Synthesis of polypropylene fiber/high-calcium fly ash geopolymer with outdoor heat exposure

Solar energy is an important source of renewable and sustainable energy. Thailand is near the equator and thus experiences hot weather throughout the year. The average maximum temperature is 35 °C and can reach 40 °C in the summer time. This outdoor heat exposure (OHE) was, therefore, used for the curing of a polypropylene (PP) fiber fiber-reinforced high-calcium fly ash geopolymer composite, in order to reduce energy consumption. Fly ash is an abundant solid waste generated from the coal-power generation process. In this research, a high-calcium fly ash was used as a source material for the geopolymer synthesis. PP fiber was also incorporated in the composites to improve tensile characteristics and control crack development. The results show that the incorporation of PP fiber in composites led to improved tensile strength, crack control, and resistance to acid solution. OHE could thus be used as an energy source for the heat curing of high-calcium fly ash PP-fiber geopolymers, resulting in a strong matrix.     

Characterization of the high-calcium fly ash geopolymer mortar with hot-weather curing systems for sustainable application

Fly ash geopolymers are an alumino-silicate material and thus enable the utilization of waste containing alumino-silicate effectively. Geopolymeric reaction occurs as a result of the activation of fly ash with alkali solutions. In Thailand, a large amount of high-calcium fly ash is available due to the use of low-grade lignite coal feedstock for pulverized coal combustion process and the calcium content becomes very high. In this study, heat curing at 35 °C as a representative of a high ambient temperature (hot weather) and low cost was investigated. Curing at temperature of 65 °C and room temperature of 25 °C were also conducted to compare the results. Geopolymeric products were tested for compressive strength and characterized by XRD, IR, SEM and TGA techniques. The results showed that heat curing enhanced the geopolymerization resulting in the formation of network product. Heat curings at 35 °C and 65 °C led to the formation of calcium silicate hydrate (C-S-H) and alumino-silicate (geopolymer bonding). Without heat curing, the product was predominantly C-S-H compound and the matrix was as strong as the heat-cured product. The immersion of samples in 3% sulfuric acid solution revealed that the performance of the heat-cured samples were better than those cured at room temperature. In addition, application of research results was to produce the geopolymer brick with outdoor heat exposure of 35 °C. Pedestrian pathway was demonstrated.     

Resistance to acid and sulfate solutions of microwave-assisted high calcium fly ash geopolymer

In this paper, 90-W microwave radiation for 5 min plus a shortened heat curing period was applied to cure the fresh geopolymer paste. Results showed that microwave radiation contributed to the dissolution of fly ash in the alkaline solution. Numerous gel formations were observed in microscopic scale. This resulted in a dense composite and strong bonding between the fly ash and the geopolymer matrix leading to high strength gain compared to those of the control pastes cured at 65 °C for 24 h. In addition, resistances to the sulfuric acid and sulfate attacks of the microwave geopolymer were superior to that of the control as indicated by the relatively low strength loss. The microwave radiation also helped the geopolymer attaining thermal stability as the dense matrices were obtained.     

Role of microwave radiation in curing the fly ash geopolymer

Fly ash geopolymer requires rather long heat curing to obtain reasonable strength development at an early age. However, the long heat curing period limits the application of the fly ash geopolymer. High strength development and a reduction in heat curing duration have been considered for energy saving. Therefore, this research proposed a process using 90-W microwave radiation for 5 min followed by conventional heat curing for high-calcium fly ash geopolymer. Results showed that the compressive strengths of geopolymer with microwave radiation followed by conventional heat curing were comparable to those of the control cured at 65 °C for 24 h. Microwave radiation gave the enhanced densification. In addition, SEM images showed that the gels formed on the fly ash particles owing to the promoted dissolution of amorphous phases from fly ash. This method accelerated the geopolymerization and gave the high compressive strength comparable to the conventional curing.     

