Alkali-activated Slag Concrete Research Articles

Probing the dual mechanism of load damage and flowing solution on sulfate attack resistance of alkali-activated slag concrete

Alkali-activated slag concrete (AASC) is a representative low-carbon green building material. To effectively characterize the sulfate attack resistance of AASC in complex environments, a self-designed simulation device for complex corrosion environments was used to study the sulfate attack mechanism and property deterioration law of AASC under the loads and flowing corrosion solutions. The results indicated that both load-induced damage and dissolution accelerated the transport of sulfate ions into the AASC. On the one hand, the load-induced damage and dissolution resulted in a decrease in the proportion of gel pores, reducing the electrostatic interaction between ions and the gel pore wall as well as the gel pore wall thinning effect. On the other hand, the load-induced damage and dissolution decreased the pH of the pore solutions in AASC, causing easier decalcification or dealumination of C-A-S-H gel under sulfate ion attack. This further accelerated the sulfate ion attack and deterioration of AASC.

Performance of self‐compacting alkali‐activated slag concrete‐filled cold‐formed steel tubular stub columns

AbstractThis study comprehensively investigates the self‐compacting alkali‐activated slag concrete‐filled cold‐formed steel tubes (SACFST) stub columns under axial compression. The innovative combination of alkali‐activated slag concrete (AASC) with concrete‐filled steel tubes (CFST) enhances both sustainability and structural performance. AASC, known for its eco‐friendly benefits compared to conventional concrete, was developed using industrial wastes, contributing to green construction practices. Based on Taguchi's L‐9 orthogonal array, nine mixes of self‐compacting alkali‐activated slag concrete (SASC) were developed, achieving impressive flowability exceeding 700 mm and compressive strengths ranging from 48 to 69 MPa. Additionally, split‐tensile strengths between 3.8 and 5.7 MPa, flexural strengths from 6.3 to 8.2 MPa, and a modulus of elasticity between 27.6 and 32.9 GPa were observed. Nine circular CFSTs containing SASC mixes were tested under axial compression, and a finite element model was developed to simulate their results. The experimental results were compared with standards from ACI 318, AIJ‐97, AISC‐360, AS 5100, and EC‐4, to evaluate the accuracy of axial capacity predictions. Among these standards, EC‐4 and AS 5100 provided the closest estimates, with mean PTest/PEC4 and PTest/P5100 ratios of 1.03 and 1.04, respectively, indicating a high level of accuracy. These findings highlight the potential of SACFST in advancing sustainable and resilient construction practices, laying the groundwork for future applications in structural engineering.

A Review on Research Progress of Corrosion Resistance of Alkali-Activated Slag Cement Concrete.

China is the largest producer and user of Ordinary Silicate Cement (OPC), and rapid infrastructure development requires more sustainable building materials for concrete structures. Portland cement emits large amounts of CO2 in production. Given proposals for "carbon peaking and carbon neutralization", it is extremely important to study alternative low-carbon cementitious materials to reduce emissions. Alkali-activated slag (AAS) cement, a new green cementitious material, has high application potential. The chemical corrosion resistance of AAS concrete is important for ensuring durability and prolonging service life. This paper reviews the hydration mechanism of AAS concrete and discusses the composition of hydration products on this basis, examines the corrosion mechanism of AAS concrete in acid, sulfate, and seawater environments, and reviews the impact of its performance due to the corrosion of AAS concrete in different solutions. Further in-depth understanding of its impact on the performance of concrete can provide an important theoretical basis for its use in different environments and provides an important theoretical basis for the application of AAS concrete, so that we can have a certain understanding of the durability of AAS concrete.

