Alkali-activated Slag Composites Research Articles

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.

Influences on the mechanical and physical properties of hot-press moulding alkali-activated slag (HP-FRAASC) composite with various fibers

ABSTRACT In this study, we will apply the hot-press moulding method for the first time to the preparation of alkali-activated slag composites (AASC) and incorporate 1vol.-%, 3vol.-%, 5vol.-%, and 7vol.-% steel, copper, and carbon fibres into the matrix in order to investigate the characteristics of the HP-AASC and fibre reinforced (HP-FRAASC) in terms of compressive strength, flexural strength, impact toughness, and bulk density of HP-AASC and HP-FRAASC. Results show that the composites have good flexural strength, compressive strength, and impact toughness when the content of steel fibres is 5 vol.-%, copper fibres are 7 vol.-%, and carbon fibres are 1–3 vol.-%. It was observed from each sample of HP-FRAASC and HP-AASC that the HP-AASC samples had lower porosity, better density, and an easy preparation process, which can be used as a suitable preparation method for this material.

The Efficiency of the Ordinary Polypropylene Fiber in Producing One-part Alkali-Activated Slag Composites with Strain-Hardening Behavior

The Efficiency of the Ordinary Polypropylene Fiber in Producing One-part Alkali-Activated Slag Composites with Strain-Hardening Behavior

Synergistic Effect of HEDP.4Na and Different Induced Pouring Angles on Mechanical Properties of Fiber-Reinforced Alkali-Activated Slag Composites

The poor flexural and damping properties of building materials damages concrete structures and affects their service life when concrete structures are subjected to dynamic loads. Three different dosages (i.e., 0%, 0.3%, and 0.6%) of organic phosphonates (HEDP.4Na) and different pouring methods (i.e., conventional pouring method, 90°-induced pouring method, and 150°-induced pouring method) were designed to improve the flexural and damping performance of fiber-reinforced alkali-activated slag composites (FR-AASC). The enhanced mechanism of HEDP.4Na was revealed by phase analysis (X-ray diffraction, XRD), pore structure analysis (Mercury Intrusion Porosimetry, MIP), the heat of hydration, and scanning electron microscopy (SEM) analysis. The results showed that 0.3% HEDP.4Na combined with the 150°-induced pouring angle can significantly improve the mechanical properties of the FR-AASC sample compared with the reference group. The sample with 0.3% HEDP.4Na cast by the 150°-induced pouring angle increased compressive and flexural strength, damping energy consumption and storage modulus by 20%, 60%, 78%, and 30%, respectively, compared with the reference sample cast by the conventional pouring methodology. HEDP.4Na reduced the early hydration heat and total porosity of the FR-AASC matrix, modified the fiber–matrix interface transition zone, and increased the frictional energy consumption of steel fibers. Overall, the synergistic effect of HEDP.4Na and the induced pouring methodology significantly improved the flexural and damping properties of FR-AASC. This study can provide a guidance for improving the flexural and damping capacity of FR-AASC and promote the application of FR-AASC in construction engineering.

Physico-mechanical and shielding properties of alkali-activated slag composites incorporating cement clinker aggregate: Effect of high temperature and particle size

The purpose of this research was to see how cement clinker aggregates (CCA) and curing temperature affected the physico-mechanical and shielding properties of alkali-activated composites (AAC) exposed to high temperature. The precursor used to create AAC was ground blast furnace slag (GBFS), which was activated using a solution of sodium silicate (Na2SiO3) and sodium hydroxide (NaOH). Quarts powder with a size range of 0–0.5 mm and CCA in two different size ranges (0–4 mm and 4–8 mm) were used as fine and coarse aggregates in the fabrication of AAC. Four different AAC mixes were produced which were cured by three curing temperatures at 25 °C (air curing), 40 °C, and 80 °C for 8 h. The impacts of curing temperature and CCA size and content were examined on the AAC's flowability, compressive strength, flexural strength, porosity, dry unit weight, and water absorption and sorptivity characteristics. The high temperature performance and shielding properties of the produced blends was also assessed. Microstructure investigations of AAC mixtures produced within the scope of the study were also carried out based on FTIR and SEM. The obtained results implied that compressive strength of up to 80.80 MPa was attained with the use of CCA at curing temperature of 80 °C. The M4 containing the highest CCA content with the sizes of 4–8 mm and the lowest CCA content with the sizes of 0–4 mm exhibited the lowest strength at all curing temperatures. All AAC mixes regardless of curing temperature lost more than 60% of their initial compressive strength after 300 °C. Finally, among the AAC mixtures, whose gamma-ray shielding properties were examined in the photon energy range of 81–661 keV and whose fast neutron dose rates were determined, 0–4 mm particle size Clinker doped M1 sample showed better nuclear shielding ability compared to other mixtures studied.