Compressive strength and microstructure of fly ash based geopolymer blended with silica fume under thermal cycle

This work aims to reveal the effects of silica fume on properties of fly ash based geopolymer under thermal cycles. Geopolymer specimens were prepared by alkali activation of fly ash, which was partially replaced by silica fume at levels ranging from 0% to 30% with an interval of 10%, by mass. Microstructure, residual strength and mass loss of fly ash based geopolymer blended with silica fume before and after exposed to 7, 28 and 56 heat-cooling thermal cycles at different target temperatures of 200 °C, 400 °C and 800 °C were assessed and compared. The experimental results reveal that silica fume addition enhances strength development in geopolymer. Under thermal cycles, the compressive strength of geopolymer decreases, and the compressive strength loss, as well as the mass loss, increases with increasing target temperature. The strength loss is the same regardless of silica fume content after thermal cycles. Microstructure analysis uncovers that pore structure of geopolymer degrades after thermal cycles. The pores of geopolymer are refined by the addition of silica fume. The incorporation of silica fume optimizes the microstructure and improves the thermal resistance of geopolymer. Silica fume increases the strength of the geopolymer and even though the strength loss is the same, the strength after heat cycle exposure is still good.     

Creep and drying shrinkage of a blended slag and low calcium fly ash geopolymer Concrete

The main purpose of this research is to study the time dependent behaviour of a geopolymer concrete. The geopolymer binder is composed of 85.2 % of low calcium fly ash and only 14.8 % of ground granulated blast furnace slag. Both drying shrinkage and creep are studied. In addition, different curing conditions at elevated temperature were used. All experimental results were compared to predictions made using the Eurocode 2. The curing regime plays an important role in the magnitude and development of both creep and drying shrinkage of class F fly ash based geopolymer concrete. A minimum of 3 days at 40 °C or 1 day at 80 °C is required to obtain final drying shrinkage strains similar to or less than those adopted by Eurocode 2 for ordinary Portland cement (OPC) concrete. Creep strains were similar or less than those predicted by Eurocode 2 for OPC concrete when the geopolymer concrete was cured for 3 days at 40 °C. After 7 days at 80 °C, creep strains became negligible.     

The role of calcium in blended fly ash geopolymers

In this research effort, the role of calcium in geopolymers was investigated through a series of syntheses where a high-calcium fly ash was blended with a low-calcium fly ash. Increased calcium content led to accelerated set-up times, increased compressive strength, and increased product formation. Powder X-ray diffraction results showed the majority of that product to be geopolymer framework with only minor contributions from calcium silicate phases. Thermal analysis confirmed the absence of a calcium silicate hydrate phase. Analysis of fly ash dissolution showed that calcium aided in aluminosilicate dissolution and therefore the geopolymerization reaction. While aiding in this reaction, calcium became incorporated into the pore structure of the geopolymer as a counter-balancing cation, according to ion exchange experiments. Thus, geopolymer synthesis with increased calcium content through the use of a high-calcium fly ash under these experimental conditions produced a quick-setting, strong, calcium incorporated geopolymer material.     

Use of OPC to improve setting and early strength properties of low calcium fly ash geopolymer concrete cured at room temperature

Most previous works on fly ash based geopolymer concrete focused on concretes subjected to heat curing. Development of geopolymer concrete that can set and harden at normal temperature will widen its application beyond precast concrete. This paper has focused on a study of fly ash based geopolymer concrete suitable for ambient curing condition. A small proportion of ordinary Portland cement (OPC) was added with low calcium fly ash to accelerate the curing of geopolymer concrete instead of using elevated heat. Samples were cured in room environment (about 23 °C and RH 65 ± 10%) until tested. Inclusion of OPC as little as 5% of total binder reduced the setting time to acceptable ranges and caused slight decrease of workability. The early-age compressive strength improved significantly with higher strength at the age of 28 days. Geopolymer microstructure showed considerable portion of calcium-rich aluminosilicate gel resulting from the addition of OPC.     

