Alkali-activated Geopolymer Research Articles

Green, non-toxic and efficient adsorbent from hazardous ash waste for the recovery of valuable metals and heavy metal removal from waste streams

As compared to alkali-activated geopolymers with phosphoric acid which may be used in high concentrations resulting in disposal concerns, acid-based geopolymers may have superior properties. A novel green method of converting waste ash to a geopolymer for use in adsorption applications such as water treatment is presented here. We use methanesulfonic acid, a green chemical with high acid strength and biodegradability to form geopolymers from coal and wood fly ashes. The geopolymer is characterized for its physico-chemical properties and tested for heavy metal adsorption. The material specifically adsorbs iron and lead. The geopolymer is coupled to activated carbon forming a composite, which adsorbs silver (precious metal) and manganese (hazardous metal) significantly. The adsorption pattern complies with pseudo-second order kinetics and Langmuir isotherm. Toxicity studies show while activated carbon is highly toxic, the geopolymer and the carbon-geopolymer composite have relatively less toxicity concerns.

Carbon nanofiber dispersion in alkali solution and its reinforcement of alkali-activated volcanic ash-based geopolymers

Volcanic ash has several negative impacts on the environment and can be used in massive to synthesize geopolymers, which are considered quasi-brittle. Carbon nanofibers (CNFs) can reinforce the geopolymers, and the dispersion of CNFs directly determines the reinforcement effect. However, the dispersion of CNFs in alkali solutions is challenging. In this paper, the effect of surfactant type, dosage, and ultrasonic time on CNFs dispersion in aqueous and alkali solutions was investigated by the quantitive method of microscopic image processing to determine the preferred dispersion scheme. Zeta potential tests and visual observation were used to analyze the mechanism and stability. Based on this, volcanic ash-based geopolymer nanocomposites with 0.1 wt% contents of CNFs were synthesized. The mechanical properties were tested, and the microstructure was characterized using Scanning Electron Microscope coupled with Energy Dispersive Spectroscopy and Mercury Intrusion Porosimetry. The results showed that methylcellulose (MC), polycarboxylate superplasticizer (PC), and polyvinylpyrrolidone (PVP) could disperse CNFs in aqueous, but evident aggregations can be observed in the alkali solution. PC performed better than MC and PVP in alkali solutions, with 4.83% aggregation area of the whole image, 86% of 0–100 μm2 aggregations, and an average aggregation area of 87 μm2, beneficial from the comb-like structures. Dispersed CNFs can improve the 28-d flexural and compressive strength of the resulting geopolymer nanocomposite by 23% and 16%, respectively, and refine the pore structure through filling, bridging, and nucleation effects, decreasing the average pore size and porosity by 33% and 15%. This paper will contribute to the resourceful use of volcanic ash to reduce pollution and the higher reinforcement of CNFs in alkali-activated geopolymers.

Characterizing a Local Sewage Sludge Ash for Developing Alkali-Activated Geopolymer Mortar

Production of ordinary Portland cement (OPC) has significant adverse effects on the environment and accounts for 5%–7% of CO2 emissions globally. Geopolymer cement is potentially a sustainable alternative to OPC and is created by the activation of aluminosilicate materials with an alkaline solution that induces geopolymerization, causing gelation of the materials. Sewage sludge ash (SSA) contains relatively high contents of aluminosilicate materials and can be a precursor for geopolymers. This study aims to characterize a local SSA as a precursor to develop geopolymer mortar. Chemical and leaching analysis found no significant adverse environmental implications of the SSA. Strength activity index tests showed that the SSA has a moderate pozzolanic activity. The SSA was then utilized to develop a geopolymer mortar in conjunction with an alkaline solution consisting of sodium hydroxide and sodium silicate. Factors that influence the geopolymerization and strength gain of geopolymers were examined, including the molar ratio between silicon dioxide and sodium oxide and the activator/binder ratio. An optimal activator/binder ratio of 0.22 and a SiO2/Na2O molar ratio of 0.80 were determined to yield the highest compressive strength of the geopolymer mortar.

