High Fly Ash Content Research Articles

Concrete with High Content of Fly Ash Intended for Constructions with Long Durability

The article deals with theme of high fly ash content concretes intended for long life constructions. Considering the still growing consumption of fly ash in construction concretes it is a live theme in the Czech Republic and abroad as well. The emphasis will be laid namely on characteristics and requirements for fresh concrete intended for construction of these specific concrete constructions. They are for instance waterproof constructions, tunnel linings, concretes for bridge and road constructions etc. Also the hardened concrete properties like compressive strength, resistance to pressure water, durability and further necessary parameters for obtainment of required properties of these concretes will be monitored.

Effects of fly ash addition on physical properties of porous clay-fly ash composites via polymeric replica technique

The porous composites of clay and fly ash have the potential to be used in many fields, such as catalyst support and gas adsorbents. In this study, various ratios of fly ash (1–2) with different percentage of suspension (50–70 wt%) were applied to produce porous clay-fly ash composites via polymeric replica technique. Fabrication process starts by mixing clay and fly ash in distilled water to form slurry. The process is followed by fully immersing polymer sponge in slurry. The excess slurry is then removed through squeezing. Finally, the sponge coated with slurry is sintered at 500 and 1250 °C for 1 h. It is found that the compressive strength of porous composites improves significantly (0.178–1.28 MPa) when the amount of clay-fly ash suspension mixture (50–70 wt%) increases. The compressive strength of porous composites is mainly attributed to the mullite, quartz and amorphous phase formations. These results are supported by X-ray diffraction analysis. On the other hand, increase in the amount of suspension reduces the apparent density (from 2.44 to 2.32 g/cm3) and porosity (from 97 to 85 %). The reduction in apparent density is believed to be caused by the presence of high fly ash content in porous composites. The melted fly ash cenospheres have closed the internal pores and increased density of samples. Higher suspension level not only reduces porosity, but also increases close pores of the porous composites. The results are justified through the observation from the structures of porous clay-fly ash composites.

Resistance of fly ash–Portland cement blends to thermal shock

Thermal-shock resistance of high-content fly ash–Portland cement blends was tested in the following ways. Activated and non-activated blends with 80–90% fly ash F (FAF) were left to set at room temperature, then hydrated for 24 h at 85°C and then for an additional 24 h at 300°C, and tested in five thermal-shock cycles (600°C heat – 25°C water quenching). X-ray diffraction (XRD) and thermal gravimetric analyses, along with calorimetric measurements and scanning electron microscope-energy-dispersive X-ray tests demonstrated that the activated blends form more hydrates after 24 h at 300°C, and achieve a higher short-term compressive strength than do non-activated ones. Sodium meta-silicate and soda-ash engendered the concomitant hydration of ordinary Portland cement (OPC) and class F fly ash (FAF), with the formation of mixed crystalline FAF–OPC hydrates and FAF hydrates, such as garranite, analcime and wairakite, along with the amorphous FAF hydration products. In sodium sulfate-activated and non-activated blends separate OPC (tobermorite) and FAF (amorphous gel) hydrates with no mixed crystalline products formed. The compressive strength of all tested blends decreased by nearly 50% after five thermal-shock test cycles. These changes in the compressive strength were accompanied by a marked decrease in the intensities of XRD patterns of the crystalline hydrates after the thermal shock. There was no significant difference in the performance of the blends with different activators.

Strength Properties of Slag/Fly Ash Blends Activated with Sodium Metasilicate and Sodium Hydroxide + Silica Fume

This study presents the strength properties of alkali activated Ground Granulated Blast Furnace Slag (GGBFS) /Fly Ash (FA) blend mortars prepared with sodium meta-silicate and, sodium hydroxide-silica fume combination mixture. The GGBFS/FA ratios were arranged at (100/0, 80/20, 60/40, 40/60, 20/80, 0/100). Water to “GGBFS+FA” ratio was kept constant at 0.5. Na concentration ratio to “GGBFS+FA” was kept constant at 6%. These ratios were in mass basis. Three different mixture parameters were used. In the first series of mixture, only sodium meta-silicate was used as activator. In second and third series, sodium hydroxide and silica fume combination was used as activator. Sodium hydroxide and silica fume was mixed with water and used directly in preparation of second series of mortar mixtures. For preparation of third series of mortar mixture, sodium hydroxide and silica fume were mixed continuously with water for three days to allow dissolution of silica fume in sodium hydroxide solution. At first, 3-days compressive strengths of all alkali activated mortar mixture were measured. After that due to the very low compressive strength of mortars made with high volume fly ash content, only 100/0 and 80/20 slag/fly ash ratios were investigated for 7, 14 and 28 days compressive and 28 days flexural strength. The results of the 3 days tests show that decreasing the slag/fly ash ratio decreases distinguishably the compressive strength of the mortars. The mortars produced with the mixture of sodium hydroxide and silica fume combination as activator showed satisfactory results when compared with those activated with sodium meta-silicate.

