Alkali-activated Slag Systems Research Articles

Influence of ground granulated blast furnace slag on the early hydration and microstructure of alkali-activated converter steel slag binder

Alkali-activated materials are low-environmental-impact binders that can be obtained from the alkaline activation of industrial wastes. In this study, converter steel slag as the major raw material and ground granulated blast furnace slag (GBFS) as the modified material were activated by water glass with a modulus of 1.5 and a Na2O dosage of 4%. The hydration process, microstructure and compressive strength of alkali-activated composite materials were investigated. The results show that adding GBFS accelerates the initial dissolution of the particles, leading to higher first exothermic peaks. But adding GBFS decelerates the formation of hydration products, resulting in the delay of the second exothermic peaks. Adding GBFS has no significant effect on the cumulative hydration heat, Ca(OH)2 content and the type of gel in the alkali-activated steel slag systems. However, with increasing GBFS content, the Ca–Si ratio in the gel decreases, and the Al–Si ratio increases. Adding GBFS can refine the pore structure and produces more Si–O–Si bonds in gels, resulting in a significant increase in the compressive strength. The improvement effect of GBFS on the compressive strength is more obvious at a later stage than at an earlier stage.

Studying the effect of alkali dosage on microstructure development of alkali-activated slag pastes by electrical impedance spectroscopy (EIS)

In this study, electrical impedance spectroscopy (EIS) is applied to investigate the effect of alkali dosage on the microstructural evolution of the alkali-activated slag (AAS) system during the curing process. Pore solution chemistry and pore structure of the paste samples were also determined to get auxiliary interpretation of the EIS results. It is shown by equivalent circuit analysis that the resistance attributed to continuous (R1) and discontinuous pores (R2) increases with curing age due to pore structure development. The value of R1 is more affected by pore solution chemistry while the value of R2 is mainly affected by pore structure. With the increase of alkali dosage, the value of R1 decreases while R2 gradually increases. The exponent n of the constant phase element (CPE), which relates to the capillary pores (n1), decreases with the increase of alkali dosage while the exponent relating to gel pores (n2) is independent of the alkali dosage of AAS paste. The bulk conductivity of AAS paste obtained from the bulk resistance gradually increases with the alkali dosage, partially because of the enhanced conductivity of pore solution. The improvement of microstructure at early age is mainly attributed to the decreased porosity, which is more affected by the pore connectivity with the increase of curing age. These results confirm that EIS is an effective technique to determine the hydration properties and pore structure features of AAS paste.

Investigation into the molecular design and plasticizing effectiveness of HPEG-based polycarboxylate superplasticizers in alkali-activated slag

In this study, a series of HPEG PCE polymers with different anionic charge density and side chain lengths was synthesized via free-radical copolymerization from acrylic acid and α-methallyl poly (ethylene glycol) ether (HPEG) as macromonomer. The resulting PCE polymers were characterized via Size Exclusion Chromatography (SEC) and anionic charge titration. Thereafter, a correlation between their molecular structures and dispersing effectiveness was established for alkali-activated slag. A broad variety of HPEG PCE polymers was found to work in the alkali-activated slag (AAS) system. Finally, the working mechanism for best performing polymer was investigated via adsorption measurements. It was concluded that HPEG PCEs of high anionicity, high Mw and short side chain length adsorb particularity high amounts on slag, thus providing enhanced dispersion capacity as compared to other HPEG polymers.

Effects of Light-Burnt Dolomite Incorporation on the Setting, Strength, and Drying Shrinkage of One-Part Alkali-Activated Slag Cement.

This study investigates the potential of light-burnt dolomite (LBD) as a supplementary cementitious material with ground granulated blast furnace slag (GGBFS) and Ordinary Portland cement (OPC). In this work, LBD was substituted for up to 20% of GGBFS in sodium sulfate-activated slag systems. The effects of LBD incorporation on the flow, setting time, compressive and flexural strength development, and drying shrinkage were explored with, X-ray diffraction and thermogravimetric analyses. LBD incorporation resulted in greater strength development of an alkali-activated slag system. The optimum LBD content for strength development was 10%, regardless of ordinary Portland cement content. In addition, LBD decreased the drying shrinkage, accelerated the hydration process, and induced hydrotalcite formation, which can be attributed to the reactive MgO inside LBD.