Study on mechanical properties and damage mechanism of alkali-activated slag concrete

The cement industry has become one of the important sources of greenhouse gas emissions, and the alkali-activated slag (AAS) has the same mechanical properties and lower carbon emissions as cement in the production process. Therefore, it can be used as an alternative binder for low-carbon buildings. The effect of alkali concentration (mass ratio of Na2O to binder) on the macro-mechanical properties and meso-damage mechanism of alkali-activated slag concrete (AASC) under uniaxial compression was investigated in this work. Six kinds of concrete with alkali concentration of 2 %, 4 %, 6 %, 8 %, 10 % and 12 % were prepared. The microscopic characteristics of AASC were characterized by nuclear magnetic resonance (NMR), scanning electron microscopy (SEM), x-ray diffraction (XRD) and x-ray energy dispersive spectroscopy (EDS); the meso-damage evolution mechanism of AASC was discussed by statistical damage theory. The results showed that the peak stress and elastic modulus of AASC specimens increased, and the microstructure became denser with the increase of alkali concentration. However, the mechanical properties of the specimen deteriorated to some extent when the alkali concentration exceeded 6 %; for example, the peak stress decreased by 24.00 MPa when the alkali concentration was increased from 6 % to 12 % at the age of 28d. Based on the statistical damage theory, the effective stress concept was introduced to quantitatively analyze the mesoscopic damage evolution of AASC. Both fracture and yielding damage modes were considered. By exploring the regular changes of the mesoscopic parameters, the connection between the macroscopic mechanical properties and the mesoscopic damage mechanism was established. Based on the above mechanical properties and mesoscopic damage results, the alkali concentration of 6 % can be used as the best mixture proportions of AASC.

Study on the Effect of Water–Binder Ratio on the Carbonation Resistance of Raw Sea Sand Alkali-Activated Slag Concrete and the Distribution of Chloride Ions after Carbonation

The excessive extraction of river sand has led to significant ecological issues. Moreover, the environmental impact and resource demand of cement production have increasingly turned the spotlight on sea sand as a viable alternative due to its abundance and ease of extraction. Concurrently, alkali-activated binders, a novel type of low-carbon cementitious material, have gained attention for their low energy consumption, high durability, and effective chloride ion fixation capabilities. However, they are susceptible to carbonation. Introducing a controlled sea sand amount can raise the materials’ carbonation resistance, although carbonation may raise the concentration of free Cl− within the structure to levels that could risk the integrity of steel reinforcements by accelerating corrosion. In this context, the current study investigates sea sand alkali-activated slag (SSAS) concrete prepared with varying water–binder (W/B) ratios to evaluate its impact on flowability, mechanical strength, performances, and chloride ion distribution post-carbonation. The results demonstrate that the mechanical property of SSAS concrete diminishes as the water-to-binder ratio increases, with a more pronounced reduction observed. The depth of carbonation in mortar specimens also rises with the W/B ratio, whereas the compressive strength post-carbonation initially decreases before showing an increase as carbonation progresses. Furthermore, carbonation redistributes chloride ions in SSAS, leading to a peak Cl− concentration near the carbonation front. However, this peak amplitude does not show a clear correlation with changes in the W/B ratio. This study provides a theoretical foundation for employing sea sand and alkali-activated concrete.

Micromechanical and chemical characteristics of interfacial transition zone in alkali-activated slag concrete containing lightweight iron-rich aggregates

This study explored the time-varying alkali-activation effects of the lightweight aggregates (LWA) on the interfacial transition zone (ITZ) of the alkali-activated slag concrete with different curing ages, molar ratios, and alkali concentrations from 28 to 300 d. The improved micromechanical performance of the ITZ at 28 d stemmed from the early-age internal curing effect of the porous LWA. However, the post-28 d internal curing effect diminished, with the alkali-activation dominating the performance of the ITZ from 28 to 300 d. Although alkali-activated cement possessed early strength characteristics, the deconvolved phase results illustrated that alkali activation persisted in the ITZ between the LWA and cement from 28 to 300 d, leading to a high proportion of calcium silica aluminate hydrate. As a marker, Fe from the lightweight iron-rich aggregates mitigated into the alkali-activated cement, demonstrating the alkali-activated reaction between the LWA and alkali-activated slag matrix. Molecular dynamics simulations further characterized that the low content of Fe did not decrease the overall strength of the ITZ. The interfacial interaction energy revealed the alkali-activation between the LWA and cement compensated for the weakening of mechanical properties caused by Fe ions on the ITZ.

