Geopolymer Gel Research Articles

Dissolution behaviour of source materials for synthesis of geopolymer binders: A kinetic approach

Controlling the initial release rate of alumina and silica from source materials is known to have a significant effect on the nanostructure of geopolymer gel and its final mechanical properties. However, most of the studies regarding the solubility of source materials take an equilibrium approach, and there is a gap in understanding of the release rates at far-from-equilibrium conditions. In the present study, the initial dissolution rate of some geopolymer precursors is characterised. The liquid to solid ratios are designed to be sufficiently high to minimise precipitation of hydration products, and the effects of solution alkalinity and milling on dissolution rates are investigated. While fly ash and blast furnace slag particles seem to release Si and Al at approximately similar rates, metakaolin shows a distinctively higher release of Si from the very early time of dissolution. Increasing solution alkalinity increases the dissolution of source materials up to some point, and the greatest effect is observed on fly ash particles. The most interesting result of milling is observed on fly ash particles where the release rate of silica has become higher than alumina, while contrasting behaviour is observed in the non-milled fly ash system. The opposite behaviour is observed in the slag system where milling rapidly increases the release rate of Al while the release rate of Si is increased slowly.

Characterisation of One-Part Geopolymer Binders Made from Fly Ash

The kinetics of one-part “just add water” geopolymerisation reaction is studied in the system of fly ash as an aluminosilicate precursor and solid sodium silicate as alkali activator. The rates of the release of Si and Al nutrients from source materials can significantly affect their availability for reaction and their extent of participation in geopolymer gel structure. The crystalline phases that usually appear in fly ash geopolymers are missing in the one-part mix binders studied here, and by increasing the Si/Al ratio the amount of Si contribution in the final geopolymer gel is decreased. The sample with lower water content sees the participation of more Si in both stages of gel formation. Adjusting the composition of raw materials can improve the mechanical properties of the final one-part mix binder. Reasonable mechanical strength with the maximum strength of 65 MPa is achieved.

Study on synthesis and characterization of ZSM-20 zeolites from metakaolin-based geopolymers

This paper presents a novel method for the synthesis of ZSM-20 zeolites. This method based on in situ transformation from a metakaolin-based geopolymer gel is more effective than the traditional hydrothermal method. The crystalline phase, micromorphology and microstructure of the geopolymers and the ZSM-20 zeolite samples were investigated using SEM, XRD and an N2 adsorption apparatus. The experimental results showed that pure ZSM-20 zeolite crystals were obtained without any by-product and with a large BET surface area of 78.52m2/g under optimal conditions. Key factors of the in situ transformation process were studied, such as the alkalinity of the geopolymer gel, the curing conditions and the hydrothermal conditions. The optimum observed conditions were as follows: the modulus of sodium silicate solution (SiO2/Na2O molar ratio) was 1.1, H2O/Na2O molar ratio=7.5, the geopolymers should be cured at 40°C for 3days, and the hydrothermal conditions should be kept at 140°C for 10h.

Synthesis of sodium waterglass from white rice husk ash as an activator to produce metakaolin-based geopolymer cements

Rice husk from source in Cameroon was leached in HCl (5M) to remove the most of metallic ingredients and then calcined at 600°C in order to produce white rice husk ash. This white ash were applied for producing sodium waterglass with different molar ratios SiO2/Na2O (0.31; 0.47; 0.62; 0.78; 0.93; 1.09 and 1.25) and then used to synthesize metakaolin-based geopolymer cements. The obtained white rice husk ash shows the loss of crystalline mineral and reveals high amorphous silica with quartz as impurity. Geopolymers GPi (i varying from 1 to 7) were obtained using different synthesis sodium waterglass (NWG) with a mass ratio NWG/MK=0.87. It could be observed that the 28 days compressive strength (4/5/7/9/32/34/36MPa) increase with increasing the molar ratios SiO2/Na2O defined in this work in the course GP1/GP2/GP3/GP4/GP5/GP6/GP7. The micrographs show the formation of more geopolymer gels when the molar ratios SiO2/Na2O in alkaline activators are between 0.93 and 1.25. Sodium waterglass from white rice husk ash proved to be an effective alkaline activator in geopolymers preparation. It can be concluded that it is possible to replace quartz sand and sodium carbonate which is responsible to greenhouse gas emitted during the production of commercial sodium silicate solution by using rice husk as silica sources.