Characterization of Alkali Activated Materials Prepared from Continuous Attrition and Ball Milled Fly Ashes

Mechanical activation is known to greatly influence the reactivity of fly ashes. In this paper, we report a comparative study of the properties of alkali-activated geopolymer materials prepared using both ball-milled and attrition-milled fly ashes. Ball milling was carried out for 30 min and 60 min while attrition milling was carried out continuously in a high-speed attritor. The surface area of the raw fly ash decreased from 4017 cm2/g to 3999 cm2/g and 3912 cm2/g after ball milling for 30 min and 60 min, respectively. By contrast, the surface area of the continuously attrition-milled fly ash increased to 5545 cm2/g. Fly ash processed by continuous attrition milling showed a 50% particle size reduction to 25–38 μm, whereas fly ash ball-milled for 30 and 60 min was reduced in size by 33.4 and 42.9%. The milled fly ash samples were activated with 8 M NaOH solution and cured at 40 °C for 68 h. After curing, the samples were maintained at room temperature, and their 7-, 14-, and 28-day compressive strengths were measured. The compressive strength of the attrition-milled 28-day geopolymer paste was 24.6 MPa; that of the geopolymers ball-milled for 30 and 60 min was 23.37 MPa and 17.58 MPa, respectively; and that of the unmilled control geopolymer fly-ash-based paste was 17 MPa. The improvement in the mechanical properties is attributed to the increased gel formation resulting from the increased surface area (decreased particle size) in the fly ash glass starting material.

The Influence of Substitution of Fly Ash with Marble Dust or Blast Furnace Slag on the Properties of the Alkali-Activated Geopolymer Paste

Worldwide, it is now known that industrial by-products rich in silicon (Si) and aluminum (Al) can be transformed by alkaline activation into so-called “green concrete”, an efficient and sustainable material in the field of construction; the geopolymer material. In this work, geopolymer materials produced using fly ash and marble dust or blast furnace slag were studied to assess the influence of these substitutions on the performances of the final product. Geopolymer materials have been characterized by physico-mechanical methods, FTIR spectroscopy and microscopically. The analysis of the results indicates the reduction of the mechanical strength performance by substituting the fly ash as the raw material.

Study on Mechanical Properties and Micro Characterization of Fibre Reinforced Ecological Cementitious Coal Gangue Materials.

Eco-gelled coal gangue materials (EGCGMs) are usually produced using coal gangue, slag, and fly ash in a highly alkaline environment. Herein, to improve the mechanical properties of such materials, polypropylene fibers were uniformly mixed with them. An unconfined compressive strength test and a three-point bending test of the fiber-reinforced EGCGMs under different conditions were conducted. Based on the performance degradation control technology of the fiber structure, the interface mechanism of the composite materials was analyzed from the micro level using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). In the mechanical test, the 28 d UCS and flexural properties of the fiber-reinforced EGCGMs were analyzed using the Box-Behnken design response surface design method and orthogonal design method, respectively. The order of significance was as follows: sodium hydroxide, fiber length, and fiber content. Within the scope of the experimental study, when the NaOH content is 3, the fiber content is 5 ‱, and the fiber length is 9 mm, the mechanical properties are the best. Based on the microscopic equipment, it was discovered that the amorphous ecological glue condensation product formed by the reaction of slag and fly ash in the alkaline environment was filled between the coal gangue particles and the fibers, and several polymerization products accumulated to form a honeycomb network topology. The distribution of fibers in the EGCGM matrix could be primarily divided into single embedded and network occurrences. The fiber inhibits the crack initiation and development of the matrix through the crack resistance effect, and improves the brittleness characteristics through the bridging effect during the failure process, which promotes the ductility of the ecological cementitious coal gangue matrix.The results presented herein can provide a theoretical basis for improving the mechanical properties of alkali-activated geopolymers.

Synthesis and Characterization of High-Performance Blended Alkali-Activated Geopolymer (FA/GBFS) from Industrial Wastes

Synthesis and Characterization of High-Performance Blended Alkali-Activated Geopolymer (FA/GBFS) from Industrial Wastes

Influence of Various Coal Energy Wastes and Foaming Agents on Foamed Geopolymer Materials' Synthesis.

