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.     

Influence of the slag content on the chloride and sulfuric acid resistances of alkali-activated fly ash/slag paste

This study aims to investigate the influence of the slag content on the chloride and sulfuric acid resistances of alkali-activated fly ash/slag (AFS) paste. A series of tests were conducted to examine the effects of reaction products and their contents on the chloride and sulfuric acid resistance capabilities. It was shown from the tests that the deterioration of the AFS binder due to a sulfuric acid attack was caused by 1) the corrosion by means of SO₄²⁻ penetration through the surface of the AFS binder, which is associated with permeable voids and a rate of water absorption and 2) the corrosion of the reaction products resulting from the different degrees of resistance to sulfuric attack between the C-(A)-S-H and the N-A-S-H. Variation of the slag content led to differences in the reaction product content of the AFS binder, clearly affecting the chloride-binding capacity and the resistance to chloride penetration.     

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.     

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.     

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.     

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.     

Reaction products and strength development of wastepaper sludge ash and the influence of alkalis

Wastepaper sludge ash (WSA) from a newsprint paper mill was investigated for its mineralogical composition and its reaction products and strength development after activation with water and sodium and potassium hydroxide solutions. The results showed the WSA to consist of calcite, free lime, gehlenite, tricalcium aluminate, belite, talc, quartz and probably a glassy phase. The principle reaction product was monocarboaluminate (CO₃–AFm) for the water- as well as for the alkali-activated WSA. Formation of monocarboaluminate and strength gain was more rapid for the alkali-activated WSA until 1 day of curing. However, afterwards reactions proceeded much slower when alkali solutions were used, leading to an about twice as high compressive strength for water-activated WSA mortars after 28 days of curing. The observed behavior is tentatively ascribed to a less uniform microstructure of the alkali-activated WSA. Significant differences between NaOH- and the KOH-activated WSA were not observed.     

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.     

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.     

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.     

Tribological investigation of Zamak alloys reinforced with alumina (Al2O3) and fly ash

ABSTRACT

In this study, zinc aluminum alloy (Zamak) (ZA-27) composites reinforced by different weight fractions of fly ash, alumina (Al₂O₃), or both particles were produced using compo-casting technique. The composites were subjected to hardness and wear tests. The hardness of the composites increases with increase of the weight fractions of reinforcements. In wear test, the composites were examined under dry sliding conditions using pin on disc apparatus. The wear results revealed that the wear resistance increases with increase of the weight fractions of reinforcements. However, the effect of fly ash particles on the wear resistance of the produced composites is more statistically significant than the effect of Al₂O₃ particles. The morphology of the composites was examined using scanning electron microscopy (SEM) after the test. The SEM images revealed the existence of adhesion and delamination wear mechanism.     

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.     

Utilization of municipal solid waste incineration (MSWI) fly ash in blended cement Part 1: Processing and characterization of MSWI fly ash

This paper is the first of a series of two articles dealing with the processes applied to MSWI fly ash with a view to reusing it safely in cement-based materials. Part 1 presents two stabilization processes and Part 2 deals with the use of the two treated fly ashes (TFA) in mortars. Two types of binder were used: an Ordinary Portland Cement (OPC) containing more than 95% clinker (CEM I 52.5R) and a binary blend cement composed of 70% ground granulated blast furnace slag and 30% clinker (CEM III-B 42.5N). In this first part, two stabilization processes are presented: the conventional process, called "A", based on the washing, phosphation and calcination of the ash, and a modified process, called "B", intended to eliminate metallic aluminum and sulfate contained in the ash. The physical, chemical and mineralogical characteristics of the two TFA were comparable. The main differences observed were those expected, i.e. TFA-B was free of metallic aluminum and sulfate. The mineralogical characterization of the two TFAs highlighted the presence of large amounts of a calcium aluminosilicate phase taking two forms, a crystalline form (gehlenite) and an amorphous form. Hydration studies on pastes containing mixed TFA and calcium hydroxide showed that this phase reacted with calcium hydroxide to form calcium aluminate hydrates. This formation of hydrates was accompanied by a hardening of the pastes. These results are very encouraging for the reuse of such TFA in cement-based materials because they can be considered as pozzolanic additions and could advantageously replace a part of the cement in cement-based materials. Finally, leaching tests were carried out to evaluate the environmental impact of the two TFAs. The elements which were less efficiently stabilized by process A were zinc, cadmium and antimony but, when the results of the leaching tests were compared with the thresholds of the European landfill directive, TFA-A could nevertheless be accepted at landfills for non-hazardous waste. The modifications of the process led to a significant reduction in the stabilization of chromium, selenium and antimony.     