The effect of heat treatment on the mechanical and structural properties of one-part geopolymer-zeolite composites

This contribution presents the results of structural and compressive strength investigations on cured and high-temperature treated silica-based one-part geopolymer-zeolite composites. The specimens were synthesized from two different silica sources, sodium aluminate and water. The phase content as well as the compressive strength of the cured composites varied depending on the starting mix-design and the silica feedstock. Besides geopolymeric gel, A-type zeolites and hydrosodalites were the major reaction products. One of the silica feedstocks yielded significantly higher compressive strength (19MPa), while the other one appears to cause less variation in phase content. Strength testing indicated an improvement on heating up to 200–400°C (28MPa) followed by a moderate decrease up to 700°C. Above 700°C the systems underwent new phase formation and shrinkage (volume decrease) deformations. After exposure at 1000°C the different mixes consisted of a mix of several stuffed silica phases, almost pure hexagonal nepheline or amorphous phase. Depending on the mix-design, the onset temperature of the high temperature phase transformations varied.

Characterizations of flax fabric reinforced nanoclay-geopolymer composites

Abstract Geopolymer composites reinforced with flax fabrics (FF) and nanoclay platelets are synthesised and studied in terms of physical and mechanical properties. X-Ray Diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Scanning Electron Microscope (SEM) techniques are used for phase and microstructure characterisation. The nanoclay platelets are added to reinforce the geopolymer matrices at 1.0%, 2.0%, and 3.0% by weight. It is found that 2.0 wt.% nanoclay enhances the density, decreases the porosity and subsequently improves the flexural strength and toughness. The microstructural analysis results indicate that the nanoclay behaves not only as a filler to improve the microstructure of the binder, but also as an activator to support the geopolymeric reaction producing higher content of geopolymer gel. This enhances the adhesion between geopolymer matrix and flax fibres, which improves the mechanical properties of the geopolymer nanocomposites reinforced with flax fabrics.

Self-compacting geopolymer concretes: Effects of addition of aluminosilicate-rich fines

The presence of alunimosilicate based fine aggregates (2.5 to 7.5wt%) effectively acted to densify the microstructure of the interfacial zone with a reduction of pore threshold and mean pore size jointly with change on the fracture mode of the Self-Compacting (SC) geopolymer concretes. From the results, pumice and recycled glass with amorphous structure improved the bi-axial four-point flexural strength from 5 to 8MPa while semi-crystalline feldspar sludge (nepheline syenite) reached 11MPa. The formation of additional geopolymer gels were responsible for the strengthening mechanism. The specimens with amorphous fines showed the tendency to delay the desorption in the laboratory conditions indicating the need for a very long curing time for the final consolidation. The semi-crystalline fines -nepheline syenite- appeared appropriate for the design of self-compacting geopolymer concretes due to their bulk composition, capacity to enhance densification and strength with low deformation rate, in a short range of curing together with a high rate of desorption, all important parameters for the prediction of the durability of concretes.

Improving compressive strength of fly ash-based geopolymer composites by basalt fibers addition

In this study, ASTM Class C fly ash used as an alumino-silicate source was activated by metal alkali and cured at low temperature. Basalt fibers which have excellent physical and mechanical properties were added to fly ash-based geopolymers for 10–30% solid content to act as a reinforced material, and its influence on the compressive strength of geopolymer composites has been investigated. XRD study of synthesized geopolymers showed an amorphous phase of geopolymeric gel in the 2θ region of 23°–38° including calcium-silicate-hydrate (C-S-H) phase, some crystalline phases of magnesioferrite, and un-reacted quartz. The microstructure investigation illustrated fly ash particles and basalt fibers were embedded in a dense alumino-silicate matrix, though there was some un-reacted phase occurred. The compressive strength of fly ash-based geopolymer matrix without basalt fibers added samples aged 28 days was 35MPa which significantly increased 37% when the 10wt%. basalt fibers were added. However, the addition of basalt fibers from 15 to 30wt% has not shown a major improvement in compressive strength. In addition, it was found that the compressive strength was strong relevant to the Ca/Si ratio and the C-S-H phase in the geopolymer matrix as high compressive strength was found in the samples with high Ca/Si ratio. It is suggested that basalt fibers are one of the potential candidates as reinforcements for geopolymer composites development.

Feasibility Study of Loess Stabilization with Fly Ash–Based Geopolymer

Loess, which primarily consists of wind-transported silt and clay particles, is widely distributed around the world, including the central Asia, central Europe, the northwestern and central United States, Alaska, and South America. The metastable structure of natural loess has resulted in construction delays and catastrophic failures. To explore the potential applications of soil improvement with green materials, this paper presents an experimental study of applying fly ash–based geopolymers for the loess stabilization. Two different precursors are employed to the investigation. It is found that potassium hydroxide renders a higher unconfined compressive strength than sodium hydroxide to the samples when the same fly ash/loess ratio is applied. This agrees with the results of the mechanical properties of the two different geopolymers. With an increasing fly ash/loess ratio, the compressive strength and Young’s modulus increase. The microstructural characterization unveils that a compact and stable microstructure has been developed in the stabilized loess. With the aid of energy dispersive X-ray spectroscopy and X-ray diffraction, it is observed that it is the binding effect of geopolymer gel that contributes to the improvements in the mechanical properties of stabilized loess samples.