Degradation of self-compacting concrete (SCC) due to sulfuric acid attack: Experiment investigation on the effect of high volume fly ash content

Concrete is susceptible to a variety of chemical attacks. In the sulfuric acid environment, concrete is subjected to a combination of sulfuric and acid attack. This research is aimed to investigate the degradation of self-compacting concrete (SCC) due to sulfuric acid attack based on measurement of compressive strength loss and diameter change. Since the proportion of SCC contains higher cement than that of normal concrete, the vulnerability of this concrete to sulfuric acid attack could be reduced by partial replacement of cement with fly ash at high volume level. The effect of high volume fly ash at 50-70% cement replacement levels on the extent of degradation owing to sulfuric acid will be assessed in this study. It can be shown that an increase in the utilization of fly ash to partially replace cement tends to reduce the degradation as confirmed by less compressive strength loss and diameter change. The effect of fly ash to reduce the degradation of SCC is more pronounced at a later age.

Influence of plasticizers on the rheology and early heat of hydration of blended cements with high content of fly ash

The dispersing effectiveness of five commercial plasticizers; lignosulfonate (LS), naphthalene sulphonate–formaldehyde polycondensate (NSF) and three polycarboxylate ethers (PCEs) were quantitatively investigated in blended cements where ordinary portland cement (OPC) was partly replaced by fly ash (FA) up to 60%. The capacity of the plasticizers in FA blended cements mimics that in the OPC system. At low replacement amounts, FA acts mainly as filler and does not impart any significant effect on the plasticizers. From 40% FA loading, PCEs showed decreased performance compared to NSF and LS relative to that in OPC systems, attributing to higher affinity of the latter polymers for cement clinkers. Retardation by plasticizers is more pronounced at higher FA contents due to lower adsorption on FA than on cement grains. Performance of plasticizers in industrial FA blended cements mimics that in OPC due to its relative higher surface area to compensate otherwise lower early strength.

Rheological and mechanical properties of cement–fly ash self-consolidating concrete incorporating high volumes of alumina-based material as fine aggregate

Portland cement and natural aggregate are the materials consumed in the greatest quantities in the production of concrete and are thus associated with environmental problems. The 3R keys principles for the production of concrete materials that are relatively environmentally friendly are reduce, reuse, and recycle. This paper presents a laboratory study on the properties of self-consolidating concrete (SCC) incorporating fly ash and recycled alumina waste. Fly ash is used as a cement replacement at a fixed 20%, and recycled alumina waste is used as a fine aggregate replacement at 0%, 25%, 50%, 75%, and 100%. The mixtures were designed to produce a controlled slump flow diameter. The properties examined in the study are workability and mechanical properties by using slump flow, J-ring flow, blocking flow assessment, V-funnel, compressive strength, and ultrasonic pulse velocity measurements. The results show that the use of fly ash and recycled alumina waste significantly improved the workability and increased the compressive strength of SCC mixtures. Further, all the mixtures showed an acceptable performance in regard to slump flow diameter and time tests. Mixes with high cement content and fly ash content into which recycled alumina waste content of up to 75% was incorporated had substantially lower V-funnel flow times and minimal blocking, flow times in the range of 4–6s, V funnel times in the range of 8–12s, and a difference in the slump flow and J-ring flow in the range of 2–3cm. These mixes also exhibited minimal or no apparent blocking compared with conventional SCC. The results further indicate that economical SCC with 28-day compressive strength up to 56MPa can be made using a high volume of recycled alumina waste, which is accompanied by a decrease of only one unit of strength.