The regularities of obtaining foamed alkali-activated geopolymer materials based on different wastes of coal power engineering (fly ash, fuel (boiler) slag, ash, and slag mixture) were considered. The phase composition of the studied waste showed the presence of a significant amount of the amorphous phase, as well as a crystalline phase. mostly in the form of high quartz. The microstructure of studied the waste showed that the fly ash consisted of monodisperse hollow aluminosilicate microspheres, the fuel slag was represented by polydisperse irregular particles, and the ash and slag mixture included both of these materials in different ratios. Blowing agents such as aluminum powder, hydrogen peroxide, and sodium hypochlorite were chosen to achieve the porous structure of the geopolymer materials. The calculations of the geopolymer precursor compositions were carried out. Samples were synthesized, and their physical and mechanical properties, such as density, strength, porosity, and thermal conductivity, were analyzed. The micro- and macrostructure of the samples, as well as the pore distribution of the obtained geopolymers were studied. Conclusions were made on the choice of the most-optimal foaming agent and the optimal coal combustion waste suitable for the synthesis of the geopolymer materials.

Atomic-scale characterisation of sodium aluminosilicate hydrates (N-A-S-H) and Mg-substituted N(-M)-A-S-H using XANES

The chemical structures of aluminosilicate hydrates presented in alkali-activated geopolymer materials underpin their performances. Mg-substituted sodium aluminosilicate hydrates (N(-M)-A-S-H) are likely to be present in alkali-activated geopolymer materials prepared using MgO-containing precursors, however, their atomic-level structures remain unclear. The lack of such knowledge made it challenging to identify and distinguish N(-M)-A-S-H from complex alkali-activated geopolymer systems (i.e., alkali-activated slag, alkali-activated Mg-rich minerals), and therefore brought challenges in understanding and predicting their durability. This study characterised for the first time the atomic structures of the synthetic N(-M)-A-S-H gels, prepared through ion-exchange or co-synthesis, using X-ray absorption near-edge spectroscopy (XANES) at Si, Al and Mg K-edge. The results suggest that the substitution of Mg in the extra-framework locations of the alkali aluminosilicate hydrates (N-A-S-H) leads to negligible changes in the coordination environments of the aluminosilicate framework. However, the Mg coordination environment is distinguishably different from other Mg-containing phases in the systems, e.g., hydrotalcite. The Mg K-edge XANES of N(-M)-A-S-H shows a 0.8–1.2 eV shift compared with hydrotalcite. The results presented in this study can be used as the fingerprint to probe the presence of N(-M)-A-S-H in alkali-activated geopolymer materials containing Mg element.

Environmental behavior and engineering performance of self-developed silico-aluminophosphate geopolymer binder stabilized lead contaminated soil

A silico-aluminophosphate geopolymer (APG) binder was developed for Pb2+ solidification/stabilization (S/S) treatment in our previous study, but the related performance of the APG stabilized Pb2+ contaminated soil was not investigated. This study investigates the environmental behavior and engineering properties of APG binder stabilized Pb2+ contaminated soil, and compares the S/S effect with cement and alkali-activated geopolymer cement by unconfined compressive strength, toxicity leaching, and pH-dependent tests, also explores the microscopic and mineral properties using scanning electron microscopy (SEM) and X-ray diffraction (XRD) tests. Results showed that the compressive strength of APG binder stabilized soil was higher than that of cement and alkali-activated geopolymer cement, the compressive strength increased with an increased APG binder content, increased first and then decreased with the Pb2+ content increasing, reaching its maximum value when the Pb2+ content was 2.0%. Under regular environment, the S/S effect of APG binder was basically consistent with that of alkali-activated geopolymer cement, in strong acid (pH = 2) and strong alkali (pH = 13) environment, the APG binder exhibited better S/S effect. The Pb2+ leaching in soil stabilized with the APG binder prepared by solid powder activator was higher at 7, 14 days, but basically the same as that of using liquid activator at 28 days. The Pb2+ leaching increased with electrical conductivity increasing, and there was a good correlation between Pb2+ leaching and electrical conductivity. SEM and XRD analysis showed that different binders stabilized contaminated soil showed different morphology and mineral composition, Lead phosphate compound was formed in APG stabilized soil. In general, the APG binder has a good application prospect in treating contaminated soil under extremely acidic environment.

