Increase In Fly Ash Content Research Articles

Mechanical Performance of Concrete Made with Recycled Aggregates from Concrete Pavements

This research aims at analysing the mechanical performance of concrete with recycled aggregates from concrete pavements. First, the characteristics of various natural and recycled aggregates used in the concrete were thoroughly analysed. The composition of the recycled aggregates was determined and several physical and chemical tests of the aggregates were performed. In order to evaluate the mechanical performance of recycled concrete, cube compressive strength and flexural tensile strength tests were performed. The effect of recycled aggregates on the strength of recycled concrete is related to the strength of recycled aggregates, the strength of natural aggregates, and the strength of old concrete. The strength of recycled concrete decreases with increasing water‐cement ratio. However, due to the water absorption of the recycled aggregate, it has a certain inhibitory effect on the strength reduction. As the replacement rate of recycled aggregates increases, the optimal sand ratio decreases. The sand ratio is controlled between 32% and 38%, which is ideal for recycled concrete. With the increase of fly ash content, the 7 d strength of recycled concrete has decreased to some extent, but the 28 d strength has been slightly improved. In addition, for compressive strength and flexural tensile strength, the optimal content of fly ash is different.

Influence of Sulphate and Chlorides Acidic Media on Mechanical and Corrosion Behavior of Fly Ash Particulate Reinforced 2024 Al/TiO2 Composites

This research studied the corrosion behavior of 2024 Al/2%TiO2 with different additions of fly ash in 0.1N H2SO4 and 0.1 N HCl media. Using a liquid metallurgy route (stir casting technique) 0, 2, 4 and 6 wt.% of fly ash were added to 2024 Al/2%TiO2 and the density and corrosion potential of the samples was evaluated by potentiodynamic polarization. The highest corrosion current density (7.437 x 10-1) was obtained in 0.1 N HCl media while the corrosion current density in 0.1N H2SO4 media was 2.276 x 10−3. The corrosion current density values of 2024 Al/2%TiO2 alloy rose with increase in the fly ash addition from 0.305 x 10−2 to1.08 x 10−2 as a result of the micro galvanic coupling between the matrix alloy and the reinforcement or conducting interfacial products. Micro-Vickers hardness of the composite increased from 120 V to 160 V with increasing fly ash content. The values of yield strength increased from (231MPa) to (279 MPa) and the values of ultimate tensile strength (UTS) were increased from (136MPa) to (184MPa) with increase in the percentage of fly ash.

Short‐Term Gradation Loading Creep Properties and Failure Characteristics of High‐Strength Fly Ash Concrete for Underground Engineering

In order to study the short‐term creep deformation of high‐strength concrete with varying fly ash replacement ratios, concrete samples with 0, 20, 35, and 50 wt% fly ash were tested using an electrohydraulic servocontrolled creep testing system and characterized using scanning electron microscopy after fracturing. Three different creep deformation behaviors were observed over time under different stress levels, namely, decelerating, isokinetic, and accelerating creep, where the creep rate increased with increasing stress. Failure of the samples occurred once isokinetic creep was achieved. The peak stress of the concrete samples exhibited a parabolic trend with increasing fly ash content, where the peak stress in the 0, 20, 35, and 50 wt% samples during short‐term gradation loading creep testing was 13.08%, 7.94%, 15.14%, and 14.50% lower, respectively, than the peak stress measured in conventional uniaxial compression testing. The accumulated creep of the samples was reported and can be used as a reference for future studies on the long‐term creep characteristics of concrete. The macro‐ and microscopic failure modes of the fly ash concrete during short‐term gradation loading creep under uniaxial compression were brittle cleavage fracturing.

Strength and Microstructure of Class-C Fly Ash and GGBS Blend Geopolymer Activated in NaOH & NaOH + Na2SiO3.