Resistance of lignite bottom ash geopolymer mortar to sulfate and sulfuric acid attack

This paper presents an investigation of the compressive strength and the durability of lignite bottom ash geopolymer mortars in 3% sulfuric acid and 5% sodium sulfate solutions. Three finenesses of ground bottom ash viz., fine, medium and coarse bottom ash were used to make geopolymer mortars. Sodium silicate, sodium hydroxide and curing temperature of 75 °C for 48 h were used to activate the geopolymerization. The results were compared to those of Portland cement and high volume fly ash mortars. It was found that the fine bottom ash was more reactive and gave geopolymer mortars with higher compressive strengths than those of the coarser fly ashes. All bottom ash geopolymer mortars were less susceptible to the attack by sodium sulfate and sulfuric acid solutions than the traditional Portland cement mortars.     

Artificial neural networks for prediction of percentage of water absorption of geopolymers produced by waste ashes

In the present work, percentage of water absorption of geopolymers made from seeded fly ash and rice husk bark ash has been predicted by artificial neural networks. Different specimens, made from a mixture of fly ash and rice husk bark ash in fine and coarse form together with alkali activator made of water glass and NaOH solution, were subjected to permeability tests at 7 and 28 days of curing. The curing regime was different: one set cured at room temperature until reaching to 7 and 28 days and the other sets were oven cured for 36 h at a range of 40–90 °C and then cured at room temperature for 7 and 28 days. To build the model, training and testing using experimental results from 120 specimens were conducted. According to these input parameters, in the neural networks model, the percentage of water absorption of each specimen was predicted. The training and testing results in the neural networks model have shown a strong potential for predicting the percentage of water absorption of the geopolymer specimens.     

Development and investigation of cellular light weight bio-briquette ash bricks

The present paper deals with the development of cellular light weight bricks using bio-briquette ash. The necessary physical and chemical tests were conducted on a bio-briquette ash sample to investigate its suitability for the development of bricks. Physico-mechanical, durability and thermal conductivity tests were conducted on cellular light weight bio-briquette ash bricks that fulfilled the requirements of Indian standard. The test results of cellular light weight bio-briquette ash bricks were compared with commercially available fly ash bricks. With reference to fly ash bricks, the cellular light weight bio-briquette ash bricks were found 43 % light in weight, having 13 % higher compressive strength and resulted in 66 % lesser thermal conductivity. A small scale model room (1 m × 1 m × 1 m) made up of fly ash bricks was designed. A similar built form for the cellular light weight bio-briquette ash bricks was also modelled. Both the models were analysed for indoor temperature control and cost. When compared with fly ash model room, cellular light weight bio-briquette ash brick model resulted in a 6 % better indoor temperature control and 29 % cost savings. Thus, the developed cellular light weight bio-briquette ash bricks were found suitable as an alternate construction material for non-load bearing walls.     

Low-temperature sintered pollucite ceramic from geopolymer precursor using synthetic metakaolin

In this article, pollucite ceramic with high relative density and low coefficient of thermal expansion (CTE) was prepared from Cs-based geopolymer using synthetic metakaolin. Crystallization and sintering behavior of the Cs-based geopolymer together with thermal expansion behavior of the resulted pollucite ceramic were investigated. On heating at 1200 °C for 2 h, the amorphous Cs-based geopolymer completely crystallized into pollucite based on crystal nucleation and growth mechanism. Selected area diffraction analysis and XRD results confirmed the resulted pollucite ceramic at room temperature was pseudo-cubic phase with superlattice structure. Compared with Cs-based geopolymer using natural metakaolin, geopolymer using synthetic metakaolin in this article showed a much lower viscous sintering temperature range, which started at 800 °C, reached a maximum value of ?7.47 × 10^?4/°C at 1121.9 °C, and ended at 1200 °C. Cesium volatilization appeared only when temperature was above 1250 °C. Therefore, densified pollucite ceramic can be prepared from Cs-based geopolymer using synthetic metakaolin without cesium volatilization. Abnormal thermal shrinkage of pollucite ceramic was observed at temperature range from 25.3 to 54.6 °C because of pseudo-cubic to cubic phase transition, and its average CTE was 2.8 × 10^?6/°C from 25 to 1200 °C.     

