Alkali-activated Composites Research Articles

Influencing factors analysis and optimized prediction model for rheology and flowability of nano-SiO2 and PVA fiber reinforced alkali-activated composites

Alkali-activated composites (AAC) has real potential as a repair material of defective concrete structures by virtue of fast setting, high strength, and recyclability. Understanding the workability of AAC is the key to realize this environmental technology as a new repair material. In this work, the effects of water-reducing admixture, nano-SiO2 (NS), and polyvinyl alcohol (PVA) fiber contents on the rheology and the flowability of AAC were investigated through the impeller-rheometer and L-box tests respectively. Based on the results of rheology and flowability, the least squares support vector regression (LS-SVR) method was used for the multi-output prediction of the rheological parameters of AAC. The test results indicate that the fresh mixtures can be regarded as a Bingham fluid, and the proper NS and water-reducing admixture contents can promote the rheology and the flowability of the mixtures, while excessive PVA fiber and NS have negative influence on the workability. The predicted results exhibit that the predicted rheological parameters of mixtures are concordant with the experimental values (the correlation coefficients of static yield stress, dynamic yield stress and plastic viscosity are 0.990, 0.963 and 0.976, respectively). Consequently, the LS-SVR method can be applied for the inverse analysis of the rheological parameters of AAC.

Influence of secondary carbon fiber on the mechanical behaviour and microstructure of alkali-activated granulated blast furnace slag-based composites

This study aimed to investigate the mechanical behaviour and microstructure of an environmentally friendly fibre-reinforce alkali-activated composites. The composites were obtained from alkali-activated granulated blast furnace slag reinforced with chopped secondary carbon fibres (SCFs) coming from the aircraft industry carbon fiber reinforced plastics waste. Three types of surfactants and two concentrations of SCFs were investigated. The compressive and bending strengths were measured to evaluate the mechanical behaviour of specimens. Moreover, the polycondensation products, pore structure, and microscopic morphology of the composites were analyzed using X-ray diffraction (XRD), method of standard contact porosimetry (MSCP), and scanning electron microscopy (SEM). It was found that tetraethylammonium bromide and a superplasticizer agent Glenium-51® increase compressive strength for reference granulated blast furnace slag-based alkali-activated matrix approximately by 60 % and lead to lower open porosity from 16 to 5 %. The experimental results showed that the incorporation of 0.7 vol. % SCFs had an optimal influence on mechanical behaviour and microstructure of composite. Based on the test results, it can be clearly said that using of Glenium-51® is improving the compressive strength of slag based alkali-activated composites reinforced SCFs.

Influence of Fly Ash Denitrification on Properties of Hybrid Alkali-Activated Composites

This article deals with the possibility of partial replacement of blast furnace slag (GGBFS) with fly ash after denitrification (FAD) in alkali-activated materials. Physical-mechanical and durability properties were tested, hydration reaction was monitored, and infrared spectroscopy was performed. Results were compared between mixtures prepared with fly ash without denitrification (FA), and also with a mixture based only on GGBFS. The basic result is that hybrid alkali-systems with FAD show similar trends to FA. The significant effect of fly ash is manifested in terms of its resistance to freeze-thaw processes. Reactions in a calorimeter show a slower development of reactions with increasing replacement of GGBFS due to the lower reactivity of the fly ash. Through testing the leaching resistance, a decrease in flexural strength was found. This may be due to the descaling of the main hydration product, C–(A)–S–H gel. After 28 days of maturation, compressive strengths of all monitored mixtures ranged from 96 to 102 MPa. The flexural strengths ranged from 6.8 to 8.0 MPa. After 28 days of maturation, the higher strengths reached mixtures without replacing GGBFS. In terms of resistance to freeze-thaw processes, the largest decrease (almost 20%) of flexural strength was achieved by a mixture with 30% of GGBFS replacement by FA. No fundamental differences were found for the mixtures in the FTIR analysis.

