Fly Ash Geopolymer Research Articles

A high-strength red mud–fly ash geopolymer and the implications of curing temperature

In response to the requirement for the comprehensive utilization of industrial waste, high-strength geopolymers were prepared from red mud (RM) and Class 1, 2, and C fly ash (FA) rich in active substances (FA1, FA2, and FAC, respectively) in the presence of an alkali activator (AA). Increasing the curing temperature from room temperature (22 °C) to 60 °C rapidly increased the strength of the geopolymer, while maintaining a certain level of humidity during curing inhibited crack formation in the geopolymer. The use of a sodium hydroxide (NaOH) solution improved the strength of the geopolymer. However, an excessively high concentration of NaOH solution induced considerable efflorescence. The strength of the geopolymer prepared with a mixed solution (MS) composed of a NaOH solution and a water glass (WG) solution increased as the concentration of the MS increased and reached 63.5 MPa when the NaOH concentration in the MS was 10 mol/L.

Determination of stress-strain relationship based on alkali activator ratios in geopolymer concretes and development of empirical formulations

Fly ash-based geopolymer has recently gained attention of researchers due to its potential application, as well as being an alternative binder with low emissions compared to ordinary Portland cement (OPC) in concrete production. Studies which are conducted on the design and mechanical properties of structural members produced from fly ash geopolymer concrete (GPC) are very important in terms of increasing the use of this concrete. The aim of this study is to obtain experimental data on the effect of sodium silicate/sodium hydroxide (SS/SH) and alkali activators/fly ash (AA/FA) ratios on the mechanical properties of a low calcium heat-cured fly ash geopolymer. In addition, it is to reveal the similarities and differences of OPC and GPC by comparing the mathematical formulations in existing regulations and concrete models with experimental data. Thus, geopolymer cylinder concrete samples were produced using 15 different mixtures with SS/SH ratios of 1.5, 2.5 and 3.5, while AA/FA ratios of 0.4, 0.5, 0.6, 0.7 and 0.8. At the end of the study, the optimum SS/SH ratio was obtained as 2.5. A decrease in the AA/FA ratio increases the compressive and splitting tensile strength, while an increment increases the ductility and consuming energy. In addition, the relationship between the experimental data and the splitting tensile strength and modulus of elasticity formulations depending on the compressive strength given in other studies and regulations as a part of literature was investigated, and then, two alternative empirical formulations considering the ratios of alkali activators were proposed at the end of the regression analysis. When the stress-strain relationship of OPC concrete models and GPC mixtures were compared, the closest unconfined concrete model for GPC concrete was the Hognestad model.

Mechanical Performance, Microstructure, and Porosity Evolution of Fly Ash Geopolymer after Ten Years of Curing Age.

This paper elucidates the mechanical performance, microstructure, and porosity evolution of fly ash geopolymer after 10 years of curing age. Given their wide range of applications, understanding the microstructure of geopolymers is critical for their long-term use. The outcome of fly ash geopolymer on mechanical performance and microstructural characteristics was compared between 28 days of curing (FA28D) and after 10 years of curing age (FA10Y) at similar mixing designs. The results of this work reveal that the FA10Y has a beneficial effect on strength development and denser microstructure compared to FA28D. The total porosity of FA10Y was also lower than FA28D due to the anorthite formation resulting in the compacted matrix. After 10 years of curing age, the 3D pore distribution showed a considerable decrease in the range of 5-30 µm with the formation of isolated and intergranular holes.

