Geopolymer Setting Research Articles

Influence of curing temperature and pressure on the mechanical and microstructural development of metakaolin-based geopolymers

This research investigates the mechanical and microstructural evolution of a metakaolin-based geopolymer (GP) subjected to combined temperature and pressure curing conditions. The study's methodology encompassed measuring compressive strengths through both ultrasonic cement analysis (UCA) and destructive uniaxial testing. To evaluate thermal stability, pore size distribution, and morphology, the study employed thermogravimetric analysis (TGA), mercury intrusion porosimetry (MIP), and environmental scanning electron microscopy (ESEM), respectively. The findings indicate that a curing temperature of 90 °C encourages the development of regular nanoporosity, while higher temperatures contribute to increased porosity. Conversely, curing pressure was found to either promote or inhibit the material's reorganization, with pressures of 20 MPa facilitating and those above 40 MPa preventing this process. An enhancement in the refinement of macropores to micropores was observed across all pressure levels. The optimal curing conditions, in terms of both mechanical and morphological characteristics, were identified as 50 °C and 20 MPa for a minimum duration of one day, which yielded a 177 % improvement in compressive strength compared to curing at 21°C under atmospheric pressure. This study significantly advances the understanding of pressurized thermal curing processes for the rapid setting of geopolymers, presenting new avenues for diverse applications.

Resistance of Chloride and Sulfate of Geopolymer based on fly ash and silica fume as alternative raw materials.

Concrete durability to salt resistance is the main factor affecting the service life of concrete used in acid or salt environment. This project aims to develop the concrete composition for salt resistance of chloride and sulfate at 50,000 mg/L as the simulated salt solution in this project. Geopolymer consisted of 10%wt ordinary Portland cement (OPC type 1) with the other alternative raw materials which were pozzolanic materials and wastes (fly ash, silica fume and rice husk ash). The various parameters which were type and content of the alternative raw materials, plasticizers content, liquid to powder ratio, etc, were studied for developing the compressive strength of geopolymer after soaking in the simulated salt solution for 14 days as comparison with those in the normal water and the ordinary Portland cement. The results found that geopolymer consisted of fly ash and silica fume with the mixture of NaOH (10M): Na2SiO3 in a ratio of 24:16 (wt./wt.) could improve the compressive strength of geopolymer cement after soaking in the simulated sulfate salt solution for 14 days as compared to the ordinary Portland cement type 1 and type 5. Moreover, this geopolymer showed low density which was beneficial for use in the engineering structure in the high salt area. In addition, the change of chemical reaction and phases after setting of geopolymer in high concentrations of chloride and sulfate solution were studied to understand the mechanism of salt resistance of geopolymer and to develop the concrete composition for salt resistance in the future.

Sodium gluconate as a retarder modified sewage sludge ash-based geopolymers: Mechanism and environmental assessment

In this study, sewage sludge ash (SSA) was utilized to prepare geopolymers at room temperature, with the aim of achieving the utilization of sewage sludge resources. The study primarily investigates the effects of alkali activators and the sodium gluconate on the setting properties and mechanical properties of geopolymers prepared using a 2:3 wt ratio of SSA and ground granulated blast-furnace slag (GGBS). Results indicated that the addition of sodium gluconate significantly inhibited the reaction of geopolymers paste setting time of the geopolymer compared to changing the alkali activators parameters. An increase in sodium gluconate dosage from 0% to 3% significantly delays the initial setting time of geopolymer pastes from 31 min to 183 min. Microstructural analyses demonstrate that sodium gluconate interferes with the dissolution of precursor materials and the growth of the hydration gel in geopolymer, leading to an inhibitory effect on the early hardening process. The introduction of 3% sodium gluconate increases the area fraction of unhydrated SSA by over 45% and unhydrated GGBS by 13%, as quantified by BSE analysis. Importantly, environmental and economic analyses suggest that delaying the setting of geopolymer through the inclusion of sodium gluconate presents a more environmentally favorable option than altering the alkali activators.