Microstructural Characterization of Alkali-Activated Composites of Lightweight Aggregates (LWAs) Embedded in Alkali-Activated Foam (AAF) Matrices.

Alkali-activated composites of lightweight aggregates (LWAs, with beneficial insulating properties) and alkali-activated foams (AAFs, higher added value products due to their production from waste materials at well below 100 °C) allow for the expectation of superior properties if a chemical bonding reaction or mechanical interlocking occurs during production. However, the interfaces between LWAs and an AAF have not been studied in detail so far. Chemical reactions are possible if the LWA contains an amorphous phase which can react with the alkaline activators of the AAF, increase the bonding, and thus, also their mechanical strengths. These, in turn, allow for an improvement of the thermal insulation properties as they enable a further density reduction by incorporating low density aggregates. This work features a first-detailed analyses of the interfaces between the LWAs’ expanded polystyrene, perlite, expanded clay and expanded glass, and the alkali-activated foam matrices produced using industrial slags and fly ash. Some are additionally reinforced by fibers. The goal of these materials is to replace cement by alkali-activated waste as it significantly lowers the environmental impact of the produced building components.

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 [...]

Role of recycled vehicle tires quantity and size on the properties of metakaolin-based geopolymer rubberized concrete

Increasing demand for concrete construction and the consequent increase in the production of conventional Portland cement is thwarting efforts towards sustainable construction practices. This is seen in the carbon dioxide footprint caused by cement manufacturing as well as natural resources consumption. Therefore, the so-called geopolymer or alkali-activated composites using byproduct or natural materials are seen as a potential substitute for cement-based binders. Similarly, recycled rubber recovered from shredded tires has been used in concrete mixes, which in addition to the environmental benefit, offers desirable properties, e.g., improved deformability and energy dissipation capacity. Therefore, this paper investigates the influence of introducing recycled rubber in metakaolin-based geopolymer mixes on the workability, compressive behavior (stress–strain components and failure mode), flexural strength, unit weight, air content, and water absorption percentage. Seven mixes were employed for this purpose with two main variables comprising rubber size (fine, coarse, or combination of both) and replacement percentage (0%, 20%, and 40%). Recycled rubber was observed to reduce the mix workability by 4%–52.5% and 40%–62.5% when fine or coarse rubber particles were added to concrete mixes, respectively. Average compressive strength of 14.3–37.7 MPa was achieved when fine and/or coarse rubber particles replaced 20% or 40% of the conventional fine and/or coarse aggregates. Furthermore, rubberized geopolymer concrete showed more deformability than plain mixes coupled with lightweight characteristics, which are desirable in many construction applications. This indicates the suitability of the rubberized geopolymer concrete for developing low to moderate-strength concrete for non-structural and structural applications.

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.

Determination of Gel Products in Alkali-Activated Fly Ash-Based Composites Incorporating Inorganic Calcium Additives

The introduction of soluble calcium additives contributes to forming calcium-containing geopolymer (N,C)-A-S-H and/or calcium (alumino)silicate hydration C-(A)-S-H in alkali-activated fly ash-based composites (AAFCs). However, the quantitative determination of the final gel product has yet been unclear due to the different chemical components, which influences the prediction of mechanical strength and microstructure. This study aims to propose a method to clarify the gel products in AAFCs by the critical elemental compositions in gel products. Furthermore, the AAFCs were prepared (by casting method) using soda residue (SR) as the calcium additive, fly ash as the aluminosilicate precursor, and sodium silicate solution as the alkaline activator. The compressive strengths of the AAFCs with different SR contents were recorded. The microstructures and gel products of AAFCs were detected through scanning electron microscope (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), and 29Si nuclear magnetic resonance spectrometer (29Si NMR). Finally, the proposed determination method was verified with the experimental results. The new proposed quantitative method refers to a limit value of 2Ca/Al = 1.00 about the critical chemical elements in gel products, which helps to determine the calcium-containing geopolymer (N,C)-A-S-H or/and calcium (alumino)silicate hydration C-(A)-S-H in stable AAFCs. Results show that the gel products are determined as single (N,C)-A-S-H gels when 2Ca/Al is lower than 1.00; meanwhile, (N,C)-A-S-H and C-(A)-S-H gels coexist when 2Ca/Al is higher than 1.00 by the above proposed quantitative determination method. Furthermore, it works that the industrial solid waste SRs regarded as calcium additive are applied in the AAFCs to verify the gel products by 2Ca/Al = 1.00 as the limit value. The results provide the theoretical and experimental basis for the performance prediction and microstructural determination.