Effect of water saturation on the electrical properties of cement and alkali-activated slag composites with graphite conductive admixture

This study investigated the influence of water saturation degree on the electrical properties of two different types of building materials – Portland cement and alkali-activated slag mortars with graphite powder as a conductive admixture. The graphite content ranged between 0 and 18 wt% of the cement/slag in order to cover the area below as well as above the percolation threshold. The properties were tested in terms of electrical resistivity measurement with an AC-frequency in the range of 40 Hz – 1 MHz, and the results were correlated with meso-scale structural changes evaluated by means of mercury intrusion porosimetry and scanning electron microscopy. The electrical resistivity of both materials decreased with increasing AC-source frequency and increasing water saturation degree. In a dry state, alkali-activated slag exhibited approximately ten times lower resistivity than cement-based material. The addition of graphite gradually decreased the resistivity until the percolation threshold was reached, but no influence of moisture on the resistivity was observed at dosages exceeding 12 wt% of graphite. The dependence of electrical resistivity on the water saturation degree was fitted for two different frequencies (50 Hz and 1 kHz) using a sigmoidal regression function.

Alkali-activated slag (AAS) and OPC-based composites containing crumb rubber aggregate: Physico-mechanical properties, durability and oxidation of rubber upon NaOH treatment

Value-added utilization of waste tires in cementitious materials has been studied for decades, whereas the synergy between rubber and alkali-activated cements deserves more investigations. To this end, we compared physico-mechanical and durability properties of alkali-activated slag (AAS) and OPC-based composites containing NaOH-pretreated/as-received crumb rubber (CR). It was found that the incorporation of CR had both positive and negative effects on specimens due to its special attributes. CR reduced electrical conductivity which was associated with the ion transport ability, since it worked as an electrical insulator and increased hydrophobicity of pore walls. The sulfate expansion decreased with CR content because the flow of sulfate ions was demobilized, and the flexibility of CR relieved the internal stress. CR also improved the freeze-thaw durability of composites if the total CR content was less than 30% in specimens. However, the low Young's modulus of CR impaired the compressive strength and failed to effectively control the total shrinkage upon drying. The composites with CR must avoid high temperature exposure which resulted in the pyrolysis of rubber. In addition, the NaOH pretreatment enhanced adhesion between CR and hydrated cement and improved overall performances of composites. The FTIR spectra confirmed the NaOH-induced accelerated oxidation of CR surface and the generation of polar functional groups including carbonyl and carboxyl groups, increasing the total surface energy of rubber. More importantly, the alkali-activation process for AAS could treat the CR aggregate in the meantime, which means the use of CR in alkali-activated cements does not require the pretreatment of CR before blending.

Correction: Park et al. Synergistic Effect of MWCNT and Carbon Fiber Hybrid Fillers on Electrical and Mechanical Properties of Alkali-Activated Slag Composites. Crystals 2020, 10, 1139

In the original article [...]

Optimization of alkali-activated ladle slag composites mix design using taguchi-based TOPSIS method

This study examines the effect of various parameters on the properties of alkali-activated composites made with unprocessed ladle furnace slag. Taguchi method was used to design the experiments. A total of five factors, each with four levels, were considered, including ladle slag content (LS), alkaline-activator solution-to-binder ratio (AAS/B), sodium silicate-to-sodium hydroxide ratio (SS/SH), sodium hydroxide molarity (M), and crushed sand replacement by dune sand (CSR). A total of 16 alkali-activated ladle slag mixtures were designed, cast, and tested. The performance responses were the workability, setting time, and compressive strength. To assess the influence of the factors on the responses, Taguchi analysis and ANOVA were employed while determining the signal-to-noise (S/N) ratios. Further, TOPSIS analysis was carried out to optimize the mixture proportions of alkali-activated ladle slag composites. The optimum mix entailed 650 kg/m3 of ladle slag, AAS/B ratio of 0.45, SS/SH of 2, and CSR of 25% to maximize strength and workability. Meanwhile, the optimum mix to maximize workability and setting time included 650 kg/m3 of ladle slag content, AAS/B ratio of 0.6, SS/SH of 2.5, and CSR of 50%. Anticipated results of the optimum mixes were experimentally verified. Microstructure analysis of the optimum mixes (isothermal calorimetry, Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy) highlighted the accelerated rate of the activation reaction, the amorphous morphology, and the formation of calcium aluminosilicate hydrate gel.

