Composition and microstructure of alkali activated fly ash binder: Effect of the activator

The alkali activation of fly ashes is a chemical process by which the glassy component of these powdered materials is transformed into very well-compacted cement. In the present work the relationship between the mineralogical and microstructural characteristics of alkaline activated fly ash mortars (activated with NaOH, Na₂CO₃ and waterglass solutions) and its mechanical properties has been established. The results of the investigation show that in all cases (whatever the activator used) the main reaction product formed is an alkaline aluminosilicate gel, with low-ordered crystalline structure. This product is responsible for the excellent mechanical-cementitious properties of the activated fly ash. However the microstructure as well as the Si/Al and Na/Al ratios of the aluminosilicate gel change as a function of the activator type used in the system. As a secondary reaction product some zeolites are formed. The nature and composition of these zeolites also depend on the type of activator used.     

Electrochemical studies on the corrosion performance of steel embedded in activated fly ash blended concrete

The use of fly ash to replace a portion of cement has resulted significant savings in the cost of cement production. Fly ash blended cement concretes require a longer curing time and their early strength is low when compared to ordinary Portland cement (OPC) concrete. By adopting various activation techniques such as physical, thermal and chemical methods, hydration of fly ash blended cement concrete was accelerated and thereby improved the corrosion-resistance of concrete. Concrete specimens prepared with 10-40% of activated fly ash replacement were evaluated for their open circuit potential measurements, weight loss measurements, impedance measurements, linear polarization measurements, water absorption test, rapid chloride ion penetration test and scanning electron microscopy (SEM) test and the results were compared with those for OPC concrete without fly ash. All the studies confirmed that up to a critical level of 20-30% replacement; activated fly ash cement improved the corrosion-resistance properties of concrete. It was also confirmed that the chemical activation of fly ash yielded better results than the other methods of activation investigated in this study.     

Effect of pumice and fly ash incorporation on high temperature resistance of cement based mortars

The effects of high temperature on the mechanical properties of cement based mortars containing pumice and fly ash were investigated in this research. Four different mortar mixtures with varying amounts of fly ash were exposed to high temperatures of 300, 600, and 900 °C for 3 h. The residual strength of these specimens was determined after cooling by water soaking or by air cooling. Also, microstructure formations were investigated by X-ray and SEM analyses.Test results showed that the pumice mortar incorporating 60% fly ash revealed the best performance particularly at 900 °C. This mixture did not show any loss in compressive strength at all test temperatures when cooled in air. The superior performance of 60% FA mortar may be attributed to the strong aggregate-cement paste interfacial transition zone (ITZ) and ceramic bond formation at 900 °C. However, all mortar specimens showed severe losses in terms of flexural strength. Furthermore, specimens cooled in water showed greater strength loss than the air cooled specimens. Nevertheless, the developed pumice, fly ash and cement based mortars seemed to be a promising material in preventing high temperature hazards.     

Alkali reactivity of mortars containing chert and incorporating moderate-calcium fly ash

This paper reports the results of an experimental program, which aimed to investigate the alkali reactivity of chert and the effect of a moderate-calcium fly ash on the alkali–silica reaction. To determine the expansions, mortar bars were cast and tested in accordance with ASTM C1260. Mortar aggregate was replaced by chert, in controlled amounts, to find out the pessimum limit, if any. To evaluate the degree of cracking, sonic pulse velocity measurements and petrographic analysis were carried out on the cracked bars and on the thin sections taken from these bars, respectively. In the next series of tests, limestone and chert were blended together as mortar aggregate and cement was replaced by different dosages of fly ash to examine the changes in the mortar bar expansion as well as in the chemistry of reaction products. Microstructural observations were done on polished sections using a scanning electron microscope, equipped with energy dispersive X-ray analysis. The results showed that the chert used in this investigation had a pessimum proportion in the range of 5–15%. Sufficient fly-ash additions suppressed the expansion caused by chert. The study also revealed out that as the CaO/Na₂O_eq of alkali–silica gel increased, the expansivity of the gel decreased.     

