Alkali-activated Slag Binders Research Articles

Effect of Dry Mix Alkali Activated Slag Binder Composite Properties Cured in Ambient Condition

<p>A dry mix (solution-free) alkali-activated slag (a primary cementitious material) and/or fly ash based binder composition is developed that can be cast in-site and consequently cured at ambient temperature. Mortar specimens were cast by mixing slag and or fly ash, river sand, powder form alkaline activators (NaOH 14M, varying solids percentage of Na2SiO3) and water were thoroughly mixed in fabricated equipment. The dry density (28 days) of all specimens showed greater than 2200 kg/m3. The compressive strength (28 days) of all mixes was resulted in higher than 40 N/mm2. Among all the four mixes, a mix F103 with 90% slag, 10% fly ash, 30% Na2SiO3 and 14M NaOH at 3, 7 and 28 days curing showed overall higher compressive strength. It is due to fewer solids content of Na2SiO3. The experimental results indicated that solution free studied binder composite can be developed under ambient conditions eliminating other curing types without compromising in the strength. The studied sustainable mixes require only 12-15 percent water which is less compared to regular used mixes. Thus reduced water quantities can be achieved thereby protects the reduction in volume of water bodies, environment hazards, reduces CO2 emissions due to use of industrial by-products as main binders.</p>

Effect of activator concentration on setting time, workability and compressive strength of sustainable concrete with alkali-activated slag binder

The global need to reduce carbon dioxide (CO2) emissions prompted studies on sustainable alternatives to conventional concrete made with ordinary Portland cement (OPC) binders. Alkali-activated ground granulated blast furnace slag (GGBS) is a promising sustainable to OPC in terms of the impact on the environment and reduction of CO2 emissions. Concretes and mortars made with alkali-activated binders demonstrate promising fresh and mechanical properties. This article presents the findings of an experimental study investigating the fresh and mechanical properties of mortar using alkali-activated GGBS binder. The activators used were sodium silicate (Na2SiO3) and sodium hydroxide (NaOH) solution with 6 M, 8 M, 10 M, and 12 M molarities. The study assessed their initial setting time, workability, and 28-day compressive strength. The study evaluated the effect of partial replacement of GGBS with 5 % silica fume. The compressive strength was determined after 28 days of curing in three different environments: 1) ambient air temperature, 2) water submersion, and 3) water immersion for 7 days followed by 21 air curing in ambient temperature. The results showed an increase in compressive strength with higher NaOH concentration but a corresponding decrease in initial setting time and flowability. Depending on the curing method, efflorescence, and blue-green pigmentation were seen during the physical assessment of the samples. The highest 28-day compressive strength was 67.5 MPa when the NaOH activator molarity was 10 mol/L which represents an optimum concentration for strength development. Replacing 5 % of the GGBS binder with silica fume resulted in a decrease in the 28-day compressive strength and workability without affecting the initial setting time.

Innovative encapsulation of alkali activators in alkali-activated slag concrete: A sustainable strategy for regulating setting time and durability

The scientific community has devoted considerable attention to alkali-activated slag binder systems, due to the low-carbon footprint and eco-friendly attributes. However, the regulation of setting time poses significant challenges to the widespread application in practical engineering environments. This study proposes an innovative approach to encapsulate alkali activators in capsules or tablets, followed by the incorporation into the alkali slag concrete matrix. This strategy aims to regulate and control the early dissolution and hydration rate of slag, thereby altering the setting time of the alkali-activated material. Specifically, this method can prolong the setting time by approximately 40%. Importantly, this intervention does not significantly damage the mechanical properties of materials over the 28 days due to the continuous outward diffusion of the active ingredients of the alkali activator. This encapsulation technique harbors the capacity for quantitative modulation of the setting time in alkali-activated slag concrete. The findings of this research offer valuable theoretical insights, paving the way for the application of alkali-activated hydration systems in diverse engineering domains.

A Comprehensive Study on Optimizing Activator Composition for Enhanced Strength and Micro-Structure in High-Strength Alkali-Activated Slag Binders

A Comprehensive Study on Optimizing Activator Composition for Enhanced Strength and Micro-Structure in High-Strength Alkali-Activated Slag Binders

Synergetic effect of superabsorbent polymer and shrinkage reducing admixture on the properties of alkali-activated slag binders

