Properties Of Fly Ash Based Geopolymer Research Articles

Synergistic effect of characteristics of raw materials on controlling the mechanical properties of fly ash-based geopolymers

This study investigates the macro- and micro-characteristics of the geopolymers to verify the feasibility of prediction of compressive strength via quantitatively adjusting the amorphous phase content (APC) and particle size distribution (PSD) of raw material. The optimization of PSD through the sieving method does not reach the intended compressive strength compared to the reference, while controlling APC by partly replacing fly ash with slag achieves the expected target. The critical reasons are well explained by the micro properties of geopolymers. The sieving process leads to the reduction in the porosity but has little effect on the Si/Al molar ratio and APC of geopolymers, which is related to the heterogeneous distribution of active constituents of fly ash. By contrast, the addition of slag significantly enhances the microstructure of geopolymers, especially by the interaction between C-A-S-H and N-A-S-H gel. As a result, the compressive strength is primarily determined by the micro index MCa, proved by the mutual positive correlation. Besides, given the low reaction rate of fly ash in the geopolymerization and relevant experimental results, the modified formula that only uses the APC and specific gravity (G20°C) of powder raw materials to establish the relationship with compressive strength owns a better linear dependence and probably more appropriate for the prediction of mechanical properties of fly ash-based geopolymers.

Fresh properties of fly ash-based geopolymer: the role of the testing conditions on the rheological measurements

Rheology is a powerful tool to assess the fresh properties of cementitious systems accurately. However, uncertainties and systematic errors in measurements can impact the obtained values, even though the rheological properties should not depend on the measurement procedure. Contrary to Portland-based systems, few reports systematically assessed the role of the testing conditions on the rheological properties of geopolymers. This study evaluates the effect of testing conditions on the measurement of the rheological properties of fly ash-based geopolymer paste. Different testing geometries, including two sets of parallel plates, two concentric cylinders, and a four-blade vane spindle, were used to obtain flow curves by continuous and stepwise shear rate increments. The results show that reducing the ratio between the spindle and container radii for coaxial geometries can induce wrong shear stress/shear rate calculations and local instability. Additionally, the differences in shear stress measured with ramp and steps protocols increase as the Ri/Ro ratio (i.e., the ratio between the spindle and the container radii) increases. Parallel plates require an extra ring to keep the paste between the plates during shearing. Increasing the maximum shear rate applied reduces yield stress and viscosity values, indicating improved particle dispersion. Comparative tests with a Portland paste demostrated that the mini-slump test was suitable for assessing the yield stress of the geopolymeric system. However, its limitation regarding viscosity assessment was evidenced. Therefore, the measurement setup must be carefully chosen for the geopolymer under evaluation, and preliminary tests are highly recommended to support this selection.

Effects of Elevated Temperature and Activation Solution Content on Microstructural and Mechanical Properties of Fly Ash-based Geopolymer

This paper examined the effects of sodium silicate to sodium hydroxide (SS/SH) and fly ash (FA) to alkaline solution (FA/AS) ratios on the thermal and mechanical properties of FA-based geopolymers. To tackle this, two factors were varied including the SS/SH (1.5, 2.5, and 3.5) and the FA to the AS (1.5, 2.5 and 3.5). The elaborated nine specimens were characterized employing several techniques including FT-IR, XRD, 29Si and 27Al MAS NMR, and SEM analysis. The thermogravimetric analysis (TGA) was utilized to study the thermal stability of geopolymer specimens. The fire resistance of the geopolymer specimen was evaluated by weight change and residual compressive strength before and after being subjected to fire testing (up to 1,000°C). A decrease in the resistance recorded when heated at 200 and 400°C was related to the vaporization of free water was observed. Meanwhile, a gain in strength was achieved for the geopolymer samples after being subjected to 600 and 800°C which was attributed to the viscous flow of silicate-rich phases that eventually bridged the fissures or defects occurring during its solidification. Samples with a high FA/AS ratio showed good fire resistance after high-temperature exposure.