Mechanical and durability properties of steel, polypropylene and polyamide fiber reinforced slag-based alkali-activated concrete

Alkali-activated composites are significant materials in reducing CO2 emissions and ensuring sustainability. With the increasing concerns about climate change globally, the interest in alkali-activated materials has also increased. Researching different fibers has very important potential in this area. This study aims to make alkali-activated concretes widespread in the concrete sector by using the materials common in conventional concretes and ensuring that alkali-activated concretes are an alternative in terms of sustainability. Experimental studies were conducted to examine the mechanical, durability, and microstructural properties (SEM) of slag-based alkali-activated concrete (AASC) reinforced with three various fibers. The fibers, polypropylene (PP), polyamide (PA), and steel (ST), were used with two ratios (%0.4 and %0.8 by vol.). Compressive, splitting tensile, and flexural strength tests were carried out at 28 and 90 days. In terms of durability properties, the samples were exposed to high temperatures (300–600–900 °C) and freeze-thaw test (250 cycles). The results showed that the addition of fibers improved the strength and durability properties; for instance, the existence of steel and polypropylene fibers increased the flexural toughness factor values by 430% and 260%, respectively. Moreover, the compressive strength of the fibrous samples exposed to 900 °C was obtained in the range of 6-23 MPa.

Mechanical performance of alkali-activatedconcrete reinforced with recycled tire steel fibers

Alkali-activated/geopolymer composites have attracted interest of the research community in recent years. The popularity of alkali-activated composites is backed by the global pressure towards producing environmentally friendly construction materials and contributing to the effort of reducing the carbon dioxide footprint associated with ordinary Portland cement production. Various by-product materials (e.g., slag, fly ash and red mud) or natural resources (e.g., metakaolin) were utilized for producing alkali-activated concrete.Previous research conducted on various alkali-activated composites have shown satisfactory mechanical properties. The addition of manufactured steel fibers (MSF) to alkali-activated concrete mixtures have also been commonly used for improving its mechanical properties. Utilizing recycled tire steel fibers (RSF) from post-consumer tires, instead of MSF, have the potential to reduce costs and positively contribute to sustainability by reducing the CO2 emissions generated from manufacturing industrial steel fibers.Therefore, this research is investigating the role of RSF dosage on alkali-activated concretes mechanical properties. The experimental program included synthesis of metakaolin-fly ash based alkali-activated concrete specimens with various percentages of RSF. The performance of the RSF mixes was also compared against a mix with MSF. The prepared specimens were tested for the compressive stress–strain behavior, bond-slip relationship and residual compressive strength after exposure to elevated temperatures. Although the addition of RSF slightly influenced the compressive strength, it significantly enhanced the strain capacity at peak strength. The addition of RSF also reduced the deterioration in compressive strength after exposure to elevated temperatures.

Comparative study of hazelnut-shell biomass ash and metakaolin to improve the performance of alkali-activated concrete: A sustainable greener alternative