This research is to investigate the synergistic effects of superabsorbent polymer (SAP) and shrinkage-reducing admixture (SRA) on autogenous shrinkage, hydration and microstructure of alkali-activated slag system (AAS). Results revealed that the addition of SRA reduced the absorption capacity of SAP, resulting in a smaller size distribution of macrovoids throughout the AAS matrix. Nevertheless, the presence of SAP affected the ability of SRA to effectively reduce the surface tension of the pore solution of AAS. Instead, the addition of SAP resulted in an increase in surface tension by up to 14%. This phenomenon can be explained by the hydrophobic interactions between SAP networks and SRA molecules. The combination of SAP and SRA in AAS exhibited greater effectiveness in controlling autogenous shrinkage compared to using SRA alone, resulting in a reduction of around 53% compared to 46%. On the other hand, incorporating only SAP proved to be more effective, leading to a higher reduction of 90%. However, this combined effect is reported to be counteractive and dependent on the content of each component. Moreover, the hybrid system of SAP and SRA demonstrates a pronounced retardation effect on AAS hydration. The XRD patterns showed no new reaction products were formed. As revealed by SEM analysis, the microstructure of AAS containing SAP and SRA exhibited a distribution of microvoids ranging from 5 to 50 μm. This can explain the reduction of the compressive strength by incorporation a high content of SAP combined with SRA. However, incorporating a lower dosage of SAP is advantageous when combined with SRA in AAS systems. Finally, the integration of SAP and SRA had a positive influence on the properties of AAS mortars, thus compensating for the limitations of each constituent.

Effect of alternative silicon sources on alkali-activated carbon steel slag binder and silicon sources substitution mechanism

The high cost and carbon footprint of conventional alkali activator (sodium silicate) limit the wide-scale adaptation of alkali-activated materials. During the production of sodium silicate(SS), carbonate and silicate need to be calcined at 1400 °C, which is not environmentally friendly. In order to improve the sustainability of the activator, 10% SS was reduced in this paper. The effect alternative silicon sources (ASS) on the mechanical properties of alkali-activated carbon steel slag (CSS) prepared at room temperature were studied when the modulus is 0.5 and 1.0, and the mechanism of silicon source substitution was discussed by means of microscopic characterization. The substitution amount of ASS was 10% of the total CSS amount. ASS included waste glass powder (WGP), rice husk ash (RHA), diatomaceous earth (DE), diatomaceous earth analytical reagent (DEAR), and silica fume (SF). Microscopic characterization methods included X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetry and differential thermogravimetry (TG-DTG), and scanning electron microscope-energy dispersive spectrometer (SEM-EDS). The results show that the effect of ASS depended on the structure of SiQn, and is affected by the initial modulus of the activator. The compressive strengths of the low modulus samples are lower than that of the high modulus samples, while the effect of silicon source substitution is the opposite. However, the effect of SF is less affected by the initial modulus, and the 90d compressive strength of low modulus and high modulus differed by only 8%. 10% SF substitution can save at least 48.28% of industrial SS. SF is an economic and effective ASS, which can provide a better ecological choice for the production of alkali-activated CSS cementitious materials.

Effects of alkali-activated slag binder and shape-stabilized phase change material on thermal and mechanical characteristics and environmental impact of cementitious composite for building envelopes

A tremendous quantity of energy has been used in the building and a huge amount of CO2 emitted to maintain comfortable indoor temperatures. In order to lower existing level of CO2 emissions, using eco-friendly alternative binders instead of ordinary Portland cement (OPC) and improving buildings’ energy efficiency are indispensable. The thermal characteristics of cementitious composite have been improved by the use of phase change materials (PCMs) via its latent heat capacity. PCMs can also be incorporated with alkali-activated binding materials of industrial by-products such as alkali-activated slag (AAS), which can result in an efficient carbon reduction compared to the use of ordinary Portland cement (OPC) as a binder. To better comprehend the practical feasibility of AAS in PCM-incorporated composites as an alternative building component, it is necessary to study the one-part AAS system holistically by assessing its fundamental material characteristics, environmental impact (CO2 emission), and energy efficiency. This work uses an experimental-computational method and a case study to evaluate the effects of one-part AAS binder and shape-stabilized PCMs on thermal and mechanical properties and environmental impact. The experimental results demonstrate that AAS binder can reduce thermal conductivity by up to approximately 20% compared to the OPC case. Incorporating thermal energy storage aggregate (TESA) in the AAS binder can further decrease the thermal conductivity. In addition, the AAS-based composite can significantly reduce CO2 emissions by more than 10% than the OPC-based case due to the improved thermal properties and the consequent reduction in carbon footprint from raw materials and annual energy consumption. The results of this study infer the feasibility of AAS binder and TESA as eco-friendly, energy-saving cementitious composites for building envelopes.

Effects of Borax, Sucrose, and Citric Acid on the Setting Time and Mechanical Properties of Alkali-Activated Slag.

