments

The search for new supplementary cementitious materials (SCMs) to reduce CO2 emissions has become increasingly vital in cement and concrete research. Non-ferrous metallurgical slag (NFMS), often overlooked due to their low pozzolanic and hydraulic reactivity, present a promising alternative, particularly given their enhanced reactivity in the presence of alkalis. This study explores the hydration behaviour of NFMS under varying concentrations of CaO, Ca(OH)2, and NaOH, with a focus on dissolution kinetics and phase evolution. 1 M and 6 M NaOH solutions were selected to examine the effect of calcium additives on NFMS hydration, representing scenarios without (1 M NaOH) and with (6 M NaOH) alkali activation reactions. In 6 M NaOH, the addition of CaO/Ca(OH)2 had a minimal impact on dissolution rates but significantly influenced phase assemblage, with CaO promoting strätlingite formation and 10 % Ca(OH)2/CaO leading to katoite. At higher replacement levels (20 % CaO/Ca(OH)2), phase formation was hindered, although microstructure densification persisted, accompanied by reduced strength. In 1 M NaOH, low concentrations of CaO/Ca(OH)2 enhanced Al and Si dissolution but had limited impact on phase assemblage. Higher CaO content favoured katoite formation, while Ca(OH)2 exhibited traces of hydrocalumite. Both additives improved microstructure, but CaO addition, associated with katoite formation, predominantly enhanced strength. The findings confirm that NFMS has significant potential as a cement replacement in hybrid systems utilizing high ratios of slag, Portland cement, and alkalis. By promoting the formation of Ca-Al phases, NFMS improves microstructure and strength, making it a viable and environmentally friendly alternative in construction materials.

Preparation of coal gasification coarse slag-based alkaline activator and its activation mechanism in alkali-activated slag

The good performance and durability of alkali-activated slag (AAS) make it to be a potential alternative to ordinary Portland cement. However, the preparation of activators such as water-glass is energy-intensive. In this paper, the feasibility of using a novel activator derived from coal gasification coarse slag (CGCS) for the preparation of AAS was investigated. The CGCS-based activators were the filtrates of CGCS and NaOH compound solution at 150 °C with different CGCS/NaOH ratios. By comparison with water glass-activated slag and NaOH-activated slag, the hydration reaction activity, mechanical property, hydrates composition, and microstructure of the CGCS-activated slag were studied. The results indicate that the compressive strength of CGCS-activated slag reached to 55.25 MPa at 28 d, much higher than NaOH-activated slag (43.08 MPa). CGCS-activated slag also performed the similar hydration dynamic to water glass-activated slag in terms of early heat release and hydrates assemblage. The amorphous silicon from CGCS dissolved in the filtrate played a crucial role in the activation effect of CGCS-based activator, which promoted the early hydration of CGCS-activated slag and contributed to the C-(A)-S-H with longer chain length. These findings proposed the mechanism of slag activated by CGCS-based activator, and provided an eco-friendly strategy for AAS production.

Unveiling the impact of carbonation treatment on BOF slag hydration activity: Formation and development of the barrier layer

Carbonation treatment is one of the most promising methods to improve the volume stability of basic oxygen furnace (BOF) slag, but the reasons for the reduced activity of heavily carbonated BOF slag have been rarely investigated. This study prepared carbonated BOF slag under the condition of CO2 concentration at 99 % and pressure at 3.0 MPa, and evaluated its activity through various methods. The mechanism behind activity inhibition was revealed. Results showed that the maximum CO2 sequestration of BOF slag was 13.4 % within 3 h. Highly carbonated BOF slag exhibits lower hydration activity than uncarbonated slag, with a 15.9 % decrease in strength activity index and a 61.0 % reduction in cumulative hydration heat release. After carbonation treatment, a 1–5μm thick CaCO3 barrier layer accumulates on the surface of BOF slag, affecting slag hydration even with accelerated hydration. This barrier layer inhibits the efflux of internal calcium ions and can persist long-term during the hydration without being affected. Furthermore, carbonation treatment increased the leaching risk of heavy metal ions, particularly Mn, by 175-fold.