Using in-situ observation to understand the leaching behavior of Portland cement and alkali-activated slag pastes

This paper presents the leaching behavior of hydration products and unhydrated particles in Portland cement (PC) and alkali-activated slag (AAS) systems with a new in-situ observation method. After leaching in NH4Cl solution, obvious decalcification can be found in unhydrated C3S and C2S in PC, while the unhydrated slag keeps stable in AAS. The Ca in the gel of AAS is more vulnerable to leaching than that in PC due to the lack of phases with buffering capability like portlandite. In addition, the Mg–Al layered double hydroxide, which mainly exists in the dark rim of AAS, is not stable in NH4Cl solution. The in-situ observation method proposed in this study provides a new methodology to investigate the leaching behavior of different phases directly and helps to understand the effect of leaching on cementitious materials from a new perspective, it can therefore be used to instruct the design of durable cementitious systems.

Incorporation of CFBC ash in sodium silicate-activated slag system: Modification of microstructures and its effect on shrinkage

Circulating fluidized bed combustion (CFBC) ash was applied in sodium silicate-activated slag system as a secondary activator and/or for the substitution of commercialized expanding admixtures. To understand the effects of this ash on such alkali-activated slag (AAS) system, a series of experiments relating to hydration kinetics, hydration products and microstructure, and engineering properties were performed. It was found that the introduction of the ash resulted in different types of hydration products in AAS, leading to increases in both the size and volume of the pores. Although the setting time was retarded, the autogenous and dry shrinkages of the AAS decreased by the increase in the ash content. The shrinkage and strength of AAS affected by the external moisture conditions during curing could be controlled by substituting the slag with the ash.

Understanding the roles of activators towards setting and hardening control of alkali-activated slag cement

Sodium silicate-activated slag often gives high early strength but sets too rapidly. Commercial retarders for Portland cement and partial replacement with other cementitious components in alkali-activated slag (AAS) system have been proven ineffective. This study investigated the approach of controlling setting by altering activator ion composition and silica polymer status. The roles of activators during the alkali-activation process were studied via pore solution chemistry analysis and microstructural analysis of hydration products. The addition of Na2CO3 does not, but NaOH does alter the polymerization of silicate ion groups in sodium silicate solution due to the increases in pH and Na2O concentration. The specific effects of Na2CO3 on the setting and hardening of AAS depend on the relative contents of NaOH and Na2O·2SiO2 in alkaline solution. The reaction degree of AAS is dependent on activator compositions, which govern the kinetics of formation and intrinsic characteristics of the reaction products calcium aluminosilicate hydrates. The setting and strength development of sodium silicate-activated slag can be controlled by manipulating the compositions of Na2CO3-NaOH-Na2O·2SiO2.

Effect of synthetic CaO-Al2O3-SiO2-H2O on the early-stage performance of alkali-activated slag

C(-A)-S-H (CaO(-Al2O3)-SiO2-H2O) gel is the main product and source of strength in alkali-activated slag systems. Previous studies on C-A-S-H have focused mostly on the characterization of the structure and composition of the products. In this study, synthetic C-A-S-H particles with a similar composition as gels produced by alkali-activated slag were prepared by a hydrothermal method and were added to the alkali-activated slag systems, and the synthetic can be regarded as a chemical intensifiers of hardening (CIH). The effects of the synthetic C-A-S-H on the hydration process, the early-stage mechanical properties, and the microstructure of the alkali-activated slag were investigated by hydration heat tests, scanning electron microscopy (SEM), mercury intrusion porosimetry tests (MIP), and other methods. The results showed that the synthetic C-A-S-H shortened the setting time, promoted the formation of an acceleration period at an early stage, and increased the hydration heat released during the acceleration period. The synthetic C-A-S-H improved the flexural and compressive strength of the alkali-activated slag to a certain extent and the improvements in the flexural strength were more significant than the improvements in the compressive strength; the flexural strength of the sample with the added 3% synthetic C-A-S-H with 3-d curing increased by 16.25%. Microstructure analysis indicated the synthetic C-A-S-H also reduced the porosity of the alkali-activated slag-hardened paste and optimized the pore size distribution. This study of the early-stage performance provides a new approach and predictions for the late-stage performance and in-depth modification mechanisms of alkali-activated slag systems.