Correlation between porous structure analysis, mechanical efficiency and gamma-ray attenuation power for hydrothermally treated slag-glass waste-based geopolymer

The main objective of this work is to compare conventional Portland cement pastes (OPC) and alkali-activated geopolymer pastes in terms of compressive strength, phase composition, micro-porosity, and attenuation ability against harmful gamma irradiation. OPC pastes were cured at 20 °C in tap water for up to 28 days, while geopolymer pastes were prepared by recycling two different industrial byproducts. Equal masses of slag (GGBFS) and recycled glass powder (RGP) were activated by two concentrations of NaOH (5% and 10% mass of total binders). The obtained geopolymer pastes were treated in different manners; some pastes were treated in high humidity at 20 °C and 80 °C for 28 days, while other pastes were cured in an autoclave for different time intervals (2, 4, 8 and 12 hrs) and different steam pressures (4, 6 and 8 bar). A Gamma attenuation capacity test was performed to determine the linear attenuation coefficient (µ) for some selected hardened pastes. Two sources of gamma-ray were used (137Cs/661.6Kev &60Co/1332.5Kev). To correlate mechanical performance and attenuation behavior, the N2-adsorption/desorption technique was conducted to analyze the micro-porosity. The results indicated GGBFS-RGP geopolymer pastes activated by 5 wt% NaOH and thermally hydrated at 4bars for 8hrs possessed the highest compressive strength (86 MPa), highest attenuation ability (µ = 0.156 and 0.126 cm−1), highest BET "Brunauer-Emmett-Teller model" surface area (10048 cm2/g) and minimum BJH "Barrett-Joyner-Halenda model"-average pore diameter (23.6 nm) as compared with OPC pastes and other all geopolymer pastes. After the autoclaving process, calcium-silicate-hydrate (C-S-Hs), calcium-alumino-silicate-hydrate (C-A-S-Hs), calcium-alumino-hydrate (C-A-Hs), sodium-alumino-silicate-hydrate (N-A-S-Hs) and hexagonal zeolite phase has been achieved via X-ray diffraction analysis (XRD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and scanning electron microscope (SEM/EDX).

Simultaneous stabilization/solidification of arsenic in acidic wastewater and tin mine tailings with synthetic multiple solid waste base geopolymer

Stabilization/Solidification (S/S) is considered as a feasible technology for the treatment of arsenic (As) in acidic wastewater (AW) and tin mine tailings (TMTs); however, high cost, high carbon footprint, and strict reaction conditions are the main limitations. Herein, a novel alkali-activated geopolymer material (AAGM) for S/S As was synthesized by combining AW, TMT, gypsum (GP), and metakaolin (MK). At room temperature, an initial As concentration of 3914 mg/L, a NaOH content of 4.98%, and an MK content of 20% decreased the As leaching concentration to 1.55 mg/L (<5 mg/L). The main S/S mechanisms of As included physical encapsulation of C-(A)-S-H and geopolymer structures, ion exchange of ettringite, and formation of Fe–As and Ca–As precipitates. Further studies showed that increasing initial As concentration and MK content facilitated the formation of Ca–As precipitates and C–(A)–S–H gels. The semi-dynamic leaching tests revealed that the leaching mechanism of As was surface wash-off. The effective diffusion coefficients of the samples were less than 10−13 cm2/s, and the respective leachability indexes were greater than 9, indicating that AAGM was effective in preventing the leaching of As. Therefore, this study provides a green and low cost solution for the synergistic utilization of AW, TMT, GP, and MK.