The setting time of alkali-activated slag (AAS) binders is extremely short, while traditional retarders of Portland cement may be invalid for AAS. To find an effective retarder with a less negative impact on strength, borax (B), sucrose (S), and citric acid (CA) were selected as potential retarders. The setting time of AAS with different admixtures dosages of 0%, 2%, 4%, 6%, and 8%, and the unconfined compressive strength and beam flexural strength of 3 d, 7 d, and 28 d AAS mortar specimens were tested. The microstructure of AAS with different additives was observed by scanning using an electron microscope (SEM), and the hydration products were analyzed by energy dispersive spectroscopy (EDS), X-ray diffraction analysis (XRD), and thermogravimetric analysis (DT-TGA) to explain the retarding mechanism of AAS with different additives. The results showed that the incorporation of borax and citric acid could effectively prolong the setting time of AAS more than that of sucrose, and the retarding effect is more and more obvious with the increase in borax and citric acid dosages. However, sucrose and citric acid negatively influence AAS's unconfined compressive strength and flexural stress. The negative effect becomes more evident with the increase in sucrose and citric acid dosages. Borax is the most suitable retarder for AAS among the three selected additives. SEM-EDS analysis showed that the incorporation of borax does three things: produces gels, covers the surface of the slag, and slows down the hydration reaction rate.

Improving the chloride binding capacity of alkali activated slag by calcium and aluminum enriched minerals

In this paper, a modified alkali activated slag with improved chloride binding capacity is proposed by introducing calcium and aluminum enriched phases in the binding system. The Ca and Al phases are added in order to promote the formation of AFm type phases in alkali activated system and then transformed into Friedel's salt (Fs) when in contact with chloride. The effects of calcium and alumina type on phase composition, chloride binding behavior, mechanical property of modified alkali activated slag binders are discussed. The results show that the addition of Ca and Al enriched minerals effectively improve the chloride binding capacity of the binder up to 44.4%. The combination of calcium hydroxide and metakaolin shows the highest binding efficiency, followed by calcium nitrite and cement. Metakaolin exhibits better modification on chloride binding capacity than nano alumina. The incorporation of Ca and Al enriched minerals together successfully promotes the formation of AFm type phases (Mc and AFm-NO2) in alkali activated system, while the addition of metakaolin increases the reaction degree and the mean chain length of C-(A)-S-H gels, contributing to the chloride binding capacity. The mechanical properties of alkali activated slag are also improved by adding calcium hydroxide, cement, and alumina enriched minerals, while calcium nitrite exhibits a slight negative effect.

Effects of chloride salts on strength, hydration, and microstructure of cemented tailings backfill with one-part alkali-activated slag

For mines located in saline areas, the use of mine water containing chloride salts to prepare cemented tailings backfill (CTB) for mined-out area treatment can realize the resource utilization of mine water and save a large amount of precious freshwater. However, the hydration behavior of binder and strength properties of CTB in the presence of chloride salts is still not well understood. This research investigates the effects of NaCl, CaCl2 and MgCl2 with different Cl- concentrations (0.50, 0.77 and 1.0 mol/L) on strength of CTB prepared with one-part alkali-activated slag binder. It analyzes the hydration process of binder and the microstructure of CTB. The results indicated that the decalcification of C-A-S-H (replaced by sodium to create N(C)-A-S-H at high Na+ concentrations) leads to numerous 0.01–0.1 μm micropores within the hardened backfill, increasing porosity while decreasing strength by 3.2–25.0%. Calcium ion (Ca2+) contributes to the formation of dense internal structures and high-strength backfill. Magnesium ion (Mg2+) reduces alkalinity by consuming Ca(OH)2, seriously impacts slag decomposition, but accelerates the dissolution of desulfurized gypsum. The study’s findings will help the application of chloride-containing mixing water in mines situated in saline regions.

Effect of polymer latex powder on shrinkage behaviors and microstructure of alkali-activated slag binder

Effect of polymer latex powder on shrinkage behaviors and microstructure of alkali-activated slag binder

Effects of biochar-amended alkali-activated slag on the stabilization of coral sand in coastal areas