Mechanical properties and microstructure evaluation of fly ash - Slag geopolymer foaming materials

The existing fly ash-slag foaming geopolymer materials generally have the shortcomings of low fly ash content and low porosity. It is urgent to develop geopolymer foaming materials with high fly ash content and high porosity. Using fly ash and slag as the main raw materials, geopolymer foaming materials were prepared by alkali activation. The effects of activator content and sodium silicate modulus on the macroscopic mechanical properties, pore structures and microstructures of geopolymer foaming materials were studied. The experimental results showed that when the activator content was 21% (wt.) and the modulus of sodium silicate was 1, the specimen exhibited the best performance. The compressive strength of the specimen reached 2.18 MPa at 28 d, the porosity was 63.07%, and the average pore sizes of macroscopic pores were 920 μm. Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) and Scanning Electron Microscopy and Energy Dispersive Spectrometer (SEM-EDS) analysis showed that when the content of activator was 21% and the modulus of sodium silicate was 1, the reaction grade of the system was the highest, reached 55.12%, meanwhile the main product Sodium silicate hydrate (N-A-S-H) gel produced the largest amount. The fractal dimension calculations showed that the spatial complexity of a specimen with large pores was greater than that of a specimen with small pores. This study can provide a basis for the design of geopolymer foaming materials with high proportion of fly ash and high porosity.     

Mechanical behaviour of various mortars made by combined fly ash and limestone in Moroccan Portland cement

Physico-chemical properties and mechanical behaviour of ternary cements made by Portland cement, fly ash and limestone are studied. The mixtures at various compositions of clinker, gypsum fly ash and limestone are intimately ground and compared to other compositions without fly ash. Blended fly ash cements are also studied. The results show that fly ash acts as grinding agent by reducing the required time to obtain the same percentage of particles retained on a 80-μm sieve as the standard cement. Fly ash cements lead to an important extension of setting time than limestone cements. The replacement of clinker by limestone gives better mechanical strengths than the mixtures containing fly ash at early days; after 28 days, the cements prepared by incorporation of fly ash gain an important strength. From mechanical point of view, an optima dosage was obtained at 77% clinker, 2% gypsum, 7.5% fly ash and 13% limestone composition.     

粉煤灰漂珠活化处理生活污水研究

根据漂珠密度小于水的特点,以水为介质从粉煤灰中成功提取漂珠,利用煅烧除炭、酸洗除杂及活化的漂珠对生活污水进行处理,研究了煅烧温度、酸洗盐酸的浓度、活化剂的种类及生活污水的浓度等对漂珠吸附性能的影响。结果表明,粉煤灰漂珠经过700℃高温煅烧后,按照液固比5∶1加入5%的盐酸洗涤,利用6 mol/L的H2SO4进行活化后其吸附性能最好,而且生活污水的COD含量越高,CODcr去除率越高。     

Synthesis and characterization of BaAl2Si2O8 using mechanically activated precursor mixtures containing coal fly ash

BaAl₂Si₂O₈ (BAS) was synthesized by solid state reaction, using coal fly ash [CFA, containing (in wt.%) 75–78% silico-aluminous glassy phase, 14.6% mullite, 5.2% quartz, 1.9% magnetite, plus other minor phases] as main raw material. A BaCO₃–CFA–Al₂O₃ powder mixture of stoichiometric BAS composition was mechanically activated in an attrition mill for up to 12 h and then sintered at 900–1300 °C. The monoclinic BAS phase (Celsian) was desired because it has better mechanical and thermal properties than the hexagonal BAS polymorph (Hexacelsian), but this tends to form first, remaining frequently metastably at low temperatures; besides, the Hexacelsian to Celsian conversion is sluggish and difficult to occur. The reaction rate, the apparent density and the mechanical properties of the synthesized materials increased with increasing milling time and sintering temperature. The mineralizing effect caused by some of the CFA impurities produced Hexacelsian to Celsian conversions higher than those previously reported for mechanically activated BAS materials.     