Isolating the effects of thixotropy in geopolymer pastes

AbstractThe rheological properties of potassium‐based geopolymers were investigated through a series of experiments intended to isolate the influence of shear rate, recovery time, and shear ramping on thixotropy for a greater understanding of geopolymer thixotropic properties within the context of the geopolymer setting reaction. It is shown that for thixotropic disruption to occur a critical shear rate of 100 s−1 must be reached or surpassed, full thixotropic restructuring occurs at around 90–100 min of total undisturbed rest time, and that reaching a state of full thixotropic disturbance heavily depends on subjected processing parameters. In addition, a consistent crossover between the storage and loss modulus within 1–3 min of oscillation during cyclical oscillatory measurements greatly indicates the repeatability and reversibility of thixotropy in geopolymers and the potential for tailorable viscosity. Overall, it is found that geopolymer pastes exhibit strong evidence of thixotropy, which is favorable for additive manufacturing, and that allotted rest time before shear and shear rate greatly influence the overall rheological properties.

Reaction kinetics and mechanical properties of a mineral-micropowder/metakaolin-based geopolymer

In this study, metakaolin was partially replaced with mineral micropowder to prepare a mineral-micropowder/metakaolin-based geopolymer was prepared under alkali activation, and the compressive and flexural strengths of various geopolymer specimens were determined. Geopolymer reaction kinetics were examined using the Johnson-Mehl-Avrami-Kolmogrov model, and the effects of the mineral-micropowder content on the properties and structure of the metakaolin-based geopolymer were investigated. Results revealed that micropowder addition significantly influenced the mechanical properties, microstructure, and reaction heat of the geopolymer. At a powder content of 30 wt%, the polymer exhibited superior mechanical properties; furthermore, the compressive and flexural strengths of the specimens cured for 28 d were 58.3 MPa and 12.6 MPa, which were 24.1% and 40% higher than those of the control group, respectively. Meanwhile, the geopolymer setting time was significantly reduced because the presence of calcium in mineral micropowder promoted the geopolymerisation reaction. Therefore, the formation of a multi-gel phase considerably enhanced the geopolymer structure.

Effect of microcrystalline cellulose on geopolymer and Portland cement pastes mechanical performance

Geopolymers are presented as a new developing construction material and a potential alternative to ordinary binders, but there is a lack of data about several of their properties and behavior – joint-effect with cellulose is one example. The present study investigated the influence of Microcrystalline Cellulose (MCC) addition on the mechanical and microstructural properties of Portland cement and geopolymer pastes. The use of MCC increases compressive strength and stiffness at 7 days to both studied binders. After 28 days, the mechanical properties of the geopolymer paste are reduced due to MCC degradation. On the other hand, MCC addition accelerated geopolymer setting time by 15%, whereas on the cement paste yield a considerable delay effect.

Investigating the suitability of fly ash/metakaolin-based geopolymers reinforced with South African alien invasive wood and sugarcane bagasse residues for use in outdoor conditions

The prevailing approach of total clearing to contain the spread of invasive plants (IP) in South Africa is generating enormous lignocellulosic wastes. This study examined the possibility of utilizing these wastes in the production of geopolymer composites for use in outdoor environments. Untreated wood particles from Acacia mearnsii and A. longifolia, as well as sugarcane bagasse residues, were incorporated into a geopolymer matrix developed from a binary precursor system of 75% fly ash and 25% metakaolin. The variables considered included precursor-activator ratio (PA), curing pattern (CP), lignocellulosic material (LM), and alkali concentration (Mcon). The production process was established using a mixed factorial experimental design. PA and CP were considered at 2 levels, while LM and MCon were considered at 3 levels. The density of the boards exceeded 1 g cm−3 and are classified as high-density boards. The boards have comparable sorption properties to the cement-bonded particleboard according to the EN 632-2: 2007 standard. However, only A. longifolia boards produced with 12 M NaOH and PA ratio of 2:1 met the mechanical strength requirements. Thermogravimetric analysis revealed that the boards are thermally stable. These results have shown that South African woody IPs are suitable for geopolymer wood composites, but there is still concern about their durability in the alkaline matrix. Scanning electron microscopy micrographs indicated mineralization of the particles and a partial degradation of hemicellulose was confirmed by Fourier transform infrared spectroscopy. Although the degraded components did not prevent geopolymer setting, there is need to further investigate the extent and means of preventing degradation as this can derail the intended use of the product.

