High Fly Ash Content Research Articles

MORPHOLOGICAL AND PROPERTIES OF PORTLAND-COMPOSITE CEMENTS WITH CLASS C FLY ASH

Portland-composite cements (PCC) were prepared with partial substitution of 5 to 25% of class C fly ash, obtained from East Java steam power plant. Properties of the composite cements was evaluated, through the setting time (initial and final) and compressive strength (3, 7, and 28 days) compared with control PCC. Setting time is retarded in Portland-composite cements with higher fly ash content. The retardation is highest in PCC with substitution of 25% fly ash. Lower compressive strength is obtained after 3 and 7 days of curing for PCC with 5-25% fly ash substitution in comparison with control cement, since pozzolanic reaction still did not show its effect. After 28 days, compressive strength was higher than that for control cement because pozzolanic reaction show its effect, highlighted by compressive strength increase of PCC substituted by 5% and 10% fly ash, 16.48 MPa and 16.52 MPa, respectively. This attributed to the differences in the pozzolanic activity of the applied fly ash. The compressive strength increase was explained by Scanning Electron Microscope (SEM), which shows that PCC with substitution of 10% fly ash had more Tobermorite presented compared to control cement.

Improving flotation decarbonization efficiency of coal gasification fly ash by mechanically breaking pore: An experimental and molecular dynamics simulation study

The high unburned carbon content of solid waste coal gasification fly ash is the main reason that it cannot be used in the construction material industry, which leads to serious waste of resources and environmental pollution. Currently, decarburization flotation has been a dominating method for coal gasification fly ash resource utilization. However, the flotation process has low efficiency and high reagent consumption because of the rich surface porosity of unburned carbon. Herein, we innovatively developed a mechanical grinding strategy to break the pore distribution on the surface of unburned carbon, significantly reducing the pore size, increasing hydrophobic functional groups and improving the flotation efficiency. The removal rate of unburned carbon is more than 95% in flotation tailings with a mechanical grinding intensity of 120 rpm and grinding time of 30 min. Furthermore, the adsorption of water on porous unburned carbon surface was studied by molecular dynamics simulation for the first time, and the results show that decreasing the particle surface porosity of unburned carbon can accelerate the diffusion rate of water molecules on the unburned carbon surface, improve the wettability of the particle surface and increase the adhesion probability between the unburned carbon particles and bubbles and remarkably improve the its floatability. This work is the first application of mechanically breaking pore to the flotation separation of unburned carbon and ash from coal gasification fly ash and provides valuable inspiration for the design of porous particles flotation, not limited to the separation of gasification fly ash.

Recycling Bio-Based Wastes into Road-Base Binder: Mechanical, Leaching, and Radiological Implications

This work presents a physical, mechanical, durability, leaching, and radiological assay of three wastes (egg and scallop shells and olive pomace ash) as road-base binders. Two different waste/Portland-cement ratios (7.5/92.5 and 80/20) were studied. Density and compressive strength decreased when different wastes were added in every proportion. Additions of 7.5% of both shells reduce the density to about 2.5% and the compressive strength to 20%, while 80% reduces the density to 20% and the compressive strength to 90%, while the addition of biomass fly ash decreases the density and compressive strength in a higher proportion than shells. The durability against acid attack is increased when the three wastes are used, and this increase is higher when the waste dosage is increased (up to 15 times more when 80% biomass ash is used). With respect to leaching, scallop and eggshells can be used as a component of hydraulic road binder, but olive pomace ash presents leaching values higher than the limits of different regulations (Se, Pb, Ni, Mo, Cu, and As). From a radiological perspective, all road-base binders present an activity concentration index lower than 1, except when olive pomace ash was used, and the binders showed higher values of 40K due to the high potassium content of fly ash.

Mechanical Performance and Acoustic Emission Characteristics during Fracture of Strain-Hardening Cementitious Composites with High-Content Fly Ash

Mechanical Performance and Acoustic Emission Characteristics during Fracture of Strain-Hardening Cementitious Composites with High-Content Fly Ash

The Effects of Supplementary Cementitious Materials (SCMs) on the Residual Mechanical Properties of Concrete after Exposure to High Temperatures—Review

Fire events remain a severe hazard despite significant progress in the construction industry. The high-temperature resistance of concrete structures highly depends on the properties of their ingredients. The present study provides an analytical review of the available experimental studies investigating the influence of high temperatures on the residual mechanical characteristics of concrete, namely compressive and tensile strength, as well as the modulus of elasticity. This paper primarily focuses on the influences of supplementary cementing materials, including silica fume (SF), fly ash (FA), and ground granulated blast furnace slag (GGBFS), as well as different aggregate types. The literature findings show that the mechanical characteristics of concrete are degraded dramatically due to heat-caused changes in concrete. In addition, the mechanical properties of concrete are highly influenced by the types of aggregate and the amount of supplementary cementing materials at both high temperatures and room temperature. The inclusion of FA and GGBFS improves the high-temperature resistance of concrete. However, high contents of FA and GGBFS result in adverse effects on the properties of concrete. This review will provide a basis for future studies and enhance the knowledge of the behaviour of reinforced concrete structures subjected to high temperatures.

