Fly Ash-based Geopolymer Paste Research Articles

Innovative shielding solutions by geopolymer paste and fly ash as effective substitution of cement materials for sustainable protection

This study aimed to evaluate gamma radiation shielding material based on fly ash (FA) type ASTM C 618 Class F with a sodium silicate solution with two molarity values. The composite geopolymer paste was synthesized by activating the alkaline solution and curing at 70 °C for 1 h. The sodium silicate solution was added to the FA matrix with two molarity values of SiO2/Na2O (2.2 and 3.3). The density of each sample GPp1 and GPp2 are 2.121 g/cm3 and 2.212 g/cm3, respectively. The XRD patterns revealed the successful preparation of the prepared samples. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) determined the samples' surface morphology and elemental composition. The compressive strength of GPp2 sample was higher than for the GPp1 sample at 7 and 28 days, respectively. The density and tensile strength of the GPp2 sample are high compared to GPp1. Also, the thermal expansion coefficients for GPp2 are 5.16 × 10-4 K-1 while the GPp1 equals 2.65 × 10-4 K-1. To evaluate the photon shielding competences of the geopolymer samples, the parameters such as half-value layer (HVL), mean free path (MFP), effective atomic number (Zeff), effective electron density (Neff), and exposure buildup factor (EBF) were calculated. The highest Zeff was obtained for the GPp2 sample, whereas the lowest values were obtained for MFP, HVL, and Neff. Additionally, μm values were compared to theoretical Phy-X/PSD. It can be concluded that the investigated fly ash-based geopolymer paste is a promising material for gamma radiation shielding.

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.

Mitigating drying shrinkage and enhancing mechanical strength of fly ash-based geopolymer paste with functionalized MWCNTs grafted with silane coupling agent

In this study, the functionalized MWCNTs grafted with silane coupling agent are obtained by means of successive chemical modification methods. The structure and dispersity of the functionalized MWCNTs are characterized using the FT-IR, XPS and HR-TEM, respectively. Then the reaction kinetics, mineralogical phases, morphology and elemental components of modified geopolymer incorporated with the functionalized MWCNTs are evaluated by means of the XRD, TAM, FT-IR and SEM-EDS analysis. In addition, the influences of functionalized MWCNTs on the mechanical strength and drying shrinkage behaviors are investigated. The MIP and 29Si NMR are employed to investigate the pore size distribution and microstructural rearrangement of gel products. The results show that the functionalized MWCNTs can be dispersed uniformly in aqueous solution without any dispersant. Importantly, the modified geopolymer incorporated with 0.2% functionalized MWCNTs-KH550 exhibits a lower drying shrinkage(0.098%) and higher mechanical strength compared with that of all geopolymer pastes. The hydrophobic modification of functionalized MWCNTs reduces significantly the mass loss of the geopolymer. The MIP results indicate that a critical pore size(ds)computed from both the pore size distributions and water loss is related with drying shrinkage, and the drying shrinkage of geopolymer pastes decreases with the increment of the ds value. A strong interaction between adjacent C(N)-A-S-H particles occurs in microstructure rearrangement period by means of the silicate polymerization and micropore closure. Moreover, the new dual interaction mechanism decreasing drying shrinkage and increasing mechanical strength of geopolymer pastes are proposed.

Research on the mechanical properties and microstructure of fly ash-based geopolymers modified by molybdenum tailings

The environmental and economic problems caused by the accumulation of molybdenum tailings (MT) have become increasingly prominent. Therefore, a cost-effective method is urgently needed to realize the resource utilization of MT. In this paper, the mechanism of action of MT on fly ash-based geopolymers were systematically researched. The effects of MT on the macro-properties of fly ash-based geopolymers were researched in terms of fluidity, setting time, drying shrinkage and mechanical strength. The reaction mechanism of fly ash-based geopolymers modified by MT were analyzed from the perspective of microstructure and micromorphology by using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetric (TG) and nitrogen adsorption/desorption (NAD) methods. The results show that the addition of MT could effectively improve the fluidity of fly ash-based geopolymer paste, appropriately delay its setting time and increase its drying shrinkage; When the content of MT was 20%, the bending strength of fly ash-based geopolymers were significantly improved, and the flexural strength at 3, 7 and 28 days were increased by 79.2%, 47.4% and 19% respectively. The SEM, FTIR and TG results confirmed that the main product was glassy sodium calcium aluminum silicate hydrate (N(C)-A-S-H) gel, which was used as a binder to connect residual raw material particles and form hardened geopolymer materials. The XRD results show that the addition of MT would not lead to the formation of new crystal phases in the fly ash-based geopolymers, and when the addition of MT was 20%, the diffraction peak intensity of the crystal phase structure of the fly ash-based geopolymers basically were not changed obviously. The NAD results show that the addition of 20% MT could significantly improve the pore structure of fly ash-based polymers, increased the volume fraction of micropores, and reduced the volume fraction of macropores. The SEM/EDS results show that the MT could be dissolved in alkali activator to form nanorods to fill the pore structure of fly ash-based geopolymers, thus reducing the cracks and defects of fly ash-based geopolymers and improving the compactness of the microstructure.