Green activating silica-alumina insoluble phase of fly ash to synthesize zeolite P with high adsorption capacity for Pb(II) in solution

Using fly ash as a raw material to synthesize zeolite is an effective way to obtain cheap adsorbent. However, the high energy consumption of activating the insoluble silica-alumina phase of fly ash limits its practical application. In this study, a low-energy, wet-grinding hydrothermal method was used to activate the insoluble phase of fly ash to synthesize zeolite P, and the adsorption capacity of zeolite for lead ion in solution was studied. Wet-grinding can reduce fly ash particle size to nanometer scale and break long-chain Q⁴(Al) into short-chain low-polymer. Without aging, the activated material could directly synthesize zeolite P with a relative crystallinity of 89.37 %. Compared with the common alkali-fusion method, the wet-grinding method reduced energy consumption by 45.93 % and increased synthetic zeolite relative crystallinity by 7.51 %. The synthesized zeolite P could effectively remove Pb²⁺ ions from solution through ion exchange and hydroxyl reaction, and the adsorption capacity reached 497.01 mg/g.     

Differential behaviour of combustion and gasification fly ash from Puertollano Power Plants (Spain) for the synthesis of zeolites and silica extraction

Coal gasification (IGCC) and pulverised coal combustion (PCC) fly ashes (FAs), obtained from two power plants fed with the carboniferous bituminous coal from Puertollano (Spain), were characterised and used as raw materials for zeolite synthesis by direct conversion (DC) and by alkaline fusion (Fu), and SiO₂ extraction (Si-Ex) at laboratory scale. The Puertollano FAs are characterised by a high SiO₂ content (59%) with respect to EU coal FAs. High zeolite synthesis yields were obtained from both FAs by using conventional alkaline activation. However, the Si extraction yields were very different. The results of the zeolite synthesis from the Si-bearing extracts from both FAs demonstrated that high purity zeolites with high cation exchange capacity (CEC, between 4.3 and 5.3 meq/g) can be produced. The solid residue arising from Si-Ex is also a relatively high NaP1 zeolite product (CEC 2.4–2.7 meq/g) equivalent to the DC products. The zeolitic materials synthesised from both FAs by Fu showed an intermediate (between the high purity zeolites and the DC products) zeolite content with CEC values from 3.4 to 3.7 meq/g. Low leachable metal contents were obtained from high purity A and X zeolites and zeolite material synthesised by Fu for PCC FA.     

Cr(VI) concentration from batch contact/tank leaching and column percolation test using fly ash with additives

Batch contact, tank leaching and column percolation tests were conducted to investigate the Cr(VI) concentration in the solution/leachate from two fly ashes (fly ash A and B) with additives. The additives used were cement, low alkalinity additive and Ariake clay. There are several factors influencing Cr(VI) concentration in solution/leachate, namely (1) properties of solid/liquid mixture (chemical composition, pH value, etc.), (2) cementation effect, (3) amount of water in contact with the solid mass (solid/liquid ratio in case of batch contact test), and (4) adsorption characteristics of the solid particles to Cr ions. The test results indicate that fly ash A has less cementation component (CaO of 1.92%) and the amount of water in contact with the fly ash played an important role. As a result, Cr(VI) concentration from the column percolation test was much higher than that of the batch contact test. Adding Ariake clay had more effect on reducing Cr(VI) concentration for fly ash A than B because the pH value of the solution from fly ash A was lower, which provided a favorable condition for Cr(VI) ions to be reduced to Cr(III) and possibly to be adsorbed by clay particles. Fly ash B has more cementation component (7.15%) and for column percolation test, curing the sample for 1 week reduced Cr(VI) concentration significantly. The test results indicate that in engineering practice, a method which closely simulates the field condition should be selected to assess possible environmental effects and corresponding countermeasure methods.     

Properties of self-compacting mortars with binary and ternary cementitious blends of fly ash and metakaolin

The link between flow properties and the formulation is actually one of the key-issues for the design of self-compacting concretes (SCC). As an integral part of a SCC, self-compacting mortars (SCMs) may serve as a basis for the design of concrete since the measurement of the rheological properties of SCCs is often impractical due to the need for complex equipment. This paper discusses the properties of SCMs with mineral admixtures. Portland cement (PC), metakaolin (MK), and fly ash (FA) were used in binary (two-component) and ternary (three-component) cementititios blends. Within the frame work of this experimental study, a total of 16 SCMs were prepared having a constant water-binder (w/b) ratio of 0.40 and total cementitious materials content of 550 kg/m³. Then, the fresh properties of the mortars were tested for mini-slump flow diameter, mini-V-funnel flow time, setting time, and viscosity. Moreover, development in the compressive strength and ultrasonic pulse velocity (UPV) of the hardened mortars were determined at 1, 3, 7, 14, and 28 days. Test results have shown that using of FA and MK in the ternary blends improved the fresh properties and rheology of the mixtures when compared to those containing binary blends of FA or MK.