Compressive strength and microstructural analysis of fly ash/palm oil fuel ash based geopolymer mortar

This paper presents the effects and adaptability of palm oil fuel ash (POFA) as a replacement material in fly ash (FA) based geopolymer mortar from the aspect of microstructural and compressive strength. The geopolymers developed were synthesized with a combination of sodium hydroxide and sodium silicate as activator and POFA and FA as high silica–alumina resources. The development of compressive strength of POFA/FA based geopolymers was investigated using X-ray florescence (XRF), X-ray diffraction (XRD), Fourier transform infrared (FTIR), and field emission scanning electron microscopy (FESEM). It was observed that the particle shapes and surface area of POFA and FA as well as chemical composition affects the density and compressive strength of the mortars. The increment in the percentages of POFA increased the silica/alumina (SiO₂/Al₂O₃) ratio and that resulted in reduction of the early compressive strength of the geopolymer and delayed the geopolymerization process.     

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.     

Effect of granite flour proportion on the strength properties of geopolymer mortar based on fly ash

Given global trends and challenges, the development of binders for the production of geopolymer concretes has become a topical area of building science. The purpose of this study is to determine whether granite can replace traditional construction aggregate, such as river sand, during geopolymer production, as well as to demonstrate the effect of the proportion of granite flour on the strength properties of fly ash-based geopolymer mortar. A combination of granite flour, quartz sand, and fly ash in various proportions was used as an aluminosilicate precursor. The scope of the study includes density measurements, compressive and flexural strength tests, abrasion by the Boehme method, and microstructural observations. Based on the obtained results, it can be concluded that granite can be successfully used as a replacement for quartz sand during the production of fly ash-based geopolymers. Moreover, the addition of granite makes it possible to improve the strength properties of geopolymers, compared to a geopolymer composite containing quartz sand.     

The effects of ground granulated blast-furnace slag blending with fly ash and activator content on the workability and strength properties of geopolymer concrete cured at ambient temperature

Inclusion of ground granulated blast-furnace slag (GGBFS) with class F fly-ash can have a significant effect on the setting and strength development of geopolymer binders when cured in ambient temperature. This paper evaluates the effect of different proportions of GGBFS and activator content on the workability and strength properties of fly ash based geopolymer concrete. In this study, GGBFS was added as 0%, 10% and 20% of the total binder with variable activator content (40% and 35%) and sodium silicate to sodium hydroxide ratio (1.5–2.5). Significant increase in strength and some decrease in the workability were observed in geopolymer concretes with higher GGBFS and lower sodium silicate to sodium hydroxide ratio in the mixtures. Similar to OPC concrete, development of tensile strength correlated well with the compressive strength of ambient-cured geopolymer concrete. The predictions of tensile strength from compressive strength of ambient-cured geopolymer concrete using the ACI 318 and AS 3600 codes tend to be similar to that for OPC concrete. The predictions are more conservative for heat-cured geopolymer concrete than for ambient-cured geopolymer concrete.     

Effect of fire exposure on cracking,spalling and residual strength of fly ash geopolymer concrete

Fly ash based geopolymer is an emerging alternative binder to cement for making concrete. The cracking, spalling and residual strength behaviours of geopolymer concrete were studied in order to understand its fire endurance, which is essential for its use as a building material. Fly ash based geopolymer and ordinary portland cement (OPC) concrete cylinder specimens were exposed to fires at different temperatures up to 1000 °C, with a heating rate of that given in the International Standards Organization (ISO) 834 standard. Compressive strength of the concretes varied in the range of 39–58 MPa. After the fire exposures, the geopolymer concrete specimens were found to suffer less damage in terms of cracking than the OPC concrete specimens. The OPC concrete cylinders suffered severe spalling for 800 and 1000 °C exposures, while there was no spalling in the geopolymer concrete specimens. The geopolymer concrete specimens generally retained higher strength than the OPC concrete specimens. The Scanning Electron Microscope (SEM) images of geopolymer concrete showed continued densification of the microstructure with the increase of fire temperature. The strength loss in the geopolymer concrete specimens was mainly because of the difference between the thermal expansions of geopolymer matrix and the aggregates.     