Use of recycled aggregates as internal curing agent for alkali-activated slag system

This study aims to investigate the feasibility of recycled aggregates (RA) as an internal curing agent for an alkali activated slag (AAS) system. The physical and chemical characteristics of the aggregates were studied through various analytical techniques including X-ray diffraction, thermogravimetry, mercury intrusion porosimetry, and indirect alkalinity measurements. The compressive strength, degree of hydration, and autogenous and dry shrinkages of AAS generated with RA were measured and compared with those of AAS made using artificial lightweight aggregates. The use of RA can reduce the autogenous shrinkage of an AAS system without a decrease in compressive strength. The degree of hydration for AAS mortar was not increased by the addition of RA owing to the dilution effect of the alkali activator, which was caused by the additional water supplied from internal curing materials.

Influence of the combination of calcium oxide and sodium carbonate on the hydration reactivity of alkali-activated slag binders

Calcium oxide (CaO) and sodium carbonate (Na2CO3) are potential activators for ground granulated blast furnace slag, but they result in slow strength development, especially at the early age. To overcome this issue and explore the application of CaO/Na2CO3 compound activators in alkali-activated slag (AAS) systems, this paper investigated the mechanical and hydration properties of CaO/Na2CO3-activated slag binders by measuring the compressive strength, pH value of pore solution and non-evaporable water content up to 28 days. The hydration products and microstructure of hardened pastes were analyzed by X-ray diffraction (XRD), thermogravimetric analysis (TGA) and scanning electron microscopy techniques. It was found that the ratio of CaO to Na2CO3 played a vital role in the performance of the AAS binders. The combination of 2.5% CaO and 5% Na2CO3 significantly increased the quantities of hydration products and thus resulted in a much denser microstructure and higher compressive strength. This increment was attributed to the generation of sodium hydrate and low crystalline calcium carbonate by the reaction of CaO with Na2CO3, which was beneficial for the formation of hydration products. XRD and TGA results showed that the main hydration products were calcium silicate hydrate gels and hydrotalcite-like phases for all samples except the one with only Na2CO3. In addition, low crystalline calcium carbonate was detected in the samples with both CaO and Na2CO3.

Apply 29Si, 27Al MAS NMR and selective dissolution in identifying the reaction degree of alkali activated slag-fly ash composites

Abstract This paper characterizes the phase composition and reaction degree of alkali activated slag, fly ash and their blends by using 29 Si, 27 Al MAS NMR and selective dissolution; with special emphasis on the effect of activator modulus and the usage of blended binders. The results revealed that in alkali activated slag systems, the increase of the activator modulus leads to a reduction of slag reactivity, together with significantly increased Q3(1Al) and reduced Q2(1Al) groups. Selective dissolution contributes to distinguish the unreacted fly ash and reaction products; 29 Si, 27 Al MAS NMR analyses were carried out on both components and their compositions were quantified in detail. Both polymerized Q4 structures and Q3 to Q0 sites were formed, and increasing the activator modulus resulted in an increased content of Q4 groups. When a blended system was applied, the slag reactivity was reduced with the increased fly ash reaction degree, the blended binder exhibited higher contents of newly formed Q4 groups with reduced Si/Al, reduced Q0 sites, shifted mean chain length and Si/Al of the formed C-A-S-H type gels, indicating gel interaction. Applying these two testing methods together brings new insights in understanding the structural composition of alkali activated slag, fly ash and their composites.

Corrosion behavior of steel bars immersed in simulated pore solutions of alkali-activated slag mortar

To study the corrosion behavior of steel bars in alkali-activated slag (AAS) system, simulated pore solutions of AAS mortar were first applied in this work. The effects of the three major species in AAS pore solutions, namely sulfur-containing species, aluminate, and silicate, were investigated. Electrochemical impedance spectroscopy and potentiodynamic polarization results showed that the simulated pore solution of AAS mortar had a stronger capacity to passivate and protect steel bars than the simulated pore solution of Portland cement (PC) mortar, mainly due to the higher concentration of silicate in the former, as confirmed by surface analysis of SEM-EDXS.

Some Issues of Shrinkage-Reducing Admixtures Application in Alkali-Activated Slag Systems.

