Alkali-activated Slag Pastes Research Articles

Mechanisms of efflorescence of alkali-activated slag

Efflorescence presents not only as a cosmetic concern but also as a structural issue, which impacts the performance of alkali-activated materials (AAMs). In this study, the mechanisms of efflorescence of alkali-activated slag (AAS) pastes are investigated. First, the efflorescence of AAS pastes with different alkali dosages (3 %, 5 % and 7 %), activator types (sodium hydroxide (NH) and sodium silicate (NS)), exposure atmospheres (ambient, N2 and 0.2 vol% CO2), and relative humidities (40 %, 60 % and 80 %) was observed. Subsequently, leaching tests were performed and the impacts of efflorescence on AAS pastes at different heights were studied. It was found that a lower relative humidity facilitated more rapid and severe efflorescence. The positioning of efflorescence products was dependent on the porosity of the matrix. Compared to NH pastes, NS pastes subjected to semi-contact water conditions were more vulnerable to cracking problems, which turned out to be exacerbated by the formation of efflorescence products. A new method to quantify efflorescence was developed and it corresponded well with both efflorescence observations and leaching experiments. Furthermore, a competitive reaction between Ca and Na in the presence of carbonate ions was identified. CaCO3, a representative product of natural carbonation, was rarely found in the regions where efflorescence products (sodium carbonate) formed. Regarding compressive strength, NS pastes were more adversely affected by efflorescence than NH pastes.

The Effects of Ester and Ether Polycarboxylate Superplasticizers on the Fluidity and Setting Behavior of Alkali-Activated Slag Paste.

This study aims to investigate the comparative performance of ester- and ether-based polycarboxylate superplasticizers in maintaining the fluidity and controlling the setting time of alkali-activated slag (AAS) paste. The experiments employed rheological tests, mini-slump tests, ultrasonic pulse velocity (UPV) measurements, and gel permeation chromatography (GPC) analysis. The results indicate that ether-based superplasticizers maintain fluidity approximately 25% longer than their ester-based counterparts and extend the setting time by about 30%. The enhanced performance of ether-based superplasticizers is attributed to their superior molecular stability in highly alkaline environments, which mitigates early polymer degradation. Additionally, the Na2O/SiO2 ratio was maintained at 1:1 throughout the experiments to ensure consistency in the activation process. The relationship between fluidity loss and the onset of setting occurs more rapidly in AAS paste than in conventional cement-based systems. These findings provide valuable insights for the development of environmentally friendly construction materials by optimizing the use of superplasticizers in alkali-activated systems.

Correction: Comparison of thermogravimetry response of alkali-activated slag and Portland cement pastes after stopping their hydration using solvent exchange method

Correction: Comparison of thermogravimetry response of alkali-activated slag and Portland cement pastes after stopping their hydration using solvent exchange method

Comparison of thermogravimetry response of alkali-activated slag and Portland cement pastes after stopping their hydration using solvent exchange method

AbstractThe critical step for any subsequent instrumental analysis of cementitious binders is to stop their hydration reactions, i.e., to remove free water. One of the most available techniques is a solvent exchange method. However, the solvents are known to be strongly bound in ordinary Portland cement (OPC) paste and alter the results of thermogravimetric analysis (TGA) and sensitive hydrates, while their effect on TGA response of alkali-activated slag (AAS) has not been comprehensively investigated. Therefore, the objective of this paper is to track the effects of fundamental aspects of the solvent exchange on the TGA response of AAS with different sodium activators (hydroxide, carbonate, waterglass) and to support these results by X-ray diffraction and effluent gas analysis. All solvents used (acetone, diethyl ether, isopropyl alcohol, ethanol, and methanol) affected the TGA response of all tested pastes, and their effect was enhanced by prolonged immersion time. All solvents induced an additional mass loss at around 800 °C and, especially for OPC paste, increased in situ carbonation, even in an inert atmosphere. Methanol and ethanol had a detrimental effect on ettringite and decreased the basal distance of the C–(A)–S–H gel, while they only marginally affected gaylussite. For AAS, hydration stoppage by washing out the alkali-rich pore solution with water was also investigated and can usually be recommended (except for its detrimental effect on gaylussite), as it is more efficient than organic solvents, which lack solubility for activators. Methanol and ethanol are the most suitable alternatives, particularly for NaOH.