Influence of heat treatment on the mechanical properties of alkali-activated fly ash-based binders with marble dust substitution

Marble waste contains a high level of calcium, which is obtained from the cutting process in marble production. The properties of geopolymer binders are influenced not only by the amount of alkali activators, their ratio, the molarity used, and the Si and Al content of the mineral additives used in the mixture, but also by the duration of the heat treatment and the heat treatment temperature. This study aimed to produce alkali-activated geopolymer binders based on fly ash and marble dust. Alkali-activated geopolymer pastes were made both with heat treatment for 24h at 70°C and without heat treatment at (23±2) °C. Structural analysis by optical microscopy revealed, in the case of heat-treated samples, the formation of pores with an average size of 1÷1.5 mm, much larger than in the case of samples without heat treatment.

Self healing of alkali active mortars with expanded perlite aggregate

The use of alkali-activated mortars instead of cemented mortars provides an advantage in terms of various properties such as high strength, fast curing, and high durability by terminating cement production. However, concrete cracks reduce durability and strength, thus, it is crucial that they be healed. This study aimed to investigate the healing of early microcracks in geopolymer mortars by using Sporasarcina pasteurii, which makes carbonate precipitation, and expanded perlite aggregate as the bacteria carrier. This is the first study to investigate the bacterial healing of an alkali-activated geopolymer mortar using expanded perlite aggregate without applying a complex encapsulation process to the bacterial agents. For this purpose, optical, physical, and mechanical changes caused by self-healing were observed ahead of and after the cure period with tests like water absorption, compressive strength, UPV, SEM, FTIR, and optical microscope images. The results showed that bacterial cells could sporulate directly on the expanded perlite aggregate, visually improving the crack closure rate of the samples by a majority to 100 %, and the unit weights of all samples healed in DM and containing bacteria increased by up to 5 %.

Molecular dynamics simulation of phosphoric acid-based geopolymer

Phosphoric acid-based geopolymer (PABG) is a new amorphous cementitious material. PABG molecular dynamics (MD) simulation is carried out for the first time, using the modified Dreiding force field and the new macromolecular model. In this study, structural and mechanical properties of PABG, such as the bulk density, bond length, angle, radial distribution function (RDF), elastic modulus, and bulk modulus, are calculated by MD simulation. The bulk density is 1.85 ​g/cm3, close to 1.82 ​g/cm3 obtained in the experiment. The bond lengths of Si–O, Al–O, and P–O bonds and the angles of ∠OSiO, ∠OAlO, and ∠OPO agree well with the experimental values. The shape of the RDF curve can reflect short-range order and long-range disorder of the PABG structure. The elastic modulus is 33.42 ​GPa, close to 31.23 ​GPa measured by the nanoindentation experiment. The bulk modulus is 20.98 ​GPa, larger than the values obtained in the alkali-activated geopolymer (AAG) experiments. These results provide a reasonable basis for further PABG MD simulation research.

Structural, Mechanical and Chemical Properties of Low Content Carbon Geopolymer

In recent years geopolymers have shown increased interest as binders with low CO2 emission compared to Portland cement. The main goal of this research is focused on connecting green and sustainable characteristics with the mechanical and chemical properties of fly ash-based geopolymer. The samples of different ratios of fly ash (FA) and metakaolin (MK) were prepared. X-ray powder diffraction (XRD) showed that in the geopolymer synthesis reaction a new amorphous phase was formed. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) confirmed characteristic bands of the Si-O and O-Si-O groups at 1045 cm−1. Compressive strength analysis revealed that the optimal ratio of FA and MK is 50:50 and exhibits the highest value. X-ray photoelectron spectroscopy (XPS) analysis revealed the total reduction of carbon content in the alkali-activated geopolymer with the optimal stoichiometry of 50:50. This indicates the possibility of obtaining a geopolymer material with an almost complete absence of carbon, which implies further application as a material with a very high environmental potential and of zero carbon emissions.