Synthesis of low-temperature calcium sulfoaluminate-belite cements from industrial wastes and their hydration: Comparative studies between lignite fly ash and bottom ash

The aim of this research was to study the production of calcium sulfoaluminate-belite (CŜAB) cement from industrial waste materials via hydrothermal-calcination process. Lignite fly ash and bottom ash were used as starting materials for comparison. Other waste materials viz., Al-rich sludge and flue gas desulfurization gypsum were also key players in raw mixes for the synthesis of CŜAB cement. For lignite fly ash as a starting material, mixed phases between ye'elimite and larnite were obtained, whereas for lignite bottom ash as starting material, only ye'elimite was obtained The hydration reaction was studied in terms of heat evolution, setting time, compressive strength and hydration product formation with various gypsum contents. The results showed a rapid formation of ettringite as a main hydration product mixed with calcium silicate hydrate, monosulfate and strätlingite phases as minority, with a fast final setting time of 24–26 min and high early compressive strength of 16.0 and 18.0 MPa in 1 day for CŜAB cements made of fly ash and bottom ash, respectively.     

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.     

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.     

Properties of geopolymer from circulating fluidized bed combustion coal bottom ash

Compressive strength, atomic ratios and microstructure of geopolymer mortars (GM) made from circulating fluidized bed combustion (CFBC) coal bottom ash (CBA) were investigated to observe the effect of air curing at ambient temperature (AC) at 20 °C and 90% RH, dry curing (DC) at 80 °C and 40% RH for 20 h. The 28-d compressive strength of GM exposed to AC (GM-AC) and DC (GM-DC) were 26.23 and 24.14 MPa, respectively. The Si/Na atomic ratio of the main reaction product (N-A-S-H gel) was close to 1. Geopolymer gel (apparently crystalline) having low Si/Na ratio (0.5) may correspond to a more advanced or developed stage of the aluminosilicate gel. It was observed that the geopolymerization was completed before the N-A-S-H gel formed when Si/Na ratio of GM is close to 2. The color of the GM changed from pink to grey and the structure became denser with almost no pores, when the temperature increased from 400 to 800 °C. The N-A-S-H gel became more amorphous due to the sintering reactions attaining Si/Al and Si/Na ratios of 4.54 and 0.98, respectively.     

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.     

Workability and strength behaviour of concrete with cold-bonded fly ash aggregate

This paper discusses the development of empirical models for workability and compressive strength of cold-bonded fly ash aggregate concrete in terms of mixture proportioning variables such as cement content, water content and volume fraction of cold-bonded aggregate through statistically designed experiments based on Response Surface Methodology. Factor level of cement is taken from 250 to 450 kg/m³ to introduce weak as well as strong matrix phase in the concrete. Apart from water content, workability of concrete is highly influenced by main and interaction effect of volume fraction of cold-bonded aggregate in the composition. Response surface indicate that increase in cement content causes to change the predominant failure mode from mortar failure to aggregate fracture and concrete strength decreases with increase in volume fraction of aggregate at higher cement contents. The models developed have been found useful in arriving typical relationship to establish a mixture proportioning methodology for cold-bonded fly ash aggregate concrete.     

Properties of geopolymer with seeded fly ash and rice husk bark ash

In the present work, compressive strength of inorganic polymers (geopolymers) produced of seeded fly ash and rice husk bark ash has been investigated. Different specimens made from a mixture of fly ash and rice husk bark ash in fine and coarse form were subjected to compressive strength tests at 7 and 28 days of curing. The curing regime was different: one set of the specimens were cured at room temperature until reaching to 7 and 28 days and the other sets were oven cured for 36 h at the range of 40-90 °C and then cured at room temperature until 7 and 28 days. The results indicate that in both 7 and 28 days regimes, the highest strengths are related to the specimens by SiO₂/Al₂O₃ ratio equals 2.99 cured at 80 °C. For these specimens, those contained finer fly ash particles show more compressive strength. Thermogravimetric analysis and Fourier transform infrared spectroscopy both also are in agreement with the obtained results from compressive strength tests. In addition, SEM micrographs of the specimens show that the finer the particle size of the utilized ashes, the denser the microstructure which confirms the results obtained by the strength tests.     

