Showing papers on "Fly ash published in 2003"

Physical and pozzolanic action of mineral additions on the mechanical strength of high-performance concrete

Abstract: Pozzolans play an important role when added to Portland cement because they usually increase the mechanical strength and durability of concrete structures. The most important effects in the cementitious paste microstructure are changes in pore structure produced by the reduction in the grain size caused by the pozzolanic reactions pozzolanic effect (PE) and the obstruction of pores and voids by the action of the finer grains (physical or filler effect). Few published investigations quantify these two effects. Twelve concrete mixtures were tested in this study: one with Portland cement (control), nine mixtures with 12.5%, 25% and 50% of replacement of cement by fly ash, rice husk ash and limestone filler; two with (12.5+12.5)% and (25+25)% of fly ash and rice husk ash. All the mixtures were prepared with water/binder ratios of 0.35, 0.50, and 0.65. The compressive strength for the samples was calculated in MPa per kg of cement. The remaining contents of calcium hydroxide and combined water were also tested. The results show that the pozzolanic and physical effects have increased as the mineral addition increased in the mixture, being higher after 91 days than after 28 days. When the results for the same strength values are compared (35 and 65 MPa), it was observed that the filler effect (FE) increased more than the pozzolanic effect. The PE was stronger in the binary and ternary mixtures prepared with rice husk ash in proportions of 25% or higher.

Mineralogical and microstructural characterisation of alkali-activated fly ash/slag pastes

Abstract: A mechanical, mineralogical and microstructural characterisation of the cement pastes obtained by alkaline activation of fly ash/slag mixtures cured at different temperatures has been carried out. The pastes obtained were characterised by XRD, FTIR, MAS NMR, SEM/EDX, atomic absorption and ion chromatography, also the insoluble residue in HCl was determined. The results obtained have proved the existence of two different reaction products in those activated pastes. The average atomic ratios in the main reaction product were Ca/Si∼0.8, Al/Ca∼0.6, Si/Al∼2–3. Such analysis corresponds to calcium silicate hydrate rich in Al, which includes Na in its structure. Other reaction product which was detected in the pastes as result of fly ash activation, was an alkaline aluminosilicate hydrate with a three-dimensional structure.

Properties of concrete incorporating fly ash and ground granulated blast-furnace slag

Abstract: This paper presents a laboratory study on the influence of combination of fly ash (FA) and ground granulated blast-furnace slag (GGBS) on the properties of high-strength concrete. A contrast study was carried out for the concrete (GGFAC) incorporating FA and GGBS, control Portland cement concrete and high-volume FA high-strength concrete (HFAC). Assessments of the concrete mixes were based on short- and long-term performance of concrete. These included compressive strength and resistance to H2SO4 attack. The microstructure of the concretes at the age of 7 days and 360 days was also studied by using scanning electron microscope. The results show that the combination of FA and GGBS can improve both short- and long-term properties of concrete, while HFAC requires a relatively longer time to get its beneficial effect.

Removal of Zinc from Aqueous Solutions Using Bagasse Fly Ash − a Low Cost Adsorbent

Abstract: Bagasse fly ash, a sugar industry waste, has been converted into an inexpensive and efficient adsorbent. The product obtained has been characterized and utilized for the removal of zinc from aqueous solutions over a wide range of initial metal ion concentration (3.06 × 10-4 to 3.06 × 10-3 M), contact time (24 h), adsorbent dose (5−20 g L-1), and pH (1.0−6.0). The removal of Zn2+ is 100% at low concentrations, whereas it is 60−65% at higher concentrations at an optimum pH of 4.0, using 10 g L-1 of adsorbent in 6−8 h of equilibration time. The uptake decreases with a rise in temperature indicating the process to be exothermic in nature. Kinetic studies have been performed to understand the mechanism of adsorption. The removal takes place through film diffusion mechanism at lower concentrations (≤1.84 × 10-3 M) and by particle diffusion at higher concentrations (≥ 3.06 × 10-3 M).

Removal of B, Cr, Mo, and Se from wastewater by incorporation into hydrocalumite and ettringite.

Abstract: Boron, chromium, molybdenum, and selenium often occur in high concentrations in fly ash leachates. During the leaching of fly ash in alkaline environments, hydrocalumite (Ca4Al2(OH)12(OH)2·6H2O) and ettringite (Ca6Al2(OH)12(SO4)3·26H2O) form as secondary precipitates. In this study, the removal of B, Cr, Mo, and Se oxyanions from high pH waters by incorporation into hydrocalumite and ettringite was examined. Experiments were performed by precipitating these minerals in solutions containing B, Cr, Mo, and Se oxyanions at conditions relevant to lime-leaching of fly ash as well as to fly ash containing concrete. The uptake of all four anions by hydrocalumite and ettringite was high. Anion uptake by hydrocalumite was larger than that by ettringite, and hydrocalumite was able to reduce anion concentrations to below drinking water standards. Ettringite showed an anion preference in the order of B(OH)4- > SeO42- > CrO42- > MoO42-. In contrast, borate was least preferred by hydrocalumite. Coordination, size, and ...