Mineral-impregnated carbon-fiber (MCF) composites made with differently sized fly-ash geopolymers for durable light weight and high temperature applications

For a wide application of carbon-fiber reinforcement in large-scaled, lightweight construction, mineral-impregnated carbon-fiber (MCF) composites provide a promising alternative to rebar or reinforcement made of non-metallic fibers and polymer matrix. The penetration ability of the aluminosilicate particles is a key factor for a stable, reproducible impregnation and the maximal reinforcing efficiency of continuous fiber geopolymer composites.The work at hand focuses on the influence of the particle sizes of geopolymer precursors on the processing quality, microstructural development and final composite performance regarding the flexural, tensile and pullout behavior. Three differently sized fly ash (FA) sources were used to impregnate a carbon fiber heavy tow before curing at 75 °C. FA particles in the same size range as the filament diameter proved to be the most efficient, enhancing both mechanical properties of MCF and its bond to concrete matrix, well beyond the effects induced by other, coarser precursors. The results of the microscopic investigation, μCT, thermal gravimetric analyzes and mercury intrusion porosimetry measurements show that the use of this particle fraction resulted in the best fiber-matrix distribution over the cross-section, densest microstructure and highest fiber volume fraction in the impregnated yarn. Furthermore, the geopolymerization kinetics of all FAs at ambient temperature and at 75 °C were determined by isothermal calorimeter, showing an increase in the reaction rate with elevated curing temperature and higher particle finesse.

Workability, rheology, and geopolymerization of fly ash geopolymer: Role of alkali content, modulus, and water–binder ratio

The performance of fresh and hardened geopolymer pastes is significantly influenced by parameters, such as alkali admixture (A/F), activator modulus (Ms), and water–binder ratio (w/b). In this work, the effects of A/F, Ms, and w/b on the properties of fresh and hardened fly ash geopolymers (FAGs) are characterized using rheological, strength, and microstructural tests. The Bingham and Herschel–Bulkley (H–B) equations were utilized to match the rheological curve of the fresh FAG paste. Experimental results showed that the rheological behavior of the FAG paste conformed to the H–B model. However, the rheological model of the FAG paste did not change with variations in A/F, Ms, and w/b. In addition, A/F, Ms, and w/b regulated the geopolymerization and microstructure of the FAG. By decreasing Ms, increasing A/F, and adjusting w/b, the properties of the FAG paste effectively improved while maintaining strength. When Ms and A/F were 1.2 and 6%, the FAG setting time decreased by 94.5%; moreover, the fluidity and 28-d compressive strength increased by 11.7% and 31.3%, respectively, compared with those when Ms and A/F were 1.4 and 8%.

Performances and microstructure of one-part fly ash geopolymer activated by calcium carbide slag and sodium metasilicate powder

As the raw material of geopolymer, F-class fly ash (FFA) often needs strong alkali such as NaOH to stimulate due to its low reactivity. In this study, FFA was refined and activated by a wet grinding process, and the activation effect of calcium carbide slag (19 %) and sodium silicate (1 %, 3 % and 5 %) was evaluated when used alone or in combination. The results show that the compound activator significantly improves the hydration products and mechanical properties of the wet-grinding FFA geopolymer. Compared with the single activation system, the 28-day hydration product content of the compound activation (co-activation) system increased by 67 %, and the 1-day compressive strength reached 24.8 MPa (at least an increase of 118 %). In addition, the co-activation system significantly improved the pore structure of the geopolymer and increased the chain length and Al/Si ratio of the C-S-H gel. The results of this study showed that co-activation and mechanical activation can significantly enhance the FFA geopolymer's mechanical characteristics and microstructure, greatly encourage the ingestion of industrial solid waste, lower the application costs of high pH activators, and efficiently provide a method for the utilization of FFA.