In this paper, class-C fly ash (FA) and ground granulated blast-furnace slag (GGBS)-based geopolymer activated in NaOH and NaOH + Na2SiO3 was studied regarding setting time, compressive strength, porosity, microstructure, and formation of crystalline phases. When comparing the effects of alkali type on the FA and GGBS geopolymer composites, results revealed that NaOH has a lesser effect in developing strength and denser microstructure than does NaOH + Na2SiO3, since the addition of Na2SiO3 provides the silica source to develop more compact structure. Incorporation of Na2SiO3 reduced the crystallinity and the paste was more amorphous compared to NaOH activated pastes. The class-C FA and GGBS blends resulted in prolonged setting time, reduced strength, and loose matrix with the increase in fly ash content. The un-reactivity of calcium in blends was observed with increasing fly ash content, leading to strength loss. It is evident from XRD patterns that calcium in fly ash did not contribute in forming C-S-H bond, but formation of crystalline calcite was observed. Furthermore, XRD analyses revealed that the reduction in fly ash leads to the reduction in crystallinity, and SEM micrographs showed the unreactive fly ash particles, which hinder the formation of a denser matrix.

Strength Characteristics of Sand Amended with Two Waste Materials - Fly Ash and Scrap Tyre

The effectiveness of the inclusion of tyre buffings, which is a scrap tyre derived material, on the strength characteristics of sand-fly ash mixtures has been investigated. At first, untreated sand, sand-fly ash, and sand-fly ash-tyre buffing specimens were subjected to compaction tests followed by unconfined compression tests. Considering compaction and unconfined compressive strength characteristics, triaxial compression tests were carried out on different sand-fly ash and tyre buffing added sand-fly ash mixtures. Sand-fly ash mixtures were modified by mixing with tyre buffing contents of 5% and 10%. Test results indicated that the increase in fly ash content in sand-fly ash mixtures improves the compressive strength. Further addition of tyre buffings to sand-fly ash mixtures decreases the compressive strength, but the strength is greater than that of the untreated sand. At 35% and 50% fly ash content, the shear strength of sand-fly ash-tyre buffing mixtures is found to be more than that of the sand-fly ash mixtures, especially at higher confining pressures. The stiffness of the sand-fly ash mixtures is affected by the inclusion of tyre buffings to the mixtures. The curing period also improves the strength of different sand mixtures. It is found that sand-fly ash-tyre buffing mixture containing 35% or 50% fly ash and 5% or 10% tyre buffing content may be used as a lightweight composite material in the construction of embankment for road and dam or as backfill material behind retaining walls. Thus, environmental problems associated with the disposal of fly ash and scrap tyres are also reduced.

Optical Sensing of pH and O2 in the Evaluation of Bioactive Self-Healing Cement.

Leakage from cementitious structures with a retaining function can have devastating environmental consequences. Leaks can originate from cracks within the hardened cementitious material that is supposed to seal the structure off from the surrounding environment. Bioactive self-healing concretes containing bacteria capable of microbially inducing CaCO3 precipitation have been suggested to mitigate the healing of such cracks before leaking occurs. An important parameter determining the biocompatibility of concretes and cements is the pH environment. Therefore, a novel ratiometric pH optode imaging system based on an inexpensive single-lens reflex (SLR) camera was used to characterize the pH of porewater within cracks of submerged hydrated oil and gas well cement. This enabled the imaging of pH with a spatial distribution in high resolution (50 μm per pixel) and a gradient of 1.4 pH units per 1 mm. The effect of fly ash substitution and hydration time on the pH of the cement surface was evaluated by this approach. The results show that pH is significantly reduced from pH >11 to below 10 with increasing fly ash content as well as hydration time. The assessment of bioactivity in the cement was evaluated by introducing superabsorbent polymers with encapsulated Bacillus alkalinitrilicus endospores into the cracks. The bacterial activity was measured using oxygen optodes, which showed the highest bacterial activity with increasing amounts of fly ash substitution in the cement, correlating with the decrease in the pH. Overall, our results demonstrate that the pH of well cements can be reliably measured and modified to sustain the microbial activity.

Influence of environmental factors on resistivity of concrete with corroded steel bar

The paper presents the experimental results of the influence of relative humidity (RH), temperature, fly ash content and chloride concentration on the relationship between concrete resistivity and the corrosion level of steel bar in concrete. The results show that the concrete resistivity decreases with the increase of corrosion level of steel bar under the same RH and temperature. The concrete resistivity decreases with the increase of RH and temperature, and the relationship between concrete resistivity and RH (temperature) is established. The concrete resistivity increases with the increase of fly ash content when the concrete cover thickness and the corrosion level of steel bar are the same, while it decreases with the increase of chloride concentration. The relationship between concrete resistivity and the corrosion level of steel bar in concrete with different chloride concentration (fly ash content) is established.