Recently, the studies on the use of environmentally harmful wastes along with the building materials in the construction sector have come to the fore with its environmentalist aspect. In particular, the use of waste materials in cement contributes to the evaluation of waste and reduces the amount of cement usage. On the other hand, the production of sustainable environmentally friendly composite materials offers economic and ecological benefits. Moreover, cementless composite building materials give a promising solution to carbon footprints by allowing the re-evaluation of different waste materials. Accordingly, the potential of using hazelnut shell ash (HA), which is agricultural waste, as a mineral additive in the production of cementless concrete has been handled in the present research. The usability of HA in the mixture has been investigated by comparing it with metakaolin and blast furnace slag (BFS), which are frequently used in the production of concrete and alkali-activated composites. Alkali activated concrete (AAC) samples are cured for 3, 7, and 28 days in this study. In addition, the effect of curing type on AAC samples is deeply discussed in three different curing conditions. Physical and mechanical tests are performed on hardened AAC samples. As a result, it is noticed that the cure types have a remarkable effect on the physical and mechanical properties of AACs. In the conclusion, the present paper reports that HA materials can be effectively used in AAC mixtures and have a respectable potential to improve the abrasion resistance of the cementless concretes.

Development of Waste-Based Alkali-Activated Cement Composites.

Nowadays, global warming and the ensuing climate change are one of the biggest problems for humanity, but environmental pollution and the low ratio of waste management and recycling are not negligible issues, either. By producing alkali-activated cements (AACs), it is possible to find an alternative way to handle the above-mentioned environmental problems. First, with a view to optimizing experimental parameters, metakaolin-based AACs were prepared, and in it, waste tire rubber was used as sand replacement (5–45 wt %). Insufficient wetting between the rubber particles and the matrix was corrected through different surface treatments of the rubber. For improving the mechanical/strength properties of the specimens, fibrous waste kaolin wool (0.5–1.5 wt %) was added to the AAC matrix. Considering the results of model experiments with metakaolin, blast-furnace-slag-based AAC composites were developed. The effects of storage conditions, specimen size and cyclic loading on the compressive strength were investigated, and the resulting figures were compared with the relevant values of classic binders. The strength (44.0 MPa) of the waste-based AAC composite significantly exceeds the required value (32.5 MPa) of clinker saving slag cement. Furthermore, following cyclic compressive loading, the residual strength of the waste-based AAC composite shows a slight increase rather than a decrease.

The Effect of Fibrous Reinforcement on the Polycondensation Degree of Slag-Based Alkali Activated Composites.

Alternative cementitious binders, based on industrial side streams, characterized by a low carbon footprint, are profitably proposed to partially replace Portland cement. Among these alternatives, alkali-activated materials have attracted attention as a promising cementitious binder. In this paper, the chemical stability of the matrix, in fiber-reinforced slag-based alkali-activated composites, was studied, in order to assess any possible effect of the presence of the reinforcement on the chemistry of polycondensation. For this purpose, organic fiber, cellulose, and an inorganic fiber, basalt, were chosen, showing a different behavior in the alkaline media that was used to activate the slag fine powders. The novelty of the paper is the study of consolidation by means of chemical measurements, more than from the mechanical point of view. The evaluation of the chemical behavior of the starting slag in NaOH, indeed, was preparatory to the understanding of the consolidation degree in the alkali-activated composites. The reactivity of alkali-activated composites was studied in water (integrity test, normed leaching test, pH and ionic conductivity), and acids (leaching in acetic acid and HCl attack). The presence of fibers does not favor nor hinder the geopolymerization process, even if an increase in the ionic conductivity in samples containing fibers leads to the hypothesis that samples with fibers are less consolidated, or that fiber dissolution contributes to the conductivity values. The amorphous fraction was enriched in silicon after HCl attack, but the structure was not completely dissolved, and the presence of an amorphous phase is confirmed (C–S–H gel). Basalt fibers partly dissolved in the alkaline environment, leading to the formation of a C–N–A–S–H gel surrounding the fibers. In contrast, cellulose fiber remained stable in both acidic and alkaline conditions.