Ultra-high early strength fly ash-based geopolymer paste cured by microwave radiation

This paper investigates the strength development of fly ash (FA)-based geopolymer paste cured by thermal and microwave radiations, together with microstructural characterisations. The results indicate that both the two methods can significantly increase the compressive strength compared to room temperature curing. Higher microwave power can promote the strength development without changing the reaction products and the final strength, owing to the accelerated formation of N-A-S-H in the reaction product under the raised temperature, characterised by the increased Si–O Q2-3 bonds and decreased Si/Al ratio. Over-time microwave radiation weakens the strength due to the formation of micro-cracks. Microwave presents higher curing efficiency than heating, producing a compressive strength of 37 MPa within 15 min. Moreover, microwave radiation can accelerate the setting progress of the fresh paste without compromising its final strength after curing. The present work can guide the preparation of FA-based geopolymers with high early strength in practice.

Properties of Calcined Oebelo Red Soil Modified Fly Ash based Geopolymer

Research on fly ash has developed rapidly in recent years, one of which is its use as an environmentally friendly geopolymer concrete material. This research study the effect of calcination temperature and duration of red soil to compressive strength and setting time of fly ash based geopolymer paste. The red soil calcination process was carried out with variations in temperature of 400°C, 600°C, and 800°C for 4 and 8 hours of calcination. The activator solution was prepared by mixing a solution of NaOH with a solution of Na2SiO3 with a 1:1 composition. Sample treatment was carried out in 2 types, with ambient curing type and dry curing type. From the results of this study, the fastest setting time and maximum compressive strength of geopolymer paste of 23.14 MPa achieved at 800°C red soil calcination at 8 hours, and dry curing.

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.

Effects of Initial Surface Evaporation on the Performance of Fly Ash-Based Geopolymer Paste at Elevated Temperatures

Geopolymer concrete is a valuable and alternative type of concrete that is free of traditional cement. Generally, geopolymer concretes require a source material, which is rich in silicon and aluminum. Furthermore, fly ash-based geopolymer concretes have been proven to have superior fire resistance, primarily due to their ceramic properties, and are inherently environmentally-friendly given their zero-cement content. This paper presents the effects on initial evaporation on the performance of fly ash-based geopolymer pastes after exposure to elevated temperatures of 400 °C and 800 °C. The fly ash (FA) samples used in the present study included: Gladstone and Gladstone/Callide. The results for sealed samples placed in the oven during curing were much more consistent than the samples that were not kept covered. In addition, Gladstone fly ash-based geopolymer samples that were sealed recorded an initial maximum compressive strength reading of ca. 75 MPa, while sealed Gladstone/Callide fly ash-based geopolymer samples, of the same mix design, only recorded an initial maximum compressive strength reading of ca. 50 MPa (both subjected to oven curing at 60 °C for 24 h). However, Gladstone/Callide fly ash-based geopolymer samples exhibited a significant strength gain, ca. 90 MPa, even after being subjected to 400 °C.