Coral sand is widely encountered in coastal areas of tropical and subtropical regions. Compared with silica sand, it usually exhibits weaker performance from the perspective of engineering geology. To improve the geomechanical performance of coral sand and meet the requirement of foundation construction in coastal areas, a novel alkali activation-based sustainable binder was developed. The alkali-activated slag (AAS) binder material was composed of ground granulated blast-furnace slag (GGBS) and hydrated lime with the amendment of biochar, an agricultural waste-derived material. The biochar-amended AAS stabilized coral sand was subjected to a series of laboratory tests to determine its mechanical, physicochemical, and microstructural characteristics. Results show that adding a moderate amount of biochar in AAS could improve soil strength, elastic modulus, and water holding capacity by up to 20%, 70%, and 30%, respectively. Moreover, the addition of biochar in AAS had a marginal effect on the sulfate resistance of the stabilized sand, especially at high biochar content. However, the resistance of the AAS stabilized sand to wet-dry cycles slightly deteriorated with the addition of biochar. Based on these observations, a conceptual model showing biochar-AAS-sand interactions was proposed, in which biochar served as an internal curing agent, micro-reinforcer, and mechanically weak point. • AAS amended with biochar could be utilized for coral sand stabilization. • The mechanical performance of stabilized sand was improved. • High biochar content resulted in superior water holding capacity of stabilized sand. • A conceptual model illustrating biochar-AAS-sand interactions was proposed.

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

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

Understanding the shrinkage behaviour of alkali-activated slag binders modified by the superabsorbent polymer

Superabsorbent polymers (SAPs) have gained attention as internal curing agents in alkali-activated slags (AAS). The absorption–desorption kinetics of SAPs in AAS pastes were monitored using X-ray computed tomography. This study examines the influence of Angular covalently crosslinked sodium polyacrylate SAPs on AAS's reaction kinetics, pore structure, and microstructure f AAS. Meanwhile, autogenous, total drying shrinkage and continuous monitoring of internal relative humidity changes were performed. The results showed that the SAPs’ desorption kinetics is crucial in influencing the efficiency to counteract the shrinkage. SAPs significantly mitigated the autogenous and total drying shrinkage up to 90% and 30%, respectively.The incorporation of the ideal content of SAPs significantly compensated for the impact of the w/b ratio AAS properties. The optimum SAPs dosage was found to be 0.6 %, which enhanced the ability to control IC water, yielding better performance in terms of shrinkage, assisting the hydration degree, and enhancing the surface quality and anti-permeability. No reaction products were detected inside the voids of SAP, and the agglomeration of SAP was observed. However, care should be taken when incorporating a high content of SAP (i.e. > 0.6 %), which can adversely affect the properties of AAS. This work confirms that IC of AAS using SAPs is a powerful mitigating strategy, particularly when traditional curing is insufficient.

Low Carbon Alkali-Activated Slag Binder and Its Interaction with Polycarboxylate Superplasticizer: Importance of Microstructural Design of the PCEs

Low Carbon Alkali-Activated Slag Binder and Its Interaction with Polycarboxylate Superplasticizer: Importance of Microstructural Design of the PCEs

Recent progress in understanding setting and hardening of alkali-activated slag (AAS) materials

Alkali-activated slag (AAS with type of slag being blast-furnace slag) binders possesses substantially different hydration mechanisms from Portland cement (PC) binders, leading to distinct setting and hardening behavior. Due to the highly amorphous nature of both of raw materials and hydration products, the characterization of hydrate assemblages and microstructural development of AAS materials is challenging. In the past decades, there has been increasing investigation interests in disclosing the nature of aluminum and sodium containing calcium silicate hydrate gel with undefined stoichiometry (C-(N-)A-S-H) along with the existence of intimately intermixed secondary phases. This paper provides an overview on the recent research progress and understanding of the complex alkali-activation reaction process from the perspectives of hydration kinetics, products and microstructure development. This is critical to the interpretation and control of setting and hardening of AAS materials at the early age. In addition, based on available information, hydration mechanisms of different AAS binders are proposed. For such a complex system, thermodynamic modelling and molecular dynamic modelling have shown their power to correlate the microstructure development and the macroscale behavior of AAS materials, which are discussed in this paper.

On the mechanisms of shrinkage reducing admixture in alkali activated slag binders

Shrinkage-reducing admixture (SRA) efficiency as a shrinkage mitigation technique has been proven for the alkali-activated slag (AAS) system. However, the controlling mechanism is a debatable subject. Moreover, it has been reported that SRA may retard strength development and slag hydration. Hence, this study explores the behaviour of SRA and its interactions with AAS's pore solution and systems. It showed that SRA affected AAS similarly to ordinary Portland cement (OPC), reducing pore solution's surface tension and maintaining a high internal relative humidity level. However, the critical micelle concentration of SRA (i.e. 9%) was identified beyond which it self-aggregated in the bulk solution, reducing its efficiency. Chemical analysis of the extracted AAS pore solution showed how the presence of SRA impacted the aqueous electrolyte AAS pore solution. Besides, SRA reduced the dissolution affinity of ions (i.e. the cation/anion concentrations), retarding the hydration reaction and extending the setting time. Nuclear magnetic resonance results confirmed that SRA addition lowered the polymerisation degree for C-A-S-H, thereby reducing shrinkage driven by C-A-S-H syneresis. The findings of this study highlighted the critical roles of AAS pore solution composition and the formation of hydration products in the presence of SRA on AAS shrinkage behaviour and the development of other properties.