Properties of one-part CaO–Na2CO3 activated ground granulated blast furnace slag

In this study, the fresh and hardened performance and durability of one-part ground granulated blast furnace slags (GGBFS) activated by solid CaO and Na2CO3 were discussed, and their hydration process and microstructure development were analyzed accordingly. Results showed that when the water-to-binder ratio was 0.32, the alkali-activated slag (AAS) paste exhibited lower flowability, higher yield stress and plastic viscosity. As the content of CaO and Na2CO3 increased, the yield stress, plastic viscosity, and compressive strength of AAS after hardening all increased. For the durability of the hardened sample, AAS with high content of activator have higher shrinkage strain and chloride ion flux. The microscopic analysis results indicated that AAS containing CaO and Na2CO3 exhibited more intense hydration heat release and rapid microstructure development in the early stage, thereby promoting the improvement of strength. The use of CaO and Na2CO3 to activate GGBFS has enormous application potential in solid waste utilization and carbon emission reduction, which can reasonably replace traditional Portland cement (PC) as a new generation of sustainable cementitious materials.

Compressive Strength and Microstructure of Carbide Slag and Alkali-Activated Blast Furnace Slag Pastes in China

The alkali-activated blast furnace slag is attracting significant attention in replacing Portland cement due to several characteristics similar to cement hydration. However, there are a few practical problems with commercial alkali activators, such as the fast setting time, relatively high costs, and significant CO2 emissions during preparation. Thus, discovering industrial residues possessing inherent alkalinity are urgent. This study proposes the use of carbide slag at levels of 0%, 5%, 10%, 15%, 20%, and 30% and alkali at levels of 1%, 2%, 3%, 4%, 5%, 6%, 8%, and 10% activated blast furnace slag. The compressive strength and microstructure of carbide slag and alkali-activated blast furnace slag (CAB) pastes were examined using X-ray diffraction analysis (XRD), Differential Scanning Calorimetry/Thermogravimetric Analysis (DSC/TG), Fourier transform infrared spectroscopy (FTIR) and Scanning electron microscopy (SEM). The results revealed that the addition of carbide slag produced more hydrotalcite-like phase as well as decreased the content of ettringite (AFt) and the calcium–silicate–hydrate (C-S-H) gel, which decreased the compressive strength of the CAB pastes. At the age of 28 days, when the dosage was 5%, 10%, 15%, 20%, and 30%, the compressive strength of CAB mixes decreased by 2.1%, 7.1%, 9.2%, 9.8%, and 28.1%, respectively. The addition of NaOH promoted the formation of AFt, and there was an optimum level of NaOH corresponding to the high compressive strength of paste. At the age of 3 days and 7 days, the compressive strength reached its maximum at the dosage of 6% NaOH, which was 24.8 MPa and 36.3 MPa, respectively. However, at the ages of 14 days and 28 days, the compressive strength increased as the dosage of NaOH increased to 5%, which was 43.3 MPa and 44.5 MPa, respectively. The water curing could both enhance the early and later strength, the compressive strength of 23.3 MPa was gained at 3 days, and this increased by 16.3%, 24.0% and 36.9% at 7 days, 14 days and 28 days, respectively. Therefore, water curing was suitable for the strength development of CAB pastes.