Preparation of glass-ceramics using high contents of red mud and fly ash

The preparation of glass-ceramics with high contents of red mud ash using TiO2 and titanium slag as nucleation agents, respectively, is proposed. The effects of different formulations on physicochemical properties, microstructure and phase composition of glass-ceramics were investigated, followed by the analysis of the crystallisation behaviours. The results showed that the primary crystalline phase was Ca2(Al(AlSi)O7) in both cases, while the secondary crystalline phase was (CaFeO6Si2) and Ca(Al2Si2O8), respectively. Also, the crystallinity increased with increasing content of nucleation agent, and condensations were observed once the nucleation agent content exceeded a critical value. With the contents of TiO2 and titanium slag to be 1 and 6·6%, respectively, the overall content of industrial solid wastes achieved 93·9 and 95·3%, respectively. The performance of as-prepared glass-ceramics can satisfy the requirements of JC/T 872–2000 standard. Therefore, this study is of great significance for the recycling of industrial solid wastes.

New technology and application of brick making with coal fly ash

China has ranked first in the coal fly ash emission in the world. The multipurpose use of the fly ash from power plant waste is always an important topic for the Chinese environmental protection, which has drawn the concern of the government, scientific research departments, manufacturing enterprises and industry experts. This paper introduces an experimental research on how to recycle fly ash effectively, a kind of new technology of making bricks by which fly ash content could be amounted to 50–80 %. The article introduces raw materials of fly ash brick, production process and key control points. It introduces how to change the technical parameters of the existing brick-making mechanical device, optimize the parameters combination and improve the device performance. High-content fly ash bricks are manufactured, which selects wet fly ash from power plants, adding aggregate with reasonable ratio and additives with reasonable dosage, and do the experimental research on manufactured products for properties, production technology and selection about technology parameters of production equipment. All indexes of strength grade, freezing-thawing resisting, and other standards of the studied bricks reached the national standards for building materials industry.

Carbonation Resistance of High Volume Fly Ash Concrete

The cement industry is responsible for a large part of the global environmental problems: is the largest consumer of natural resources; the most responsible for the emission of greenhouse gases, including about 1.8 Gt of CO2; and requires huge amounts of energy, corresponding to between 12 and 15% of industrial energy use. The cement is also not used in the most appropriate manner, since 40% of the consumption of concrete is due to the renovation and repair of buildings, making concrete structures inefficient because its durability is relatively low. However, in the future, concrete can and should evolve in order to improve its eco-efficiency, with a smaller amount of cement in its composition, replacing it with high quantities of mineral additions, particularly fly ash. Nevertheless, current technology may not allow this type of concrete to be very efficient, because its long-term durability may be compromised. In fact, with increasing dosage of pozzolanic mineral additions, alkali paste components are consumed in the reaction leaving it vulnerable to concrete carbonation which may compromise the passivation layer needed for steel rebar protection against corrosion. This article explores a promising approach to mitigate this problem, which consists in the careful addition of hydrated lime in the concrete composition, highlighting the synergy of its components, significantly enhancing its carbonation resistance. It is proposed, therefore, to manufacture a concrete with high volume of fly ash, low cement content and high service life period: an efficient and sustainable concrete. In this context, an experimental campaign was developed with the aim of characterization of pastes behavior with high fly ash content, in particular with respect to its durability. The results will be presented and properly analyzed.

Economic Calculation of Binder in Concrete with a Higher Content of Fly Ash

The company is still trying to the sustainable development. This trend also applies to the construction industry. One way to contribute is to use waste materials. Such a material with untapped potential is also fly ash. Fly ash is already partly used, mainly just in construction, but this use has still reserves. Fly ash is problematic material, when used correctly, also has positive benefits. One of the important of questions for the use of fly ash is the price. This paper provides an economic analysis of the binder in concrete with high content of fly ash and also comparing with other types of binder in concrete.

Microstructural characterization and mechanical property of Fly Ash/Al-25Mg composites

Fly ash/A1-Mg composites are fabricated by powder metallurgical method. The morphology and structure of fly ash/A1-Mg composites are characterized by scanning electron microscope (SEM) and X-ray diffraction, respectively. The influences of different fly ash content on the friction and wear behavior of the composites are investigated at a constant sliding velocity of 400 r/min and the worn mechanism of composites is discussed. The results indicate that the friction coefficient is steadily lower than that of Al alloy matrix at the lower fly ash content and loads. For the fly ash/A1-Mg composites, the wear mechanism is characterized as abrasive wear and adhesive wear under small normal load and at low fly ash content, and it is characterized as delamination wear and abrasive wear transferred onto the counterpart under high normal load and at high fly ash content.