Self-cleaning superhydrophobic fly ash geopolymer

Building materials with hydrophobic surfaces can exhibit increased service life by preventing moisture absorption or diffusion through their surfaces. For concrete used in construction, this hydrophobicity can prevent the corrosion of reinforcing steel bars. Geopolymers are a new cement-free binding material that have been extensively studied to replace Portland cement. However, similar to normal concrete, geopolymers are susceptible to the intake of moisture. This paper presents the fabrication of a superhydrophobic and self-cleaning surface on a fly ash geopolymer as a method to prevent moisture intake. A composite coating of polydimethylsiloxane (PDMS) solution containing dispersed polytetrafluoroethylene (PTFE) or calcium stearate (CS) microparticles was applied by dip-coating to form the hydrophobic surface. Additionally, fly ash was incorporated with the PTFE and CS microparticles to increase surface roughness and reduce material cost. The experimental results showed that the coating containing CS microparticles yielded a hydrophobic surface with a contact angle of 140°, while those containing PTFE microparticles provided a superhydrophobic surface with a contact angle of 159°. The incorporation of fly ash resulted in increased surface roughness, leading to a larger contact angle and a smaller sliding angle. A contact angle of 153° with a sliding angle of 8.7° was observed on the PTFE/fly ash-coated surface. The cleaning process was demonstrated with a test whereby dust was removed by water droplets rolling off the surface. The tested coating exhibited self-cleaning and waterproofing properties and could thus improve the sustainability of materials in building construction.

Shear strength characteristic of geopolymer fly ash and egg shell powder stabilized clay soil

The clay soil can present various issues as a construction and infrastructure material. Clay soil is susceptible to shrinkage and swelling, limited bearing capacity, poor drainage, and erosion. Several techniques, including soil stabilization, drainage systems, and excavation techniques, may alleviate these issues and control the risks associated with clay soil. There is a growing interest in soil stabilization and developing environmentally friendly alternatives to Portland cement. Alternative utilization of waste with pozzolanic properties as a stabilizing material to replace cement and lime is increasing. In this field, the use of waste materials such as fly ash and eggshell powder is increasing. This study examines the properties of clay soil shear strength treated with a mixture of fly ash and eggshell powder geopolymer. The unconsolidated, undrained triaxial test was performed to determine the shear strength of the treated soil. In addition, the effects of sample variation, such as NaOH molarity, curing duration, and the ratio of eggshell powder to replaced fly ash, were studied. The results demonstrated that 5% eggshell powder might boost the shear strength of treated soil. In addition, increasing the NaOH molarity resulted in higher shear strength values, showing that the geopolymer has a more substantial binding effect. However, extending the curing period impacts the shear strength of stabilized soil. This study established the viability of eggshell powder geopolymer as a stabilizing agent for clay soil. Including 5% eggshell powder in the fly ash geopolymer mixture can enhance the shear strength of the soil.

Effects of Curing Conditions on the Self-Healing of Geopolymer Paste

Geopolymers are inorganic polymers produced by the reaction of aluminosilicates and alkaline solutions and show good mechanical and thermal features with a huge potential for use in the construction industry. Furthermore, geopolymers with self-healing properties can considerably decrease repair or maintenance costs and support economic and environmental sustainability. This study aimed to observe the self-healing capability of sodium silicate-containing agents in geopolymer pastes. Polyethylene glycol (PEG) and epoxy resin were used to encapsulate sodium silicate. The prepared healing agent was introduced into fly ash and metakaolin-based geopolymer samples. After curing for 28 days under room conditions, the samples were subjected to compressive loading to introduce cracks. The cracked samples were stored under three conditions: at 60°C under 100% relative humidity (RH), at room temperature under 100% RH, and at room temperature in water. After various durations, optical images were captured using a light microscope to determine the closure of the cracks. X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR) were used to identify healing products. Additionally, the change in compressive strength was used as an indicator of self-healing. The results showed that the sodium silicate agents could heal the cracks with narrow widths (<15μm) under all conditions and supported strength recovery. XRD and FT-IR results indicated that the main healing materials were aluminosilicate phases.