Ge extraction from gasification fly ash

Water-soluble germanium species (GeS₂, GeS and hexagonal-GeO₂) are generated during coal gasification and retained in fly ash. This fact together with the high market value of this element and the relatively high contents in the fly ashes of the Puertollano Integrated Gasification in Combined Cycle (IGCC) plant directed our research towards the development of an extraction process for this element. Major objectives of this research was to find a low cost and environmentally suitable process. Several water based extraction tests were carried out using different Puertollano IGCC fly ash samples, under different temperatures, water/fly ash ratios, and extraction times. High Ge extraction yields (up to 84%) were obtained at room temperature (25 °C) but also high proportions of other trace elements (impurities) were simultaneously extracted. Increasing the extraction temperature to 50, 90 and 150 °C, Ge extraction yields were kept at similar levels, while reducing the content of impurities, the water/fly ash ratio and extraction time. The experimental data point out the influence of chloride, calcium and sulphide dissolutions on the Ge extraction.     

Fly ash effects: I. The morphological effect of fly ash

The morphological effect is an important part of fly ash effects. The paper analyzes emphatically this effect and points out that it is composed of the filling role, surface role and lubricating role. For different fly ash, these roles are different. They must be considered synthetically when the morphological effect is analyzed. Analyzing result shows that the filling role is relative to the particle size, the surface role is relative to the specific surface area and the water affinity and the lubricating role is relative to the shape of particle. The morphological effect of fly ash is the synthetical embodiment of these roles.     

Prediction of compressive strength of fly ash concrete by new apparent activation energy function

A new prediction model using apparent activation energy is proposed to estimate the variation of compressive strength of fly ash concrete with aging. After analyzing the experimental result with the model, fly ash replacement content and water-binder ratio influence on apparent activation energy was investigated.According to the analysis, the model provides a good estimation of compressive strength development of fly ash concrete with aging. As the fly ash replacement content increases, limiting relative compressive strength and initial apparent activation energy increase. Concrete with water-binder ratio smaller than 0.40 gives nearly constant limiting relative compressive strength and initial apparent activation energy when analyzed with various water-binder ratios. However, concrete with water-binder ratio larger than 0.40 increases limiting relative compressive strength and initial apparent activation energy.     

Hydraulic conductivity of compacted cement-stabilized fly ash

When combined with portland cement and compacted, fly ash is a high-strength material. In some instances, it may also be desirable to control the hydraulic conductivity (k) of the compacted mixture. Therefore, a study was performed to assess the effects of water content (w), cement content, curing time, and compaction effort on the hydraulic conductivity of compacted cement-stabilized fly ash. When compacting relatively dry mixtures (w < 20%), k is independent of compaction effort, and is on the order of 10⁻⁵ cm/s. When compacting between w of 20% and optimum water content (w_opt), compaction effort affects k, and, at a given w, k decreases by about an order of magnitude when increasing from standard to modified proctor effort. When wet of w_opt, k is on the order of 10⁻⁶ cm/s regardless of compaction effort or water content. With respect to curing time, extended curing time has relatively little effect on k within a 60-day time frame. Based on the results of this study, an approach to construction quality assurance testing can be applied to estimate k based on in situ measurement of dry density (ρ_d) and w.     

Variation in fly ash properties with milling and acid leaching

Coal-burning power plants that consume pulverized solid fuels produce large amounts of fly ash as a residue. Fly ashes have been used in construction, agriculture, metal recovery and water pollution control. This paper considers the variation in properties of fly ashes to be used in the field of construction.The fly ashes produced in two coal-burning power plants in Asturias (Spain) were physically and chemically characterized in order to determine their pozzolanic activity and reactivity. Fly ashes are used as hydraulically active additives as they are a finely divided inorganic material. They were examined to determine whether they could be used as special preservatives in cements and concretes. To improve their properties, they are mechanically activated by wet milling as well as chemically activated by leaching with 30% by weight sulfuric acid at different temperatures and times.     