Strain Hardening of Polypropylene Microfiber Reinforced Composite Based on Alkali-Activated Slag Matrix.

A comparative study of the fracture features, strength and deformation properties of pseudo strain-hardening composites based on alkali-activated slag and Portland cement matrices with polypropylene microfiber was carried out. Correlations between their compositions and characteristics of stress–strain diagrams under tension in bending with an additional determination of acoustic emission parameters were determined. An average strength alkali-activated slag matrix with compressive strength of 40 MPa and a high-strength Portland cement matrix with compressive strength of 70 MPa were used. The matrix compositions were selected for high filling the composites with polypropylene microfiber in the amount of 5%-vol. and 3.5%-vol. ensuring the workability at the low water-to-binder ratios of 0.22 and 0.3 for Portland cement and alkali-activated slag matrices, respectively. Deformation diagrams were obtained for all studied compositions. Peaks in the number of acoustic signals in alkali-activated slag composites were observed only in the strain-softening zone. Graphs of dependence of the rate of acoustic events occurrence in samples from the start of the test experimentally prove that this method of non-destructive testing can be used to monitor structures based on strain-hardening composites.

Crack self-healing in alkali-activated slag composites incorporating immobilized bacteria

Decarbonizing concrete manufacturing calls for sustainable construction materials since cement production contributes about 8% of global anthropogenic CO2 emissions. Alkali-activated materials (AAMs), usually derived from the activation of industrial by-products, have demonstrated great potential as alternatives to ordinary portland cement (OPC) concrete. However, AAMs are susceptible to cracking due to their inherent brittleness, which may compromise their service life and severely hinder wider acceptance and applications of AAMs. While bacteria-based crack self-healinghas attracted enormous research interests in OPC-based materials, only limited research has explored such self-healing in AAMs. Accordingly, this study aims at investigating the self-healing efficiency of alkali-activated slag-based composites using bacteria immobilized in expanded glass granules made from recycled glass. The effect of different calcium sources on the reaction kinetics, mechanical properties, capillary water absorption, and final self-healing compounds was investigated. The results reveal that the bio-based specimens attained a remarkably enhanced crack healing ratio, while no obvious self-healing was observed in abiotic control samples. Furthermore, the sorptivity coefficient decreased significantly by incorporating bacteria. Scanning electron microscopy with energy dispersive X-ray (SEM-EDX) analysis and Raman spectroscopy confirmed that calcite was the dominant self-healing product. This study suggests that using calcium lactate as the calcium source is advantageous owing to its economic and sustainability considerations.

High strength fiber reinforced one-part alkali activated slag composites from industrial side streams

This paper details the effect of fiber reinforcement on mechanical characteristics of by-products based one-part alkali activated material (AAM). Two different binder matrices were considered in this study, such as, plain slag, and ternary blended slag, to understand their efficiency in fiber reinforced system. These matrices were reinforced with 1% v/v of three different fibers (steel, glass and basalt) to improve the flexural performance of high strength mortar blends. Steel fiber reinforced one-part AAM outperforms mineral fibers in compressive strength contribution. The fracture energy of steel fiber reinforced compositions was roughly 4 times higher than that of mineral fiber reinforced materials. In addition, the flexural performance of ternary blended matrix was higher than that of slag-based composition regardless fiber types and properties. Finally, preliminary finite element modelling was considered to assess the applicability of the concrete damage plasticity constitutive model in predicting the nonlinear behavior of the developed composites. The numerical predictions proved the accuracy of the model with good agreement between experimental and numerical results.