Influence of pre-curing time on the hydration of binder and the properties of concrete under steam curing condition

There is a pre-curing period before the freshly made concrete elements were exposed to steam curing in the steam curing process. In this paper, the influence of pre-curing time on the hydration of binder and the properties of concrete under steam curing condition was investigated. Three binders were used: the pure cement, the binder containing high content of GGBS, and the binder containing high content of fly ash. Three pre-curing times (1, 3, and 6 h) and one steam curing period at 60 °C (over 8 h) were adopted. Results show that pre-curing time has limited influence on the hydration degree of binder, and compressive strength and pore structure of paste. The influence of pre-curing time has limited influence on the compressive strength and chloride permeability of the pure cement concrete and the concrete containing high content of GGBS at whether early or late ages, indicating that the proper pre-curing time can be as short as 1 h for these two concretes. Increasing pre-curing time enhances the late-age compressive strength of the concrete containing high content of fly ash significantly, but it has limited influence on the late-age permeability.

Setting Time, Strength, and Bond of High-Calcium Fly Ash Geopolymer Concrete

AbstractIn this paper, setting time, strength and bond of high-calcium fly ash geopolymer concrete were investigated. The high-calcium fly ash was from Mae Moh power plant in northern Thailand. Both sodium silicate solution and sodium hydroxide solution were used as alkali activators in every mix. Sodium hydroxide solution with 10 M, 15 M, and 20 M concentrations, sodium silicate to sodium hydroxide ratios of 1.0 and 2.0, alkaline liquid to fly ash ratio of 0.5 and two curing regimes viz., heat curing at 60±2°C for 24 h and room temperature curing at 23±2°C were used. The results indicated that fresh geopolymer concrete had short setting time of 28–58 min due to the presence of high calcium content of fly ash. In general, strengths and modulus of elasticity increased with the increase in NaOH concentration. For compressive strength, the optimum Na2O content was around 12% of fly ash. The high-strength geopolymer concrete with 28-day compressive strength of 54.4 MPa was obtained for mix with 15M NaOH. The ...

Concrete with High Content of Fly-Ash for Common Use in the Czech Republic

Concrete with high substitute of the cement with fly-ash (FAC) is favourably used for structures with undesirable development of hydration heat or where reduction of the share of Portland clinker in the adhesive provides better resistance of concrete against impacts of acidic aggressive environment. Due to pozzolanic reaction fly-ash participates on formation of cement or adhesive stone and contributes to increased strength of concrete. As the pozzolanic reaction process is gradual its impact on the increased concrete strength is shown within longer time horizon (i.e. after lapse of standard age) and fly-ash concretes are therefore characterized by a short-term low strength which is one of its disadvantages mostly during winter season. However during summer season concretes with partial substitute of cement with fly-ash are beneficial solution for common use even from the aspect of reduced material costs on concrete production. Reduced cement volume in the concrete is nevertheless limited by requirements of ČSN EN 206-1 on concrete composition which is making difficult launch of concrete with higher cement substitutions with fly-ash on the Czech market. FAC allow effective use of the fly-ash, which is otherwise a waste product representing environmental load on spoil heaps as a substitute of clinker, the production of which is also environmentally loading from the power and raw materials aspect. This environmental input of FAC would be hopefully more considered for design of building structures in the future.

Effect of Carbon Fibers on Thermal and Mechanical Properties of Metakaolin-Fly Ash-Based Geopolymers

This paper presents results from a set of experiments carried out to evaluate the effect of carbon fibers on thermal and mechanical properties of metakaolin fly-ash-based geopolymers. Bending and compression tests were conducted at ambient temperature and after exposure to elevated temperatures on fiber-reinforced geopolymers with varying proportions of carbon fibers, metakaolin, and fly ash. Also, crack propagation, dimensional changes, and temperature gradients in geopolymers with different carbon fiber content were recorded during and after heating. Data from these tests show that the addition of short carbon fibers in geopolymers provides an effective mechanism for controlling temperature-induced cracking and thermal deformation, and this in turn enhances bending strength under high temperatures. The enhancement in bending strength under high temperatures, due to the presence of carbon fibers, is higher in geopolymers with higher fly ash content than that with lower fly ash content. Further, the addition of carbon fibers has no significant effect on compressive strength of geopolymers both at ambient temperature and after exposure to high temperatures.