Significant drying shrinkage is one of the main limitations for the wider utilization of alkali-activated slag (AAS). Few previous works revealed that it is possible to reduce AAS drying shrinkage by the use of shrinkage-reducing admixtures (SRAs). However, these studies were mainly focused on SRA based on polypropylene glycol, while as it is shown in this paper, the behavior of SRA based on 2-methyl-2,4-pentanediol can be significantly different. While 0.25% and 0.50% had only a minor effect on the AAS properties, 1.0% of this SRA reduced the drying shrinkage of waterglass-activated slag mortar by more than 80%, but it greatly reduced early strengths simultaneously. This feature was further studied by isothermal calorimetry, mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). Calorimetric experiments showed that 1% of SRA modified the second peak of the pre-induction period and delayed the maximum of the main hydration peak by several days, which corresponds well with observed strength development as well as with the MIP and SEM results. These observations proved the certain incompatibility of SRA with the studied AAS system, because the drying shrinkage reduction was induced by the strong retardation of hydration, resulting in a coarsening of the pore structure rather than the proper function of the SRA.

Effect of the activator dose on the compressive strength and accelerated carbonation resistance of alkali silicate-activated slag/metakaolin blended materials

The effects of activator dose on the properties of alkali-activated slag/metakaolin blends, were studied in fresh and hardened states: heat evolution, strength and accelerated carbonation. High activator concentrations affect the slag dissolution rate, reducing compressive strength when this is the sole precursor. An increased activator concentration favours metakaolin reaction, promoting high strengths and reduced permeability. Metakaolin addition, and increased activator concentrations reduce the susceptibility to carbonation, associated with the refinement of the pore network under extended CO2 exposure. The effect of adding an aluminosilicate precursor to an alkali-activated slag system is strongly dependent on the activator concentration.

Intrinsic differences in atomic ordering of calcium (alumino)silicate hydrates in conventional and alkali-activated cements

The atomic structures of calcium silicate hydrate (C–S–H) and calcium (–sodium) aluminosilicate hydrate (C–(N)–A–S–H) gels, and their presence in conventional and blended cement systems, have been the topic of significant debate over recent decades. Previous investigations have revealed that synthetic C–S–H gel is nanocrystalline and due to the chemical similarities between ordinary Portland cement (OPC)-based systems and low-CO2 alkali-activated slags, researchers have inferred that the atomic ordering in alkali-activated slag is the same as in OPC–slag cements. Here, X-ray total scattering is used to determine the local bonding environment and nanostructure of C(–A)–S–H gels present in hydrated tricalcium silicate (C3S), blended C3S–slag and alkali-activated slag, revealing the large intrinsic differences in the extent of nanoscale ordering between C–S–H derived from C3S and alkali-activated slag systems, which may have a significant influence on thermodynamic stability, and material properties at higher length scales, including long term durability of alkali-activated cements.

Microstructural, Mechanical, and Durability Related Similarities in Concretes Based on OPC and Alkali-Activated Slag Binders

Alkali-activated slag concretes are being extensively researched because of its potential sustainability-related benefits. For such concretes to be implemented in large scale concrete applications such as infrastructural and building elements, it is essential to understand its early and long-term performance characteristics vis-à-vis conventional ordinary portland cement (OPC) based concretes. This paper presents a comprehensive study of the property and performance features including early-age isothermal calorimetric response, compressive strength development with time, microstructural features such as the pore volume and representative pore size, and accelerated chloride transport resistance of OPC and alkali-activated binder systems. Slag mixtures activated using sodium silicate solution (SiO2-to-Na2O ratio or Ms of 1–2) to provide a total alkalinity of 0.05 (Na2O-to-binder ratio) are compared with OPC mixtures with and without partial cement replacement with Class F fly ash (20 % by mass) or silica fume (6 % by mass). Major similarities are noted between these binder systems for: (1) calorimetric response with respect to the presence of features even though the locations and peaks vary based on Ms, (2) compressive strength and its development, (3) total porosity and pore size, and (4) rapid chloride permeability and non-steady state migration coefficients. Moreover, electrical impedance based circuit models are used to bring out the microstructural features (resistance of the connected pores, and capacitances of the solid phase and pore-solid interface) that are similar in conventional OPC and alkali-activated slag concretes. This study thus demonstrates that performance-equivalent alkali-activated slag systems that are more sustainable from energy and environmental standpoints can be proportioned.