Sustainable utilization of pre-treated and nano fly ash powder for the development of durable geopolymer mortars

Developments in geopolymer construction are gaining more interest nowadays due to the elimination of cement and the consequent effects such as carbon dioxide emission, greenhouse effect, etc. Although the use of fly ash as a binder in the geopolymer system acts as a key solution for the major hazardous effects like land dumping, soil contamination, groundwater pollution, and respiratory diseases, the slow reactivity of the fly ash resulted in the considerable reduction in the strength. In this paper, a novel pretreatment method was employed on the fly ash binder in terms of thermal and mechanical means. Also, a cost-effective nano fly ash powder was synthesized and used as filler material on the geopolymer system. The efficiency of the fabricated geopolymer mortar was assessed by examining the workability, compressive strength, and resistance against chloride ion penetration. The geopolymer mortars with pre-treated fly ash exhibited a highly workable mix of 130% improved flow rate without adding any superplasticizer. Further, the addition of 1% nano fly ash, exhibited the highest compressive strength of 71.22 MPa, confirmed almost nil chloride ion permeability, and sustained 90% residual strength after immersing in the brine solution for 60 days which explored the development of sustainable and cost-effective geopolymer construction in the marine environment.     

Effects of light crude oil contamination on the physical and mechanical properties of geopolymer cement mortar

Fly ash and oil contaminated sand are considered as the two waste materials that may affect environment. This paper investigated the suitability of producing geopolymer cement mortar using oil contaminated sand. A comparison between physical and mechanical properties of mortar produced using geopolymer and Ordinary Portland Cement (OPC), in terms of porosity, hydration and compressive strength, was conducted. The results showed that heat curing can increase the compressive strength of geopolymer mortar up to 54% compared to ambient curing situation. The geopolymer mortar with 1% of light crude oil contamination yielded a 20% higher compressive strength than OPC mortar containing sand with a saturated surface dry condition. Furthermore, the formation of efflorescence decreased as the level of oil contamination decreased. Moreover, the heat curing method increased the kinetic energy and degree of reaction for geopolymer cement mortar, which cause an increment of the density of the pore system and improving the mechanical properties of the resulting composites. From the results of this study, it was demonstrated that geopolymer mortar has the potential of utilizing oil contaminated sand, and reducing its environmental impacts.     

Alkali-activation potential of biomass-coal co-fired fly ash

Co-fired fly ash, derived from the co-combustion of coal and biomass, is examined as a potential precursor for geopolymers. Compared to a coal fly ash, two co-fired fly ashes have a lower vitreous content and higher carbon content, primarily due to differing combustion processing variables. As a result, binders produced with these co-fired fly ashes have reduced reaction potential. Nevertheless, compressive strengths are generally highest for all ashes activated with solutions with a molar ratio of SiO₂/(Na₂O + K₂O) = 1, and these mixes reach the highest extent of reaction among those studied. Activation with sodium hydroxide solution forms zeolitic phases for all ashes. The thermal and dilatometric behavior of the coal and co-fired fly ash geopolymers is similar between equivalent mix designs. These results indicate that co-fired fly ashes can be viably used to form alkali-activated geopolymers, which is a new beneficial end-use for these emerging waste materials.     

