Stability Of Geopolymers Research Articles

Preparation, characterization, and determination of mechanical and thermal stability of natural zeolite-based foamed geopolymers

This study is the first attempt to evaluate the effect of two foaming agents, aluminum powder and hydrogen peroxide (30 wt% water solution) on zeolite-based geopolymers and their possible use as building material according to Ecuadorian technical standards. These foamed geopolymers were prepared by alkali activation of an Ecuadorian natural zeolitic tuff and by alkali activation using NaOH (10 M), Ca(OH)2 and Na4Si5O12, with the use of two different foaming agents mentioned. In the case of aluminum powder, the experimental range was between 0.05 and 0.25 wt%; on the other hand, the percentages of hydrogen peroxide ranged from 0.5 to 4.0%. For characterization of the foamed geopolymer samples, quantitative X-ray diffraction (QXRD), thermogravimetric (TGA), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM) techniques were used. The results of both foaming agents in zeolite-based geopolymers met Ecuadorian technical standards NTE INEN 643 and NTE INEN 638; nevertheless, the use of aluminum powder as foaming agent exhibited a better pore size distribution, suggesting a better performance. On the basis of these results, foamed geopolymers are suitable construction materials for the application of manufactured for masonry wall in houses.

Alkali activation as new option for gold mine tailings inertization

The mining industry produces a huge quantity of sulphidic mine tailings, which cause several short- and long-term environmental problems when disposed by landfilling in impounding lakes. The possibility of immobilizing several heavy metals from gold mine tailings by reactive geopolymerization technique has been investigated in the present study. The chemical stability of geopolymers synthetized by the alkali activation of metakaolin and blast furnace slag and the addition of 40–50 wt% gold mine tailings is demonstrated. The geopolymers were cured at room temperature, and the effects of different Si/Al and Na/Al molar ratios and curing times were investigated. The inertization effectiveness was evaluated by means of leaching tests carried out according to standard EN 12457 after 7 and 28 days and after 18 months. The samples were immersed into the water for 1 day, and the leachable metals in the test solution were determined by ICP-OES. The results show that various elements (Cr, Cu, Ni, Zn and Mn) from gold mine tailings are able to immobilize almost completely by alkali activation with proper co-binder material. The immobilization efficiency were highly improved with longer curing period also for the problematic elements As, V, Sb and B.

Effects of Si/Al molar ratio on strength endurance and volume stability of metakaolin geopolymers subject to elevated temperature

Good structural performance in a fire scenario necessitates that the structural material possesses chemical stability, deformation resistance and strength endurance. Excellent chemical stability for geopolymers has been reported in literature at a microscale. However, their performance at macroscale has not yet been systematically explored and the underlying mechanisms remain unexplained. In current study, effect of variation in Si/Al molar ratio on the meso- and macro-scale thermal stability of metakaolin geopolymers has been comprehensively investigated to discover the underlying mechanisms governing the performance. Results show that all the geopolymer samples experienced reduction in compressive strengths after exposure to high temperature up to 900°C. Although, the geopolymer mixes exhibited good chemical stability at microscale, they possessed poor volume stability at mesoscale with very high thermal shrinkage. It was observed that thermal shrinkage induced crack formation dominates the residual strength for geopolymer mixes with Si/Al molar ratio ≤ 1.50, while densification of matrix is the governing factor of the residual strength for geopolymer mixes with Si/Al molar ratio > 1.50. Re-crystallization of nepheline at high temperature adversely affect the strength by inducing expansion and cracking of the geopolymer matrix. Geopolymer sample with Si/Al ratio 1.75 retained highest strength (6MPa) because viscous sintering of geopolymer mixes with high Si/Al ratio at temperature beyond 600°C enables localized healing of micro-cracks and densification of matrix which favored compressive strength gain after exposure to 900°C. At an even higher Si/Al of 2.0, foaming of unreacted silica upon heating can lead to expansion and cracking of the sample which reduce the strength. It was observed that due to high degree of cracking damage and low residual strength retention, it is essential to improve the macro-scale stability of metakaolin geopolymers for structural fire resistance applications.

