Reaction Of Fly Ash Research Articles

Mechano-chemical activation of high-Ca fly ash by cement free blending and gypsum aided grinding

This paper presents a laboratory study exploring the pozzolanic reactivity of a calcium rich fly ash by blending and grinding in a cement free system. A synergistic physico-chemical composition was achieved through individual grinding and ternary blending with another alkali sulphate rich fly ash and silica fume. The beneficial role of Flue gas desulfurisation gypsum as a grinding aid as well as a sulphate activator for fly ashes was also reported. The new blend’s parameters sensitivity revealed the physico-chemical features analogous to cement. The dispersion of the ground reactive blend particles by super plasticiser was found more effective than further alkali activation by NaOH. The potential of the developed blend as a new cementitious material is highlighted in this study. Progressive mortar compressive strength up to 90days and 46MPa after 28days of curing was reported by the cement free system of activated fly ashes.

Influence of SO2 in incineration flue gas on the sequestration of CO2 by municipal solid waste incinerator fly ash

The influence of CO2 content and presence of SO2 on the sequestration of CO2 by municipal solid waste incinerator (MSWI) fly ash was studied by investigating the carbonation reaction of MSWI fly ash with different combinations of simulated flue gas. The reaction between fly ash and 100% CO2 was relatively fast; the uptake of CO2 reached 87 g CO2/kg ash, and the sequestered CO2 could be entirely released at high temperatures. When CO2 content was reduced to 12%, the reaction rate decreased; the uptake fell to 41 g CO2/kg ash, and 70.7% of the sequestered CO2 could be released. With 12% CO2 in the presence of SO2, the reaction rate significantly decreased; the uptake was just 17 g CO2/kg ash, and only 52.9% of the sequestered CO2 could be released. SO2 in the simulated gas restricted the ability of fly ash to sequester CO2 because it blocked the pores of the ash.

Field investigation of high-volume fly ash pavement concrete

Field investigation of high-volume fly ash (HVFA) concrete in pavement construction was carried out. Test results performed on cores drilled from pavement after 270 days of concrete age showed that use of HVFA results in production of pavement concrete with improvements in: strength; moisture barrier qualities; and abrasive resistance characteristics. These improvements are brought about by the pozzolanic reaction of fly ash with the hydrates of cement that favorably changes the microstructure and interfacial transition zone in the resulting concrete.Use of high volume of fly ash in pavement concrete as partial replacement for cement is estimated to produce major energy and environmental gains and is a practice that is aimed at producing durable and sustainable concrete-based infrastructure. The use of HVFA concrete can significantly economize the construction of concrete pavements and improve the service life of transportation infrastructure.

Modeling of chloride diffusion in concrete containing low-calcium fly ash

Low-calcium fly ash (FL) is a general product from the combustion of anthracite and bituminous coals and has been widely used as a mineral admixture to produce high strength and high performance concrete. And in marine and coastal environments, penetration of chloride ions is one of the main mechanisms causing concrete reinforcement corrosion. In this paper, we proposed a numerical procedure to predict the chloride diffusion in low-calcium fly ash blended concrete. This numerical procedure includes two parts: a hydration model and a chloride diffusion model. The hydration model starts with mix proportions of low-calcium fly ash blended concrete and considers Portland cement hydration and low-calcium fly ash reaction respectively. By using the hydration model, the evolution of properties of low-calcium fly ash blended concrete is predicted as a function of curing age and these properties are adopted as input parameters for the chloride penetration model. Furthermore, based on the modeling of physicochemical processes of diffusion of chloride ion into concrete, the chloride distribution in low-calcium fly ash blended concrete is evaluated. The prediction results agree well with experiment results.

Micromechanical Properties of Calcium Silicate Hydrate

The indentation modulus of several cementitious materials is discussed with the assumption that the C-S-H gel is an aggregation of precipitated, colloidal-sized particles. At least two kinds of structurally distinct but compositional similar phases are found existent during the hydration process. In addition, the C-S-H originated from the pozzolanic reaction of fly ash is found to be the same to that of cement hydration in micromechanical properties; however, the C-S-H gel formed from the secondary hydration is inclined to develop into high density packing configuration, due to the limitation of reaction zone available.

Effect of Fly Ash on Resistance to Sulfate Attack of Cement-Based Materials

The resistance to sulfate attack of mortars containing 0%, 20%, and 40% of fly ash cured in 5 wt. % sodium sulfate solution at 20°C was investigated in this paper. Visual appearance, cracking analysis, velocity of ultrasonic wave and length change were applied to evaluate the sulfate resistance of mortars. The phases and microstructure of the reaction products due to sulfate attack were examined by XRD and SEM, and the pore structure of the mortars was analyzed by MIP. The effects of fly ash on the sulfate attack of mortars were analyzed. Results indicated that the addition of fly ash improved the resistance of sulfate attack significantly, this probably contributed to the pozzonlanic reaction of fly ash.