Raw earth stabilised by alkaline activation of slag: Effect of clay on slag hydration

Alkali-activated slag is used as a cement substitute to stabilise poured earth materials, aiming to enhance their mechanical properties and decrease their water sensitivity. However, the interaction between electrically charged clay particles contained in the earth and the alkaline solution can modify the consistency and, more significantly, lead to the absorption of chemical elements. This can have a detrimental effect on the stabilisation efficiency. In order to optimise slag activation, it is essential to understand the interaction between the clay and the alkaline medium, and its effect on slag hydration. This study examines the microstructural effects of the interaction between the alkaline solution and clays on slag hydration, through microstructural characterization (SEM, FTIR, and DTA-TGA analyses). The impact of three distinct types of earth and two levels of sodium concentration are investigated. Clayey activity causes changes in alkaline conditions, leading to the formation of hydrates that are typically produced at very high sodium concentrations, such as hydrotalcite and depolymerised C-A-S-H. Only formulations with a sodium concentration higher than 1.6 M provide robustness with respect to the three types of raw earth studied. This alkaline context has a limited effect on the microstructure of the clay particles.

Experimental Investigation on Hydrophobic Alteration of Mining Solid Waste Backfill Material

To address the issues of corrosion weakening of solid-waste-based backfill material caused by mine water, a novel hydrophobic solid waste backfill (HSBF) material was developed using polydimethylsiloxane (PDMS) and a silane coupling agent (SCA) as hydrophobic modification additives, and NaOH (SH) and sodium silicate (SS) as alkali activators. Fly ash and slag were chosen as the primary raw solid waste materials. The rheological properties of the hydrophobic-treated backfill slurries were measured, and the resulting physicochemical properties were compared with the unmodified reference group. This study reveals that the fresh HSBF slurry follows a Modified Bingham (M-B) model with shear-thinning characteristics. The addition of PDMS causes an increase in the water contact angle of the hardened HSBF material with F8S2 to up to 134.9°, indicating high hydrophobicity. Morphological observations indicated that PDMS mainly attaches to the inorganic particles’ surface through the bridging action of SCA for the hydrophobic modification of the backfill material. The overall strength of the HSBF materials was further ensured via fly ash–slag ratio optimization, and was found to be enhanced up to 98% by increasing slag content from 20% to 50%. This is mainly attributed to the hydration of slag, forming C-S(A)-H gel, which contributes to the increased strength. The novel HSBF material enables the elimination of cement in mine backfilling applications, demonstrating good economic benefits. Its excellent mechanical and hydrophobic properties can not only prevent overburden displacement in goaf areas, but can also mitigate water resource loss from overlying strata and simultaneously reduce the safety risks associated with long-term mine water deterioration.

Calcined alunite-modified alkali-sulphate-activated slag as a novel binder for high-performance cemented paste backfill

The free water content and pore structure significantly affect the mechanical properties of cementitious materials. Because of high water-to-cement ratio and low binder dosage, cemented paste backfill (CPB) is characterized by high free water content and high porosity, which lead to a low strength. In this study, calcine alunite (CA) was used to modify the alkali-sulphate-activated slag (ASAS) to synthesis high-strength CPB with low porosity and low free water content. The results show that at dosage ≤3 %, CA can significantly enhance the strength of ASAS-CPB, and the strengthening effect becomes more prominent as both the dosage and curing time rise. An addition of 3 % CA leads to a 184 % increase in strength at 14 days. However, excessive CA (5 %) reduces 3-day strength by 65 %, but provides the greatest strength at 56 days. As the dosage increases up to 3 %, the promoting impact of CA on early hydration of ASAS-CPB gradually increases, and the duration of induction stage and acceleration stage gradually decreases. Nevertheless, 5 % CA prolongs the duration of induction stage by 921 %. Electrical conductivity and volume water content can be used to assessing the hydration process of ASAS-CPB. Besides, the addition of CA can promote the hydration of slag, resulting a greater formation of C–S–H and ettringite. The hydration products fill the pores and transform the macropores into micropores. Furthermore, mechanisms of CA-induced enhancement in strength is discussed. The results of this study can offer a reference for the synthesis of high performance CPB.