Influence of raw material properties on microscopic and mechanical characteristics of alkali-activated materials

Alkali-activated fly ash and slag pastes are considered a new low-carbon cementitious material with good mechanical characteristics and durability. However, the mechanical characteristics of the alkali-activated materials are uncertain due to the multiplicity of the raw material properties. Few studies clarify how the raw material properties affect the mechanical characteristics of hardened pastes. The correlation between the raw material properties and the reaction rate, product composition, gel content, and pore size distribution is discussed in this study. The increase in the CaO content of the precursor will accelerate the degree of alkali activation. In this case, the reaction production becomes relatively more, and the pore size distribution becomes relatively denser, improving the compressive strength of the pastes. Moreover, the effects of the activator modulus and alkali content on the compressive strength of the pastes are not monotonically increasing. The excessive alkali will cause the surface of raw material particles to form a gel layer quickly, which could prevent the further alkali activation of the particles. This case is not conducive to gel formation and will cause the pore structure of the hardening pastes to be relatively loose. Furthermore, the influence of activator modulus on the compressive strength of pastes is lower than that of alkali content. Excess activator modulus is also unfavourable for pore structure densification.

Effect of Steel Slag on Hydration Kinetics and Rheological Properties of Alkali-Activated Slag Materials: A Comparative Study with Fly Ash.

The effects of steel slag (SS) and fly ash (FA) on hydration heat, fluidity, setting time and rheological properties of alkali-activated slag (AAS) pastes with different silicate modulus (Ms) values were comparatively investigated. The results show that the incorporation of SS shortens the induction period, increases the cumulative hydration heat, improves the initial fluidity and decreases the setting time at low Ms, but the opposite trend is found at high Ms. FA significantly retards the reaction, reduces the hydration heat, increases the fluidity and prolongs the setting time. The addition of SS or FA reduces the yield stress and plastic viscosity of AAS paste. SS improves the rheological properties of AAS paste more significantly than that of FA at high Ms. The yield stress and plastic viscosity of AAS paste with SS or FA rise with the increasing Ms and decline with the increasing water/binder (w/b) ratio.

Performance evolution of alkali-activated phosphorus slag paste filling material: Effect of hemihydrate phosphogypsum content

The stockpiling issues of phosphorus slag (PS) and hemihydrate phosphogypsum (HPG) have become vital technical obstacles limiting the economic development of enterprises. The synergistic substitution of PS and HPG for ordinary Portland cement in preparing alkali-activated phosphorus slag paste filling material (AAPS-PM) is essential to realize the high-value utilization of PS and HPG and reduce carbon emissions. HPG was selected as a sulfate activator for this research due to its metastable qualities and chemical composition benefits. The effect of HPG on the properties of AAPS-PM was systematically investigated by setting time, fluidity, strength development, water resistance, and pore solution pH experiments. The evolution of hydration products and pore size characteristics was elaborated employing X-ray diffractometer (XRD), scanning electron microscopy (SEM) and low-field nuclear magnetic resonance (NMR). The results demonstrated that the addition of HPG displayed remarkable procoagulant thickening characteristics on AAPS-PM. Additionally, HPG was conducive to developing early uniaxial compressive strength (UCS), whereas 90% content of HPG had a negative impact on the enhancement of later UCS. The softening coefficient and water absorption exhibited duality with 80% HPG content as the critical point, obtaining a maximum value of 0.73 and a minimum value of 6.63%, respectively. It is noteworthy that the decrease in pore solution pH was primarily correlated with the acidity carried by HPG. Herein, the alkali activator accelerated the depolymerization of PS vitreous and generated H3SiO4- and H3AlO42- and Al(OH)63-. Furthermore, the active ions combined with Ca2+ and SO42- supplied by HPG, which contributed to the formation of dihydrate gypsum, ettringite and C-(A)-S-H gels. Encouragingly, microscopic analysis confirmed the macroscopic performance changes of AAPS-PM. The gel network created by the interweaving of hydration products refined the pore structure and improved the microstructure morphology, which is the primary source of mechanical strength. Nevertheless, the high content of HPG provided a driving force for the development of capillary pores, resulting in a rapid reduction in later strength. Overall, this research can contribute new ideas to the large-scale integrated disposal of solid waste.