Geopolymer with improved thermal stability by incorporating high-magnesium nickel slag

The present study investigates the thermal stability of binary geopolymers prepared with fly ash and high magnesium nickel slag (HMNS). TGA, XRD and SEM techniques are adopted to investigate the chemical composition and microstructure of the binder gel phase exposed to high temperatures. The results show that incorporating HMNS mitigates dehydration induced mass loss and volume shrinkage of the geopolymer samples between room temperature and 250°C, and increases their residual strengths when exposed to high temperatures between 600°C and 800°C. The key role that HMNS substitution of improving the thermal stability of geopolymers is associated with the formation of sodium-alumina(magnesia)-silica hydrate (N-A(M)-S-H) gel phase, which turns to be a less porous microstructure after thermal exposure when compared with the N-A-S-H gel phase, which is formed in the pure fly ash sample. The binary geopolymers exhibit higher residual strengths and better maintaining of volume stability in the range from 250°C to 550°C compared to Portland cements. However, both the geopolymer and Portland cement show severer volume shrinkage after exposure of 600°C, because of the viscous sintering of geopolymer gels and the decomposition of cement hydration products, respectively.

An investigation of the effect of migratory type corrosion inhibitor on mechanical properties of zeolite-based novel geopolymers

The effects of migratory type corrosion inhibitor and curing time on the thermal stability and mechanical properties of Ecuadorian natural zeolite-based geopolymers were evaluated. Geopolymer samples were prepared by alkali activation of the natural zeolite by 8 M NaOH solution and calcium hydroxide Ca(OH)2 1–3 wt%, with an activator/binder ratio of 0.6. The geopolymer samples cured for 24 h at 40 °C and then for 6 days more at room temperature showed the compressive strength values in a range of 3–5,5 MPa. Mineralogical analysis of natural zeolite obtained by XRD is as follows: Mordenite (∼67%), quartz (∼27%) and amorphous (∼6%). SEM-EDS micrographs analysis of geopolymers revealed the presence of Na and Ca which proves the incorporation of the activators, NaOH and Ca(OH)2. The compressive strength values obtained indicate that the use of alkali activation of natural zeolites is an effective method for the synthesis of geopolymers. The mechanical properties of geopolymers were slightly but not adversely affected by the addition of the migratory corrosion inhibitor, MCI-2005 NS. These results will be used in future research on geopolymer concrete with embedded reinforcing steel.

Influence of Fly Ash–Based Geopolymer Binder on the Sedimentation Behavior of Dredged Mud

This laboratory study investigated the sedimentation behavior of a fly ash–based geopolymer-stabilized dredged-mud slurry extracted from the Port of Townsville, Queensland, Australia. Settling-column tests were conducted to study the sedimentation behavior of 400%-water-content dredged mud of untreated and geopolymer-stabilized slurries at 6, 12, and 18% content by weight. Dredged-mud slurry interface height movements with elapsed time were recorded, and their settling patterns were observed. Mineralogical and microstructural characteristics of dried, untreated, and geopolymer-stabilized dredged-mud sediments were determined using X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) analysis. The study found that a fly ash–based geopolymer gel coating the dredged-mud particles in the slurry led to a flocculated settling behavior, and the geopolymer stabilization reduced the overall dredged-mud slurry sedimentation duration. XRD and SEM/EDS analysis showed that the geopolymer stabilization altered the microstructure of stabilized dredged-mud sediment and reduced its desiccation shrinkage cracks.

Thermal stability of geopolymers used as supporting materials for TiO2 film coating through sol-gel process: Feasibility and improvement

Thermal stability of geopolymers synthesized with fly ash, metakaolin as aluminosilicate sources was investigated in order to explore the feasibility of using geopolymers as supports for nano-titanium dioxide film coating via sol-gel process. Structural and compositional changes of geopolymers heated up to 800°C were examined by X-ray diffraction, Fourier transform infrared spectroscopy, mercury intrusion porosimetry and thermogravimetric analysis. Below 800°C, the loss of water is the main cause for the strength degradation and thermal shrinkage of geopolymers. Geopolymers synthesized with appropriate fly ash content possess good thermal stability because of high permeability arising from low reactivity of fly ash, filler effect provided by unreacted fly ash particles and original occurring minerals as quartz, mullite and hematite. TiO2 films were coated onto geopolymer derived from fly ash containing sources via sol-gel process. The grazing-incidence X-ray diffraction patterns of the films calcined at different temperatures show that anatase crystalizes well at 600°C. The study reveals the feasibility of using geopolymers with improved thermal stability as supports for TiO2 film coating via sol-gel process.

Thermal Behaviour of Metakaolin/Fly Ash Geopolymers with Chamotte Aggregate.