Role of additives in improved thermal activation of coal fly ash for alumina extraction

The extraction of alumina from coal fly ash is a good direction for its value-added utilization. The presence of the inert matters with high degree of polymerization, such as mullite and other aluminosilicates, makes the reactivity of coal fly ash very poor. The activation of coal fly ash is necessary before its utilization. The thermal activation calcination with the addition of NaOH and Na2CO3 was carried out in this research. The results showed that the addition of NaOH and Na2CO3 improved the alumina extraction evidently. The maximum alumina extraction reached ~60% when calcination at 600–900°C with the addition of NaOH and that could reach 82% at 900°C with the addition of Na2CO3. Detailed analysis and characterization was carried out by using thermal gravimetric and differential scanning calorimetric analysis (TG-DSC) and X-ray diffraction (XRD). The results indicated that NaOH and Na2CO3 facilitated the decomposition of the polymeric phases. At lower temperatures (<600°C), NaOH played a main role while Na2CO3 did at higher temperatures (> 700°C). As a result, the mixed additives containing NaOH and Na2CO3 made alumina extraction attain 95% at 700°C.

Quantitative study of the reactivity of fly ash in geopolymerization by FTIR

Fourier transform infrared (FTIR) spectroscopy has been applied to analyse the environments of Al–O and Si–O bonds in fly ash, which are used as raw materials of geopolymer synthesis. It is noted that the relative intensities of the bands at around 1000, 910 and 700 cm−1 are much higher in fly ash with higher reactivity, as reflected by the compressive strength of geopolymer. Deconvolution analysis of the band from 400 to 1400 cm−1 shows that the cumulative area of these three resolved bands, together with the band at ∼1090 cm−1, which is assigned to the asymmetric stretching of Si(Al)–O–Si, is proportional to the reactivity of fly ash. If it is assumed that the area of the resolved bands is proportional to the concentration of the corresponding bonds, a general indication is therefore that fly ash containing more reactive bonds will exhibit higher reactivity in geopolymerisation. FTIR spectroscopy in combination with particle size analysis provides a fast approach to predict the reactivity of fly ash, from the perspective of aluminosilicate glass chemistry.

An examination of the reactivity of fly ash in cementitious pore solutions

Fly ash is frequently used to replace cement in concrete, but it is difficult to predict performance based only on the oxide composition, which is typically the only compositional information available. In order to better utilize fly ash in concrete, it is important to develop more meaningful characterization methods and correlate these with performance. The research presented here uses a combination of analytical methods, including X-ray powder diffraction, scanning electron microscopy coupled with multispectral image analysis, and solution analysis to determine the compositions of the glassy phases in a specific fly ash and to examine the fly ash’s reactivity in late- and early-age cement pore solutions, ultrapure water, and sodium hydroxide. The dissolution of individual glassy phases in the fly ash was tracked over time and the precipitation of reaction products monitored. A high-calcium aluminosilicate glass was the most reactive, a low-calcium aluminosilicate glass was of intermediate reactivity and a medium-calcium aluminosilicate glass had the lowest reactivity in the solutions tested for a specific fly ash. This result suggests the glass composition has a strong effect on reactivity, but that that there is not a strict correlation between calcium content and glass reactivity.

A comparison between CO2 capturing capacities of fly ash based composites of MEA/DMA and DEA/DMA

Two classes of amine fly ash based composites (FAA13 and FAA23) are prepared and evaluated as CO2 adsorbents. FAA13 is prepared from primary and tertiary amines (MEA/DMA) while FAA23 (DEA/DMA) is prepared from secondary and tertiary amines. Composites were characterized by elemental analyses such as Scanning electron microscope (SEM-EDX), UV–Visible, Fourier Transform Infra Red (FT-IR), carbon, hydrogen, nitrogen (CHN) and Thermo-gravimetric Analysis (TGA). A temperature-programmed degradation study shows a much higher stability in comparison with other known simple amine solvents. Carbamate-amine is formed by the reaction of fly ash–amine composite with CO2. Results showed the maximum adsorptions of CO2 of 6.89 and 5.99mmol/g on FAA13 and FAA23 respectively. Kinetics of CO2 on the composites was described by the linear driving force (LDF) model. The diffusion coefficients and activation energies of CO2 in the composites ranged between 6.33×10−12±7×10−12cm2/s and 0.377±0.184kJ/mol respectively at atmospheric pressure and 306K. The prepared composites are thermally stable. A comparison of material cost and adsorption capacity showed that they are cheap and efficient for CO2 capturing.