Effects of fiber hybridization on mechanical properties and autogenous healing of alkali-activated slag-based composites

This paper investigated the effects of hybrid fiber-reinforcement on the mechanical properties and autogenous healing ability of alkali-activated slag (AAS) composites. Three composites with varying quantities of fibers were investigated using a comprehensive set of experiments including mechanical behavior characterization, autogenous healing measurement, and microscopic characterization of healing products were performed. The experimental results showed that for the same total fiber volume fraction, the AAS composite using hybrid PE-PVA fibers achieved higher tensile strain capacity than the mixture using only the PE fibers. In terms of crack width reduction, stiffness restoration, and tensile recovery, the composite reinforced by hybrid PE-PVA fibers showed superior healing performance than that of the composite reinforced with only PE fibers. Even at a small volume fraction of 0.25%, the PVA fibers had an influence on the nature of the healing products.

Optimization of Alkali-Activated Municipal Slag Composite Performance by Substituting Varying Ratios of Fly Ash for Fine Aggregate.

This study investigates the effect of varying ratios of fly ash as a partial replacement for fine aggregate on the performance of alkali-activated municipal slag composites. The strength and other properties of alkali-activated cementitious material (AACM) composites can be optimized by selecting the appropriate mix proportion. In this study, we used fly ash as a substitute for fine aggregate (FA/S) at varying ratios of 0.0, 5.0, 10.0, 15.0, 20.0, 25.0, and 30.0%, mixed with 50% water (W/SL), and 20% alkali activator (AL/SL) content instead of municipal slag (SL) as a core binder, cured in steam conditions. The effects of these substitutions on the initial mixing temperature, slump flow, compressive and splitting tensile strengths, and microstructure analysis of composites cured in steam conditions were investigated at 1, 7, 28, and 91 days. The evaluation of the experimental results revealed that increasing the ratio of fly ash substitution to fine aggregate by up to 20.0% led to a higher strength attributable to the composites, whereas when the extra substitution ratio of FA/S ranged from 25.0–30.0%, significant decreases in strength were observed. The composites’ strengths were estimated using the ACI 209 and ACI 318 design equations and compared to the measured strengths.

Strength Characteristics of Alkali-Activated Slag Mortars with the Addition of PET Flakes.

The production of ordinary Portland cement is associated with significant CO2 emissions. To limit these emissions, new binders are needed that can be efficiently substituted for cement. Alkali-activated slag composites are one such possible binder solution. The research programme presented herein focused on the creation of alkali-activated slag composites with the addition of PET flakes as a partial substitute (5%) for natural aggregate. Such composites have a significantly lower impact in terms of CO2 emissions in comparison to ordinary concrete. The created composites were differentiated by the amount of activator (10 and 20 wt.%) and curing temperature (from 20 to 80 °C). Their mechanical properties were tested, and a scanning electron microscope analysis was conducted. Compressive and flexural strengths ranging from 29.3 to 68.4 MPa and from 3.5 to 6.1 MPa, respectively, were achieved. The mechanical test results confirmed that a higher amount of activator improved the mechanical properties. However, the influence of the PET particles on the mechanical properties and microstructure varied with the curing temperature and amount of activator. Areas that require further research were identified.

Investigation of Graphene Derivatives on Electrical Properties of Alkali Activated Slag Composites.

Reduced graphene oxide (rGO) has been widely used to modify the mechanical performance of alkali activated slag composites (AASC); however, the mechanism is still unclear and the electrical properties of rGO reinforced AASC are unknown. Here, the rheological, mechanical, and electrical properties of the AASC containing rGO nanosheets (0, 0.1, 0.2, and 0.3% wt.) are investigated. Results showed that rGO nanosheets addition can significantly improve the yield stress, plastic viscosity, thixotropy, and compressive strength of the AASC. The addition of 0.3% wt. rGO nanosheets increased the stress, viscosity, thixotropy, and strength by 186.77 times, 3.68 times, 15.15 times, and 21.02%, respectively. As for electrical properties, the impedance of the AASC increased when the rGO content was less than 0.2% wt. but decreased with the increasing dosage. In contrast, the dielectric constant and electrical conductivity of the AASC containing rGO nanosheets decreased and then increased, which can be attributed to the abundant interlayer water and the increasing structural defects as the storage sites for charge carriers, respectively. In addition, the effect of graphene oxide (GO) on the AASC is also studied and the results indicated that the agglomeration of GO nanosheets largely inhibited the application of it in the AASC, even with a small dosage.

