Suitability of Sarawak and Gladstone fly ash to produce geopolymers: A physical,chemical, mechanical,mineralogical and microstructural analysis

Two types of fly ash sourced from Sarawak, Malaysia and Gladstone, Australia reflect differences in chemical compositions, mineral phase and particle size distributions. In this paper, the Sarawak fly ash was used to produce geopolymer in comparison to the well-developed Gladstone fly ash-based geopolymer. Characteristics of fly ash and mixtures proportions affecting compressive strength of the geopolymers were investigated. It is found that the variations of both fly ash types on particle size distributions, chemical compositions, morphology properties and amorphous phase correspond to the compressive strength. The results obtained show that after 7 days, geopolymer using Sarawak fly ash has lower compressive strength of about 55 MPa than geopolymer using Gladstone fly ash with strength of about 62 MPa. In comparison with Gladstone fly ash-based geopolymer, it showed that Sarawak fly ash-based geopolymer can be a potential construction material. Moreover, the production of Sarawak fly ash-based geopolymer aids to widen the application of Sarawak fly ash from being treated as industrial waste consequently discharging into the ash pond.     

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

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

Synthesis of heat and ambient cured one-part geopolymer mixes with different grades of sodium silicate

The use of solid activators in the manufacture of geopolymer enhances its commercial viability as it aids the development of a one-part “just add water” geopolymer mixture, similar to the conventional Portland cement-based materials. This study is aimed to synthesize heat and ambient cured one-part geopolymer mixes. Appropriate combinations of low calcium (Class F) fly ash, slag and hydrated lime as the aluminosilicate source materials were activated by three different grades of sodium silicate and a combination of sodium silicate and sodium hydroxide powders. A conventional two-part geopolymer mix with the commonly used sodium hydroxide and sodium silicate solutions was also made for comparison. Effects of the type and amount of the solid activator, the amount of fly ash replacement with slag and hydrated lime and water content on short term mechanical properties of the heat cured one-part geopolymer mixtures including workability of the fresh mix, hardened density and compressive strength were evaluated. Subsequently, effects of ambient curing on the properties of the developed one-part geopolymer mixes were also investigated. Moderate to high compressive strength of over 37 MPa developed for the heat and ambient cured one-part geopolymer mixes. The 28-days compressive strengths of the ambient cured one-part geopolymer mixtures, regardless of the type of activator and geopolymer source materials, were comparable to those of the counterpart heat cured one-part geopolymer mixes. Such one-part geopolymer mixes could enhance the commercial viability and large-scale applications of the geopolymer in the construction industry.     

Response of alkali activated fly ash mortars to microwave curing

Volumetric heating provided by microwave curing results in faster property development as compared to conventional heat curing that relies on heat conduction from the skin to the core. This paper discusses the compressive strength and microstructure development of microwave cured NaOH activated fly ash mortars, and relates them to the microwave energy absorption by the material which is a function of its dielectric properties. Microwave curing parameters are chosen so as to eliminate the effects of thermal runaway. Strengths that are comparable to or greater than those of mortars heat cured for 48 h at 75 °C are obtained in less than 120 min of microwave curing. The rate of energy absorption by the mortars is found to be relatively constant for a considerable fraction of the microwave curing duration, attributable to the compensation for the drop in dielectric loss factor as a result of moisture loss by the increase in internal electric field. Compressive strength is shown to be related to the microwave energy absorbed by the specimens, especially during the time when free water is present in the system.     

Synthesis of fly ash based geopolymer mortar considering different concentrations and combinations of alkaline activator solution

Geopolymerisation is a process that can transform alumina and silica rich waste materials into valuable binding materials, having excellent mechanical properties. The present experimental study shed a light on the variation in compressive strength of fly ash based geopolymer mortar by varying the molarity of sodium hydroxide as 12 M, 14 M, 16 M and accompanying by sodium silicate (Na₂SIO₃) in 2:1 (Na₂SIO₃/ NaOH) with same molarities. All the geopolymer mixes were oven cured at 80 °C for 24 h and after that kept at room temperature up to the time of testing. The compressive strength was checked subsequently at the ages of 3, 7, 14 and 28 days. The experimental results reveal that the addition of sodium silicate enhances the strength development in geopolymer mortar. The ultimate compressive strength of 40.42 MPa was obtained by incorporating sodium silicate along with 16 M concentrated sodium hydroxide. Furthermore, increasing trend of the compressive strength was found with increasing molar concentration of sodium hydroxide and curing period.     