The improvement of strength and microstructural properties of fly ash-based geopolymer by adding elemental aluminum powder

The improvement of strength and microstructural properties of fly ash-based geopolymer by adding elemental aluminum powder

Influence of Fe2O3, MgO and Molarity of NaOH Solution on the Mechanical Properties of Fly Ash-Based Geopolymers.

The use of waste from industrial activities is of particular importance for environmental protection. Fly ash has a high potential in the production of construction materials. In the present study, the use of fly ash in the production of geopolymer paste and the effect of Fe2O3, MgO and molarity of NaOH solution on the mechanical strength of geopolymer paste are presented. Samples resulting from the heat treatment of the geopolymer paste were subjected to mechanical tests and SEM, EDS and XRD analyses. Samples were obtained using 6 molar and 8 molar NaOH solution with and without the addition of Fe2O3 and MgO. Samples obtained using a 6 molar NaOH solution where Fe2O3 and MgO were added had higher mechanical strengths compared to the other samples.

Impact of micro characteristics on the formation of high-strength Class F fly ash-based geopolymers cured at ambient conditions

• The Class F fly ash-based geopolymer pastes with 28 d’s strength over 75 MPa is produced under ambient conditions. • The mechanical properties are strongly related to the synergistic action of amorphous phase content, reactive phase’s Si/Al molar ratio, and porosity. • Water content plays a critical role among the options of adjusting mix proportions in improving the micro characteristics. The high-strength Class F fly ash-based geopolymer pastes cured at ambient conditions with 28 d compressive strength over 75 MPa is rarely reported. Herein based on such reference, the geopolymer pastes synthesized by cognate fly ash with lower activity are produced to investigate the combined effects of geopolymer’s micro characteristics, such as amorphous phase content, Si/Al molar ratio of reactive phase, and porosity, on the enhancement of mechanical properties. The fly ash-based geopolymers are prepared with different Si/Al molar ratios, Na/Al molar ratios, and water/fly ash mass ratios. The results at the micro-level indicate that the development of mechanical properties of fly ash-based geopolymers is mainly affected by the reactive phase’s Si/Al molar ratio, followed by the porosity and then amorphous phase content. In addition, experiments show that decreasing the water/fly ash mass ratio leads to the positive promotion in all the micro characteristics with small reactive phase’s Si/Al molar ratio, low porosity, and high amorphous phase content. Therefore, the dramatic increase in the compressive strength and Young’s modulus of geopolymer specimens can be obtained. In summary, it can be concluded that, given the workability of precursor preparation, the application of a water/fly ash ratio lower than 22.5 % can increase the strength close to 75 MPa, which is beneficial to the potential application of fly ash-based geopolymers in practical engineering. The other related characteristics of high-strength fly ash-based geopolymers will be investigated in future studies to further reveal its latent practicability.

A Comprehensive Review of Development and Properties of Flyash-Based Geopolymer as a Sustainable Construction Material

A Comprehensive Review of Development and Properties of Flyash-Based Geopolymer as a Sustainable Construction Material

Setting behavior and mechanical properties of concrete rubble fly ash geopolymers

Due to concerns about the very high primary raw material consumption and CO2 emissions of the economically important construction sector, the demand for “green” binders is growing. One option that is receiving particular attention is the material class of “geopolymers”, which could be used as a substitute for Portland cement. This new group of binders not only exhibits improved mechanical properties, but is also characterized by particularly low carbon dioxide emissions in the course of its production. This work focuses on the influence of concrete rubble on the setting behavior and microstructural properties of fly ash-based geopolymers. In the course of the investigations, the manufactured geopolymer samples are examined for the material parameters relevant to building materials, namely compressive strength, raw density and thermal conductivity. The setting behavior and the forming structures are investigated by infrared spectroscopy, X-ray diffraction analysis and scanning electron microscopy. The present work is intended to contribute to the development of a suitable recycling strategy for the material recycling of concrete rubble in novel substitute construction materials, the geopolymers.