Examination of alkali-activated material nanostructure during thermal treatment

The key nanostructural changes occurring in a series of alkali-activated materials (AAM) based on blends of slag and fly ash precursors during exposure to temperatures up to 1000 °C are investigated. The main reaction product in each AAM is a crosslinked sodium- and aluminium-substituted calcium silicate hydrate (C-(N)-A-S-H)-type gel. Increased alkali content promotes the formation of an additional sodium aluminosilicate hydrate (N-A-S-(H)) gel reaction product due to the structural limitations on Al substitution within the C-(N)-A-S-H gel. Heating each AAM to 1000 °C results in the crystallisation of the disordered gels and formation of sodalite, nepheline and wollastonite. Increased formation of N-A-S-(H) reduces binder structural water content after thermal treatment and correlates closely with previous observations of improved strength retention and reduced microcracking in these AAM after heating to 1000 °C. This provides new insight into thermally induced changes to gel atomic structure and thermal durability of C-(N)-A-S-H/N-A-S-H gel blends which are fundamental for the development of new fire-resistant construction materials.     

Utilization of chitosan biopolymer to enhance fly ash-based geopolymer

This paper investigates the enhancement of fly ash-based geopolymer with chitosan biopolymer. Unconfined compression and split tensile tests were carried out to investigate the effect of addition of small amount of N-carboxymethyl chitosan (0.05, 0.1, 0.15, and 0.2 wt% of fly ash) on the mechanical performance of fly ash-based geopolymer. Scanning electron microscopy (SEM) imaging was also conducted to study the microstructure of the chitosan enhanced fly ash-based geopolymer. The results indicated that the inclusion of N-carboxymethyl chitosan led to slight increase of the unconfined compressive strength and substantial increase of the tensile strength, the displacement at the peak tensile load and the pre-peak toughness, with the maximum increases at 0.1 wt% chitosan content. The SEM imaging indicated that the added N-carboxymethyl chitosan biopolymer coated and bridged the (geopolymerized) fly ash particles and led to the formation of a more condensed geopolymer network structure, thus enhancing the mechanical behavior of the geopolymer–biopolymer composite. However, when too much N-carboxymethyl chitosan was used, the excessive coating and encapsulation of un-reacted and partially hydrolyzed fly ash particles hindered their geopolymerization and adversely affected the mechanical behavior of the geopolymer–biopolymer composite.     

Mechanical properties of lightweight concrete made with coal ashes after exposure to elevated temperatures

This study investigated the thermal resistance of lightweight concrete with recycled coal bottom ash and fly ash. Specimens were exposed to temperatures up to 800 °C then cooled to room temperature before conducting experiments. Compressive strength test, FF-RC test, TG analysis, and XRD analysis were performed to analyze the physicochemical effects of coal ashes on the thermal resistance of concrete. Test results indicated that both bottom ash and fly ash were associated with a substantial increase in the residual strength of thermal exposed concretes. The results were attributed to the surface interlocking effect and the smaller amount of SiO₂ for bottom ash. For fly ash, the formation of pozzolanic C-S-H gel and tobermorite retained water at high temperatures, and the consumption of Ca(OH)₂ lowered stress from rapid recrystallization after exposure to 600 °C. It was concluded that the incorporation of coal ashes allows for lightweight concrete with good thermal resistance.     

Workability and strength of coarse high calcium fly ash geopolymer

In this paper, the basic properties viz., workability and strength of geopolymer mortar made from coarse lignite high calcium fly ash were investigated. The geopolymer was activated with sodium hydroxide (NaOH), sodium silicate and heat. The results revealed that the workable flow of geopolymer mortar was in the range of 110 ± 5%–135 ± 5% and was dependent on the ratio by mass of sodium silicate to NaOH and the concentration of NaOH. The obtained compressive strength was in the range of 10–65 MPa. The optimum sodium silicate to NaOH ratio to produce high strength geopolymer was 0.67–1.0. The concentration variation of NaOH between 10 M and 20 M was found to have a small effect on the strength. The geopolymer samples with high strength were obtained with the following practices: the delay time after moulding and before subjecting the sample to heat was 1 h and the optimum curing temperature in the oven was 75 °C with the curing duration of not less than two days.     