Effect of binder and activator composition on the characteristics of alkali-activated slag-based concrete

Alkali Activated Slag Concrete (AASC) has been a sustained research activity over the past two decades. Its promising characteristics and being environmentally friendly compared to Ordinary Portland Cement made AASC of exceptional interest. However, there is still no firm mix design, for the AASC, that can provide desirable fresh and hardened properties based on the composition of the binder and activator. This research specifically aims to investigate the affecting parameters on the slump and compressive strength of alkali-activated slag/lime-based concrete and provide a better understanding of the potential reasons for these characteristics. The experimental program consisted of two stages; the first stage studied the effect of different binder and activator compositions, and the second stage studied the water-to-binder ratio and binder content effects on the slump and compressive strength of alkali-activated slag/lime-based concrete. The binder and activator compositions were defined through two main parameters, the hybrid factor (HF = CaO/Si2O + Al2O3) and the solution modulus (Ms = SiO2/Na2O). The compressive strength, initial slump, and slump loss were measured to evaluate the different mixes and specify the optimum range of compositions. Based on the studied parameters, the effective range to achieve desirable slump and concrete compressive strength is from HF 0.6 up to 0.8 at Ms 1.5, this would achieve a compressive strength of more than 30 MPa and a slump of 100 mm after 90 min.

Chloride ingress analysis on alkali-activated slag concrete: A validated modelling framework for long-term assessment

Amidst global commitment to reduce CO2 emissions, the forthcoming decades expect to witness the prevalence of alkali-activated materials. As a predominant trigger for reinforcement corrosion, the accurate prediction of chloride ingress is paramount for the widespread application of the emerging building material. In this regard, this paper presents a novel framework tailored for analysing long-term chloride penetration into alkali-activated slag (AAS) concrete. An innovative technique for modelling chloride binding is developed and integrated into the framework. The technique can spontaneously assess time-dependent chloride binding based on the detailed phase assemblage, thereby rendering the framework applicability for the long-term analysis. The effectiveness of this model is rigorously validated against experiments. Furthermore, a series of virtual experiments are conducted by using the verified model to investigate the impact of different activators, where the distinction of resulting AAS concretes in resisting chloride ingress is elucidated. Overall, the present model is promising in delivering comprehensive understanding and robust prediction concerning long-term behaviours of next-generation AAS concrete buildings.

Investigation on the anti-carbonation properties of alkali-activated slag concrete: Effect of activator types and dosages

In this paper, three alkaline activators, namely sodium hydroxide (SH), calcium oxide and sodium carbonate (COSC), calcium hydroxide and sodium sulfate (CHSS), and the influence of different alkali equivalents on the carbonation resistance of alkali-activated slag concrete (AASC) were investigated. By testing the mechanical properties at different carbonation ages of AASC with three types of activators at varying alkali equivalents (4%, 6%, and 8%), along with the changes in carbonation depth, this study aims to characterize hydration products and porosity through microstructural analysis. The objective is to explain the mechanism by which carbonation affects the AASC structure. The findings showed that the carbonation rate (i.e., the growth rate of the carbonation depth) of AASC was CHSS > COSC > SH when the exciter dosages were the same, and the carbonation rate of all samples tended to slow down as the alkali components increased. Calcite and spherulite were the calcium carbonate morphologies after SH, COSC, and CHSS carbonation according to XRD, TG/DSC, and pore structure tests; the calcium carbonate content decreased with increasing alkali equivalent after carbonation. Since the pore-increasing effect of carbonation due to the destruction of C–S–H is weaker than the filling effect produced by calcium carbonate, the total pore volume of SH, COSC, and CHSS decreased due to the activator effect after carbonation. Specifically, the calcium carbonate created by carbonation repairs pores and cracks of matrix, but its presence also reduces the calcium hydroxide concentration, leading to a decrease in the alkalinity of the pore solution, which disrupts the structure of the hydration products and loosens the matrix. The focus is on which aspect is more decisive. Interestingly, small pores however decreased, while there was an increase in larger pore sizes resulting in a change in pore size distribution. However, this negative impact on pore distribution decreased with increasing alkali dosage.