Structural, Mechanical and Chemical Properties of Low Content Carbon Geopolymer

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

Comprehensive review of the properties of fly ash-based geopolymer with additive of nano-SiO2

Abstract Nano-SiO2 is a non-toxic, tasteless, and pollution-free material with hydroxyl groups that facilitate the adsorption of water on its surface. Nano-SiO2 is characterized by small particle size, high purity, low density, large surface area, and good dispersion properties. In addition, nano-SiO2 has excellent stability, reinforcement, thixotropy, and optical and mechanical properties. The additive of nano-SiO2 can enhance the mechanical properties and microstructure of concrete. Therefore, nano-SiO2 is widely used as an additive in the field of building materials. Geopolymers have excellent mechanical properties, acid–alkali resistance, fire resistance, and high-temperature resistance. In addition, mineral waste and construction waste can be used as raw materials for geopolymers. Therefore, geopolymers have the potential to substitute ordinary Portland cement and have good prospects for application as construction materials. The application of nanomaterials in geopolymer products has shown that nano-SiO2 is effective in increasing the rate of geopolymerization reaction and reducing the setting time of geopolymers in a controlled quantity. Related results indicate that an appropriate quantity of nano-SiO2 can make the microstructure of fly ash-based geopolymers denser and produce higher mechanical strength. In this study, based on the mechanism of geopolymerization, the effects of nano-SiO2 on the properties of fly ash-based geopolymers including compressive strength, microstructure, hardening properties, shear bond strength, durability, and practical applications have been summarized. This study can provide a basis for understanding the effects of nano-SiO2 on the mechanical properties and durability of fly ash-based geopolymers.

The effect of glass waste as an aggregate on the compressive strength and durability of fly ash-based geopolymer mortar

Geopolymers have been introduced to limit the use of ordinary Portland cement (OPC), as its production contributes to the emission of about 7% of the world’s carbon dioxide, which has a negative effect on the environment. The present study aimed to investigate the effect of glass-waste aggregate on the mechanical properties of fly ash-based geopolymer and OPC mortars. In the study, fly ash geopolymer and OPC mortar mixtures were prepared using glass-waste as fine aggregate. In addition, geopolymer and OPC mortars were also prepared using silica sand as control mixes. A blended solution comprising sodium silicate and sodium hydroxide was used as an alkali activator in fly ash geopolymer mixtures. Fresh mixtures were subjected to workability measurements, while 50 mm cubes were made for compressive strength testing. Mortar prisms of 25 x 25 x 285 mm were prepared and subjected to drying shrinkage test. From the results, the use of glass-waste aggregate negatively affected the compressive strength of the mortars, regardless of the binder type. Geopolymer mortars made using glass-waste aggregate gave 55% lower compressive strength than those made using silica sand. However, mixtures made using glass waste aggregate exhibited better performance in drying shrinkage than those made using silica sand.

Influence of Silica-Alumina Modified Materials on Mechanical Properties of Fly Ash-Based Geopolymers

Influence of Silica-Alumina Modified Materials on Mechanical Properties of Fly Ash-Based Geopolymers

Effects of Molybdenum Tailing on Properties of Fly Ash-Based Geopolymers from Macroscopic and Microscopic Perspectives

Effects of Molybdenum Tailing on Properties of Fly Ash-Based Geopolymers from Macroscopic and Microscopic Perspectives

An Experimental Study of the Effects of Low-Calcium Fly Ash on Type II Concrete

In this study, the compressive strength and the permeation properties of fly ash-based Geopolymer were experimentally investigated. Type 2 Portland cement (T2PC) was partially or entirely replaced with 0, 10, 20, 30, 50, 70, and 100% of fly ash (FA). The laboratory tests were conducted for compressive strength at 7, 28, and 90 days, and permeation properties such as water absorption at 7 and 28 days. The main goal was to produce eco-friendly concrete with high strength and low permeability through blending cementitious materials including low Calcium (Ca) (T2PC and FA) for protecting concrete against sulphate attacks and other chemically destructive compounds in the environment. This study focused on the effectiveness of the curing period, combinations of chemical activators by varying the molarity of alkaline solutions between 4.16 and 12.96 M and keeping the sodium silicate (SS) to sodium hydroxide (SH) by the weight ratio of 2.5. Lab observations from this study demonstrated that the compressive strength was enhanced with the increment in fly ash content at all ages, with optimum being at 20% as the replacement of T2PC.