Geopolymers are generally appreciated for their good resistance against high temperatures. This paper compares the influence of thermal treatment with temperatures ranging from 200 to 1200 °C on the mechanical properties and microstructure of geopolymers based on two different aluminosilicate precursors, metakaolin and fly ash. Moreover, the paper is also aimed at characterizing the effect of chamotte aggregate on the performance of geopolymers subjected to high temperatures. Thermal treatment leads to a deterioration in the strength of metakaolin geopolymer, whereas fly ash geopolymer gains strength upon heating. The formation of albite above 900 °C is responsible for the fusion of geopolymer matrix during exposure to 1200 °C, which leads to the deformation of the geopolymer samples. Chamotte aggregate improves the performance of geopolymer material by increasing the thermal stability of geopolymers via sintering of the aggregate particles with the geopolymer matrix in the contact zone.

Stabilization of Demolition Materials for Pavement Base/Subbase Applications Using Fly Ash and Slag Geopolymers: Laboratory Investigation

AbstractThe use of recycled construction and demolition (C&D) materials in unbound and cement stabilized pavement base/subbase applications has generated growing interest in recent years. C&D materials consisting of crushed brick (CB), recycled crushed aggregate (RCA), and reclaimed asphalt pavement (RAP) have been investigated in unbound and cement stabilized pavement base/subbase applications. However, the high carbon footprint of using cement for pavement base/subbase stabilization has led to this research to seek alternative low-carbon binders. This study evaluates the behavior of C&D materials when stabilized with geopolymers. Fly ash (FA) and ground granulated blast furnace slag (S) were used as pozzolanic binders and a different alkaline activator solution to pozzolanic binder ratio was tested. A maximum of 4% of dry weight of soil was used for geopolymer stabilization of the C&D materials. The binders used were either 4% FA, 2% FA+ 2% S, or 4% S. The geotechnical engineering and strength propertie...

Spent Coffee Grounds–Fly Ash Geopolymer Used as an Embankment Structural Fill Material

AbstractThe drinking of coffee forms a deep-rooted pastime in many communities worldwide. However, the culture of coffee drinking generates vast quantities of organic waste that ends up in landfills. Current research trends are inclined towards recycling of waste materials into alternative construction materials, hence the need to research sustainable uses for spent coffee grounds. Coffee grounds (CG) are highly organic with a very high percentage of biodegradable material. The objective of this research was to study the strength development of CG when used as a geopolymer stabilized embankment structural fill material aiming for a better understanding of geopolymer stabilization of highly organic material. Fly ash (FA), being a silica and alumina rich material, was used as a precursor. A liquid alkaline activator, L, being a sodium hydroxide-sodium silicate solution was used for alkali activation of FA in the CG-FA geopolymer. Factors found to affect strength development of the CG-FA geopolymer were: (1)...

Fire resistant melamine based organic-geopolymer hybrid composites

Hybrid composite materials based on geopolymer and epoxy melamine resins have been designed to produce a novel material showing high compatibility between the organic and inorganic phases. This has been realized through a synthetic method based on the co-reticulation in mild condition of the two phases. These materials show good interpenetration of the resin into the inorganic matrix up to nanometric level and significantly improved mechanical properties in respect to un-modified geopolymer without compromising the good thermal stability and fire resistance typical of unmodified geopolymers. These properties make the novel materials very attractive for the realization of thermo-resistant and thermo-insulating panels.

Influence of the activating solution composition on the stability and thermo-mechanical properties of inorganic polymers (geopolymers) from volcanic ash

The influence of the activating solution composition on the stability and thermo-mechanical properties of geopolymers made from a Cameroonian volcanic ash is investigated. NaOH, KOH solutions and silicate solutions with low modulus were tested.The samples are cured at 90°C and the mechanical strength increases up to 21days. With NaOH the strength development is faster, with a dry and wet compressive strength respectively around 40MPa and 20MPa at 21days. The largest particles only act as reactive filler while smaller particles dissolve in the activating solution as seen by SEM. Pure volcanic ash and synthesized materials consisted of mostly X-ray amorphous material with some newly formed crystalline phases. KOH specimens were found to be thermally more stable, shrinking less than 3% after heating till 1000°C.The obtained results suggest the possible use of the synthesized materials for building applications and low temperature refractories up to 700–800°C.