Influence of recycled concrete aggregate on the foam stability of aerated geopolymer concrete

The effect of recycled concrete aggregates (RCA) in enhancing the foam stability and hardened properties of aerated geopolymer concrete (GFC) produced by a mechanical foaming method is investigated. The RCA was used as a partial or full replacement to fine aggregates, and approximately the same amounts of foam were used in all mixes to achieve a targeted density of 800 kg/m3. Despite the same water/binder and activator/binder ratios in all mixes, the fresh and hardened properties of GFC significantly varied between mixes. The instability of fresh geopolymer mix, caused by the collapse of air bubbles and the settlement was identified as a major issue in the control GFC mix. The use of RCA in GFC has significantly improved the foam stability with a significant reduction in settlement. The foam stability criterion was correlated to the yield stress development of base mixes, where the addition of RCA increases the initial yield stress and yield stress development over time. This has significantly influenced the stability of bubbles in GFC. On the other hand, the setting time of the geopolymer has reduced with the increasing RCA content. Nevertheless, the foam instability and corresponding settlement were stabilized well ahead of the initial setting time, indicating the setting of geopolymer has less impact on the foam instability in GFCs. The XRD and TGA analyses detect the presence of Ca(OH)2 in RCA, which increases the yield stresses of fresh geopolymer pastes. The variation in foam stability was well reflected on the hardened properties of GFC, such as porosity, pore size distribution, and thermal properties.

Review on characteristics of metakaolin-based geopolymer and fast setting

The setting of metakaolin-based geopolymer depends on the raw materials and mix proportions. Setting, when material is cured at room temperature, takes about 1 day and is longer than setting time of Portland cement. For the fast setting of geopolymers, some studies increased the curing temperature or used raw materials with high CaO content. Also, Ca2+ and Mg2+ compounds were used as additives. Setting can be easily controlled and accelerated by adding Ca2+ compounds. However, it has been reported that knowledge of the reaction mechanism and final products between Ca2+ and geopolymers is still limited. In this study, we investigated the characteristics of metakaolin-based geopolymers and methods for fast setting of geopolymers, and made hypotheses about the reaction mechanism between Ca2+ and geopolymers.

How to control the geopolymer setting time with the alkaline silicate solution

Geopolymers are new binders resulting from the activation of an aluminosilicate source by an alkaline solution. It is well-established that the precursors that are used affect geopolymer formation. This study focuses on how to control the geopolymer setting time with an alkaline silicate solution. To conduct this investigation, various geopolymer mixtures based on different alkaline silicate solutions were characterized in terms of viscosity and setting time. To understand the link between geopolymer formation and the properties of the solution, the ionic conductivity and the viscosity of the solutions were measured. The solutions were also characterized by Raman spectroscopy. The characterization of the geopolymer mixture shows that the properties of the alkaline silicate solution highly depend on the viscosity and setting time of the geopolymer. Therefore, the alkali molar concentration (3 < [M] < 10 mol·L−1) influenced the properties of the solution (silicate species, ionic conductivity, and viscosity). A relationship among the chemical composition, the silicate species of the solution and the geopolymer setting time is highlighted depending on the nsinAl+nM ratio. For example, a ratio of nsinAl+nM>1.1 permits a slower geopolymerization reaction, and the setting time of the geopolymer mixture is higher.

The Use of Borax in Deterring Flash Setting of High Calcium Fly Ash Based Geopolymer

Geopolymer that was made with high CaO content fly ash was found to have higher compressive strength than the low CaO fly ash, using the same mixture composition. This effect could be due to the physico-chemical properties of the fly ash, in respect to its particle size or the chemical composition. Although it was not widely published, the occurrence of flash setting of geopolymer was known to occur when using high CaO content fly ash as the precursor. Geopolymer paste may solidify within minutes after the addition of alkali activators, making it very difficult to cast in big volume. This paper investigate the effect of borax addition to the high calcium fly ash-based geopolymer mixture to reduce the occurrence of flash setting. It was found that the setting time can be extended significantly, with the addition of 5% borax, by mass, of fly ash. The addition of borax also have positive effect on increasing the compressive strength of geopolymer.