Properties and Microstructure of Alkali-Activated Slag Paste Modified by Superabsorbent Polymers

Properties and Microstructure of Alkali-Activated Slag Paste Modified by Superabsorbent Polymers

Chloride binding by layered double hydroxides (LDH/AFm phases) and alkali-activated slag pastes: an experimental study by RILEM TC 283-CAM

Chloride binding by the hydrate phases of cementitious materials influences the rate of chloride ingress into these materials and, thus, the time at which chloride reaches the steel reinforcement in concrete structures. Chloride binding isotherms of individual hydrate phases would be required to model chloride ingress but are only scarcely available and partly conflicting. The present study by RILEM TC 283-CAM ‘Chloride transport in alkali-activated materials’ significantly extends the available database and resolves some of the apparent contradictions by determining the chloride binding isotherms of layered double hydroxides (LDH), including AFm phases (monosulfate, strätlingite, hydrotalcite, and meixnerite), and of alkali-activated slags (AAS) produced with four different activators (Na2SiO3, Na2O·1.87SiO2, Na2CO3, and Na2SO4), in NaOH/NaCl solutions at various liquid/solid ratios. Selected solids after chloride binding were analysed by X-ray diffraction, and thermodynamic modelling was applied to simulate the phase changes occurring during chloride binding by the AFm phases. The results of the present study show that the chloride binding isotherms of LDH/AFm phases depend strongly on the liquid/solid ratio during the experiments. This is attributed to kinetic restrictions, which are, however, currently poorly understood. Chloride binding by AAS pastes is only moderately influenced by the employed activator. A steep increase of the chloride binding by AAS occurs at free chloride concentrations above approx. 1.0 M, which is possibly related to chloride binding by the C–(N–)A–S–H gel in the AAS.

Monitoring the internal structure behaviour of alkali-activated slag paste: effect of the curing mode

This paper deals with the monitoring of the internal structure behaviour of an alkaliactivated slag (AAS) paste. The slag was activated with a 4M solution of sodium hydroxide. The behaviour of the internal structure of the paste was regularly monitored through the changes in the resonant frequency and the mechanical properties, until the paste reached the age of 90 days. The main aim of the article is to show the long-term maturation and degradation process of an AAS paste under different curing modes. The results obtained suggest that the curing mode of the specimens has a significant effect on the behaviour of the internal structure of the paste based on the AAS. The development of both the dynamic properties and the flexural strength indicates the occurrence of a higher porosity in the internal structure of the paste, especially when the free drying process is started earlier. Insufficient hydration of the binder system is also a likely cause of cracks. The reduction in the relative dynamic moduli values ranging from 50 % to 80 % was observed for drying specimens at the age of 90 days. What is very interesting is that the occurrence of cracks was not prevented even by intensive moist curing of the paste as, between the 21st and the 28th day of maturing, there was a significant decrease of about 20 % in the relative dynamic modulus of elasticity and also a 50 % reduction in the flexural strength.

Characterizing two types of zonation within slag rims of aged alkali-activated slag pastes through SEM and TEM

In this paper, the authors characterized two types of zonation within slag rims in aged alkali-activated slag (AAS) systems through SEM and TEM technology. These two elemental zonation were closely related to the pore structure of AAS pastes, thus providing strong implication for the transport- and durability-related performance of AAS systems. The first type of zonation occurred in the rims of AAS specimens under sealed curing. It was found that lath-like hydrotalcite-like phase accumulated near the boundary while a generally homogeneous and foil-like C-(N-)A-S-H gel phases precipitated in the following sub-zone. When slag rims were thick enough, a new Mg-rich region occurred. The second type of zonation was noticed in the carbonated AAS pastes. For this kind of distribution pattern, C-(N-)A-S-H gel phases were observed near the boundary. Following, the accumulation of Mg and Ca occurred alternatively. Additionally, transformation mechanism between these two types of zonation was also proposed.