In-situ thermo-mechanical testing of fly ash geopolymer concretes made with quartz and expanded clay aggregates

The mechanical and microstructural properties of geopolymer concretes were assessed before, during and after high temperature exposure in order to better understand the engineering properties of the material. Fly ash based geopolymer concretes with either quartz aggregate or expanded clay aggregate were exposed to various temperatures up to 750 °C using a thermo-mechanical testing apparatus. Microstructural investigations were also undertaken to better understand the measured changes in the mechanical properties. It was found that dehydration of capillary water caused cracking and strength losses at temperatures ≤ 300 °C, an effect that was more severe in the quartz aggregate geopolymer due to its lower permeability. At higher temperatures (T ≥ 500 °C) sintering promoted strength increases which enabled both concrete types to yield significant strength advantages over conventional materials. Stress–mechanical strain curves, which form the basis of the fire design of concrete structures, are reported.     

Effect of transient creep on compressive strength of geopolymer concrete for elevated temperature exposure

The strength and transient creep of geopolymer and ordinary Portland cement (OPC)-based material (paste and concrete) were compared at elevated temperatures up to 550 °C. The strength properties were determined using an unstressed hot strength test and unstressed residual strength test for paste and concrete, respectively. At 550 °C, compared with the original strength, the strength of geopolymer was increased by 192% while the strength of OPC paste showed little change. However, after exposure to 550 °C, the residual strength percentage of both geopolymer and OPC concretes was similar. Transient creep data show that geopolymer had little change in transitional thermal creep (TTc) between 250 and 550 °C while OPC paste developed significant TTc in this temperature range. In comparison with OPC concrete, a higher strength loss of geopolymer concrete is thus believed to be due to the absence of TTc to accommodate nonuniform deformation during thermal exposure.     

Effect of relative humidity on the reaction products of alkali activated fly ash

The alkali activation of fly ash (AAFA) is a chemical process in which the ash is mixed with an alkaline activator and cured at a mild temperature to generate compact solids. Both the curing conditions (temperature, time, relative humidity, etc.) and the nature and concentration of the alkali activator play a key role in the development of AAFA micro- and nanostructure, and consequently the properties of these materials. In the present study, fly ash was activated with a 15% water-glass (Na₂SiO₃) + 85% 10-M NaOH solution at 85 °C for 12 h, 7 and 30 days. Two curing methods were used, in which the variable was the relative humidity (RH).     

Characterisation and properties of geopolymer composites. Part 2: Role of cordierite-mullite reinforcement

Our previous research paper on geopolymer-mullite composites showed promising results on compressive strength and fire resistance. However, no improvement in thermal shock resistance was observed in the afore mentioned study. In this study, further attempts to improve thermal shock resistance of the geopolymer were explored. The research was performed by compositing a fly ash-based geopolymer with cordierite-mullite at 20, 40 and 60 wt% replacement. X-ray diffraction (XRD) of the cured geopolymer composite specimens showed the existence of cordierite, mullite, quartz, cancrinite and lazurite. It was found that compressive strength and strength retention after thermal exposure at 400 °C were improved in the geopolymer composite specimens, especially those with 20–40 wt% replacement. Upon further heating to 600 °C, all geopolymer specimens showed insignificant differences in compressive strength. Fire resistance was found to improve with increasing proportion of replacement contents.     