Four-point bending tests of ECC: Mechanical response and toughness evaluation

This work investigated the mechanical response of engineered cementitious composites (ECC) with high content fly ash by four-point bending tests, and a suitable method was recommended for the flexural toughness evaluation of ECC. The test variables were fiber volume fraction (for polyvinyl alcohol (PVA) fiber), fiber type (PVA and polyethylene (PE)), curing age (56d, 28d and 7d), water-to-binder ratio, sand-to-binder ratio and fly ash content. The results of the tests showed that the crack pattern, deflection-hardening behavior and energy dissipation capacity of ECC were mainly affected by fiber volume fraction, fiber type and curing age. The larger the proportion of fiber pulling out during crack bridging was, the higher the deflection and energy dissipation of ECC exhibited. The commonly used JSCE-SF4 method, ASTM C1609 method and post crack strength (PCS) method for evaluating the flexural toughness of fiber reinforced concrete were not applicable to evaluate the toughness of ECC, because the significant deflection-hardening behavior of ECC could not be considered. An improved method was proposed for flexural toughness evaluation of ECC materials, which included two modified indexes (equivalent initial flexural and post-peak flexural toughness index) and a new index (initial energy factor). The large deformation of ECC was taken into account and the determining the first crack point was eliminated in this new method.

The Transformation and Leachability of Fly Ash/Cement Waste Forms Subjected to the Simultaneous Effect of Heat and Chemistry

The utilization of cement waste forms with high content of fly ash is a potential method in large-volume immobilization of low- and intermedium-level radioactive waste at near-surface. However, the migration behavior of radionuclides in fly ash/cement waste forms under geological environment is still unclear. This study researched the transformation of products, pore structure as well as the leachability of Ca2+, Na+, and simulated radioactive nuclides (Sr2+ and Cs+) in fly ash/cement waste forms under simultaneous effect through X-ray diffractometer (XRD), mercury intrusion porosimetry (MIP), and inductively coupled plasma optical emission spectroscopy (ICP-OES). Furthermore, the relationship between the decalcification and the leaching of Sr2+ and Cs+ was established by the fitting curves of apparent diffusion coefficient. The results indicated that the products of fly ash/cement waste forms were kept stable under simultaneous effect and the pore structure has excellent resistance to temperature. The increasing salt concentrations of leachant and temperature promoted the leaching of radionuclides while the protection layer formed at 60–80°C inhibited this phenomenon. The relationship of apparent diffusion coefficient between Sr2+ and Ca2+ was quadratic nonlinear, while the relationship between Cs+ and Ca2+ showed a linear relationship. Compared to cement waste forms, fly ash/cement waste forms were more stable. In addition, the long-term retention ability of fly ash/cement waste forms to Cs+ was more outstanding than Sr2+.

Study on macro-meso mechanical properties of cemented tailings backfill with high fly ash content.

The use of cement and fly ash (FA) to prepare cemented tailings backfill (CTB) lowers backfill mining costs while also reducing pollution caused by the accumulation of waste materials like tailings and FA, making it a green backfill mining process. While adding FA to CTB may reduce costs, too much FA might weaken CTB's strength property. Mechanical tests were used to explore the effects of FA content and curing time on the uniaxial compressive strength (UCS) and deformation modulus of CTB in this research. The effects of FA content on failure modes, strain energy, and crack evolution of CTB were studied using a numerical model that considered FA content and particle contact mode. The influence mechanism of different FA contents on CTB was also revealed at the microscopic level. The results demonstrate that the UCS of CTB has a quadratic polynomial and a linear relationship with FA content and curing time respectively, and that the elasticity modulus and secant modulus of CTB increase and then decrease with FA content under different curing times. The peak strain energy of CTB increases and subsequently declines with the FA content, and crack propagation inside CTB may be limited by regulating the FA content. A reasonable content of FA can optimize the size and distribution of CTB microscopic defects, enabling them to exhibit superior strength property. This study systematically explores the mechanism of different FA contents on the strength property of CTB from a macro-micro perspective, providing an essential reference value for improving the recycling of FA and waste residues such as tailings.