Development of Bottom Ash as Pozzolanic Material

Abstract: This research studies the potential of using bottom ash from the Mae Moh power plant in Thailand as a pozzolanic material. Bottom ash, which was rarely used in concrete due to its inactive pozzolanic reaction, improved its quality by grinding until the particle size retained on Sieve 325 was less than 5% by weight. Bottom ashes before and after being ground were investigated and compared for their physical and chemical properties. The bottom ashes were used to replace portland cement type I in mortar and concrete mixtures. The results indicated that the particle of bottom ash was large, porous, and irregular shapes. The grinding process reduced the particle size as well as porosity of the bottom ash. Compressive strengths of mortar containing 20–30% of bottom ash as cement replacement were much less than that of cement mortar at all ages, but the use of ground bottom ash produced higher compressive strength than the cement mortar after 60 days. When ground bottom ash was used at a 20% replacement of cemen...

Sulfate resistance of plain and blended cements exposed to varying concentrations of sodium sulfate

Abstract: Concrete deterioration due to sulfate attack is the second major durability problem, after reinforcement corrosion. This type of deterioration is noted in the structures exposed to sulfate-bearing soils and groundwater. Though concrete deterioration due to sulfate attack is reported from many countries, the mechanisms of sulfate attack have not been thoroughly investigated, particularly the effect of sulfate concentration and the cation type associated with the sulfate ions on concrete deterioration. This study was conducted to evaluate the performance of plain and blended cements exposed to varying concentrations of sodium sulfate for up to 24 months. Four types of cements, namely Type I, Type V, Type I plus silica fume and Type I plus fly ash, were exposed to five sodium sulfate solutions with sulfate concentrations of 1%, 1.5%, 2%, 2.5% and 4%. These concentrations are representative of the sulfate concentration in highly saline soils. The sulfate resistance was evaluated by visual examination and measuring the and reduction in compressive strength. The maximum deterioration, due to sulfate attack, was noted in Type I cement followed by silica fume and Type V cements. The performance of Type V, Type I plus silica fume and Type I plus fly ash was not significantly different from each other. The enhanced sulfate resistance noted in the Type I cement blended with either silica fume or fly ash indicates the usefulness of these cements in both sulfate and sulfate plus chloride environments.

High-Volume Fly Ash Concrete with High Strength and Low Drying Shrinkage

Abstract: In this work, a laboratory investigation was carried out to evaluate the strength and particularly the shrinkage properties of concrete containing high volumes of fly ash. The concrete mixtures made with 50 and 70% replacement (by mass) of ordinary portland cement (OPC) with fly ash were prepared. Water-cementitious material ratios ranged from 0.28 to 0.34. Some concrete mixtures were also made with superplasticizer. The strength and shrinkage properties of the concrete mixtures cured at 20°C temperature with 65% relative humidity are reported. The laboratory test results show that high-volume fly ash (HVFA) concrete attained satisfactory compressive and tensile strength at 1 day of age. It also showed that 50% replacement HVFA concrete developed higher strength than OPC concrete at 28 days and beyond. The inclusion of high volumes of fly ash in concrete with a low water-cementitious material ratio resulted in a reduction in the shrinkage values of up to 30% when compared to OPC concrete. The concrete mixtures made with superplasticizer showed higher shrinkage values of up to 50% when compared to the concrete made with no superplasticizer.

Reusing sewage sludge ash as adsorbent for copper removal from wastewater

Abstract: The sewage sludge ash (SSA) can potentially be used for removal of heavy metals from wastewater due to its similar chemical composition to that of fly ash and blast-furnace slag. This study investigated the adsorptive characteristics of SSA including specific surface area, cation-exchange capacity (CEC), and pH of zero point of charge (pHZPC). In addition, the adsorption isotherms of SSA with synthetic wastewater containing copper ions were tested. Experimental results indicated that the CEC and pHZPC of SSA were 24.1–25.7 meq/100 g and 3.1–3.4, respectively, which exhibited the capability of removing heavy metals from wastewater. The adsorption test of applying SSA into synthetic wastewater revealed that the adsorption isotherm of SSA for copper ions generally followed the Langmuir model. The estimated maximum capacity of copper adsorbed by SSA was 3.2–4.1 mg/g and close to that of fly ash. The primary mechanisms of copper removal by SSA included electrostatic attraction, surface complex formation, and cation exchange. The precipitation of copper hydroxide occurred only when the dosage of SSA and the equilibrium pH of wastewater were at a high level (30–40 g/l and greater than 6.2, respectively). In general, similar to fly ash and blast-furnace slag, the SSA can be reused as an adsorbent for copper removal from wastewater and the removal efficiency was greater than 98%.