Effect of steel slag as alkaline exciter on the properties of supersulfated cement

In order to investigate the feasibility of steel slag as an alkaline exciter for supersulfated cement, the effects of different steel slag dosages on the properties of supersulfated cement were studied. The results show that when steel slag is used as alkaline exciter, the compressive strength and softening coefficient of supersulfate cement increase and then decrease with the increase of steel slag dosage, and the increase of steel slag dosage accelerates the hydration of slag, forming more hydration products of Ettringite and C-S-H gel. Steel slag dosage in 10% of the best results, excessive steel slag will generate expansion of Ettringite, resulting in expansion of the cement matrix structure cracking, is not conducive to the development of strength.

Experimental study on performance, hydration, and sustainability of coal gasification coarse slag based geopolymers for sustainable environment

The large-scale application of coal gasification technology generates a large amount of coal gasification coarse slag (CGCS) difficult to treat. In this study, low-carbon geopolymers (CGRC) were produced using CGCS and ground granulated blast furnace slag (GGBS) as precursors, whereas red mud (RM) and carbide slag (CS) served as solid waste activators. The effects of the binder/activator, RM/CS and CGCS/GGBS ratios and the curing age on the compressive strength, setting time, heavy metal leaching, XRD, SEM-EDS, TG-DTG, FTIR, and LF NMR of CGCS-based geopolymers were investigated, and the hydration mechanism of CGRC geopolymer was disclosed. The results showed that the binder/activator ratio had a negligible effect on the ultimate compressive strength (UCS), the RM/CS ratio had a noticeable impact on the compressive strength and setting time, and increasing the GGBS content significantly improved the performance of CGCS-based geopolymers. Therefore, the synergistic effect of RM and CS led to the activation of CGCS and GGBS. The main hydration products were N-A-S-H, N/C-A-S-H, C-A-S-H gel, CH, and CaCO3. Furthermore, the decrease of the RM/CS ratio which promoted the gel transformation from metastable N-A-S-H to N/C-A-S-H configuration and then to stable C-A-S-H state, finally forming a denser gel matrix. The amounts of various heavy metals leached from CGCS-based geopolymers were even lower than those required by the national standards; therefore the material met the requirements for environmental protection. Furthermore, all raw materials belong to solid waste, which reduces carbon emissions.

Effect of Calcium Aluminate and Carbide Slag on Mechanical Property and Hydration Mechanism of Supersulfated Cement

Supersulfated cement (SSC), a low-carbon, energy-efficient, eco-friendly cementitious material, is mainly made from industrial byproducts. However, SSC’s slow early strength development leads to inadequate initial hardening and reduced durability, which restricts its practical application. This study investigated the potential enhancement of SSC by incorporating calcium aluminate (CA) and carbide slag (CS) alongside anhydrite as activators to address its slow early strength development. The effects of varying CA and CS proportions on the mechanical property and hydration mechanism of CA-CS-SSC were examined. Results indicate that employing 1% CA and 4% CS as alkaline activators effectively activates slag hydration in the 1CA-4CS-SSC, achieving a compressive strength of 9.7 MPa at 1 day. Despite the limited improvement in early compressive strength of other mixtures with higher CA and lower CS proportions in the CA-CS-SSC system, all mixtures exhibited enhanced compressive strength during long-term hydration. After 90 days, ettringite formation in the CA-CS-SSC system decelerated, whereas anhydrite remained. Concurrently, the formation of C-S-H continued to increase, promoting late compressive strength. The mechanism for enhancing the early compressive strength of the CA-CS-SSC system is attributed to the swift hydration of CA with anhydrite, dissolution of fine slag particles, and reaction with anhydrite under conditions with suitable alkali content to augment the ettringite production. This process also generates a C-S-H and OH-hydrotalcite to fill the void in the skeleton structure formed by ettringite, resulting in a dense microstructure that improves early compressive strength.