Sustainable high-strength alkali-activated slag concrete is achieved by recycling emulsified waste cooking oil

To mitigate the shrinkage of high-strength alkali-activated slag concrete (AASC), this paper introduces emulsified cooking oil (ECO) and emulsified waste cooking oil (EWCO) into the AASC system. The effects of admixing ECO and EWCO on the compressive strength, drying shrinkage, autogenous shrinkage, carbonation, and sulfuric acid resistance of the AASC are systematically explored. The optimization mechanism is also proposed based on the surface tension and microstructural analysis. The experimental results show that the admixing ECO and EWCO slightly reduce the compressive strength of the AASC by 7.8%. Interestingly, the admixing ECO and EWCO significantly reduce the drying shrinkage and autogenous shrinkage, simultaneously improving the resistance to carbonation and sulfuric acid of the AASC. Specifically, the introduction of 2 wt.% ECO and EWCO can reduce the autogenous shrinkage of the AASC by 66.7% and 41.0%, respectively. Microstructural observations reveal that the addition of ECO and EWCO can reduce the internal surface tension of the AASC, improve the transport and diffusion of the pore solution, and increase the absorbable free water of the slag, which in turn reduces the shrinkage of the composites. It also increases the ionic concentration in the pore solution, resulting in a more complete reaction of the AASC, which can optimize the pore structure and thus improve the durability of the AASC. This study proposes a promising way to develop sustainable alkali-activated slag concrete achieved by recycling waste materials.

Effects of mixing conditions and activator anionic species on the rheology of silicate-activated slag concrete

The proper control of the rheological performance of silicate-based alkali-activated slag (AAS) mixtures is problematic, as the conventional superplasticizers become less effective in alkaline media. Nevertheless, several methods have been proposed to improve the workability of silicate-activated AAS, such as by extending the mixing time, and replacing sodium silicate with sodium carbonate activators. However, the underlying fluidizing mechanism is not yet well understood in the literature, which is crucial knowledge to achieve proper rheology control of silicate-activated AAS.In this study, the effects of mixing conditions and activator anionic species on the rheology of silicate-activated AAS concrete have been assessed. The reaction products, particle size and interparticle interactions, as well as the reaction kinetics in AAS, have been further investigated to understand the distinct fluidizing mechanisms. By using a longer mixing time, it was found that the solid particles formed at early ages are broken down into smaller particles, accompanied by a slight increase in the amount of reaction products to improve the fluidity. With the sodium carbonate substitution, the calcium ions dissolved from slag particles are entrapped into calcium carbonate precipitates to slow down the accumulation of C-(A)-S-H phases, leading to a better dynamic flow. However, the interparticle interactions are intensified due to the formation of larger particles and the declined dispersing effect induced by silicate activators.

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.

Mixture optimization of alkali activated slag concrete containing recycled concrete aggregates and silica fume using response surface method

In this study, response surface method (RSM) was used to predict a quadratic model for mechanical and durability properties of alkali activated slag concrete containing silica fume (SF) and recycled coarse and fine aggregates (RCA, RFA). Mechanical, durability, economic, and environmental aspects of the mixes were evaluated. The results showed that substituting natural aggregates with recycled ones had minimal impact on the final price. Different alkali activated mixes did not make a significant difference in the CO2 emission. Although the optimum mix design was the one with 0% RCA, 0% RFA, and 8.1% SF, other environmentally friendly concretes had comparable desirability factors.

Metaheuristic optimization of machine learning models for strength prediction of high-performance self-compacting alkali-activated slag concrete

Metaheuristic optimization of machine learning models for strength prediction of high-performance self-compacting alkali-activated slag concrete

Chloride Permeability of Alkali-Activated Slag Concretes after Exposure to High Temperatures.