The study of different additives on a geomaterial reformulated mixture

Geopolymers are new binders resulting from the activation of an aluminosilicate source by an alkaline solution. These ecomaterials are economically and environmentally profitable because they have the advantage of ensuring adhesion between mud bricks and wood. In this context, this paper focuses on the study of the effect of various additives on the kinetic of the setting of geopolymers to facilitate adhesion between various supports. For this purpose, the chemical composition of a laboratory formulation was modified by the addition of aluminous (boehmite) and amorphous silica. Characterization of various mixtures showed that the rate of polycondensation is controllable by the addition of a different aluminosilicate source. Preservation of the working properties of the formed materials was observed. Finally, the addition of a delaying mineral compound allowed the setting time of the geopolymer binder to be adjusted while preserving the mechanical properties.

Effects of Parameters on the Setting Time of Fly Ash Based Geopolymers Using Taguchi Method

Geopolymers are the inorganic polymer materials possessing properties superior than the properties of conventional materials. Geopolymers are also environment friendly materials. This paper determines the effects of parameters such as silica to alumina (Si/Al) ratio, sodium to alumina (Na/Al) ratio, water to solid (W/S) ratio, and temperature on the setting of fly ash geopolymers. The experiments were designed using Taguchi model. The setting time was determined by Vicat Needle apparatus. The Si/Al - 2.2, Na/Al - 1.4, W/S - 0.30, and temperature of 40 °C increased the setting time and workability of the geopolymers. The Si/Al ratio of 1.8, Na/Al ratio of 1.0, W/S ratio of 0.20, and temperature of 80° C caused fast setting of geopolymers. The setting time of geopolymers can be optimized using Taguchi method for particular applications.

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.

Monitoring Setting of Geopolymers

AbstractThe setting behavior of geopolymer pastes as a function of time was studied using two methods: penetration resistance and ultrasonic shear wave reflection. Several starting materials were included—metakaolin, Class C and Class F fly ashes, and slag—and chemical parameters known to affect set were varied. The geopolymers showed a wide range of set times compared to a reference Portland cement paste: some were much more rapid, some were similar, and some were much slower. Although most geopolymers formed gels that were both solid and had measurable strength, some, initially, formed a soft gel that had no measurable strength. Therefore, to fully characterize setting behavior, it is necessary to use both types of tests. It was seen that setting behavior was sensitive to chemical parameters, with setting delayed somewhat with increasing silica/alumina and increasing water/alkali, and accelerated substantially with calcium hydroxide substitution.

Inorganic polymers from alkali activation of metakaolin: Effect of setting and curing on structure

Geopolymers, obtained by chemical reaction between aluminosilicate oxides and silicates under highly alkaline conditions, are studied in this paper. The proposed mechanism of geopolymer setting and hardening or curing consists of a dissolution, a transportation or an orientation, as well as a polycondensation step. The aim of this paper is to investigate the influence of the curing time and temperature, the relative humidity and the reagents temperature on the geopolymerization process in order to obtain a resistant matrix usable for inertization of hazardous wastes. The evolution of the process from the precursors dissolution to final geopolymer matrix hardening has been followed by FTIR spectroscopy, X-ray diffractometry, SEM/EDS and leaching tests. The results show the significant influence of both curing temperature in the curing stage and of the mould materials on the matrix stability. The easy-to-run preparation procedure for a chemically stable metakaolin geopolymer individuated can be summarized as reagents setting and curing at room temperature and material mould which permits moisture level around 40%.

Fire-resistant geopolymer produced by granulated blast furnace slag

This paper describes research into the use of granulated blast furnace slag as an active filler in the making of geopolymers. During this work it was found that geopolymer setting time correlates well with temperature, potassium hydroxide concentration, metakaolinite and sodium silicate addition. The physical and mechanical properties of the geopolymer also correlated well with the concentration of alkaline solution and the amount of metakaolinite that is added. The highest compressive strength achieved was 79 MPa. For fire resistance tests, a 10 mm thick geopolymer panel was exposed to a 1100 °C flame, with the measured reverse-side temperatures reaching less than 350 °C after 35 min. The products can be fabricated for construction purposes and have great potential for engineering applications.