固硫灰渣和粉煤灰的特性对比分析

介绍了固硫灰渣的形成过程,并将固硫灰渣与粉煤灰的物化特性进行对比,分析表明:固硫灰渣与粉煤灰的化学成分类似,矿物组成差异较大;固硫灰渣与粉煤灰一样,具有较高的火山灰活性,用于建筑材料的生产完全可行.     

The leachability of heavy metals in hardened fly ash cement and cement-solidified fly ash

The effect of mix proportion, leachant pH, curing age, carbonation and specimen making method etc. on the leaching of heavy metals and Cr(VI) in fly ash cement mortars and cement-solidified fly ashes has been investigated. In addition, a method for reducing the leaching of Cr(VI) from cement-solidified fly ashes is proposed. The results mainly indicate that: (1) either Portland cement or fly ash contains a certain amount of heavy and toxic metals, and the leaching of them from hardened fly ash incorporated specimens exists and is increased with fly ash addition and water to cement ratio; (2) the leachability of some heavy metals is greatly dependent on leachant pH; (3) when carbonation of cement mortars occurs the leaching of chromium ions is increased; (4) the amount of heavy metals leached from cement-solidified fly ashes depends more on the kind of fly ash than their contents in fly ash; and (5) with ground granulated blast furnace slag addition, the leaching of Cr(VI) from solidified fly ashes is decreased.     

Ceramic tiles derived from coal fly ash: Preparation and mechanical characterization

Coal fly ash (CFA) accounts for a large fraction of the solid waste produced in China. Hence, there is an urgent need for the effective utilization of CFA, for example, as a raw material for ceramics production. In this study, clay- and feldspar-like materials fabricated by alkali activation pre-treatment of CFA were mixed with untreated CFA (regarded as a quartz-like material) and sintered to prepare fully ash-based ceramic tiles. The obtained tiles exhibited excellent sintering properties, e.g., low firing temperature and a wide sintering range; further, they showed better green strength (due to hydrogen bonding) and post-sintering performance (due to fluxing and mullite skeleton effects) than ceramic tiles produced exclusively from untreated CFA. The fully ash-based ceramic tiles sintered at 1100 °C exhibited optimal post-sintering properties (bulk density, 2.5 g/cm³; rupture modulus, 50.1 MPa; and water absorption, 0%). Thus, the proposed method is well suited for preparing a novel kind of ceramic tiles completely derived from CFA, highlighting its importance in the field of fly ash ceramics.     

Triboelectrostatic separation of fly ash and charge reversal

Triboelectrostatic separation has been investigated as a method for separating unburned carbon from coal combustion fly ash. It was found that when a fly ash is exposed to moisture before it undergoes separation, the charging properties of the components of the fly ash change significantly. The mineral and carbon components of the fly ash appear to charge oppositely to how they were charged before exposure to moisture. A correlation was found between the degree of charge reversal and the relative amounts of leachable ions, especially calcium and sodium ions, present on the surface of the ash.     

Enhanced mechanical properties of wood ash and fly ash as supplementary cementitious materials

The incorporation of fly ash (FA) and wood ash (WA) in concrete as supplementary cementitious materials (SCM) was studied. The chemical composition of ordinary Portland cement, FA and WA was determined according to ASTM C-114. SEM and optical microscopy were used for the analysis of concrete. Setting time, compressive strength, water absorption and acid resistance of the concrete with different percentages of SCM ranging from 0 to 60% were evaluated. The results obtained showed that setting time and rate of water absorption increased with the increase in percentage of SCM. After 7 and 28 days, the compressive strength of concrete with 20% FA as SCM was higher than that with substitution with 20% WA. Resistance of concrete against sulphate attack increased with an increase in the percentage of FA. It was found that incorporating more than 20% WA resulted in a decrease in sulphate attack resistance.