A study on the effect of nano silica on compressive strength of high volume fly ash mortars and concretes

This paper presents the effect of nano silica (NS) on the compressive strength of mortars and concretes containing different high volume fly ash (HVFA) contents ranging from 40% to 70% (by weight) as partial replacement of cement. The compressive strength of mortars is measured at 7 and 28days and that for concretes is measured at 3, 7, 28, 56 and 90days. The effects of NS in microstructure development and pozzolanic reaction of pastes containing above HVFA contents are also studied through backscattered electron (BSE) image and X-ray diffraction (XRD) analysis. Results show that among different NS contents ranging from 1% to 6%, cement mortar containing 2% NS exhibited highest 7 and 28days compressive strength. This NS content (2%) is then added to the HVFA mortars and concretes and the results show that the addition of 2% NS improved the early age (7days) compressive strength of mortars containing 40% and 50% fly ash by 5% and 7%, respectively. However, this improvement is not observed at high fly ash contents beyond 50%. On the other hand, all HVFA mortars exhibited improvement in 28days compressive strength due to addition of 2% NS and the most significant improvement is noticed in mortars containing more than 50% fly ash. In HVFA concretes, the improvement of early age (3days) compressive strength is also noticed due to addition of 2% NS. The BSE and XRD analysis results also support the above findings.

Assessment of Hydration Degree of Cement in the Fly Ash-Cement Pastes Based on the Calcium Hydroxide Content

The hydration degree of fly ash and the calcium hydroxide (CH) content were measured. Combined with the equilibrium calculation of cement hydration, a new method for assessment of the hydration degree of cement in the fly ash-cement (FC) pastes based on the CH content was developed. The results reveal that as the fly ash content increase, the hydration degree of fly ash and the CH content decrease gradually; at the same time, the hydration degree of cement increase. The hydration degree of cement in the FC pastes containing a high content of fly ash (more than 35%) at 360 days is as high as 80%, even some of which hydrates nearly completely. The effect of water-cement ratio to the hydration degree of cement in the FC pastes is far less distinct than that of the content of fly ash.

Strength and resistance to sulfate and sulfuric acid of ground fluidized bed combustion fly ash–silica fume alkali-activated composite

Fluidized bed combustion (FBC) is an environmentally friendly process for burning of coal and is used in many small factories located in urban area. The FBC fly ash is an environmental problem and needs good disposal or utilization. This research studied the strength and resistance to sulfate and acid of alkali-activated FBC fly ash–silica fume composite. The FBC fly ash was interground with silica fume (at the dosage levels of 1.5%, 3.75% and 5.0%) to make the source material homogenous with increased reactivity. Addition of silica fume enabled the adjustment of SiO2/Al2O3 ratios (6.55-7.54) of composite and improved the strength and resistance to sulfate and acid of composite. The composite with 3.75% silica fume showed the optimum strength with 28-day compressive strength of 17.0MPa. The compressive strengths of composite with 3.75% silica fume immersed in 5% magnesium sulfate solution and 3% sulfuric acid solutions were substantially higher than the control. The strength loss was from the high calcium content of FBC fly ash and incorporation of silica fume thus increased the durability of the composite.

Some durability aspects of hybrid alkaline cements

Blended cements that contain a high content of fly ash and a low content of Portland cement typically suffer from low early strength development and long setting times. Recently, one method of overcoming these problems has been to use an alkali activator to enhance the reactivity of fly ash particles at early ages. Such cements can be grouped under the generic term “hybrid alkaline cements”, where both cement clinker and fly ash, encouraged by the presence of alkalis, are expected to contribute to cementitious gel formation. The work presented here examines some of the durability aspects of high fly ash content hybrid alkaline cement. Specifically, the aspects investigated were: exposure at high temperatures (up to 1000°C), resistance to immersion in aggressive solutions and susceptibility to the alkali aggregate reaction. All tests were repeated with a commercially available sulfate resistant Portland cement for comparison. When exposed to high temperatures, the hybrid alkaline cement showed strikingly different behaviour compared to the control Portland cement, showing fewer micro-cracks and maintaining residual compressive strengths at least equal to original strengths. Beyond 700°C, the hybrid alkaline cement began to sinter, which resulted in shrinkage of around 5% and a 100% increase in residual compressive strengths. No such sintering event was noted in the control Portland cement, which showed a drastic loss in residual compressive strengths upon heating. In immersion tests, the hybrid alkaline cement possessed excellent resistance to sulfate and seawater attack, similar to the control sulfate resistant cement. Both cements were however severely degraded by immersion in 0.1M HCl for 90 days. Both binders complied with the accelerated alkali-aggregate test but when this test was extended, the hybrid alkaline binder showed much greater dimensional stability. Possible reasons for the differences in durability behaviour in both cements are discussed, based on experimental evidence provided.