Alkali-Activated Mortars Modified by Epoxy-Carbon Fiber Composites Wastes

Short chopped fibers coated by epoxy resin of different length (5 to 10 mm length) were added at low volume content (about 4.6% on the composite) to alkali-activated fly ash or metakaolin mortars. These uncured scraps derive from the production of carbon fiber-reinforced polymer composites and they are not presently recycled, despite their outstanding mechanical properties. The workability, microstructure, porosity, and physical and mechanical properties (mainly flexural strength) of the derived materials were investigated. Superior flexural strength and increased toughness were obtained. An acid treatment of the scraps further improved the mechanical properties of the mortars by changing the chemical structure of the surface, thus increasing the interaction with the inorganic phase. These results foster the use of these wastes to improve the performance of low carbon footprint building materials such as alkali-activated composites in the building industry.

Molecular dynamics and experimental study on the adhesion mechanism of polyvinyl alcohol (PVA) fiber in alkali-activated slag/fly ash

This paper aims to study the adhesion mechanism of polyvinyl alcohol (PVA) fiber within alkali-activated slag/fly ash (AASF) matrix using molecular dynamics (MD) simulation in combination with systematic experimental characterization. The adhesion of PVA to C-(N-)A-S-H gel with different Ca/(Si+Al) and Al/Si ratios was modeled using MD simulation, with the related adsorption enthalpy calculated and the adhesion mechanism explored. The experimentally attained chemical bonding energy of PVA fiber in AASF coincides well with the simulation results. In both cases, the adhesion enhances primarily with increasing Ca/(Si+Al) ratio of C-(N-)A-S-H gel. Additionally, MD simulation indicates preferential element distributions of Ca around PVA molecule, which was confirmed experimentally by the detection of the Ca-rich C-(N-)A-S-H gel in the interfacial transition zone (ITZ).This study provides further insights into the adhesion mechanism of PVA fiber to C-(N-)A-S-H gel formed in AASF, which is particularly valuable for the future development of PVA-based high-performance alkali-activated composites.

Mix design optimization for fresh, strength and durability properties of ambient cured alkali activated composite by Taguchi method

The Alkali Activated Composite (AAC) is gaining the attention of the researchers and material scientists worldwide owing to its improved sustainable and mechanical attributes over the conventional concrete. Despite the acclaimed strength improvement, the lack of experimental data on the durability of the material has restrained the usage of AAC in the construction field. The strength and durability of AAC primarily depend on the mix design and adequate proportions of the constituents. To obtain the optimum proportions of the materials, enormous trial-mixes are required, and despite many efforts, the desired properties are likely to vary during the testing. To streamline the mix design by an improved method of preparing the mixes the present work is focused on utilizing the Taguchi method and the design guidelines of IS: 10262:2019 for the appropriate mix proportions of the constituents. The primary input parameters namely the molar content of NaOH, binder proportions, the dosage of plasticizer, and sodium silicate to sodium hydroxide ratio have been considered as the signal parameters suggested by the Taguchi analysis. The workability and the strength performance of the mixes have been evaluated by considering the higher is a better signal to noise ratio and the durability by considering signal to noise ratio as‘lower is better’. The test results and the predicted values of the proportions of the mix design showed good agreement and effectiveness when prepared as per the code provisions.

Development of alkali-activated composites from calcined iron-rich laterite soil

An experimental investigation was carried out to investigate the feasibility of using calcined iron-rich aluminosilicate (i.e. laterite soil) as a precursor for making alkali-activated mortars (AAMs). Laterite soil was calcined at a temperature of 600 °C and activated with an alkali solution in order to produce the AAMs. The developed AAMs were cured at 27 °C and 80 °C for 4 h followed by continuous curing at 27 °C until testing. The corresponding mechanical and durability performance of the AAMs were evaluated at 7, 28 and 90 days.The findings from this study showed that the use of initial elevated curing (i.e. at 80 °C) enhanced the performance of the AAMs. AAMs cured at 80 °C exhibited compressive strength and flexural strength up to 32.2 MPa and 12.1 MPa, respectively at 28 days. It was also found out that the mechanical performance of the AAMs can be correlated with the apparent porosity and water absorption. The microstructure evaluation of the AAMs revealed that the binder phases are a mixture of amorphous Na-aluminosilicate and iron silicate binder phases surrounding the sand particles. Similar to the mechanical performance, AAMs cured at 80 °C exhibited better resistance to the acid environment (H2SO4 solution of pH=1) than the ones cured at 27 °C. The higher degree of deterioration in AAMs cured at 27 °C was due to the lower rate of reaction and higher porosity in the AAMs. The properties of the end products showed that calcined laterite is a decent alternative precursor for alkali-activated materials.