The effects of phase changes on the bonding property of geopolymer to hydrated cement

Using geopolymers as bonding materials could be an alternative solution to the concrete repair. This study reports the effects of phase changes on the bonding property of geopolymer to hydrated cement. A geopolymer binder was prepared by activation of heated kaolin with sodium silicate solution, bonded with 28days aged cement paste and cured under different conditions. The experimental results demonstrated that under the 20°C air curing conditions, the bond strength achieved 1.3MPa at 7days and increased to 1.5MPa at 28days. While under the 80°C steam and water curing conditions, the bond strength decreased by 31% and 37% respectively. The XRD, FTIR and SEM analysis of the geopolymers and hydrated cement pastes show that the strength loss is due to two factors: (1) the increased porosity in cement paste due to the water loss and/or crystallization of C–S–Hs; and (2) the mineralogical change (crystallization) in geopolymer binder which becomes more ordered structures. The increased porosity and the crystallization either in cement or geopolymer generate local stress and weak regions at the interface. This study suggests that the stability of metakaolin-based geopolymers should be taken into consideration when they are used as bonding or coating materials for concrete repair, particularly at the conditions with elevated temperatures.

Influence of Na2O/Al2O3and SiO2/Al2O3ratios on the immobilization of Pb from electric arc furnace into the fly ash based geopolymers

Electric arc furnace dust (EAFD) is by-product of iron and steel scrap remelting in electric arc furnace. This waste is classified as a hazardous material due to the leaching of heavy metals into the environment. Geopolymeryzation of fly ash into construction materials called - geopolymer is a considering as a possible way of immobilization of heavy metals from EAFD. Mechanical properties and chemical stability of geopolymers are mostly determined by the ratio between main oxides, Na2O, Al2O3 and SiO2. In this paper we have investigated possibility of Pb immobilization from EAFD into the fly ash based geopolymer. Geopolymers are synthesized at different Na2O/Al2O3 and SiO2/Al2O3 ratios. Compressive strength and chemical stability of geopolymers with respect to the leaching of Pb in water according to the EN 12457 regulation were tested. The results have shown that Na2O/Al2O3 and SiO2/Al2O3 ratios have influence on the compressive strength of geopolymers and Pb release in the water environment.

Chemical stability of geopolymers containing municipal solid waste incinerator fly ash

Municipal solid waste incinerators every year produce tons of fly ashes which, differently from coal fly ashes, contain large amounts of toxic substances (heavy metals, dioxins, furans). The stabilization/solidification (S/S) technology known as geopolymerization is proposed with the purpose to bond physically and chemically incinerator fly ashes (IFA) in a solid matrix, in order to reduce pollutant mobility. The chemical stability of geopolymers with Si/Al ratio of 1.8–1.9 and Na/Al ratio of 1.0, synthesized by alkali activation of metakaolin and the addition of 20 wt% of two different kinds of IFA, is presented. The concentration of the alkaline solution, water to solid ratio and curing process have been optimized. The room temperature consolidation of IFA containing geopolymers has been tested for leachability in water for 1 day, accordingly to EN 12457 regulation and extended to 7 days to increase the water attack on solid granules. Leachable metals in the test solution, determined by ICP_AES, fall within limit values set by regulation for non-dangerous waste landfill disposal. Geopolymeric matrix evolution with leaching time has been also evaluated in terms of pH and electrical conductivity increase in solution.

Effects of Anions on the Formation of Aluminosilicate Gel in Geopolymers

This work demonstrates that an increase in soluble silicate concentration over ∼200 mM in an aqueous alkaline solution (pH = ∼13.95) can significantly increase the reactivity of a Class F fly ash at room temperature. The greatly increased dissolution of the fly ash primary phase(s) and the subsequent secondary precipitation are responsible for the formation of an aluminosilicate gel similar to the major binding phase of a geopolymer. On the basis of this observation, a reaction model was developed to simulate aluminosilicate gel formation in a real geopolymeric system. In this work, potassium salts of chloride, carbonate, oxalate, and phosphate (KCl, K2CO3, K2C2O4·H2O, and K2HPO4) were used as solution contaminants. It was found that the anions (Cl-, CO32-, C2O42-, and PO43-) significantly affected the formation kinetics as well as the nature of the aluminosilicate gel formed in the reaction model. This observation could be successfully used to mechanistically explain the retardation effects of the various salts on the gel solidification time of a geopolymer. Aluminosilicate crystallization in the chloride-affected and the oxalate-affected reaction models implies that similar processes could also occur in geopolymers. This could affect the chemical stability and durability of geopolymers and thoroughly deserves a careful future investigation.