Influence of ligands as chemical admixtures in the early hydration of sodium carbonate-activated blast furnace slag

Insufficient strength performance at early age impedes the use of sodium carbonate (SC) activator to produce sustainable blast furnace slag-based binders. To solve this issue, this study aimed to accelerate the early hydration kinetics of SC-activated slag system using ligand admixtures: triethanolamine (TEA), triisopropanolamine (TIPA), trisodium nitrilotriacetate (NTA), tetra potassium pyrophosphate (TKPP), and 2,3-dihydroxynapthalene (DHNP). Effect of ligands were investigated by isothermal calorimetry, compressive strength determinations, workability tests, XRD analysis, TGA, and batch dissolution tests. Results showed that the effect of the ligand on the reaction mechanism and strength development depends on the ligand functional group, concentration, and dissolution and metal complexing properties. Between the investigated ligands, 25 mM of DHNP accelerated the hydration kinetics by 28 h and produced 2-day strength of 41 MPa compared to the reference of only 2 MPa. Mechanism behind this acceleration is by affecting the ion activity product (IAP) of layered double hydroxides (LDHs) or C-(A)-S-H phases and not hindering the underlying carbonate salts (gaylussite, calcite) precipitation kinetics. These findings show the potential of ligands as admixtures in binder forming reactions and holds the key to potentially develop and fine tune sustainable low-CO2 binders.

Impact of ettringite seeding on hydration, strength and shrinkage of Na2SO4 activated slag

Na2SO4 activated slag (NSAS) is accepted as one of the most promising low carbon cementitious materials, while its slow strength development seemed the most prominent defect. In this study, nanoscale ettringite (NE) was prepared, and used as nucleation seed to stimulate the hydration of NSAS, thereby promoting the strength development. Compressive strength, hydration process, pore structure, and shrinkage were studied, and the underlying mechanism was revealed. Results indicated that 5% NE enhanced the compressive strength of NSAS paste by 74% at 7 d and by 98% at 28 d, and the reason was that NE significantly facilitated the hydration of slag and the formation of hydrates, including ettringite and C-(A)-S-H gel, especially in the early stage. Furthermore, the presence of NE hastened the dissolution of aluminum phases, and these aluminum phases were inclined to precipitate as ettringite rather than C-(A)-S-H gel. Meanwhile, NE caused the size miniaturization of ettringite crystal formed in hydration. The reason might be that NE provided more nucleation sites, which led to the dispersed growth of ettringite. The small ettringite was harmless to pore structure, and compensated for autogenous shrinkage. In the presence of 1% NE, the porosity and autogenous shrinkage at 28 d decreased from 9.62% to 8.24% and 5209 μm/m to 3992 μm/m, respectively. In addition, NE-NSAS showed great potential in low-carbon cementitious materials.

Analysis of Hydration Mechanism of Steel Slag-Based Cementitious Materials under Saline–Alkaline-Coupled Excitation

In order to realize the resourceful, large-scale, and high-value utilization of steel slag, which is a bulk industrial solid waste, and to reduce the use of cement-based cementitious materials, this study adopted the coupled excitation effect of sodium carbonate–magnesium oxide–desulfurization gypsum to excite steel slag-based cementitious materials, and it preliminarily investigated the hydration process of the steel slag-based cementitious system by the analysis of the heat of hydration of the cementitious materials and the pH value of the pore solution. The hydration products and microscopic morphology of the steel slag-based gelling material were initially investigated by XRD and SEM technical means on the gelling system. The results showed that the hydrolysis of the exciter and the dissociation of the active components in the steel slag provided an alkaline environment and relevant ions for the gelling system, which promoted the generation of the AFt and hydrotalcite phases. Subsequently, the AFt provided ungenerated sites for C-S-H gels as well as calcites, and the hydrotalcite phase accelerated the transformation of the carbonate phase in the gelling system, which promoted the synergistic effect of the hydration of the steel slag and mineral slag. Eventually, a large number of C-S-H gels, calcites, and other hydration products were generated in the gelling system under the synergistic effect of the hydration of the steel slag and slag, which was manifested in the improvement in the mechanical properties at the macrolevel. In addition, this study also standardized 28 d steel slag-based gelling for carbonization maintenance, and the data show that a carbonization temperature of 70 °C, CO2 pressure of 0.7 MPa, and carbonization time of 30 min achieved the best results, with a strength of up to 51.22 MPa, illustrating that steel slag-based gelling materials are safe and can be used for the green storage of CO2.