The effect of grinding process on mechanical properties and alkali–silica reaction resistance of fly ash incorporated cement mortars

The effect of fineness of fly ash on mechanical properties and alkali–silica reaction resistance of cement mortar mixtures incorporating fly ash has been investigated within the scope of this study. Blaine fineness of fly ash has been increased to 907 m²/kg from its original 290 m²/kg value by a ball mill. Test samples were prepared by replacing cement 20, 40 and 60%, with finer and coarser fly ashes and kept under standard and steam curing conditions until testing. Test results showed that grinding process improved the mechanical properties of all samples significantly. The beneficial effect of grinding fly ash, may increase utilization of this by-product in precast and ready-mix concrete industries. Incorporation of fly ash with different fineness values and ratios also decreased the expansions to harmless levels of cement mortars due to alkali–silica reaction.     

Metakaolin and fly ash alkali-activated mortars compared with cementitious mortars at the same strength class

Alkali-activated and cementitious mortars belonging to R1 ≥ 10 MPa, R2 ≥ 15 MPa and R3 ≥ 25 MPa strength classes were tested and compared in terms of workability, dynamic modulus of elasticity, porosimetry, and water vapor permeability. Capillary water absorption, drying shrinkage, resistance to sulfate attack, and corrosion behavior of embedded bare and galvanized reinforcements were also investigated.In alkali-activated mortars, drying shrinkage is higher than that of cementitious mortars but restrained shrinkage is lower due to lower modulus of elasticity. Pore dimensions affect water vapor permeability, more pronounced in alkali-activated mortars, and capillary water absorption, much lower in fly ash ones. The high alkalinity of fly ash and metakaolin mortars delayed the achievement of the passive state in particular for the galvanized reinforcements but after 1 month of curing they reached the same corrosion rates of those embedded in cementitious mortars.     

Development of a High Strength Geopolymer by Novel Solar Curing

Geopolymer is a popular construction material derived from different sources of aluminosilicates known for its environmental benefits and excellent durability in harsh conditions. However, the curing of fly-ash based geopolymer normally requires a thermal treatment that increases the manufacturing cost and carbon footprint. This paper explored a new economical and environmentally-friendly alternative, i.e. solar curing, that harnesses solar radiation to achieve accelerated geopolymerization process. Geopolymer mortars coated in two different greyscales namely solar curing black (SCB) and 40% black (grey, SCG) were prepared to study the effect of solar radiation absorption ability on the strength of the specimens, along with ambient cured specimens (ATC) for comparison. Mechanical properties such as workability, compressive strength, stress-strain relationship from 1 day to 28 days were tested. The SCB specimens that can easily reach 65 °C under the sun showed a substantial improvement of the compressive strength especially at the early age, i.e. 49.2 MPa at 1-day compared with 25.5 MPa for the ATC ones. At 28-day, SCB reached 92 MPa in compressive strength which is 17.8% (13.9 MPa) higher than that of ATC. SCG showed a moderate enhancement in strength. Through in-depth physical and chemical characterizations, the structure and morphology of geopolymers were identified through X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS). It was found that geopolymer cured by solar radiation had more calcium aluminate silicate content hence leading to a higher mechanical strength. Furthermore, a titration study that determines the conversion rate of the activators inside geopolymers suggested a faster geopolymerization process in the solar cured specimens.     

Dry pressed ceramic tiles based on fly ash–clay body: Influence of fly ash granulometry and pentasodium triphosphate addition

Fly ash from brown coal (70 wt.%) and stoneware clay (30 wt.%) were used for the dry pressed ceramic tiles (according to EN 14411) raw materials mixture. The effects of fly ash milling and pentasodium triphosphate addition as a deflocculant and fluxing agent on the properties of green body (flexural strength, bulk density) and fired body (EN ISO 10545—water absorption, bulk density, true density, apparent porosity, flexural strength, frost resistance) were studied and explained as a function of the firing temperature (1000–1150 °C). Fly ash milling (corresponding to 5 wt.% residue of fly ash grains on 0.063 mm sieve) increased the sintering abilities of the fly ash–clay body. A similar effect was achieved by 1.3 wt.% pentasodium triphosphate (PST) addition with an increase in green body flexural strength and a decrease in water content of the granulate. Fly ash–clay bodies can be frost resistant with water absorption above 10% due to positive pore size distribution, which were examined using the high-pressure mercury porosimetry method.     