Comparative study on the effect of fiber type and content on the fire resistance of alkali-activated slag composites

Alkali-activated slag (AAS) binder has been recognized as a suitable material for construction applications owing to its low carbon footprint and good mechanical and durability performance. As a promising alternative to the conventional Portland cement binder, it is important to maximize the performance of AAS composites under normal and harsh environmental conditions such as exposure to fire. The use of fibers in a brittle matrix is a well-known approach to enhance the mechanical strength and cracking behavior under thermal loading. In this study, polypropylene fiber (PPF), glass fiber (GF), and basalt fiber (BF) are used at volume fractions of 0.5%, 1%, and 1.5% in AAS mortar mixes. First, the workability, compressive strength, and flexural strength of mixes were evaluated at ambient temperature. Then, the mortar mixes were exposed to elevated temperatures of 200, 400, 600, and 800 °C, and the residual flexural and compressive strengths and the mass loss as well were determined. Furthermore, the changes in the matrix after heat treatment were investigated utilizing scanning electron microscopy (SEM) analysis. Based on the test results, the fiber type and dosage did not have a considerable influence on the mass loss of mixes, and in general, fiber-reinforced mixes showed a higher mass loss due to the melting of fibers. The optimum PPF, GF, and BF content based on the compressive strength measured at the ambient and elevated temperatures was found equal to 0.5%, 0.5%, and 1%, respectively. Whereas, the optimum percentages to achieve the highest flexural strength were equal to 0.5%, 1%, and 1%, respectively. It was found that the GF-reinforced mix containing 0.5% and 1% GF exhibited 33% and 160% higher residual compressive and flexural strengths at 800 °C, respectively, compared to that of the control mix.

Effects of crumb rubber particles on mechanical properties and sustainability of ultra-high-ductile slag-based composites

This study investigated the effect of the addition of crumb rubber (CR) particles in a range of 5% to 10% of binding materials on the compressive strength and tensile behavior of alkali-activated slag (AAS) composites reinforced by polyethylene (PE) fiber. The effect of silica fume (SF), as a partial binder replacement, on the properties of the composite was also investigated. Four mixtures were designed including two with different amounts of CR, one with a combination of CR and SF, and one control. Test results showed that the tensile strain capacity of the AAS composites can be increased by incorporating CR; however, the compressive strength and tensile strength decreased with an increase in the amount of CR. The mixture using CR to replace 5% of the binder amount achieved ultra-high-ductile behavior with a tensile strain capacity of 10.7% and saturated cracking patterns (average crack spacing: 0.70 mm). In addition, the robust greenness of the rubberized composites was completely demonstrated, making this a sustainable model.

Synergistic Effect of MWCNT and Carbon Fiber Hybrid Fillers on Electrical and Mechanical Properties of Alkali-Activated Slag Composites

Herein, we investigated the synergistic effect of multi-walled carbon nanotube (MWCNT) and carbon fiber (CF) hybrid fillers on electrical and mechanical characteristics of alkali-activated slag (AAS) composites. Many studies on AAS composites have been conducted in the past; however, not much progress has been made regarding characteristics of AAS composites with hybrid conductive fillers. The specimens with different mix proportions were fabricated in the present study, and numerous material characteristics, including flowability, electrical resistivity, and compressive strength of AAS composites were measured. In addition, the synergistic effects were investigated through scanning electron microscopy and thermogravimetric analysis. It was found that the 0.5 wt.% of MWCNTs and CFs lead the effects of the bridging and reducing crack propagation, thereby improving its electrical and mechanical performances. The filler exceeding a percolation point improved the electrical performance of the AAS composites; however, it interfered with the hydration process during the curing period, and caused a decrease in compressive strength of AAS composites.

Composite properties of calcium-based alkali-activated slag composites reinforced by different types of polyethylene fibers and micromechanical analysis

This paper presents an experimental study exploring the effects of different types of polyethylene (PE) fibers and curing methods on the compressive strength and tensile behavior of calcium-based alkali-activated slag composites. Three types of PE fibers were used and a matrix mixture was designed using slag and calcium hydroxide. A series of experiments was performed to measure and compare the compressive strength and tensile properties of each mixture, and single fiber pullout test and fiber-bridging analysis were also performed. It was found that the types of PE fibers and curing methods significantly affected the properties of the composites. The single fiber pullout test results showed that the interfacial properties between matrix and PE fiber depend on the types of PE fibers. The tension test results were explained by performing fiber-bridging analysis.