Gamma Radiation Shielding Properties of Slag and Fly Ash-based Geopolymers

Industrial waste-based geopolymer cement is a greener alternative to Ordinary Portland Cement (OPC) for radiation shielding with comparable mechanical properties without the production of CO 2 during synthesis. In this paper, the linear attenuation coefficient of slag and fly ash-based geopolymers, unmodified by aggregates, is measured and used to calculate the mass attenuation coefficients, half-value layer (HVL), and tenth-value layer (TVL) of the geopolymers. Narrow Beam Gamma Spectrometry with gamma energy of 0.662 MeV, 1.173 MeV, and 1.332 MeV was used to irradiate a series of slag and fly ash-based geopolymer paste of cylindrical shape with a diameter of 7.5 cm and height of 9.5 cm. S lag geopolymer has linear attenuation coefficient of 0.1642/cm, 0.1237/cm, 0.1150/cm, mass attenuation coefficient of 0.0782 cm 2 /g, 0.0589 cm 2 /g, 0.0548 cm 2 /g, the HVL of 4.222 cm, 5.609 cm, 6.056 cm, and TVL of 14.025 cm, 18.633 cm, 20.118 cm, respectively. Fly ash geopolymer has linear attenuation coefficient of 0.1387/cm, 0.1075/cm, and 0.0964/cm, mass attenuation coefficient of 0.0761 cm 2 /g, 0.0589 cm 2 /g, 0.0529 cm 2 /g, HVL of 4.998 cm, 6.453 cm, 7.202 cm, and TVL of 16.603 cm, 21.437 cm, 23.926 cm, respectively. Test samples made from slag-based geopolymers have a better shielding capability compared to fly ash-based geopolymers due to having higher attenuation coefficients as well as lower HVL and TVL, thus requiring less material to absorb radiation of the same energy level.

Experimental study of the effect of graphene on properties of ambient-cured slag and fly ash-based geopolymer paste and mortar

This paper investigates the effect of graphene on the properties of slag and fly ash-based ambient cured geopolymer paste and mortar. Graphene was added with five percentage (0.0%, 0.1%, 0.5%, 1.0% and 1.5%) of aluminosilicate materials by weight. The initial and final setting times of geopolymer pastes were reduced. The workability of geopolymer paste and mortar was slightly decreased with graphene. The 7-day and 28-day compressive strength of geopolymer mortar with 1.0% addition of graphene increased by 25% and 10%, respectively. The addition of 1% graphene could be considered as optimum proportion to improve the compressive strength of geopolymer mortar.

Durability of Moroccan fly ash-based geopolymer binder

This study investigates the durability of fly ash-based geopolymer pastes (FA-GP). Thermal behavior was evaluated by heating the FA-GP pastes at 1000 °C for 2 h (FA-GP-1000 °C). Acid resistance was studied by immersing specimens in a 5% sulfuric acid solution for a period of 4 months (FA-GP-H2SO4). Physicochemical, microstructural and mechanical changes after heat and acid exposure were studied using several analysis methods such as Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Scanning electron microscopy (SEM) and compressive strength. The results showed that the visual appearance of fly ash-based geopolymer samples was well preserved after high temperature exposure, and kept about 37% of its initial compressive strength value. SEM micrographs of acid exposed samples showed very porous structure with some micro-cracks, inducing a decrease of the residual compressive strength of about 33.33 %.

Performance evaluation of basalt fiber-reinforced geopolymer composites with various contents of nano CaCO3

High carbon footprint of cement production is the major drawback of plain cement concrete resulting in environmental pollution. Geopolymer composites paste can be effectively used as an alternative to Portland cement in the construction industry for a sustainable environment. The demand for high-performance composites and sustainable construction is increasing day by day. Therefore, the present experimental program has endeavored to investigate the mechanical performance of basalt fiber-reinforced fly ash-based geopolymer pastes with various contents of nano CaCO3. The content of basalt fibers was fixed at 2% by weight for all specimens while the studied contents of nano CaCO3 were 0%, 1%, 2%, and 3%, respectively. The compressive strength, compressive stress-strain response, flexural strength, bending stress-strain response, elastic modulus, toughness modulus, toughness indices, fracture toughness, impact strength, hardness, and microstructural analysis of all four geopolymer composite pastes with varying contents of nano CaCO3 using scanning electron microscopy (SEM) were evaluated. The results revealed that the use of 3% nano CaCO3 in basalt fiber-reinforced geopolymer paste presented the highest values of compressive strength and hardness while the use of 2% nano CaCO3 showed the highest values of flexural strength, impact strength, and fracture toughness of composite paste. The SEM results indicated that the addition of nano CaCO3 improved the microstructure and provided a denser geopolymer paste by refining the interfacial zones and accelerating the geopolymerization reaction.