Development of building materials based on a high content of fly ash and polycondensation products from Chinese heat and power plants

Abstract The addition of fly ash from thermal power plants (TPP) and chemical additives, such as polycondensation products of acetone and formaldehyde, is an effective and economical method for increasing the strength and durability of building materials, in particular concrete. Fly ash added to the concrete and mortars at 10%–15% does not reduce their technical properties, while polycondensation products of acetone and formaldehyde plasticize and accelerate the hardening process of concrete. The study aims to substantiate the possibility of obtaining concrete on dense aggregates with a high content of fly ash through the use of polycondensation products of acetone and formaldehyde as additives, which are highly soluble in water and polyfunctional. The strength indicators were determined using standard methods. The study has shown quantitative changes in the properties of the fly ash-concrete mixture, improvement of its physical and technical characteristics, and durability of the fly ash concrete with the addition of polycondensation products of acetone and formaldehyde. In the present study, the authors determine the effect of fly ash addition on the properties of a concrete mixture, as well as the maximum possible content of fly ash that can be added to concrete to maximize certain properties required of the mixture; additionally, the same action is conducted with the addition of polycondensation products of acetone and formaldehyde concomitant with the addition of fly ash.

Durability of concrete containing gypsum powder recycled from waste drywalls combined with high content of fly ash as supplementary cementing materials

To address the negative contribution of cement production on global carbon emissions, this study aims to reduce the cement content typically required in concrete by using partial replacement by gypsum powder recycled from waste drywalls (15%) and fly ash (50%), by weight. The durability of concrete cylinders made of the mixture was evaluated by testing the cylinders in compression after exposure to various environmental conditions for 1000, 3000, and 5000 h. A total of 45 specimens under 5 exposure conditions were considered. The conditions included dry, submerged in fresh water, submerged in seawater, and two groups rotated weekly between dry and submerged in either fresh water or seawater. Overall, specimens in both freshwater and seawater conditions after 5000 h showed strength higher than control specimens. Results indicate that the strength of the concrete specimens containing recycled gypsum powder and fly ash was not adversely affected by exposure to the conditions.

Experimental Study on Strength Development and Engineering Performance of Coal-Based Solid Waste Paste Filling Material

To explore the strength development characteristics and engineering performance of different coal-based solid waste filling materials cemented into filling body, coal gangue was used as coarse material, fly ash, desulfurization gypsum, gasification slag, and furnace bottom slag as fine material, and cement as a gelling agent. The uniaxial compressive strength (UCS) and bleeding rate of coal-based solid waste cemented backfill (CBSWCB) were tested by an orthogonal experiment, and the influencing factors of mechanical properties and strength development were analyzed. The multiple generalized linear model of strength and bleeding rate was established, and the optimal filling material ratio was determined. The engineering performance index of CBSWCB with the optimal ratio was tested. The results show the following points: (1) the concentration and content of desulfurization gypsum had a great influence on the early compressive strength of CBSWCB, while fly ash, gasification slag, and furnace bottom slag had little influence on the early compressive strength. (2) High concentration, high content of fly ash and furnace bottom slag, low content of desulfurization gypsum, and gasification slag can significantly improve the early strength. High concentration and high content of fly ash, low content of gasification slag, furnace bottom slag, and desulfurization gypsum are beneficial to the later strength increase. (3) Under the optimal ratio scheme, the bleeding rate of CBSWCB was 1.6%, the slump was 16.6 cm, the cohesion was general, the segregation resistance was good, the initial setting time was 5.42 h, the final setting time was 7 h, and the early strength after curing for 8 h reached 0.24 MPa.

Influence of ion chelator on pore structure, water transport and crack-healing properties of cement pastes incorporating high-volume fly ash and blast-furnace slag