Mix Proportion and Mechanical Properties of One-Part Alkali-Activated Geopolymer Concrete

Mix Proportion and Mechanical Properties of One-Part Alkali-Activated Geopolymer Concrete

Fly Ash Selection and Mix Proportioning for Ambient Cured High Strength In-situ Cast Mono Component Geopolymer Concrete

Potential valorisation of fly ash powders is examined for scalable production of in-situ cast structural member with ambient cured alkali-activated geopolymer concrete. The critical issue of strength variation has been investigated by correlating with the physical and chemical properties of the original fly ash affecting water demand in mix proportion. A Class-F fly ash having fineness of 680 m2/kg Blaine’s specific surface area (SSA) and a low w/b down to 0.16 showed the 28-day strength of 99 MPa. In contrast, medium sized (416-464 m2/kg) fly ash-based mono component concrete produced the strength between 40-47 MPa and that of the coarse (157-326 m2/kg) grade &lt; 20 MPa in the 28-day test. Lower strength was observed in Class-C fly ash containing high proportion of quartz sand exhibiting higher water demand. Results of mix proportioning suggest a higher strength gain with 20-30% increase in fly ash over the minimum cement content specified in the mix design guidelines of IS 456 : 2000 while replacing 15-20% of coarse aggregates by fine aggregates. Unconfined flexural load applied on the three M40 reinforced concrete (RC) beams, viz. mono component, bi-component geopolymer and Portland pozzolana cement showed similar deflection up to 20 kN load.

XRD and TG-DTA Study of New Phosphate-Based Geopolymers with Coal Ash or Metakaolin as Aluminosilicate Source and Mine Tailings Addition.

Coal ash-based geopolymers with mine tailings addition activated with phosphate acid were synthesized for the first time at room temperature. In addition, three types of aluminosilicate sources were used as single raw materials or in a 1/1 wt. ratio to obtain five types of geopolymers activated with H3PO4. The thermal behaviour of the obtained geopolymers was studied between room temperature and 600 °C by Thermogravimetry-Differential Thermal Analysis (TG-DTA) and the phase composition after 28 days of curing at room temperature was analysed by X-ray diffraction (XRD). During heating, the acid-activated geopolymers exhibited similar behaviour to alkali-activated geopolymers. All of the samples showed endothermic peaks up to 300 °C due to water evaporation, while the samples with mine tailings showed two significant exothermic peaks above 400 °C due to oxidation reactions. The phase analysis confirmed the dissolution of the aluminosilicate sources in the presence of H3PO4 by significant changes in the XRD patterns of the raw materials and by the broadening of the peaks because of typically amorphous silicophosphate (Si–P), aluminophosphate (Al–P) or silico-alumino-phosphate (Si–Al–P) formation. The phases resulted from geopolymerisation are berlinite (AlPO4), brushite (CaHPO4∙2H2O), anhydrite (CaSO4) or ettringite as AFt and AFm phases.

Solidification/stabilization of landfill leachate concentrate contaminants using solid alkali-activated geopolymers with a high liquid solid ratio and fixing rate

Landfill leachate concentrate (LLC) is a highly toxic wastewater that contains many refractory contaminants. One of the technical and economic treatment methods is solidification/stabilization (S/S), where the contaminants of LLC can be sealed in one step to achieve zero wastewater discharge. This study presents the S/S of LLC contaminants using solid alkali-activated geopolymers prepared from blast furnace slag (BFS) and powdery sodium silicate. The stability of the formed geopolymer was studied through unconfined compressive strength (UCS) and leaching tests. The strongest UCS was obtained when the modulus of the activator was 1.16 with a high liquid/solid ratio of 0.64. BFS-based geopolymers presented excellent LLC S/S efficiency. The S/S rates of TOC, CODCr, NH3–N, Cl−, and SO42− were 81%, 89%, 97%, 97%, and 78%, respectively. The S/S rates of heavy metals, i.e., Cd and Pb, were all more than 99%. The results of microstructure characterization showed that the S/S mechanism of LLC pollutants was the dual effect of physical closure and chemical stability. Cl− and SO42− were respectively stabilized in the crystal lattice by Friedel's salt and calcium sulfate, respectively, while organic matter and NH3–N were physically encapsulated in the dense structure of the geopolymer. Overall, BFS based geopolymers demonstrated high treatment capacity and excellent S/S efficiency, and have a potential application prospects in LLC treatment.