One-part alkali-activated slag binder for cemented fine tailings backfill: proportion optimization and properties evaluation.

In this study, a one-part alkali-activated slag (AAS) composed of ground-granulated blast furnace slag, desulfurized gypsum, and hydrated lime is proposed as alternative to cement for the production of cemented fine tailings backfill (CFTB), which is an environmentally friendly binder consisting of 93.72 wt.% industrial solid waste. Results show that AAS with 67.83 wt.% slag, 25.92 wt.% desulfurized gypsum, and 6.25 wt.% hydrated lime yields the highest strength, which is 1.7-3.2 times that of ordinary Portland cement (OPC). Aside from calcium silicate hydrate gel, appreciable quantity of ettringite characterized by interlocking needles structure and high bound water is also produced during the AAS hydration process. In addition, the hydration heat of the AAS binder is 48% less than that of OPC. Moreover, CFTB made of AAS provides better workability than that of CFTB with OPC up to 20h. The findings of this study will contribute to the production of more cost-effective, durable, and environmental-friendly cemented fine tailings backfill.

Antimicrobial performance and biodeterioration mechanisms of alkali-activated slag

Eco-friendly antimicrobial alkali-activated slag (AAS) binders show great promise in newly designed and existing wastewater infrastructure threatened by microbial induced concrete corrosion (MICC). This work investigates the antimicrobial performance and biodegradation mechanisms of a new benzoate (BZ)-modified AAS binder. The bacterial activity, corrosion morphology, and material degradation mechanisms, of the novel BZ-modified antimicrobial AAS post-simulated MICC tests are analysed. The bacterial activity assessments (pH value, sulphate concentration, and live/dead staining) results demonstrate the remarkable antimicrobial efficacy of BZ-modified AAS in killing sulphur-oxidising bacteria A. thiooxidans and inhibiting biogenic acidification. The BZ-modified AAS demonstrates excellent microbial corrosion resistance with greatly suppressed production of deleterious gypsum and corrosion depth. The mechanisms of biodeterioration of antimicrobial AAS matrix are revealed, suggesting the nanoscale rupture of calcium-aluminosilicate-hydrate (C-A-S-H) in short chains and networking of aluminosilicate unites upon the removal of interlayer CaO polyhedral. The removal of Al-tetrahedra from bridging sites of C-A-S-H is more susceptible than that of Si-tetrahedra in AAS pastes under microbial corrosion. This study has demonstrated the potentials of novel antimicrobial AAS-based concrete as the next-generation construction materials for wastewater infrastructure.

Sustainable Alkali-Activated Slag Binders Based on Alternative Activators Sourced From Mineral Wool and Glass Waste

In the present study, four different locally available waste glass materials (bottle glass-BG, glass wool-GW, stone wool-SW and cathode-ray tube glass-CRTG) were treated with hot concentrated potassium hydroxide (KOH) in order to obtain alternative alkali activators (AAAs). We evaluated the suitability of the solutions obtained for use as AAAs in the production of AAMs. AAMs were prepared using electric arc furnace slag and selected AAAs with a higher content of dissolved Si. We evaluated the performance of the AAMs in comparison to that of slags activated with KOH or potassium-silicate (K-silicate). The compressive strength of the AAMs prepared with KOH-based AAAs were high when Si and Al were simultaneously abundant in the AAA (9.47 MPa when using the activator sourced from the CRTG), and low with the addition of KOH alone (1.97 MPa). The AAM produced using commercial K-silicate yielded the highest compressive strength (27.7 MPa). The porosity of the KOH-based AAM was lowest when an alternative BG-based activator was used (24.1%), when it was similar to that of the AAM prepared with a K-silicate. The BG-based activator had the highest silicon content (33.1 g/L), and NMR revealed that Si was present in the form of Q0, Q1 and Q2. The concentrations of toxic trace elements in the AAAs used for alkali activation of the slag were also determined, and leaching experiments were performed on the AAMs to evaluate the immobilisation potential of alkali-activated slag. In the SW AAAs the results show acceptable concentrations of trace and minor elements with respect to the regulations on waste disposal sites, while in the activators prepared from BG, CRTG and GW some elements exceeded the allowable limits (Pb, Ba, Sb, and As).