Water absorption and compressive strength of various coal fly ash-based geopolymer pastes

This paper discusses the effects of the silicon dioxide to aluminium oxide molar ratio, sodium oxide to aluminium oxide molar ratio and concentration of sodium hydroxide on the water absorption and compressive strength of synthesised fly ash-based geopolymers. Geopolymer pastes were prepared using fly ash mixed with an alkaline activator consisting of sodium hydroxide solution and sodium silicate solution. The phase composition, microstructure and chemical bonding of the synthesised products were characterised after 7 and 28 days using X-ray diffraction, scanning electron microscopy and Fourier-transform infrared spectroscopy, respectively. The synthesised geopolymer with a silicon dioxide to aluminium oxide molar ratio of 5 and a sodium oxide to aluminium oxide molar ratio of 1.2, activated with 10 M sodium hydroxide, was found to have the highest strength and the lowest water absorption. The compressive strength was observed to increase with increasing silicon dioxide to aluminium oxide molar ratio. A high initial molar ratio of sodium oxide to aluminium oxide enhances the water absorption and decreases the strength of the geopolymer sample. The dissolution of silica and alumina from the starting materials becomes more efficient as the concentration of sodium hydroxide increases.

Effect of Elevated Temperature on Mechanical Properties of High-Volume Fly Ash-Based Geopolymer Concrete, Mortar and Paste Cured at Room Temperature

This paper presents results from experimental work on mechanical properties of geopolymer concrete, mortar and paste prepared using fly ash and blended slag. Compressive strength, splitting tensile strength and flexural strength tests were conducted on large sets of geopolymer and ordinary concrete, mortar and paste after exposure to elevated temperatures. From Thermogravimetric analyzer (TGA), X-ray diffraction (XRD), Scanning electron microscope (SEM) test results, the geopolymer exhibits excellent resistance to elevated temperature. Compressive strengths of C30, C40 and C50 geopolymer concrete, mortar and paste show incremental improvement then followed by a gradual reduction, and finally reach a relatively consistent value with an increase in exposure temperature. The higher slag content in the geopolymer reduces residual strength and the lower exposure temperature corresponding to peak residual strength. Resistance to elevated temperature of C40 geopolymer concrete, mortar and paste is better than that of ordinary concrete, mortar and paste at the same grade. XRD, TGA and SEM analysis suggests that the heat resistance of C–S–H produced using slag is lower than that of sulphoaluminate gel (quartz and mullite, etc.) produced using fly ash. This facilitates degradation of C30, C40 and C50 geopolymer after exposure to elevated temperatures.

Simple Model for Alkali Leaching from Geopolymers: Effects of Raw Materials and Acetic Acid Concentration on Apparent Diffusion Coefficient.

This paper investigates alkali leaching from geopolymers under various concentrations of acetic acid solutions. The effects of the raw metakaolin purity as well as fly ash-based geopolymer mortars and pastes are considered. A new methodology for (acetic) acid attack is proposed, adapting standard approaches, where the concentration of the leached alkali in the exposure solution is measured over time. The applicability of a simple diffusion-based mathematical model to determine the apparent diffusion coefficient (Dapp) for geopolymer pastes and mortars was validated. At the end of the paste tests, microstructural alterations of the specimens’ cross-sections were analyzed microscopically, revealing occurrence of degradation across the outermost surface parts and, especially under acid attack, the formation of long cracks that connect the surface with the intact inner zone. Drastically different Dapp are discussed in terms of the differences in the mix designs, principally resulting in different alkali-binding capacities of the geopolymers, while the acid promoted dissolution and increased porosity. As a result of this interpretation, it was concluded that Dapp is governed mainly by the chemistry of the alkali release from the gel, as it overruled the effects of porosity and cracks.

Tailored design of properties of Hongsa fly ash-based geopolymer paste via an adjustment of the alkali activator composition

The composition of the alkali activator was proven to significantly affect the properties of Hongsa fly ash-based geopolymer paste, which, in turn, was found to be beneficial to the product application design. In this research, the composition of the alkali activator was selected by adjusting Na2SiO3/NaOH ratio. The results showed that the compressive strength of the geopolymer paste increased when the ratio of Na2SiO3/NaOH increased and reached its maximum of 26.54 MPa at the ratio of 0.6 before being decreased with further increasing the ratio. The high-temperature resistance of the geopolymers prepared with two conditions, Na2SiO3-free (M − 00) and 0.6 Na2SiO3/NaOH ratios (M − 06), was assessed. The results showed that the geopolymer M − 06 started to deform and crack after heating at ≥ 600 °C while the geopolymer M − 00 did not show significant deterioration. The geopolymer M − 00 could retain its compressive strength much better than the geopolymer M − 06, making it suitable as a lightweight high temperature resistant-component in construction. The geopolymer M − 06 was marked to provide an excellent low-temperature compressive strength so it can be used as a geocement binding material.