Effect of fibres on the strength and behaviour of GPC columns

Geopolymers are alternative binders to ordinary Portland cement binder, and in geopolymer concrete (GPC) geopolymer gel is used as a binder. In the present study, the effect of steel fibres on the strength and behaviour of GPC columns under monotonic axial compression is studied. In total, 30 GPC columns with l/d ratios of 6 and 9 were cast and tested. The main variables considered in this study include different values of volume fraction of fibres such as 0·25% (19·62 kg/m3), 0·5% (39·24 kg/m3), 0·75% (58·86 kg/m3) and 1% (78·48 kg/m3) for each l/d ratio. Experimental results reveal that the addition of steel fibres slightly improves the axial strength and significantly improves the stress–strain behaviour of columns. A theoretical model for the stress–strain behaviour is proposed, and this model predicts the response of plain and fibre-reinforced GPC columns satisfactorily.

Synthesis of ternary geopolymers based on metakaolin, boiler slag and rice husk ash

Ternary mixtures of geopolymers obtained from the alkaline activation of metakaolin (MK), boiler slag (BS), and rice husk ash (RHA) using a solution of potassium hydroxide were mechanically, thermally, and microstructurally characterized. The geopolymer properties and final microstructures indicate that the addition of BS, despite containing large amounts of unburned material (16.36%), allows for greater densification and greater homogeneity of the geopolymeric gel, which results in greater stability in strength at long curing ages. Substitution of 30% of MK by BS results in an increase in compressive strength of up to 21% and 122% after 28 and 180 days of curing, respectively. These results demonstrate the possibility of the construction sector using geopolymers based on MK and adding BS and RHA to obtain cementitious materials with a lower environmental impact.

The transition of fly ash-based geopolymer gels into ordered structures and the effect on the compressive strength

This paper reports the investigation of microstructural properties associated with the transition of amorphous gels into more ordered structures in fly ash-based geopolymers. Three typical geopolymer mixes are synthesized with Class F fly ash and slag. Accelerated ageing scheme is performed at the 80°C hydrothermal conditions, lasting for 90days. It is observed that under the hydrothermal conditions, the two geopolymer pastes containing solely fly ash activated with sodium hydroxide and sodium silicate exhibit exhibit a common tendency to partial crystallization after 28days. Crystalline zeolitic phases (including analcime) are detected by XRD and FTIR techniques. The SEM observation shows that the transition from the amorphous gels into more ordered structures takes place in the binder matrix randomly, rather than in pores or from particular structures. The blending of slag can reduce the transition (crystallization) trend, which is attributed to the formation of calcium containing gels. The decrease of compressive strength of geopolymer at late ageing stage can be correlated to the phase transition. Proper molecular designs of geopolymers to kinetically control the transition must be considered for the applications of this family of materials.

Metakaolin-, fly ash- and calcium hydroxide-based geopolymers: effects of calcium on performance

Effects of calcium on the performance of geopolymers prepared from metakaolin, class C fly ash (CFA) and calcium hydroxide are studied, including setting times and compressive strength testing. Leaching concentrations of calcium from prepared geopolymers are investigated by inductively coupled plasma atomic emission spectrometry. Relative contents of geopolymeric products and hydration products are studied by acid dissolving testing. The mineral composition, morphology and chemical groups of geopolymeric products and hydration products are analysed by X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopy. Geopolymer setting time is much faster when calcium content increases. Calcium has a positive effect on compressive strength of geopolymers. By acid dissolving testing, the ratio of geopolymer value/ordinary hydration product value in metakaolin-based geopolymer was 4·4 : 1, whereas in CFA-based and CFA + 6% calcium hydroxide-based geopolymers the values were 1 : 1·7 and 1 : 1·5, respectively. Thus, the main product in metakaolin-based geopolymer is geopolymeric products, whereas in CFA-based geopolymer, hydration products also occur. In the alkali-activated condition, CFA-based geopolymer is a composite system with geopolymeric gel and calcium–silicate–hydrate gel co-existing.