The Influence of Short Coir, Glass and Carbon Fibers on the Properties of Composites with Geopolymer Matrix.

The aim of the article is to analyze the influence of short coir, glass and carbon fiber admixture on the mechanical properties of fly ash-based geopolymer, such as: flexural and compressive strength. Glass fiber and carbon fibers have been chosen due to their high mechanical properties. Natural fibers have been chosen because of their mechanical properties as well as for the sake of comparison between their properties and the properties of the artificial ones. Fourth series of fly ash-based geopolymers for each fiber was cast: 1, 2, and 5% by weight of fly ash and one control series without any fibers. Each series of samples were tested on flexural and compressive strength after 7, 14, and 28 days. Additionally, microstructural analysis was carried out after 28 days. The results have shown an increase in compressive strength for composites with fibers—an improvement in properties between 25.0% and 56.5% depending on the type and amount of fiber added. For bending strength, a clear increase in the strength value is visible for composites with 1 and 2% carbon fibers (62.4% and 115.6%). A slight increase in flexural strength also occurred for 1% addition of glass fiber (4.5%) and 2% addition of coconut fibers (5.4%). For the 2% addition of glass fibers, the flexural strength value did not change compared to the value obtained for the matrix material. For the remaining fiber additions, i.e., 5% glass fiber as well as 1 and 5% coconut fibers, the flexural strength values deteriorated. The results of the research are discussed in a comparative context and the properties of the obtained composites are juxtaposed with the properties of the standard materials used in the construction industry.

Combined effect of curing temperature, curing period and alkaline concentration on the mechanical properties of fly ash-based geopolymer

This paper presents a comprehensive study of the impacts of three governing factors: Alkaline concentration, curing temperature and curing period on the mechanical properties of a geopolymer that include the uniaxial compressive strength, Poisson’s ratio, and Young’s modulus of elasticity. As literature review indicates, most of the previous research studies were focused on the effect of single factor, while there is a lack of comprehensive studies to simultaneously investigate the combined effects of all these factors, especially in a systematic experimental setup. Acknowledging this rarely exploited area of geopolymer research, this work developed a sequential experimental setup to perform a detailed study to understand the mechanical behaviors of the geopolymer as a function of alkaline concentration, curing temperature and curing period. The test results show that the compressive strength of geopolymer is mostly governed by the curing temperature as compared to alkaline concentration and curing period. Geopolymer samples cured at 60 °C and 80 °C give a competitive compressive strength of about 30 MPa even when prepared with less alkaline activator and cured for a shorter period. This observation is justified by the microscopic analysis of samples at each curing temperature that explains the formation of microcracks, amount of unreacted fly ash, sodium-aluminosilicate hydrate crystals (NASH), calcium-silicate hydrate (CSH) crystals and efflorescence.

Effects of initial SiO2/Al2O3 molar ratio and slag on fly ash-based ambient cured geopolymer properties

Geopolymers cured at ambient temperature are of high interest since they provide promising environmentally friendly alternative to cement. This paper discusses the effect of the initial molar ratio of SiO2/Al2O3 and fly ash replacement with ground granulated blast furnace slag (GGBFS) on the properties of fly ash-based geopolymers. Geopolymer mixtures developed in this study are user-friendly (SiO2/Na2O larger than 1.4) and suitable for producing self-compacting geopolymer concrete at ambient temperature. Results showed that the increase in SiO2/Al2O3 ratio accelerated the setting of fly ash-slag geopolymer systems, which was quite different from that reported for metakaolin-based geopolymers. An increasing–decreasing trend in compressive strength was observed by increasing the SiO2/Al2O3 ratio. The maximum compressive strength achieved when the SiO2/Al2O3 ratio was 3.37 (Si/Al = 1.68). The higher compressive strength at this ratio could be related to the alumina-silica bondings in the amorphous area, which comprised more than 81%-84% of geopolymer microstructure. Furthermore, the study revealed that the decrease in fly ash replacement with GGBFS prolonged the setting of geopolymer mixtures and reduced their compressive strength significantly.