Effects of fly ash on the mechanical and impact properties of recycled aggregate concrete after exposure to high temperature

Experiments were conducted on 180 specimens to investigate the mechanical and impact properties of recycled aggregates (RAs) concrete with different fly ash (FA) contents after exposure to temperature at 20 °C (room temperature), 200 °C, 400 °C, 600 °C and 800 °C. The compressive strength, elastic modules, peak strain, ultimate strain, Poisson’s ratio and recovering effect of air curing time on the tested specimens were analysed. The impact numbers and impact energy under the repeated drop-weight test were obtained. Test results showed that the numbers of blows causing the first and final crack decreased gradually with the increasing temperature and FA contents, and the temperature of 200 °C had a great influence on the impact resistance of RA concrete. The weight loss and compressive strength of the specimens suffering from high temperature alone decreased with the increasing FA content. The compressive strength of unheated specimens for 118 days curing was about 0.045–0.196% higher compared to that for only 28 days curing. The Poisson’s ratio of RAs concrete at 400 °C was less than half of that at 20 °C. Results of this study provided certain theoretical basis for research on fire-resistant design of concrete structures.

Tribo‐performance assessment of milled fly ash brake friction composites

AbstractThe milled fly ash (MFA) was attained by mechanical ball milling of raw fly ash (RFA) and was characterized for changes in particles morphology, size, and specific surface area (SSA). The RFA and MFA, varying in wt% from 20 to 35, were exclusively used for composite fabrication using compression molding machine. The milled fly ash composite (MFC) specimens were assessed for the tribo‐performance in comparison with raw fly ash composite (RFC) and commercial truck brake composite (CTBC) specimens under the loads ranging from 125 to 200 N at 3.3 and 5 m/s of sliding velocity. The characterization study showed that the glassy and spherical RFA particles of average size 86 μm changed to rough and irregular size of 15 μm, after ball milling. The SSA per unit bulk volume was found to increase from 138.2 to 1009 m2/m3. The tribo‐study revealed that the fly ash milling significantly improved the friction coefficient of MFC specimens, especially at 35 wt% of MFA, as compared to RFC and CTBC specimens. The increase in fly ash content (raw as well as milled) to 35 wt% greatly increased the specific wear rate (SWR) at both the sliding velocities. The worn surfaces of RFC specimens were found to be more damaged than the MFC specimens as revealed from SEM results. The appreciable frictional characteristics of MFC specimens encourage the use of MFA in brake friction composites; however, the changes in the friction formulation are required from SWR point of view.

Correlation between Compressive Strengths and Water Absorption of Fly Ash Cement Mortar Immersed in Water

AbstractThe compressive strength and water absorption of cement mortars with different water-binder ratio (0.35, 0.45 and 0.55) and fly ash content (0, 10%, 20% and 30%) under water immersion were investigated, and the correlation between them was further analyzed. The internal microstructure and phase composition of mortar was studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The results show that the inside of mortar mixed with fly ash displayed the loose and porous microstructure. Therefore, the incorporation of fly ash reduced the compressive strength of mortar, especially the early strength, and the strength decreased with the increase of fly ash content, and the water absorption of mortar also increased. There was a linear correlation between the compressive strength and water absorption of mortar with the equation: fc = −3.838β + 62.332, where fc and β represented the compressive strength and water absorption, respectively. Therefore, when the water absorption of mortar immersed in water was measured, its corresponding compressive strength could be preliminarily inferred through this equation, which was of great significance for detecting and identifying the stability and safety of hydraulic structures.