Metakaolin-, fly ash- and calcium hydroxide-based geopolymers: effects of calcium on performance

Effects of calcium on the performance of geopolymers prepared from metakaolin, class C fly ash (CFA) and calcium hydroxide are studied, including setting times and compressive strength testing. Leaching concentrations of calcium from prepared geopolymers are investigated by inductively coupled plasma atomic emission spectrometry. Relative contents of geopolymeric products and hydration products are studied by acid dissolving testing. The mineral composition, morphology and chemical groups of geopolymeric products and hydration products are analysed by X-ray diffraction, Fourier transform infrared spectroscopy and scanning electron microscopy. Geopolymer setting time is much faster when calcium content increases. Calcium has a positive effect on compressive strength of geopolymers. By acid dissolving testing, the ratio of geopolymer value/ordinary hydration product value in metakaolin-based geopolymer was 4·4 : 1, whereas in CFA-based and CFA + 6% calcium hydroxide-based geopolymers the values were 1 : 1·7 and 1 : 1·5, respectively. Thus, the main product in metakaolin-based geopolymer is geopolymeric products, whereas in CFA-based geopolymer, hydration products also occur. In the alkali-activated condition, CFA-based geopolymer is a composite system with geopolymeric gel and calcium–silicate–hydrate gel co-existing.

Flood Mud as Geopolymer Precursor Materials: Effect of Curing Regime on Compressive Strength

This paper studies the effect of curing temperature and curing duration to the flood mud based geopolymer on compressive strength properties. Flood mud was used as a raw material for geopolymer and geopolymer samples were synthesized by using sodium silicate and sodium hydroxide 14M solution. These samples were cured at different temperature (100°C, 150°C, 200°C and 250°) for different curing duration (6h, 12h and 24h) respectively. Compressive strength tests were carried out at after 28 days. The compressive strength and SEM analysis of geopolymer products were evaluated. Result showed that the maximum compressive strength was 24 MPa at temperature of 150°C for 24 hours. With increasing ageing day, densification of geopolymer gel was observed.

The Effects of Aggressive Environments on the Properties of Fly Ash based Geopolymers

This paper analyzes the effects of two different aggressive environments, concentrated ammonium nitrate solution (480 g/dm3) and sodium sulphate solution (50 g/dm3), on the structure and mechanical strength of fly ash based geopolymers. Geopolymer samples were subjected to the aggressive solutions over a period of 365 days. It was found that exposure to the NH4NO3 and Na2SO4 solutions caused small decrease in geopolymer strength (10-20%). The most valuable insight into the structural changes caused by testing of the geopolymer samples in the aggressive solutions was provided by means of 29Si MAS NMR. It was found that the immersion of geopolymer samples in the NH4NO3 solution caused breaking of Si-O-Al bonds in the aluminosilicate geopolymer gel structure. On the other hand, treatment of the geopolymer samples with the Na2SO4 solution resulted in breaking of Si-O-Si bonds in geopolymer gel structure and leaching of Si. It was concluded that the major changes in the geopolymer structure were associated with the changes in the pH values of aggressive solutions during the testing.

Transformation of the geopolymer gels to crystalline bonds in cold-setting refractory concretes: Pore evolution, mechanical strength and microstructure

Two K2O–MgO–Al2O3–SiO2 based geopolymer gels with bulk chemical composition corresponding to cordierite (Co) and 1:1 mullite-cordierite (MuCo) were successfully transformed to crystalline bonds in high temperature service of cold-setting made refractory concretes. Kyanite aggregates changed the flexural strength of the gels from 11 to 28MPa due to the development of good adhesive bonds. Under thermal cycles, up to 1250°C, the cumulative pore volume remained at 0.09mL/g, as from the absence of important densification/shrinkage. However, the behavior of the cumulative pore volume curves changed from that of a matrix with a wide range of distribution of pore sizes to that, of matrix, consisting of relatively coarse grains. The latter exhibits a rise at 10μm as void spaces created around the contact points among the coarse kyanite grains and that at 0.054μm as pores within the crystalline phases (cordierite, kalsilite, leucite, mullite, enstatite) formed. The microstructural observations confirmed the transformation of gel pores (size around 0.01μm) to interparticle and intergranular pores due to the crystallization. The flexural strength of refractory concretes increased from 28MPa to 40MPa in agreement with the increase in the elastic modulus from 9 to 30GPa. The crystallization was enhanced by the MgO content (being important in Co compared to MuCo) and the kyanite concentration as particles of kyanite effectively acted as phase separation and nucleation sites.