Effects of NaOH concentrations on physical and electrical properties of high calcium fly ash geopolymer paste

The effects of sodium hydroxide (NaOH) concentration on setting time, compressive strength and electrical properties at the frequencies of 100 Hz–10 MHz of high calcium fly ash geopolymer pastes were investigated. Five NaOH concentrations (8, 10, 12, 15 and 18 molar) were studied. The liquid to ash ratio of 0.4, sodium silicate to sodium hydroxide ratio of 0.67 and low temperature curing at 40 °C were selected in making geopolymer pastes. The results showed that NaOH concentration had significant influence on the physical and electrical properties of geopolymer paste. The pastes with high NaOH concentrations showed increased setting time and compressive strength due to a high degree of geopolymerization as a result of the increased leaching of silica and alumina from fly ash. The dielectric constant and conductivity increased with NaOH concentration while tan δ decreased due to an increase in geopolymerization. At the frequency of 10³ Hz, the dielectric constants of all pastes were approximately 10⁴ S/cm and decreased with increased frequency. The relaxation peaks of tan δ reduced with an increase in NaOH concentration and ranged between 2.5 and 4.5. The AC conductivity behavior followed the universal power law and the values were in the range of 3.7 × 10⁻³–1.5 × 10⁻² at 10⁵–10⁶ Hz.     

Organic carbon leaching behavior from incinerator bottom ash

The understanding of the leaching behavior of organic carbon from incinerator bottom ash is an important aspect for the control of organic carbon emissions from landfills in order to minimize their potential risk to the environment. The leaching behavior of organic carbon from incinerator bottom ash samples, obtained from two different solid waste sources, as well as the effects of calcium (Ca) on organic carbon (DOC) leaching associated with pH were therefore investigated in this paper. The effect of pH on the leaching of DOC and other major elements from bottom ash was ascertained through pH-stat leaching experiments. Column leaching experiments were carried out to evaluate the dependence of the leached amount of DOC on Ca leaching. It was found that the bottom ash produced by incineration of municipal solid waste (MSW) was composed of two DOC fractions: DOC leached independent (fraction I) of Ca leaching, observed at alkaline-neutral pH, and DOC leached dependent (fraction II) on Ca leaching, observed at acid pH. However, the bottom ash produced by incineration of industrial solid waste (ISW) was composed of only DOC fraction I. The addition of calcium oxide during incineration and the differences in the paper to plastic ratio in the physical composition of the solid wastes incinerated would explain the distinct organic carbon leaching behaviors of bottom ash samples.     

Effect of NaOH concentration on properties and microstructure of a novel reactive ultra-fine fly ash geopolymer

This study used reactive ultra-fine fly ash (RUFA) as the primary raw materials in the preparation of a novel RUFA geopolymer. Note that the solution to binder weight ratio was maintained at the same level by varying the concentration of the NaOH solution. Extensive analysis was conducted to characterize the flowability and mechanical properties. X-ray diffraction (XRD), Scanning Electron Microscope-Energy Dispersive Spectrometer (SEM-EDS), Fourier transform infrared spectroscopy (FT-IR), mercury intrusion porosimetry (MIP), thermogravimetric (TG), and zeta potential analyses were used to examine the microstructure of RUFA geopolymers. Increasing the concentration of NaOH also led to an increase in compressive strength. A high NaOH concentration of 12 mol/L resulted in compressive strength of 97.6 MPa at 28 days. Finally, increasing the concentration of NaOH increased the formation of the primary reaction geopolymerization product, N-A-S-H gel, resulting in a denser microstructure with lower porosity.     

Biotoxicity evaluation of fly ash and bottom ash from different municipal solid waste incinerators

Different types of municipal solid waste incinerator (MSWI) fly and bottom ash were extracted by TCLP and PBET procedures. The biotoxicity of the leachate of fly ash and bottom ash was evaluated by Vibrio fischeri light inhibition test. The results indicate the following: (1) The optimal solid/liquid ratio was 1:100 for PBET extraction because it had the highest Pb and Cu extractable mass from MSWI fly ash. (2) The extractable metal mass from both fly ash and bottom ash by PBET procedure was significantly higher than that by TCLP procedure. (3) The metal concentrations of fly ash leachate from a fluidized bed incinerator was lower than that from mass-burning and mass-burning combined with rotary kiln incinerator. (4) The TCLP and PBET leachate from all MSWI fly ash samples showed biotoxicity. Even though bottom ash is regarded as a non-hazardous material, its TCLP and PBET leachate also showed biotoxicity. The pH significantly influenced the biotoxicity of leachate.