Characterization of coal fly ash for possible utilization in glass production

The recycling of three different fly ashes obtained from the coal fired thermal power plants has been studied. Coal fly ashes were vitrified by melting them at 1773 K for 5 h without any additives. After the glass production, glass samples were subjected to a heat treatment process to be able to see whether or not the glasses could be transformed into a microcrystalline structured materials. Produced glass samples were heated to 1423 K and held at this temperature for 2 h to determine the effect of heat treatment process on the properties of glasses. The properties of glass and the heat treated glass samples produced from coal fly ash were investigated by means of differential thermal analysis (DTA), X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. DTA study indicated that there were only inflection points of the endothermic peaks in the DTA curves of the glass samples. XRD analysis showed the amorphous state of the glass samples and also the presence of only the diopside phase in the heat-treated glass samples. SEM investigations revealed that small amount of crystallites occurred in the microstructure of the heat treated glass samples in contrast to the amorphous structure of the glass samples. The mechanical, physical and chemical properties of the heat-treated glass samples are found better than those of the glass samples. Toxicity characteristic leaching procedure (TCLP) results showed that the heavy metals of fly ashes were successfully immobilized into both glass and heat treated glass samples. It can be said that glass and heat treated glass samples obtained by the recycling of coal fly ash can be taken as a non-hazardous material. Overall, results indicated that the vitrification technique is an effective way for the stabilization and recycling of coal fly ash.     

Effects of fly ash on the properties of environmentally friendly dam concrete

The utilization of a solid waste – fly ash (FA) in the construction of concrete dams was investigated in this paper, which contained its effects on the strength, shrinkage and expansion strain of dam concrete with and without 8% of a novel MgO-bearing expansive agent. The results are shown a relationship between the content of fly ash replacing cement and the above properties of dam concrete.The compressive strengths of dam concrete with 50% fly ash in 90 d are higher than that of dam concrete with 30% fly ash or without fly ash slightly. Fly ash may decrease the deformation of dam concrete in that with 50% fly ash, and the shrinkage and expansive strain was reduced significantly – about 33% and 40% less than that of the specimens without fly ash respectively.     

Recycling of harmful waste lead-zinc mine tailings and fly ash for preparation of inorganic porous ceramics

The porous ceramics were prepared by directly sintering of lead-zinc mine tailings and fly ash as the raw materials without any additional sintering and foaming agent. The effects of fly ash addition on the crystalline phases, pore structure, physical–chemical porosities and mechanical strength were investigated. The results showed that the bulk density decreased firstly and then increased while the porosity and water absorption presented the opposite tendency with the increase of fly ash content. Meanwhile, the chemical stability improved and the flexural strength had the same variation tendency of the bulk density. The phase evolution of sample with 60 wt% fly ash addition indicated that anorthite phase was formed at low temperature (1000 °C). The thermal behavior illustrated that the foaming process was initiated by the reaction of internal constituents in the lead-zinc mine tailings. Different pore structures indicated different foaming mechanisms that probably occurred at different temperatures. The porous ceramics with 60 wt% fly ash addition exhibited excellent properties, including bulk density of 0.93 g/cm³, porosity of 65.6%, and flexural strength of 11.9 MPa.     

Hydration of Portland cement with high replacement by siliceous fly ash

The effects of two different low calcium fly ashes on the hydration of ordinary Portland cement (OPC) pastes containing 50 wt.% of fly ash were investigated over a hydration time of 550 days. The results were compared with a reference blend of OPC containing 50 wt.% of inert quartz powder allowing the distinction between "filler effect" and pozzolanic reaction.Until 2 days, no evidence of fly ash reaction was measured and its influence on the hydration is mainly related to the “filler effect”. From 7 days on, the effects of the pozzolanic reaction were observed by the consumption of portlandite, the change of the pore solution chemistry, the formation of a presumably water-rich inner hydration product and the change of the C–S–H composition towards higher Al/Si ratio compared to the C–S–H of neat OPC. Additional strength due to the pozzolanic reaction developed after 28 days of hydration.     