Hydration and Strength Evolution of Ternary-Blend High-Volume Fly Ash Concretes

The increase of carbon emissions due to the annual growth of portland cement (PC) production has promoted research into the development of sustainable green concrete using a range of readily available industrial waste materials. The present study is focused on developing two high-volume fly ash (HVFA) concretes with cement replacement levels of 65% (HVFA-65) and 80% (HVFA-80). The required lime for both HVFA concrete mixtures was initially determined and the optimized mixture designs identified, based on the 28-day compressive strength, by varying the low-calcium Class F fly ash-hydrated lime composition. The optimized concrete mixtures achieved a compressive strength of 53 and 40 MPa (7.69 and 5.80 ksi) for HVFA-65 and HVFA-80 concretes, respectively. The early-stage strength development is dependent on the matrix produced in the specific HVFA concrete, which is itself dependent on the number of unreacted fly ash spheres. The increase of fly ash and hydrated lime dosage in HVFA concrete increases the rate of hydration of the C3A and C4AF phases, but decreases the hydration of the C3S phase, which resulted in lower early-age strength development than occurs in PC concrete. It was noted that the initial setting time of HVFA concretes increase with an increase of fly ash content. However, addition of hydrated lime accelerates the hydration and decreases the final setting time for HVFA concretes.

Properties of concrete containing fly ash and temperature control measures used during construction

In recent years, fly ash cement has been applied in engineering with the continuous application of concrete admixtures. Although we have been using fly ash concrete and have done a lot of experimental research, but the understanding of its thermophysical properties was not complete, and further research was needed. This paper presented results of a study into important fundamental thermal and physical properties of both fly ash mortar and fly ash concrete. Different replacement percentage of OPC by FA ranging from 0 to 60% by mass were devised. Increasing fly ash content was found to delay the initial setting time and final setting times, decrease both compressive and flexural strengths and reduce hydration heat. It also can effectively reduce the hydration heat evolution of cement-fly ash binder. The adiabatic temperature rise of cement fly ash material in engineering can be estimated by reduction coefficient k. In fly ash concrete tests, thermal properties, including thermal diffusivity, conductivity, specific heat, ultimate tensile strain and static elasticity modulus were also measured and reported. There also appeared the relationship between compressive strength and flexural strength showed similar linear change with the replacement rate of fly ash rising from 30% to 60%. Most importantly, pipe cooling tests of fly ash concrete were carried out and from the perspective of temperature control of mass concrete, dynamic pipe cooling measures were investigated.

Experimental analysis on the relationship between pore structure and capillary water absorption characteristics of cement‐based materials

AbstractCapillary absorption capacity has an important influence on the durability of concrete and is closely related to pore structure. In this study, the pore structure and capillary water absorption of samples were determined by low‐field nuclear magnetic resonance spectroscopy and the gravimetric method, respectively. The test results show that the most probable pore diameter and equivalent pore diameter of cement‐based materials increase with increasing water to cement ratio (w/c) and fly ash (FA) content and decrease with increasing curing age and cement to sand ratio (c/s). The porosity of cement‐based materials increases with increasing w/c and c/s, decreases with increasing curing age, and first decreases and then increases with increasing FA content. The curves for the amount of capillary water absorption per unit area in specimens with the square root of time for cement‐based material under different influence factors are similar and show obvious linear and stable stages. The capillary absorption coefficient is nearly linearly related to w/c or c/s, and nonlinearly related to curing age or FA content, observation that are well correlated with the porosity and square root of the equivalent pore radius. A modified model is proposed for the capillary absorption coefficient with consideration of the pore structure, and the theoretical results of the model are in good agreement with the test results.

Hydration Behavior of Magnesium Oxysulfate Cement with Fly Ash via Electrochemical Impedance Spectroscopy