Influence of chemical composition of soda ash activated fly ash and copper slag geopolymer pastes on compressive strength

The effect of the chemical composition of geopolymer pastes on compressive strength was investigated in high-calcium fly ash and copper slag blends. In synthesizing the pastes, soda ash at activator to binder ratio of 0.4 was used. The characterization of material samples and the hardened fly ash-copper slag pastes was conducted through X-ray fluorescence (XRF), and X-ray diffraction (XRD) for the major oxide and phase composition. The hardened paste cubes which were cured at 80 °C were tested for compressive strength at ages 3, 7, and 28 days to obtain the mechanical performance property of each respective mix. The findings establish the impact of variation in the individual material and paste composition on the compressive strength of fly ash-copper slag geopolymer. The result shows that increase in the SiO2/Al2O3 and Na2O/Al2O3 ratios of paste products of samples corresponded to an increase in compressive strength. Whilst Fe2O3 wt.% increase in products from slag addition and positively influenced the compressive strength as fillers. However, CaO had no positive influence on the matrix of the activated product. The optimal blend mix design was 40 wt.% of copper slag which achieved a 28-day compressive strength of 24.66 MPa.

The Effect of Sodium Silicate to NaOH Ratio on Strength Development of Fly Ash Geopolymer Mortar in Marine Environment

This paper presents the effect of sodium silicate to NaOH ratio on the strength performance of fly ash geopolymer mortar subjected to continuous immersion in a simulated marine environment during 1-year observation. The geopolymer mortar specimens were prepared using class C fly ash activated with sodium silicate and NaOH 15 Molar combination. The sodium silicate to NaOH ratio varied from 1.4 to 2.7 to keep the water-to-solid ratio and Na2O content at 0.37 and 10, respectively. The strength properties were measured based on compressive strength test, weight, and porosity changes. The compressive strength was investigated at 90, 180, and 360 days after immersion. The results showed that the sodium silicate to NaOH ratio significantly affected the long-term performance of geopolymers in simulated marine environments. A ratio between 1.6 to 2.3 provides better durability by reducing the deterioration rate, i.e., strength, weight, and porosity, of geopolymer specimens in simulated marine environments. However, increasing the ratio by more than 2.30 tends to decrease the performance of the geopolymer. It can be concluded that geopolymer can be used as an alternative construction material in marine environments in a specific sodium silicate to NaOH ratio.

Compressive Strength Study based on Fly Ash Geopolymer Concrete at the age of 28 days under very High Temperature

Due to creeping process at the burning material, material’s elastic modulus and yield strength will decrease.Three fire protection system are exist in handling the fire problem: active fire protection, passive fire protection, and safety management. In correlation with material problem due to fire, passive fire protection is a strategy to counter it. This paper presents the results of fire protection by using Suralaya fly ash geopolymer concrete. Three molaritas of geopolymer concrete mixture design are 2M, 4M, and 6 M. Sodium Silicate (Na2SiO3) and Sodium Hydroxide (NaOH) are used as alkaline activator. The results of the experiment were analysed by compressive strength for the optimum values. This study found that the value of compressive strength of steam curing is higher than that of water curing, the value of geopolymer concrete compressive strength of 2M is higher than that of 4M, and the value of geopolymer concrete compressive strength of 6M is higher than that of 4M.

Unconfined compressive strength test on geopolymer fly ash stabilized clay shale

This paper discusses the problem of clay shale in construction and the need for its improvement. The use of geopolymer as a sustainable and environmentally friendly solution for soil stabilization is presented as an alternative to cement-based stabilization. Geopolymers are inorganic aluminum-silicates formed by silicon and aluminum and can be easily found in materials such as fly ash mixed with sodium hydroxide and sodium silicate. The successful use of geopolymer as a soil stabilizer has been reported in several studies, and the alkali activators used significantly impact the mechanical properties and durability of the geopolymer-stabilized soil. This paper aims to explore how geopolymer works for soil stabilization and its corresponding unconfined compressive strength value. The study's findings indicate that the concentration of the alkali activator had a significant influence on the stabilized soil's compressive strength, brittleness index, and secant modulus. The research identified an optimal range of 12-14 M for Na2SiO3+NaOH mixtures in fly ash-based geopolymers, and emphasized the importance of the alkali activator ratio in stabilizing soil with geopolymer materials. However, the study suggests that further research is necessary to investigate the impact of different ratios and molarities of alkali activator on stabilized soil properties.