Paraffin-polyglycerol fatty ester composite as a coating material for delaying the hydration of carbide slag

Carbide slag, mainly composed of hydroxyl calcium (Ca(OH)2), could be used to produce CaO expansive agents. However, the reaction of calcined carbide slag (CCS) up contact with water happens in the fresh state of cement, which belongs to invalid expansion, thus limiting the wide application of CCS. This paper prepared paraffin-polyglycerol fatty ester (PPFE) for coating CCS to slow down its hydration. The physicochemical performance of PPFE was studied at different polyglycerol fatty ester (PGFE) content of 0, 5, 10, 20, and 30 %. For PPFE-coated CCS (PPFE/CCS) capsules, the hydration characterization and mechanisms were analyzed considering the variables of PGFE content. Meanwhile, the hydration characteristics, free expansion (FE), and mechanical performance of mortar with PPFE/CCS were explored. Results showed that the hydrophilicity of PPFE increased with PGFE content. However, the chemical structure of PPFE did not change, suggesting that PPFE was the physical combination of paraffin and PGFE. Furthermore, PPFE considerably delayed the hydration of CCS by forming a dormant period due to the adsorption of PPFE on the surface of CCS, compared to uncoated CCS. PPFE-20 % exhibited the optimal delayed effect on CCS hydration. In addition, the hydration of cement with PPFE/CCS, compared to mortar, is inhibited before 5 h due to the hydrophobicity of PPFE. The FE of mortar with PPFE/CCS was expressively enhanced while the compressive and flexural strengths were reduced, compared to mortar without PPFE/CCS.

Strength development and self-desiccation of saline cemented paste backfill.

Given that many mines around the world are located in areas where fresh water is scarce, and companies are being held to increasingly stringent sustainability and environmental responsibility standards, many mines are looking to use locally available saline groundwater or seawater as mixing water in cemented paste backfill (CPB). However, the impacts of this decision on key engineering properties of CPB (e.g. strength and self-desiccation) that affect its mechanical stability need to be better understood to allow confident selection of this practical and more sustainable solution. Thus, the effect of mixing water salinity and binder type on the strength (unconfined compressive strength, UCS) development and self-desiccation (measured by suction and volumetric water content) of CPB is explored in this research. NaCl concentrations from 0 to 300g/L were used in CPB made with silica tailings and Portland cement type I (PC). Concentrations of 10 and 35g/L were found to moderately increase UCS, while a concentration of 100g/L had comparable UCS to non-saline CPB and a concentration of 300g/L was found to significantly decrease UCS over all curing times. The overall trend is 10g/L > 35g/L > 0g/L > 100g/L > 300g/L. The UCS of the 60-day-old CPB with a NaCl of 300g/L is significantly lower, registering a 26% decrease compared to the UCS of the 60-day-old CPB without salt. In contrast, the UCS of the 60-day-old CPBs containing 10g/L and 35g/L of salt exhibits a notable improvement, being 15% and 10% higher, respectively, than the UCS of the 60-day-old CPB without salt. Water content and suction monitoring were conducted up to 28days of curing time, and it was found that suction only slightly contributed to UCS gain of the saline CPB, and high salt contents (100 and 300g/L) significantly inhibited the self-desiccation ability of CPB due to inhibition of cement hydration by the excessive amount of salt. The increase in strength of both saline and non-saline samples was attributed primarily to the increase in cement hydration products, while the increased strength of the samples with salinities of 10 and 35g/L was mainly attributed to the enhancement of the binder hydration due to the low amount of salt and the presence of Friedel's salt in the pores. The effect of PC replacement by 25 to 75% with slag on CPB with 35g/L mixing water salinity was also studied. Slag replacement of 50% and higher resulted in significantly higher UCS over most curing times. Suction likely moderately contributed to UCS of the saline CPB with slag, in addition to the presence of Friedel's salt in the pores and the acceleration of cement and slag hydration by the presence of NaCl.