The number of fires in buildings and on bridges has increased worldwide in recent years. As a structural material, the strength of alkali-activated slag (AAS) concrete after exposure to high temperatures has been given much attention. However, research of its durability is still lacking, which limits the application of this type of concrete on a larger scale. In this context, as one of the most important aspects of durability, the chloride permeability of AAS concretes after exposure to high temperatures was examined in this study. The influence of the alkali concentration (Na2O%) and the modulus (Ms) of the activator, as well as the influence of heating regimes, including the heating rate, duration of exposure to the target temperature, and cooling method, was also discussed. The results show that the chloride permeability of the AAS concretes increased with temperature elevation. Due to the interference of pore solution conductivity, the influence of the Na2O% and the Ms of the activator on the chloride permeability of the AAS concretes was not made clear by using the ASTM C 1202 charge passed method; however, after exposure to high temperatures, AAS with a lower Na2O% and lower Ms has lower porosity and may have lower chloride permeability, which needs further investigation. Faster heating for a longer duration at the target temperature and water cooling reduced the resistance of the AAS concretes to chloride permeability as a result of their increased porosity.

Experimental study on flexural mechanical properties of steel fiber reinforced alkali-activated slag concrete beams

As an environmentally friendly alternative to ordinary concrete, slag concrete is subject to limitations such as drying shrinkage and micro-cracking during its promotion and application. In order to address these challenges, steel fibers, known for their excellent tensile, shear, crack-resistance, and toughness properties, have been introduced to enhance the ductility of alkali-activated slag concrete. This study utilized steel fiber content as a variable and produced eight steel fiber-reinforced alkali-activated slag concrete beams to investigate their flexural mechanical properties. By exploring the influence of steel fiber content variation on the mechanical behavior of alkali-activated slag concrete beams and conducting validation through finite element analysis, the study unveiled the impact of steel fibers on the performance of alkali-activated slag concrete beams. The research findings demonstrate a significant enhancement in the flexural mechanical properties of alkali-activated slag concrete beams with the addition of steel fibers, leading to a reduction in surface cracking and an improvement in the durability of the elements. The outcomes of this study hold crucial theoretical implications for the widespread application of steel fiber-reinforced alkali-activated slag concrete.

Effects of alkali activator on the chloride-ion permeability of one-part alkali-activated nickel slag concrete

Herein, the effects of the ionic types and content of alkali activator on the chloride-ion permeability of one-part alkali-activated nickel slag concrete were examined. The NT Build 492 method was adopted to measure the Cl− transport performance. In general, the total Cl− concentration in concrete decreases with the increase of penetration depth; however, the enrichment of Cl− concentration in the sample is not obvious. Anions have more effect on 28-d compressive strength, while cations have more effect on chlorine-ion permeability. For the same Na2O content, SiO32−-activator and Na+-activator perform better than other anions and cations, while OH– and K+ perform worse than other ions. The chloride-ion permeability coefficient (DRCM) of concrete with Na2SiO3 is the lowest and that with KOH is the highest. The DRCM of concrete prepared with KOH is 1.93 times higher than that of concrete prepared with Na2SiO3. When the activator is Na2SiO3, the DRCM of concrete decreases with the increase in Na2O content when the Na2O content is less than 7%. However, when the Na2O content exceeds 7%, the DRCM of concrete increases with the increase in Na2O content.

Experimental and numerical study on self-compacting alkali-activated slag concrete-filled steel tubes

This study presents a comprehensive experimental and numerical investigation of axially loaded self-compacting alkali-activated slag concrete-filled steel tubular (ACFST) stub columns. Integrating alkali-activated slag concrete (AASC) with concrete-filled steel tubes (CFST) can yield a novel construction composite material that offers advantageous outcomes for both AASC and steel tube. AASC, an environmentally friendly alternative to traditional concrete, offers improved sustainability, while CFST can provide enhanced structural capabilities. To investigate the performance of ACFST columns, nine different AASC mixtures were designed, each varying in binder content, water-to-binder ratio, Na2O %, and activation modulus. In this study, eighteen ACFST specimens were tested under axial compression, evenly divided into nine circular and nine square CFST sections. A numerical model is developed to evaluate the stress-strain behaviour of confined AASC filled in circular and square steel tubes. The proposed numerical model is first applied to analyse ACFST specimens tested in this study and then used to analyse ACFST specimens tested by another researcher to verify its accuracy. The results, including ultimate load capacity, axial load-displacement graphs, and failure modes obtained from the proposed numerical model, have shown strong agreement with the test results. This suggests that the proposed model can be effectively utilized to predict the axial compressive behaviour of both circular and square ACFST columns.