Improving early-age performance of alkali-activated slag paste backfill with calcium salts at low temperature

The hydration of alkali-activated slag (AAS) is highly sensitive to the curing temperature, especially at early ages. In low temperature environment, cemented paste backfill made of AAS (AAS-CPB) exhibits notably reduced early-age uniaxial compressive strength (UCS), attributable to the inhibitory effect of low temperature on slag hydration. This study aims to improve the performance of AAS-CPB at low temperatures by incorporating chemical additives. The results shows that calcium salts, specifically CaCl2, CaSO4, and Ca(COOCH3)2, can increase the early-age UCS of AAS-CPB by up to 1050%, depending on the specific anion involved. Overall, CaCl2 provides the greatest enhancement in the UCS. However, the presence of these salts result in strength degradation at later ages. A pronounced exponential relationship is evident between UCS and ultrasonic pulse velocity. The change of conductivity and moisture content are valuable indicators of hydration process at low temperatures. C(N)-A-S-H, hydrotalcite and portlandite are the primary hydration products. Anions play a decisive role in the morphology, precipitation and quantity of C(N)-A-S-H. The slag hydration degree and UCS basically exhibit a consistent trend.

Hydration characteristics and microstructure development of sodium hydroxide slag pastes exposed under different curing environments

AbstractThe aim of this research is to investigate the effect of curing environments on strength development and microstructure changes of alkali activated slag pastes (AAS). Mixtures of AAS are prepared using 5% NaOH content at a water to binder ratio of 0.35. Samples are exposed to six different curing environments, including heat treatment in different temperatures (30, 60 and 90 °C) for 2, 7 and 28 days. Their properties, as heat of hydration, compressive strength as well as porosity and morphology have been investigated on both fresh and hardened pastes via XRD, NMR, MIP, TGA and ESEM/EDX. Heat treatment was found to be very effective in promoting high early strength, accelerating the strength development of AAS pastes. Samples cured at 30 °C showed the best results in terms of compressive strength and porosity development. The compressive strength of 28 days samples is slightly reduced as the curing temperature increased. There is no significant difference in the types of hydration products present in the AAS pastes cured at different temperatures for up to 28 days.

Superabsorbent polymer in alkali-activated slag paste: Swelling-deswelling behaviour and effects on surrounding matrix

Superabsorbent polymer (SAP) has been applied in alkali-activated slag (AAS) for internal curing, but its interaction mechanism remains unclear. This study employed SAP of a large size (∼2 mm) to achieve in-situ and continuous observation of SAP behaviors in AAS paste and their impacts on the reaction kinetics, reaction products, and microstructure of surrounding matrix for 90 days. Results showed that the behaviors of SAP in AAS changed over time: swelling (0–10 h), fast deswelling (10 h–48 h), and slow deswelling (48 h-90 d). SAP swelling in AAS paste is subjected to the additional physical restriction by surrounding matrix. SAP postponed the hydration of AAS but hardly affected the long-term reaction degree due to the ion release during deswelling. Some Ca ions were retained in SAP, altering the reaction products of AAS by forming N-A-S-H gels. The effects of SAP on AAS diminish over distance, ultimately forming an hollow, annular affected zone with a dense microstructure.

Comparison of stearic acid and oleic acid for shrinkage-mitigating of alkali-activated slag composites