Fast microwave syntheses of fly ash based porous geopolymers in the presence of high alkali concentration

Microwave synthesis of porous fly ash geopolymers was achieved using a household microwave oven. Fly ash paste containing SiO₂ and Al₂O₃ component was mixed with sodium silicate (Na₂SiO₃) solutions at different concentrations of sodium hydroxide (NaOH) of 2, 5, 10, and 15 M, which were used as NaOH activators of geopolymerization. The mass ratio of Na₂SiO₃/NaOH was fixed at 2.5 with SiO₂/Al₂O₃ at 2.69. After the fly ash and alkali activators were mixed for 1 min until homogeneous, the geopolymer paste was cured for 1 min using household microwave oven at different output powers of 200, 500, 700, and 850 W. Porous geopolymers were formed immediately. Micro X-ray CT and SEM results showed that the porous structure of the geopolymers was developed at higher NaOH concentrations when using 850 W power of the microwave oven. These results derive from the immediate increase of the temperature in the geopolymer paste at higher NaOH concentrations, meaning that aluminosilicate bonds formed easily in the geopolymers within 1 min.     

Development of fly ash and iron ore tailing based porous geopolymer for removal of Cu(II) from wastewater

This work aims to verify the feasibility of utilizing iron ore tailing (IOT) in porous geopolymer and intends to broaden the application of porous geopolymer in heavy metal removal aspect. Porous geopolymer was prepared using fly ash as resource material, which was partially replaced by IOT at level of 30%, by weight, with H₂O₂ as foaming agent and removal efficiency, adsorption affecting factors, adsorption isotherms and thermodynamics of Cu²⁺ by the developed porous geopolymer were investigated.The experimental results uncover that the porous amorphous geopolymer was successful synthesized with total porosity of 74.6%. The transformation of fly ash and IOT into foaming geopolymer leads to the formation of porous structure encouraging Cu²⁺ sorption. Batch sorption tests were carried out and geopolymer dosage, Cu²⁺ initial concentration, pH, contact time and temperature were the main concern. Both Langmuir and Freundlich models could explain the adsorption of Cu²⁺ on the porous geopolymer due to the high fitting coefficients. The uptake capacity reaches the highest value of 113.41 mg/g at 40 °C with pH value of 6.0. The thermodynamic parameters ΔHº, ΔSº and ΔGº suggests the spontaneous nature of Cu²⁺ adsorption on porous geopolymer and the endothermic behavior of sorption process.     

Mechanical and microstructural evolutions of fly ash/slag-based geopolymer at high temperatures: Effect of curing conditions

Designing a building material with excellent heat resistance is crucial for protection against catastrophic fires. Geopolymer materials have been investigated as they offer better heat resistance than traditional cement owing to their ceramic-like properties. Curing temperature and conditions are crucial factors that determine the properties of geopolymers, but their impacts on the heat resistance of geopolymers remain unclear. This study produced geopolymers from fly ash and ground granulated blast furnace slag by using sodium silicate and sodium hydroxide solutions as alkaline solutions. To examine the effect of curing conditions on the high-temperature performance of geopolymer, four different curing conditions, namely, heat curing (70 °C for 24 h), ambient curing (20 °C), water curing, and the combination of heat and water curing (70 °C for 24 h followed by water curing), were applied. At 28 d, the specimens were subjected to high temperatures (500 °C, 750 °C, and 950 °C), and their mechanical and microstructural evolutions were studied. The results revealed that the curing condition significantly affects the properties of the unexposed geopolymer; the effect on its high-temperature performance is insignificant. Furthermore, all the specimens could maintain adequate compressive strength after exposure to the maximum temperature of 950 °C, promising the use of geopolymer for structural applications.