Material Properties of Structurally Viable Alkali-Activated Fly Ash Concrete

AbstractAs the concern for the environment and need for sustainable construction practices continues to grow, development of portland cement–free binders is gaining wide interest in the concrete research and engineering community responsible for design and construction of civil infrastructure. In this work, microstructural (as determined by quantitative X-ray diffraction, scanning electron microscopy, and energy dispersive spectroscopy techniques) and material properties of alkali-activated fly ash concrete were evaluated to verify the material’s performance and structural viability. Results show that the heat-cured products of reaction of fly ash in an aqueous alkaline solution form a concrete binder with adequate design properties and promising durability aspects.

Chemical activation of calcium aluminate cement composites cured at elevated temperature

The influence of sodium sulfate, as an activator, on the hydration of calcium aluminate cement (CAC)–fly ash (FA)–silica fume (SF) composites was investigated. Different mixes of CAC with 20% pozzolans (20% FA, 20% SF and 10% FA + 10% SF) were prepared and hydrated at 38 °C for up to 28 days. The hydration products were investigated by XRD, DSC and SEM. The results showed that sodium sulfate accelerated the hydration reactions of calcium aluminate cement as well as the reactions of FA and SF with CAH10 and C2AH8 to form the strätlingite (C2ASH8). The later reactions prevent the strength loss by preventing the conversion of CAH10 and C2AH8 to the cubic C3AH6 phase. The acceleration effect of Na2SO4 on the reactivity of fly ash was more pronounced than on the reactivity of silica fume with respect to reaction with CAH10 and C2AH8 phases.

Strength and water permeability of concrete containing various types of fly ashes and filler material

Abstract In this study, the strength and water permeability of concretes containing various types of fly ashes and filler material were investigated. Five types of fly ashes with two levels of fineness and ground-river sand were used to partially replace Portland cement at the dosages of 20 and 40% by weight. Results indicated that pulverized coal combustion fly ash produced concrete with good strength and low permeability. Normal fluidized bed combustion fly ashes containing very irregular particles with pores produced concrete with low strength and relatively low permeability. Fly ash from fluidized bed combustion of coal with bio-mass materials and fly ash with high amount of CaO and SO3 produced poor concretes with low strength and high permeability. The increase in fly ash fineness helped improve both strength and permeability of concrete due to the increase in the reactivity of fly ash. Filler material could also be used to replace Portland cement at low replacement level of 20% with small effects on strength and water permeability.

Fly ash influences on sulfate-heave in lime-stabilised soils

The paper describes a study carried out to investigate how fly ash influences sulfate-heave in lime-stabilised soils. Five different clays having a range of total potential sulfate contents were tested. Quicklime was added at 3·0, 4·5 and 6·0% (by mass of dry soil) and two fly ashes with different characteristics were used at levels of 6, 12, 18 and 24%. A ground granulated blastfurnace slag was also considered for comparative purposes. Swelling tests followed the BS EN 13286-49:2004 accelerated (40°C) volumetric method. Mellowing period, exposure temperature, mineralogical development, unconfined compressive strength and porosity of the test mixes were also considered in studying fly ash effects on the process. Swelling generally increased with overall sulfate content in the lime-stabilised mixes and reduced with increasing fly ash level and coarseness. The mechanisms involved appeared to depend on the characteristics of the fly ash used. In the case of coarse fly ash, this was attributed to a mainly physical phenomenon associated with increased pore space in the system to accommodate expansive sulfate products forming, which contrasted with enhanced fly ash reactivity and increased strength of the lime-stabilised structure for fine material.

Experimental Investigation on Reaction Rate and Self-healing Ability in Fly Ash Blended Cement Mixtures

This paper investigates the hydration rate in fly ash blended cement paste and self-healing ability in mortar incorporating fly ash for long-term period. The hydration rate of fly ash and consumption of calcium hydroxide in fly ash paste containing calcium hydroxide reagents were examined at different ages and curing temperatures. Five types of fly ash blended cement paste, each of which with 10% by mass fly ash replacement ratio, were prepared for the acceleration test at 80&degC. Four fly ash cement pastes, with two types of fly ash and with fly ash replacement ratios by mass 10% and 30%, were tested to measure the rate of reaction of fly ash in the mixtures. Ten mixtures were tested to evaluate the self-healing ability of mortar incorporating fly ash, considering different types of cement and fly ash. Compressive strength, bending strength, accelerated carbonation depth, after applying freeze/thaw cycling until 60% and 80% relative dynamic modulus of elasticity, and porosity were examined. The experimental results revealed that incorporating fly ash in cement paste would affect the hydration rate of fly ash and consumption of calcium hydroxide. Curing temperature can affect hydration rate, acceleration rate and velocity of reaction rate in fly ash cement paste. Moreover, it is confirmed that self-healing ability, carbonation rate coefficient and the pore volume modification in mortar incorporating fly ash rely on the curing conditions. Finally, it is suggested that the practical fly ash replacement ratio in concrete could be 10% to 15% and 11% to 20% for water to cement ratios 0.50 to 0.55 and 0.55 to 0.60, respectively.