Fresh and Mechanical Characteristics of Alkali Activated Fly Ash Slag Concrete Activated with Neutral Grade Liquid Glass

Abstract: The development and use of geopolymer concrete continues to evolve as researchers and engineers refine mix designs, develop standardized guidelines, and explore new applications. One of the key benefits of geopolymer concrete is its lower carbon footprint compared to traditional Portland cement concrete. This reduction in carbon emissions is due to the elimination of clinker production, which is energy intensive and emits significant amounts of carbon dioxide. This sustainable alternative to Portland cement concrete plays an important role in reducing the environmental impact of the construction industry. Many prior studies on fly ash-slag based geopolymers have primarily focused on the characteristics of concrete that has undergone heat curing. This approach is seen as a constraint when it comes to in-situ casting applications in ambient conditions. Ambient curing of geopolymer concrete is a more energy-efficient and environmentally friendly option compared to heat curing, as it doesn't require the use of specialized curing chambers or high temperatures. However, it may require longer curing times to achieve the desired strength and making it important to plan the construction schedule accordingly. An experimental study was carried out using neutral grade liquid glass with a silica modulus (SiO2/Na2O) of 2.92 coupled with combinations of fly ash and Ground Granulated Blast Furnace Slag (GGBS) in proportions of 30:70, 50:50. The primary aim is to explore how altering the ratios of fly ash and GGBS impacts the mechanical characteristics of the resultant concrete for different solution/ binder ratios (0.6, 0.65 and 0.7) with a fixed binder quantity of 400 and 500 Kg/m3 under ambient curing conditions. The experimental program employed the fractional factorial method of experimentation to minimize the number of mix variations. In general, the findings indicated that higher levels of neutral-grade sodium silicate led to improvements in all mechanical properties. Conversely, an increase in the alkaline solution-to- binder ratio and curing temperature had a detrimental impact on the alkaliactivated slag concrete. Optimal results for hardening of concrete for demolding were obtained after two days of casting. The implications of this research could lead to the development of more sustainable and environmentally friendly alternatives in the field of construction materials.

Drying shrinkage properties of fiber-reinforced alkali-activated slag and their correlations with microstructure

The superior strength, durability, and environmental sustainability of alkali-activated slag (AAS) systems renders it a viable alternative to those based on Portland Cement (PC). However, drying shrinkage in AAS systems is a highly complex process requiring extensive testing. Several studies have reported the incorporation of fibers to AAS systems as an effective way not only to enhance its strength but also to reduce its drying shrinkage, but the information is scattered. In pursuance of this objective, the present study investigates the impact of incorporating steel, polypropylene, polyvinyl alcohol and glass fibers at varying volume fractions from 0.1% to 0.3% on both mortar and concrete AAS specimens. According to the findings, incorporating fibers to AAS concrete increased its compressive strength by up to 13% when compared to the plain mix. A higher volume fraction of 0.3% for steel and glass fiber inclusions significantly minimized the drying shrinkage of AAS concrete by 27% and 31%, respectively. However, due to the increased heterogeneity and subsequent void formation in fiber-reinforced AAS systems, it was noted that the mass loss resulting from the loss of moisture is greater in these cases. Despite the greater mass loss at earlier ages, the fibers in AAS specimens demonstrated their ability to restrain the shrinkage strains. The microstructural studies, which distinctly depict the morphology of AAS paste and the interaction between the fibers and matrix, further validate these conclusions.