One-step facile synthesis of bulk zeolite A through metakaolin-based geopolymer gels

A one-step facile method without any surfactants and autoclaves has been proposed to synthesis bulk zeolite A through metalkaolin-based geopolymer gels. The geopolymer gels could be in situ transformed to zeolite A during geopolymerization process when the samples were kept in the oven for a period of time. Influence of the synthesis temperature, curing time, Na/Al ratio and water–binder ratio on the structures, morphology and mechanical properties of the bulk zeolite A has also been studied. Results indicated that highly purified and crystallined zeolite A with Na/Al ratio of 1.0 and water–binder ratio of 0.7 could be obtained at 70 °C for 16 h. The non-crystallined geopolymer gels would bond the micro-particles of zeolite A with each other, which make the bulk samples have certain strength. The formation mechanism of the bulk zeolite A has also been discussed. Micropores and mesopores in the bulk zeolite A have been observed by nitrogen adsorption because of the coexistence of zeolite A phase and geopolymer phase. Meanwhile, the mercury intrusion porosimetry also confirmed the existence of macro-pores with 0.9 μm of pore-size and 39.2 % of porosity in the bulk body.

Degree of reaction and phase content of silica-based one-part geopolymers investigated using chemical and NMR spectroscopic methods

One-part geopolymers were synthesized from two different silica materials (a silica-rich residue from chlorosilane production and a commercial microsilica) and sodium aluminate at three different SiO2/Al2O3 ratios and a nominal water/solids ratio of 0.5. The degree of reaction of the silica in the cured geopolymers (i.e. the fraction of silica dissolved to form aluminosilicates and minor products) was determined using two different methods: chemical attack with HCl to dissolve the reaction products and evaluation of peak areas of 29Si MAS NMR spectra. It was found that the degree of reaction of the silica decreases with increasing the silica content of the starting mix, and that it is almost constant after 1 day of curing and almost independent from the kind of starting silica. From the results of the NMR-based method, the mean SiO2/Al2O3 ratio of the reaction products (aluminosilicates and minor products) can be estimated to be ca. 2.0, nearly independent of the starting composition of the geopolymers. The dissolution method is biased, but of sufficient precision to be useful for following changes of the degree of reaction. Major crystalline phases in the cured geopolymers are zeolite A and/or hydrosodalite. Depending on the starting composition, the relative amounts of these zeolites vary; additionally, sodalite (only for the residue from chlorosilane production with >1 wt% Cl-), faujasite, and zeolite EMT can appear in the geopolymers. The 29Si and 27Al MAS NMR results indicate mainly Si(4Al) and Al(4Si) sites, in line with the presence of zeolite A, hydrosodalite, sodalite, and geopolymeric gel of comparatively low SiO2/Al2O3 ratio.

Strength and microstructural properties of fly ash based geopolymer concrete containing high-calcium and water-absorptive aggregate

In this research, performance of limestone and oil palm shell (OPS) as the alternative aggregates in fly ash based geopolymer concrete was evaluated based on the fresh and hardened concrete test. Fly ash and oil palm shell were studied due to their abundant availability as industrial and agricultural waste. Experimental works were conducted by replacing the portion of coarse and fine aggregate in concrete with limestone and oil palm shell in various percentages. Evaluation of its performance was done based on slump, compressive strength, and porosity test. Microstructure analysis was added via Scanning Electron Microscope (SEM) and Energy Dispersive X-Ray (EDX) test to support the generated hypothesis. From the fresh properties test, the increasing amount of limestone and OPS have shortened the setting time of fresh geopolymer concrete, even though they presented small variations in the slump's height and diameter results. The availability of Calcium Aluminate Silicate Hydrate (C–A–S–H) structure in limestone based specimen and higher polycondensation rate due to optimum water content in OPS based specimens have stimulated the rapid production of geopolymer gel and improved the strength performance of geopolymer concrete in ambient curing condition. Moderate water absorption from OPS fibers and additional dissolution of Al and Si elements due to formation of C–A–S–H gels from limestone inclusion have presented 10% OPS and 25% limestone as the optimum replacement to river sand and coarse granite aggregate, respectively. In ambient curing, 25% limestone presented a 35.23% higher strength while 10% OPS had 11.01% higher strength than control. An escalation to the curing temperature reduced the efficiency of both aggregates, where only 25% limestone was able to produce specimen with 2.34% higher strength than control A supportive analysis from SEM and EDX has also revealed the existence of poly(sialate), poly(sialate-siloxo), poly(sialate-disiloxo) and sialate link in the geopolymer matrix. From this research, the applications of limestone and oil palm shell in geopolymer concrete have presented a nurturing result, particularly as the alternative aggregates in ambient-cured application.