Hydration of Portland cement with high replacement by siliceous fly ash

The effects of two different low calcium fly ashes on the hydration of ordinary Portland cement (OPC) pastes containing 50wt.% of fly ash were investigated over a hydration time of 550days. The results were compared with a reference blend of OPC containing 50wt.% of inert quartz powder allowing the distinction between "filler effect" and pozzolanic reaction.Until 2days, no evidence of fly ash reaction was measured and its influence on the hydration is mainly related to the “filler effect”. From 7days on, the effects of the pozzolanic reaction were observed by the consumption of portlandite, the change of the pore solution chemistry, the formation of a presumably water-rich inner hydration product and the change of the C–S–H composition towards higher Al/Si ratio compared to the C–S–H of neat OPC. Additional strength due to the pozzolanic reaction developed after 28days of hydration.

Effects of colloidal nanoSiO2 on fly ash hydration

The influences of colloidal nanoSiO2 (CNS) addition on fly ash hydration and microstructure development of cement–fly ash pastes were investigated. The results revealed that fly ash hydration is accelerated by CNS at early age thus enhancing the early age strength of the materials. However, the pozzolanic reaction of fly ash at later age is significantly hindered due to the reduced CH content resulting from CNS hydration and the hindered cement hydration, as well as due to a layer of dense, low Ca/Si hydrate coating around fly ash particles. The results and discussions explain why the cementitious materials containing nanoSiO2 had a lower strength gain at later ages. Methods of mitigating the adverse effect of nanoSiO2 on cement/FA hydration at later ages were proposed.

Dilatometry of geopolymers as a means of selecting desirable fly ash sources

The use of dilatometry as a screening tool for the analysis of fly ash reactivity in geopolymers is presented and validated by testing six fly ash sources across a range of mix designs. In particular, a prediction of geopolymer mechanical performance can be obtained from the temperature at which the high-temperature expansion peak attributed to the release of strongly chemically-bound water is observed; high-strength samples generally display this expansion at a higher temperature than low-strength samples. Samples made either with a poorly reactive fly ash or an excessive SiO2 content in the activating solution also expand by up to 10–15% below 200°C, which indicates a low extent of crosslinking in the silicate gel phase. The second derivative of the dilatometric data can be helpful in identifying the onset temperature of the high-temperature expansion peak, as it is sometimes overlaid on a shrinkage phenomenon attributed to viscous sintering.

Development of eco-friendly binder using metakaolin-fly ash–lime-anhydrous gypsum

This paper presents the properties of a cementless mortar developed to produce eco-friendly building material without carbon dioxide emissions and with various beneficial effects. The binder used in this mortar was developed by the reaction of inorganic minerals fly ash (FA), lime (L) and activated kaolin clay, which is rich in silica and alumina. The Metakaolin (MK) was prepared by thermal activation of kaolin clay for 3h at 750°C. The ratio of FA:MK:L was 30:40:30 mass%. Anhydrous gypsum was added as a chemical activator in varying proportions of 0, 2.5, 5, 7.5 and 10% by weight of the binder. The mortar was prepared using a binder:sand ratio was of 1:1.5 mass% and a water/binder ratio of 0.50. The mortar specimens were cured at 20°C for 24h in their moulds and then demoulded. The demoulded mortar specimens were cured at room temperature up to 28days. Compressive strength and apparent density of the hardened mortar specimens were systematically measured. On the basis of the test results, it was concluded that the activator addition of 10% had an appreciable influence on the compressive and flexural strength. The compressive and flexural strength increased by 56% and 45% more than that of the control mortars at 28days of hydration. It was also concluded that the microstructure of the binder contain a cross link of hydration products and exhibits dense morphology.

ASSESSMENT OF PROPERTIES AND DURABILITY OF FLY ASH CONCRETE USED IN KOREAN NUCLEAR POWER PLANTS

Since the opening of the Shin-Kori #1,2 in 2005, fly ash mixed concrete has been used for NPP concrete structures under construction in Korea with the aim of preventing aging and improving durability. In this paper, the quality suitability of fly ash manufactured in Korea is assessed and the basic physical properties of fly ash mixed concrete and its durability against primary causes of aging are verified through experimental methods. Because of the internal structure filling effect from the pozzolanic reaction of fly ash and the resulting improvements in mechanical performance in such areas as strength and salt damage resistance, the durability of fly ash mixed concrete is shown to be superior. It is judged that this result can be applied in measures not only for improving the safety of NPP structures in operation in Korea but also for implementing effective structure life management should extending the life of structures be needed in the future.