A new type of material, magnesium oxysulfate cement (MOSC), has attracted widespread attention as an environmentally friendly and low-energy inorganic green cementitious material due to its outstanding performance. In this study, electrochemical impedance spectroscopy (EIS) and in situ X-ray diffraction (XRD) were used to characterize the hydration behavior and process of MOSC. EIS was also used to investigate the effect of fly ash (FA) on the hydration behavior of MOSC. Mercury intrusion porosimetry (MIP), backscattered scanning electron microscopy (BSE), and scanning electron microscopy (SEM) were used to study the evolution of the pore structure and micromorphology of MOSC incorporated with FA. The results show that the resistance values (R1) in the equivalent circuit for MOSC increase with increasing curing time. The incorporation of FA can affect the resistance values for each element in the equivalent circuit. In the early stages of hydration (1 day and 2 days), high-volume FA can reduce the degree of hydration of MOSC paste. At the later stages of hydration, the resistance values (R1) increase with increasing FA content. The MIP, BSE, and SEM results also show that the porosity of MOSC decreases with increasing FA content; the incorporation of FA can significantly improve the morphology of MOSC, resulting in a denser microstructure. This study shows that the electrochemical impedance spectroscopy is a powerful technique to investigate the hydration behavior and process of MOSC, and the results also indicate that FA is a good mineral admixture used as a partial substitute for MOSC cementitious materials.

Swell and compressibility of fly ash−clay blends in lumps and powders: effect of 4% lime

This paper describes the influence of fly ash on the swell-compressibility characteristics of an expansive clay passing through a 4·75 mm sieve. The free-swell index (FSI), heave, swell potential, swelling pressure, coefficient of compressibility (av), the compression index (Cc) and linear shrinkage (LS) of fly ash–clay blends were studied. Fly ash content was varied from 0, 5, 10, 15 to 20%. Swelling and compressibility of the clay decreased with increasing fly ash content. Heave was the lowest at 15% fly ash. The paper compares the effect of 15% fly ash on the swell-compressibility behaviour of clay lumps passing a 4·75 mm sieve and clay powder passing a 425 μm sieve. While the swell potential was found to be higher for clay powder than for clay lumps at 15% fly ash, the swelling pressure was higher for clay lumps. As 4% lime resulted in the highest reduction of the FSI, a series of swell-consolidation tests and a series of FSI tests were conducted on fly ash–clay blends to which 4% lime was added. FSI, heave and swelling pressure decreased on the addition of fly ash. They further decreased when 4% lime was added. The parameters av, Ccand LS also reflected a similar trend.

Study on Fire Resistance Ability and Mechanical Properties of Composites Based on Epikote 240 Epoxy Resin and Thermoelectric Fly Ash: An Ecofriendly Additive

In this study, fly ash was tested as a filler in epoxy with concentrations of 5, 10, and 20 wt.%. Fly ash particles were modified by chemical treatments (using NaOH and HCl) to enhance the compatibility and adhesion, making mechanical properties and flame retardancy of materials better. Flexural strength, tensile strength, and impact resistance decrease as fly ash content increases. The compressive strength is further increased by the addition of fly ash (compressive strength of the materials including 5, 10, 20 wt.% of fly ash modified with NaOH is 176.01, 189.90, and 197.07 MPa, respectively). The interface between fly ash and epoxy matrix plays an important role in determining the mechanical strength and flame retardancy of synthetic materials. The results of UL-94HB and LOI test method for composite materials including 20 wt.% fly ash (modified by NaOH) reached 13.45 mm/min and 22.4%, respectively. These results showed that fly ash is an efficient additive as a flame retardant which decreases the amounts of additives in products and improves their efficiency. Fly ash was also dispersed into epoxy resin to enhance its resistance to oxidation.

Relationship between Density and Strength for High-volume Fly Ash Lightweight Porcelanite Concrete

In concrete construction, there are many advantage in reducing total load on the structure. One of the methods to reduce the load is using lightweight concrete, LWC. The concrete production is not sustainable due to its consuming a lot of Portland cement, which is responsible for the emission of greenhouse gases that cause global warming. One of the effective methods to reduce the impact of negative environmental concrete industry is using by product materials such as fly ash as a partial replacement by weight of cement. In the present study, the ability of producing high-volume fly ash structural lightweight aggregate concrete was verified. This concrete was produced by using Porcelanite aggregate and a large amount of type F fly ash as partial replacement by weight of cement, 50, 60 and 70 %. The studied characteristics are: density, water absorption, compressive and splitting tensile strengths at different ages. Lightweight concretes containing high-volume fly ash, 50 and 60 % by weight of cement, have proven themselves as structural lightweight concrete with compressive strength fc, more than 17 MPa at 28 days age. The results in this study showed that increasing fly ash content up to 70 % will decrease the compressive strength but on the other hand it will significantly reduce density and increasing water absorption.