The large drying shrinkage is one of the most serious factors limiting the practical application of alkali-activated slag concrete (AASC). In this study, based on the successful shrinkage reduction of waste cooking oil for alkali-activated slag paste (AASP), the influence of stearic acid (saturated fatty acids) and oleic acid (unsaturated fatty acids) on fresh properties (setting time and fluidity), physical characteristics (density and water absorption), mechanical strengths (flexural strength and compressive strength), and durability (drying shrinkage, self-shrinkage, and carbonation resistance) are systematically compared. The experimental results reveal that admixing stearic acid and oleic acid can effectively inhibit the drying shrinkage and self-shrinkage of the AASP. Specifically, adding 1 wt% stearic acid (oleic acid) can reduce the drying shrinkage of the AASP by 20% (25%) and self-shrinkage by 32% (55%), respectively. Remarkably, the combined usage of the two additives can maximize the performance of the AASP, such as obtaining satisfactory mechanical strengths, extending the initial setting time, and improving the carbonation resistance. The developed AASP can be regarded as a low-cost and environmentally-friendly product, which can promote practical applications of AASC.

On the role of alkali content on one-part alkali activated slag pastes produced with tri- blend solid activators

The paper investigates the effect of tri-blend solid alkaline activators, based on sodium metasilicate pentahydrate, potassium hydroxide and sodium carbonate, on the physico-chemical features, the fresh and hardened performances of activated slag pastes prepared with the one-part methodology. Specifically, the alkali to slag ratio (Ac) and the total amount of the selected activators were varied at a fixed water/binder ratio, molar silica modulus (Ms) and relative mass activators ratio. Microstructures and reaction products were analyzed by means of X-ray diffraction, thermogravimetry, scanning electron microscopy and FT-IR. Fresh and elasto-mechanical properties of pastes were also evaluated. Amorphous calcium carbonate (ACC) as well as metastable vaterite were found in low reacted systems, produced by the preferential early-age reaction between dissolved carbonate ions present in the pore solution and the calcium ions released by the partial dissolution of the slag and stabilized by a sufficient high concentration of magnesium in the pore solution, not consumed to the form hydrotalcite. Sodium carbonate failed in extending the pot life of the investigated systems. The optimal alkali content was found to be close to 0.075 as it determines the development of compact and dense microstructure (2025 kg/m3), the maximum elasto-mechanical properties (elastic modulus of 26.1 GPa and compressive strength of 102.8 MPa at 28 days) and it allows to produce flowable mixtures (200 mm).

Revealing the difference between creep behavior of hydration products of Portland cement and alkali-activated slag paste at early age

This study investigates the early-age creep behavior of various gels in Portland cement (PC) and alkali-activated slag (AAS) pastes. The objective is to reveal the correlation between the composition and creep behavior of hydration products in these two systems. The results indicate that the creep modulus of C-A-S-H in AAS is significantly lower than that of outer products (Op) and inner products (Ip) in PC paste, even though Op exhibits higher elastic modulus than C-A-S-H. The intermixed CH has a more pronounced effect on the creep modulus of Op compared to its elastic modulus, which should be mainly attributed to the pinning effect of CH during the deformation of Op during the creep process. The lack of enhancement by high stiffness crystal phases in the matrix of AAS leads to the relatively low creep modulus of C-A-S-H.

Role of limestone powder in alkali-activated slag paste with superabsorbent polymer

Superabsorbent polymer (SAP) has been commonly used to mitigate the shrinkage of alkali-activated slag (AAS) pastes through internal curing. However, the effectiveness of internal curing might be affected by the microstructure of the pastes. Therefore, the effects of incorporating limestone powder (LP) on the microstructure and shrinkage of AAS pastes containing SAP are studied. The results reveal that incorporating LP mitigates the autogenous shrinkage of the pastes with SAP. This is because the incorporation of LP reduces the fraction of mesopores in the pastes containing SAP, while adding SAP maintains the internal relative humidity and increases the size of minimum pores that can drive autogenous shrinkage. In contrast, adding LP tends to enhance the driving force of autogenous shrinkage in the pastes without SAP due to the conversion of mesopores to gel pores. Furthermore, incorporating LP simultaneously enhances the reaction and generates more macropores in the pastes, causing a negligible influence on their mechanical properties and an increase of drying shrinkage. Overall, incorporating LP promotes the internal curing effectiveness in decreasing the autogenous shrinkage of the pastes.