Influence of elevated temperatures on the mechanical properties of foamed geopolymer materials

Current research focuses heavily on geopolymer concrete as possible applications for insulation materials. The aim of the research is to test the strength properties of lightweight geopolymer concrete after exposure to high temperatures. Waste material from the Wieczorek mine (Poland) was used to produce the foamed geopolymers. Alkaline activation took place by mixing the mine powder with an aqueous solution of sodium hydroxide combined with an aqueous sodium silicate with a concentration of 10 M. Prepared geopolymer samples after temperature curing at 75 °C for 24 hours in a laboratory dryer, they were seasoned for 28 days, after which the strength properties were determined. Mechanical tests: compressive strength and bending strength were carried out at temperatures: 20 °C, 200 °C, 600 °C, 800 °C, 1100 °C. Research has shown the precursor activation with the presence of hydrogen peroxide enabled the manufacturing of foamed geopolymers. Heating in the temperature range up to 1100 °C influenced, to some extent, the total porosity of the tested foams. The geopolymer foams based on coal gangue present stable mechanical properties in the range up to 800 °C. No sharp mechanical performances decrease or material chipping was observed. Only colour change of heated samples occurred.     

Assessing the suitability of three Australian fly ashes as an aluminosilicate source for geopolymers in high temperature applications

Fly ash characteristics cannot be assumed to be constant between power stations as they are highly dependent on the coal source and burning conditions. It is critical to understand the characteristics of fly ash in order to produce geopolymers suitable for high temperature applications. We report on the characterisation of fly ash from three Australian power stations in terms of elemental composition, phase composition, particle size, density and morphology. Geopolymers were synthesised from each of the fly ashes using sodium silicate and sodium aluminate solutions to achieve a range of Si:Al compositional ratios. Mechanical properties of geopolymer binders are presented and the effect of the source fly ash characteristics on the hardened product is discussed, as well as implications for high temperature applications. It was found that the twenty eight day strength of geopolymers is largely dependent on the sub 20 μm size fraction of the fly ash. Strength loss after high temperature exposure was found to be dependent on the concentration of iron in the fly ash precursor and the Si:Al ratio of the geopolymer mixture.     

Evaluations on heat resistance of curtains with water film in a fire

Fire tests of fireproof and non-fireproof curtains were conducted to investigate the cooling performance of the proposed water film system. The experimental results showed that although commercial fireproof curtains without a water film system had good flame resistance, they had limited heat resistance. The maximum temperature on the unexposed surface of the tested commercial fireproof curtains without a water film system reached 693 °C, and the curtains failed in 30 min. In the cases of curtains with a water film system, the temperature of the unexposed surface was able to remain below 45 °C for the fireproof curtain and 55 °C for the non-fireproof curtain. The integrity of both curtains was conserved for the entire 60-min test duration. Using the proposed water film system, the heat resistance and fire integrity of curtains were greatly improved.     

An application of blended fly ash and residual rice husk ash for producing green building bricks

The present study investigates the possibility of using a blended class-F fly ash (FA) and residual rice husk ash (RHA) in the production of green building bricks through the application of densified mixture design algorithm (DMDA) in order to provide a new use for solid waste materials. This study uses unground rice husk ash (URHA) as a partial fine aggregate substitution (10–40%) in the studied cementitious mixtures. Solid bricks of 220 × 105 × 60 mm in size were prepared under forming pressure of 25–35 MPa, a curing temperature of 90 °C, and a relative humidity of 50%, for tests that assessed: compressive strength, flexural strength, bulk density, void volume, and water absorption. The test results showed that all brick samples demonstrated excellent properties. Compressive strength and flexural strength ranged, respectively, between 20.2–33 MPa and 5.4–6.9 MPa. Additionally, up to 30% of URHA content, the values of water absorption and void volume ranged, respectively, between 8.8–15.7% and 1.5–2.1%. All of these values not only conformed well to the requirements of the Vietnamese codes but also demonstrated great potential for using a blended FA–RHA in producing green building bricks.