Mitigating of drying shrinkage in alkali-activated slag composites

Alkali activated slag composites are promising alternatives to be used as a replacement of Portland cement composites for different construction applications. However, despite the evolution of these composites over the years, its high drying shrinkage still poses a limitation on its application. The increasing interest in alkali-activated slag composites by the research community has resulted in the use of various methods and materials to mitigate the drying shrinkage. This current paper explores the different major types of mitigation techniques that can be used to reduce the drying shrinkage in alkali-activated composites. The mitigation techniques explored are in terms of the use of various materials and curing methods. Discussions presented in this paper showed that a significant reduction in the drying shrinkage can be achieved by partially replacing slag with mineral admixtures or incorporating chemical admixtures specifically made to reduce shrinkage. The use of appropriate internal or external curing method for alkali-activated slag was also found to reduce drying shrinkage effectively. However, it is recommended to carry out further research in order to fully understand the mechanism of drying shrinkage in alkali-activated slag composites in order to develop effective ways to mitigate it.

Evaluation of Physical Properties of a Metakaolin-Based Alkali-Activated Binder Containing Waste Foam Glass.

Foam glass production process redounds to large quantities of waste that, if not recycled, are stockpiled in the environment. In this work, increasing amounts of waste foam glass were used to produce metakaolin-based alkali-activated composites. Phase composition and morphology were investigated by means of X-ray powder diffraction, Fourier-transform infrared spectroscopy and scanning electron microscopy. Subsequently, the physical properties of the materials (density, porosity, thermal conductivity and mechanical strength) were determined. The analysis showed that waste foam glass functioned as an aggregate, introducing irregular voids in the matrix. The obtained composites were largely porous (>45%), with a thermal conductivity coefficient similar to that of timber (<0.2 W/m∙K). Optimum compressive strength was achieved for 10% incorporation of the waste by weight in the binder. The resulting mechanical properties suggest the suitability of the produced materials for use in thermal insulating applications where high load-bearing capacities are not required. Mechanical or chemical treatment of the waste is recommended for further exploitation of its potential in participating in the alkali activation process.

Performance Evaluation of Cementless Composites with Alkali-Sulfate Activator for Field Application.

This study analyzed the performance evaluation of alkali-activated composites (AAC) with an alkali-sulfate activator and determined the expected effects of applying AACs to actual sites. Results revealed that when the binder weight was increased by 100 kg/m3 at 7 days of age, the homogel strength of ordinary Portland cement (OPC) and AAC increased by 0.9 and 5.0 MPa, respectively. According to the analysis of the matrix microstructures at 7 days of age, calcium silicate hydrates (C–S–H, Ca1.5SiO3.5·H2O) and ettringite (Ca6Al2(SO4)3(OH)12·26H2O) were formed in AAC, which are similar hydration products as found in OPC. Furthermore, the acid resistance analysis showed that the mass change of AAC in HCl and H2SO4 solutions ranged from 36.1% to 88.0%, lower than that of OPC, indicating AAC’s superior acid resistance. Moreover, the OPC and AAC binder weight ranges satisfying the target geltime (20–50 s) were estimated as 180.1–471.1 kg/m3 and 261.2–469.9 kg/m3, respectively, and the global warming potential (GWP) according to binder weight range was 102.3–257.3 kg CO2 eq/m3 and 72.9–126.0 kg CO2 eq/m3. Therefore, by applying AAC to actual sites, GWP is expected to be 29.5 (28.8%)–131.3 (51.0%) kg CO2 eq/m3 less than that of OPC.