Compressibility Characteristics of an Organic Soil Treated with Fly Ash and Fly Ash-Based Geopolymer

Organic soils are problematic soil for various engineering applications due to their high compressibility and low shear strength which need to be improved. For many soil improvement techniques, using waste materials, such as fly ash (FA), is a practical and sustainable process. In this research, FA and geopolymer were used e used to reduce organic soil's compressibility. A one-dimensional consolidation test was performed to evaluate the organic soil's consolidation and compressibility properties. The geopolymer was prepared using 20% FA and of sodium hydroxide ratio and sodium silicate alkali solutions. The geopolymer specimens were first cured for 2 hours at 45 and 65 oC, then cured for further 28 days at room temperature. The consolidation test results showed that FA-based geopolymer is effective in stabilizing organic soils due to the observed improvement in the compressibility, consolidation, and permeability characteristics. The compression index decreased by 98.16%, and the permeability decreased by 95%.

Unit Weight, Strengths and Thermal Conductivity of Cellular Lightweight Fly Ash Geopolymer Mortar Reinforced with Polyvinyl Alcohol

Lightweight materials are the most popular building wall construction materials because of their fast installation, high insulation, and low cost. However, these materials use cement as a binder. Cement production releases carbon dioxide emissions resulting in environmental issues. The research investigated the use of fly ash geopolymer and polyvinyl alcohol (PVA) to enhance the mechanical and thermal properties of cellular lightweight mortar. The effects of PVA concentrations, foam content (Fc), and curing time on properties of the cellular lightweight fly ash geopolymer (CLFAG) mortar reinforced with PVA specimens were evaluated. The results revealed that the PVA concentrations and Fc had an effect on the unit weights of the CLFAG reinforced with PVA specimens. The lowest unit weight of the specimens was 10.10 kN/m³ at a PVA concentration of 20% and Fc of 2%. According to the Thailand industrial standard for C12 and C14 CLC block types, all mixed ingredients met the unit weight and compressive strength requirements. The correlation between unit weight and thermal conductivity of CLFAG reinforced with PVA specimens was represented by a linear function, which was a useful equation for estimating the thermal conductivity of specimens.

Physical, Mechanical and Radiological Characteristics of a Fly Ash Geopolymer Incorporating Titanium Dioxide Waste as Passive Fire Insulating Material in Steel Structures.

This research analyzes whether a titanium dioxide waste (TiO2 waste) can be used as a source material for geopolymers with good fire resistance properties. Samples with different proportions were prepared, replacing fly ashes with titanium dioxide waste on geopolymers (0, 20, 30, 40 and 100% w/w). The activating solution has a Na2O/SiO2 molar ratio of 0.98. Physical (bulk density, moisture content and water absorption) and mechanical (superficial hardness and compressive strength) characteristics have been evaluated. In addition, their thermal behavior at high temperatures (fire resistance, compressive strength at elevated temperature and absorbed energy) has also been evaluated to see if they can be used as fire insulating materials. This work also studies the radiological activity of geopolymer materials. The replacement of FA with WTiO2 increases the bulk density due to its higher specific bulk density. The highest compressive strength values were obtained with a TiO2 waste content between 30 and 40% w/w. The compressive strength decreases at high temperatures, especially when more TiO2 waste is added. When the amount of TiO2 waste is increased, so is the plateau of evaporation, and this, in turn, increases the resistance to fire. Geopolymers containing FA and TiO2 waste do not present radiological problems, although, when the TiO2 waste is increased, the activity index of the geopolymer also rises.