Enhancement of the Thermal Durability of Fly Ash-Based Geopolymer Paste by Incorporating Potassium Feldspar

Enhancement of the Thermal Durability of Fly Ash-Based Geopolymer Paste by Incorporating Potassium Feldspar

Mechanical and fracture behaviour of micro steel fibre-reinforced fly ash-based geopolymer paste containing nano CaCO3

The present study has investigated the mechanical properties of micro steel fibre-reinforced fly ash-based geopolymer pastes by adding different contents of nano CaCO3. Three contents (0%, 1%, 2%, and 3% by weight of fly ash) were evaluated for nano CaCO3 while the content of micro steel fibres was kept constant at 0.5% by weight of paste. A control mix containing 0.5% micro steel fibres without any content of nano CaCO3 was fabricated for comparison purposes. The evaluated mechanical characteristics of micro steel fibre-reinforced geopolymer pastes were the compressive strength, flexural strength, elastic modulus, Charpy impact strength, hardness, fracture toughness, toughness modulus, bending stress–strain response, bending load-deflection behaviour, compressive stress–strain response and toughness indices. The outcomes of the present study showed that the utilization of 2% nano CaCO3 in micro steel fibre-reinforced GPC introduced the optimum mechanical performance for compressive strength, flexural strength, fracture toughness, elastic modulus, toughness modulus, and hardness. .

Properties of fly ash geopolymer modified with red mud and silica fume: a comparative study

Alkali-activated geopolymer has emerged as a sustainable alternative to highly polluting conventional cement. The present paper investigates the activation of fly ash-based geopolymer paste modified by red mud and silica fume. Three geopolymer mixes are considered: (1) fly ash; (2) fly ash—red mud; and (3) fly ash—silica fume. Mass proportions of modifiers in the respected mixes varied by 0, 5%, 10%, 15%, 20%, and 25% of total binder. Heat curing (60 °C) provided for 24 h just after casting. Alkaline activator consists of sodium hydroxide and sodium silicate solutions. Workability results reveal that both red mud and silica fume triggered the reduction in workability with the later one had a greater impact. A flash set was obtained with the silica fume content of more than 15%. Both the modifiers cause a decrease in the setting time at ambient temperature, and silica fume proved to be more productive with the reduction of more than 65% of setting time. Mixes containing red mud exhibit slightly higher density. Water absorption of fly ash geopolymer increases with red mud. Compressive strength increases with the addition of modifiers, and geopolymer containing silica fume possesses the highest compressive strength among the three mixes considered. Compressive strength exhibited a linear correlation with molar ratios (Si/Al and Na/Si) of source materials. XRD and SEM methods showed that the geopolymerization reaction is affected by the red mud and silica fume. It is recommended that the flash set and setting time at higher contents of modifiers must be examined for the design of a modified geopolymer mix.

Effect of Porosity and Water Absorption on Compressive Strength of Fly Ash based Geopolymer and OPC Paste

The fly ash based geopolymer is a promising binder by activation of fly ash with an alkaline activating solution. The fly ash based geopolymer prepared was characterized by several methods. The experimental result, studies effect of the porosity and water absorption on compressive strength of fly ash based geopolymer and Ordinary Portland Cement paste for comparison. The porosity studies were determined using the Brunauer, Emmett and Teller method included nitrogen adsorption / desorption plots. Then followed by water absorption and compressive strength tested at 7 and 28 days curing time. The result shows that the porosity of fly ash based geopolymer paste was in the lower surface area, pore volume and pore size compared to Ordinary Portland Cement paste. The small pore size of the fly ash based geopolymer had a significant proportion of a micropores whilst Ordinary Portland Cement paste pores were mostly mesopores. The highest compressive strength of fly ash-based geopolymer can be achieved up to 76.723 MPa at 28 days when less of pore size and water absorption. Therefore, the paste based on geopolymeric materials is a better durability and high resistance to aggressive environment compared Ordinary Portland Cement paste.