Effect of environmental aging conditions on the properties of fly ash filled epoxy composites

The effect of environmental aging conditions on the structural and mechanical properties of fly ash (FA) filled epoxy composites with four different volume fraction of FA (i.e. 0.1, 0.2, 0.3 and 0.4) has been investigated under four aggressive aging environments and aging periods. The crystallite size and microstrain of crystalline particle in the composite matrix is quantified from XRD analysis. The 10% FA filled epoxy composite under fuel solution retains the higher order of crystallinity and glass transition temperature. It is observed that the void content and actual density of composites decreases with increasing FA content. The mechanical behavior of FA epoxy composite deteriorated with increasing aging duration and reinforcement percentage. 10% FA-containing specimen gives the highest mechanical properties under fuel environment for any given aging duration. The 20% FA filled epoxy composite can also be used in low load bearing applications.

Effect of nano silica on compressive strength and microstructures of high volume blast furnace slag and high volume blast furnace slag-fly ash blended pastes

This paper presents the effect of nano silica (NS) on compressive strength and microstructure of cement paste containing high volume blast furnace slag (BFS) and high volume BFS-fly ash (FA) blend as partial replacement of ordinary Portland cement (OPC). Results show that high volume BFS pastes containing 60% and 70% BFS exhibited 4% higher and comparable compressive strength, respectively than control cement paste. However, about 9% and 37% reduction in compressive strength in high volume BFS pastes is observed due to use of 80% and 90% BFS, respectively. The high volume BFS-fly ash pastes containing total BFS and FA content of 60% exhibited about 5%–16% higher compressive strength than control cement paste. However, significant reduction in compressive strength is observed in higher BFS-FA blends with increasing fly ash contents. Results also show that the addition of 1–4% NS improves the compressive strength of high volume BFS paste containing 70% slag by about 9–24%. However, at higher BFS contents of 80% and 90% this improvement is even higher e.g. 11–29% and 17–41%, respectively. The NS addition also significantly improves the compressive strength of high volume BFS-FA pastes. The thermogravimetric analysis (TGA) results confirm the reduction of calcium hydroxide (CH) in high volume BFS and combined BFS-FA pastes containing NS indicating the formation of additional calcium silicate hydrate (CSH) gels in the system. The addition of NS also significantly reduced the pore volume of high volume BFS and combined BFS-FA pastes. The X-ray diffraction (XRD) analysis also confirms the reduction of intensity of CH peaks indicating its consumption in pozzolanic reaction. The scanning electron microscopy (SEM) images also confirm denser microstructure of high volume BFS and combined BFS-FA pastes containing NS compared to those without NS. By combining slag, fly ash and NS in high volumes e.g. 70–80%, the carbon footprint of cement paste is reduced significantly while maintains the similar compressive strength of control cement paste.

The Behavioural Framework of a Lightly Cemented High Plasticity Clay Under Low Effective Stresses

This paper presents the findings of an experimental study on the primary yielding and failure mechanisms of an artificially prepared lightly cemented high plasticity clay, using fly ash as the cementation binder and on the influence of its presence in the soil matrix. The study was conducted for a series of triaxial isotropically consolidated compression and extension tests in the low stress range. Studies on clays with low effective stresses at yield and failure are limited, mostly because of the experimental difficulties. The results showed that the difference between the clay samples, with or without the addition of fly ash, was the shape of the yield locus, resulting in a flattening of the primary yield locus of the fly ash treated samples compared to the wide arch shape of primary yield locus of the untreated sample. Experimental results showed that the yield locus expanded with the presence or increase of fly ash content and this was due to kinematic softening induced by the breakage of inter-aggregate bonds or destructuration. To corroborate this interpretation, the microstructure of untreated and treated mixtures was investigated by polarised light microscopy analysis. Testing also revealed that the mode of failure for the fly ash treated clay is different in compression (shear failure) and extension (detachment of bonds). Shear resistance in extension was found to be less dependent on the normal stress and was mostly due to the cementation bonds.