Durability of fly-ash geopolymer binder in deep-mixed expansive soils

This study investigates the durability and leachate behaviour of an alkali-activated fly ash (FA) as a binder to stabilise expansive soils in deep-mixing applications. To activate FA, a liquid alkali activator (LAA) to binder ratio (LAA–FA) was introduced and varied from 1.0 to 1.5. The effects on swelling–shrinkage, consolidation and unconfined compressive strength (UCS) characteristics under wetting–drying cycles (durability) were studied on deep-mixed expansive soils. Leachate studies were conducted to determine the environmental impact of alkali-activated FA. Heavy metal and sodium ion concentrations were measured in each leachate cycle. To examine the permanent microstructural and phase changes in the mixes, scanning electron microscopy (SEM) and x-ray diffraction (XRD) studies were conducted. The swelling–shrinkage behaviour of soil could be marginally controlled for LAA/FA = 1.0 and 1.25 due to heavy weight loss during the wetting–drying cycles. However, even after 12 durability cycles, the UCS of soil with LAA/FA = 1.5 retained 1.0 MPa and showed non-critical swelling–shrinkage behaviour. The heavy metals and sodium ion concentrations were in acceptable range for LAA/FA = 1.25 and 1.5. XRD and SEM studies revealed that soil treated with a binder ratio of LAA/FA = 1.5, showed high crystalline peaks and aggregated structures.

Bonding Strength between Geopolymer Fly Ash to Ordinary Portland Cement Concretes using Mohr-Coulomb Theory

This paper presents an experimental study to analyse the bonding strength between geopolymer concrete (GC) and normal concrete (NC) bond substrate. Three different strengths of GC which were 80 MPa, 85 MPa and 90 MPa were bonded to 30 MPa NC and denoted asNC30-GC80, NC30-GC85 and NC30-GC90. Slant shear and split tensile tests were conducted to investigate the bonding strength between two different substrates. The effect of bonding was then determined by using Mohr-Coulomb theory where critical bonding condition (smooth surface) was created. From the analysis, it was found that NC30-GC80 had the most powerful self-adhesion between GC towards NC. This result indicated the highest bonding strength of GC to NC at critical condition. Such strong bond was obtained by the effect of self-adhesive from GC to NC. The self-adhesive characteristic which represented by pure shear strength was obtained from Mohr-Coulomb theory.

Self-Sensing Properties of Carbon Black or Graphite Modified Fly Ash Geopolymer under Flexure

Alkaline activated materials and geopolymers are recently widely involved in studies concerning their potential for advanced composites with enhanced electrical properties. This paper reports self-sensing behaviour of fly ash mortar doped with carbon black or graphite conductive admixture subjected to loading in three-point flexural test. Development of self-sensing properties and deformations of all mortars was initially investigated under flexural loading cycles with constant amplitude, final stage of the experimental part consisted in loading till fracture. Both reference and modified geopolymers failed to achieve satisfactory self-sensing performance during the cyclic loading but certain differences in fractural properties were observed.

Using Artificial Neural Networks for the Prediction of the Compressive Strength of Geopolymer Fly Ash

Geopolymers are promising cement replacement materials as their use results in a considerable reduction of CO2 emissions. Geopolymer Fly ash (GF) is a widely used geopolymer due to its low cost and waste management achievement. The compressive strength of GF depends on variables such as curing time, curing temperature, NaOH molarity, the ratio of sodium silicate to sodium hydroxide, the ratio of fly ash to alkaline solution, etc. Artificial Neural Networks are employed to predict the strength of GF due to their accurate prediction capability as well as saving time and cost of experimental work. The obtained Root Mean Square Error (RMSE) and correction coefficient (R2) values were 4.47 and 0.972 respectively. The results illustrate the ability of the ANN model to be used as an efficient tool in predicting the compressive strength and determining the optimal values of GF parameters. The maximum strength of GF was observed for 2 hours curing time at 100°C, molarity of 10, fly ash to alkaline solution ratio of 3, and sodium silicate to sodium hydroxide ratio of 1.