Physico-chemical, mechanical, microstructure and durability characteristics of alkali activated Egyptian slag

The aim of the present work is to study the characteristics of alkali activated Egyptian slag (GBFS) mixed with different contents of Na2O and SiO2. The rate of the kinetic of activation of GBFS has been studied by electrical conductivity, FTIR, DTA/TGA, XRD and SEM techniques. Electrical conductivity of alkali activated slag systems depends mainly on the binder composition, the activator type and its concentration. The increase in alkali activator contents affected the location and intensities of the conductivity maximum. As the hydration time increases the amount of hydration products increase, hence the chemically combined water and combined slag contents increase. SEM images showed the presence of (N,C)ASH gel with low porosity. With the increase of the content of Na2O leads to form a denser closed microstructure, leading to higher compressive strength values. The activated GBFS showed good durability in 5% MgSO4 or 5% MgCl2 solution, i.e., the compressive strength increased gradually with immersing time up to 180days. The total chloride and total sulphate contents decrease with alkali activated GBFS due to the forming hydrated products that fill some available open pores, thereby inhibiting Cl− or SO42− ions penetration; this effect leads to a decreased accessibility of Cl− or SO42− ions towards the more dense with low capillary pore structure. It can be concluded that alkali activated GBFS are more durable in 5% MgSO4 or 5% MgCl2 than OPC pastes.

Comparative Analysis of the Influence of Sodium and Potassium Silicate Solutions on the Kinetics and Products of Slag Activation

Abstract This paper primarily explores the influence of the alkali cation (Na or K) on the reaction kinetics, product formation, gel structure, and mechanical properties of alkali activated slag systems. For the same activator Ms, i.e., molar SiO2–M2O ratio (M = Na or K), a shorter induction period, a larger acceleration peak, and consequently, a higher amount of total heat release under isothermal conditions is observed for the K-silicate activated slag pastes. The early-age compressive strengths in these systems roughly relate to the heat release response. The later-age (7 days and beyond) compressive strengths are observed to be higher for the Na-silicate activated systems, which is corroborated by: (1) higher amounts of C–(A)–S–H gel in this system indicated by a thermal analysis-based approximate quantification method, and (2) higher combined intensities of Q1 and Q2 structures that point to increased degrees of reaction, and lower amounts of unreacted slag obtained from 29Si magic-angle-spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. The NMR spectra also show evidences of Al-substituted C–S–H gel, with a higher amount of substitution when Na-silicates are used.

Gel nanostructure in alkali-activated binders based on slag and fly ash, and effects of accelerated carbonation

Binders formed through alkali-activation of slags and fly ashes, including ‘fly ash geopolymers’, provide appealing properties as binders for low-emissions concrete production. However, the changes in pH and pore solution chemistry induced during accelerated carbonation testing provide unrealistically low predictions of in-service carbonation resistance. The aluminosilicate gel remaining in an alkali-activated slag system after accelerated carbonation is highly polymerised, consistent with a decalcification mechanism, while fly ash-based binders mainly carbonate through precipitation of alkali salts (bicarbonates at elevated CO2 concentrations, or carbonates under natural exposure) from the pore solution, with little change in the binder gel identifiable by nuclear magnetic resonance spectroscopy. In activated fly ash/slag blends, two distinct gels (C–A–S–H and N–A–S–H) are formed; under accelerated carbonation, the N–A–S–H gel behaves comparably to fly ash-based systems, while the C–A–S–H gel is decalcified similarly to alkali-activated slag. This provides new scope for durability optimisation, and for developing appropriate testing methodologies.

Hydration of alkali-activated slag: thermodynamic modelling

The composition of solid and liquid phases of alkali-activated slag systems was investigated by thermodynamic modelling. The calculations predict the formation of C(–A)–S–H, stratlingite and hydrotalcite in different alkali-activated slag systems, which agrees well with experimental findings. In addition traces of FeS and ettringite are predicted to precipitate. The type of alkaline activator (Na-silicate, NaOH) has no influence on the type of hydrates formed but influences the Ca/Si ratio of the C–S–H. The addition of sodium silicate leads to the formation of C–S–H with a higher Ca/Si ratio (1.1–1.2), whereas NaOH results in a Ca/Si ratio of 0.8to1.1. In contrast, the kind and concentration of the activator has strong influence on the composition of the pore solution. Low concentrations of the alkaline activator result in lower Na, Si and Al concentrations and lower pH values. In all pore solutions a high concentration of sulphide (HS–) is expected to be present.