Using cellulose nanocrystals to improve the mechanical properties of fly ash-based geopolymer construction materials

Ordinary Portland cement production is one of the biggest emitters of carbon dioxide. Consequently, there is a strong need for construction materials with lower environmental footprints. However, the development of alternative green construction materials requires a standardized framework. Although cellulose nanocrystals have shown considerable reinforcement potential in conventional construction materials, its effect on the mechanical properties of fly ash-based geopolymers as green construction materials is not known. Consequently, a detailed database outlining the cellulose nanocrystals interactions on the compressive strength, density, and corrosion resistance properties of geopolymers can optimize and guide further research efforts. The aims of this study were to firstly determine the effect of cellulose nanocrystals on the mechanical properties of fly ash-based geopolymers. Secondly, to produce a database of the effects of cellulose nanocrystals concentration and activator concentration on the mechanical properties of the formed geopolymers. Finally, to formulate an empirical framework to develop green construction materials. An empirical framework was developed alongside the cellulose nanocrystals-reinforced geopolymers, which were optimized using a statistical experimental design. The experimental results yielded the geopolymer property database. It was found that low cellulose nanocrystals concentrations (less than 0.5%) favoured the geopolymer mechanical properties. Using industrial wastes to produce green construction materials can divert industrial wastes from landfills and minimize the widespread use of environmentally degrading conventional construction materials. The framework developed in this study can facilitate the commercialization of green construction materials in industry.

Prediction of engineering properties of fly ash-based geopolymer using artificial neural networks

Fly ash-based geopolymer has been studied extensively in recent years due to its comparable properties to Portland cement and its environmental benefits. However, the uncertainty and complexity of design parameters, such as the SiO2/Na2O mole ratio in alkaline solution, the alkaline solution concentration in liquid phase, and the liquid-to-fly ash mass ratio (L/F), have made it very difficult to create a systematic approach for geopolymer mix design. These mix design parameters, along with fly ash properties and curing conditions (temperature and time), significantly influence key properties of the material, such as setting time and compressive strength. In this study, an artificial neural network (ANN) was used to develop models for predicting the key properties of high-calcium fly ash-based geopolymer according to its mix design parameters. The correlations between experimental measurements and ANN model predictions of setting time, compressive strength, and heat of geopolymerization were established based on the results of tests on 36, 273, and 72 geopolymer mixes, respectively. The results show that the correlations between the experimental measurements and ANN model predictions of the properties studied are all strong. ANN modeling was found to be a suitable computing method to analyze the effects of design parameters on geopolymer properties and showed that L/F exhibited the greatest effect on setting time, alkaline solution concentration had the greatest influence on compressive strength, and a mole ratio larger than 1.5 significantly impacted heat at the geopolymerization peak. The developed ANN models can be used as guidance for mix design of high-calcium fly ash geopolymer in engineering applications.

Rheology of fly ash-based geopolymer: Effect of NaOH concentration

The objective of this paper is to study the flow and viscoelastic properties of fly ash-based geopolymers activated at different concentrations of NaOH. Although the investigated geopolymers exhibited a shear thinning behavior, their rheological properties are greatly affected by the NaOH concentrations. Increasing the concentration of NaOH up to 7 mol/L resulted in an increase of the yield stress, the storage modulus, and the rigidification rate of the geopolymers. However, the use of higher concentrations resulted in lower yield stress, storage modulus, and rigidification rate. The competition between dissolution and geopolymerization processes seems to be the key parameter affecting the evolution of rheology of the investigated geopolymers. In the case of a NaOH concentration lower than 7 mol/L, the geopolymerization process is accelerated. However, when the concentration exceeds 7 mol/L, the geopolymerization is delayed despite the higher dissolution. This is probably due to the dominance of the repulsive forces between particles, thus preventing the network formation. Geopolymers activated at a NaOH concentration of 2 mol/L and a temperature around 40 °C exhibited comparable rheological and viscoelastic properties than those activated at higher concentration of 7 mol/L and a temperature of 25 °C.