This paper focuses on the influence of ion chelator (CA) on pore structure, water transport and self-healing ability of cement pastes incorporating high content fly ash (FA) and blast-furnace slag (BFS). The pore structure of pastes was measured by low-field nuclear magnetic resonance (LF-NMR). The water transport property was tested by water absorption test. The self-healing ability was assessed by water permeability and crack healing test. Results showed that the compressive strength of hardened pastes was promoted by CA, the increase rates in compressive strength of pastes were 21.9% (50%BFS), 15.7% (pure cement), 9.9% (30%FA) and 8.8% (16%FA24%BFS). CA decreased the proportion of small capillary pores (0.01–0.1 μm) in cement pastes which were more pronounced for the water absorption, the reduction rates of small capillary pores were 49.0% (50%BFS), 41.5% (pure cement), 20.2% (30%FA) and 13.0% (16%FA24%BFS). The self-healing performance of pastes with 50% BFS was most obviously improved that crack with width of 0.43 mm could be completely healed within 21d, while the relative water permeability coefficient dropped by around 80%, followed by pure cement, 30%FA and 16%FA24%BFS pastes. Notably, the morphology of the crack healing product in the blend pastes with FA and BFS was changed with the addition of CA, a large amount of lumpy and clustered calcite was generated, which promoted the compactness and content of healing products. The results of this study showed that CA could significantly improve pore structure, water transport and self-healing properties of high-volume BFS cementitious materials, which meant that CA had a better application prospect in high-volume BFS cement-based materials.

Effect of fly-ash as fine aggregate on the workability and mechanical properties of cemented paste backfill

An experimental study was conducted to investigate the effect of fly-ash(FA) as a fine aggregate on filling performance. The effect of FA content on the gradation distribution and compactness of the dry mixture is tested. Slump, bleeding rate, and unconfined compressive strength (UCS) of cemented waste rock-FA backfill (CWFB) samples were also tested at different FA content, solids content, and cement-sand ratio conditions. Results show that FA as a fine aggregate can effectively improve the mixture gradation performance. Short-term (3d/7d) strength increases with FA content, while long-term (28d) strength and slump first increase and then decrease, reaching a maximum value of 8.58 MPa and 27.4 cm respectively at 30% FA content. The bleeding rate first decreased and then stabilized (about 5.2%) with FA content. The UCS showed higher sensitivity to high solid content. The slump reduction rate of samples with high FA content decreases with solid content. Larger cement content will weaken the fluidity. When the cement-sand ratio increased from 1:6–1:4, the slump decreased from 27.5 cm to 23.5 cm, and the bleeding rate decreased to 4.05%. However, the mechanical properties are significantly improved with cement content. This study has positive implications for recycling the FA.

Damage mechanism of engineered cementitious composites after exposed to elevated temperatures: Experimental and molecular dynamics study

Replacing cement with large amounts of fly ash in engineered cementitious composites (ECC) is one of the win-win measures to enhance matrix ductility and effectively reduce carbon emissions. However, the high content of fly ash undoubtedly makes the mechanical properties of ECC lower, resulting in its performance at high temperature to be questioned. To solve this problem, this paper combines experiments and molecular dynamics study to fully explore and innovatively explain the performance of ECC exposed to different temperatures (20 °C, 200 °C, 400 °C, 600 °C and 800 °C) from the aspects of macroscopic mechanical properties, microstructural characteristics and molecular-scale interfaces. From 20 °C to 800 °C, the mechanical properties first increase and then decrease significantly. SEM shows the changes in the microscopic morphology at elevated temperatures, such as the increase of cracks and pores in the matrix, and the melting of the PVA fibers. TG/DSC and XRD illustrate the phase transformation at different temperatures. 29Si NMR demonstrates the decomposition of hydration products and the depolymerization of silicate chains at higher temperatures. Additionally, molecular dynamics study is carried out to characterize the interaction between PVA fibers and C–S–H gel at a molecular scale as a function of temperature. With the increase of temperature, the morphology of C–S–H gradually changes from layered structure to spherical structure, the water molecules between the C–S–H layers gradually diffuse to the C–S–H channel, and PVA chains are present by disordered distribution. The simulated results have a good agreement with experimental results and further explain the damage mechanism of ECC at elevated temperatures on the nano-scale level.

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.

Compressive Strength Estimation and CO2 Reduction Design of Fly Ash Composite Concrete

Fly ash is broadly utilized to produce concrete materials. This study presents a strength estimation model and a CO2 reduction design method for concrete with fly ash. First, a hydration-based strength (HBS) model is proposed for the evaluation of strength development at different ages of fly ash composite concrete with different mix proportions. Second, CO2 emissions for 1 MPa strength were evaluated. The analysis results show that, as the fly ash-to-binder ratio (FA/B) increased, the CO2 emissions for 1 MPa strength decreased. For concrete with a low water-to-binder ratio (W/B), the addition of high content of fly ash had an obvious dilution effect, which increased the reaction degree of cement and reduced CO2 emissions for 1 MPa strength. Moreover, the extension of the design age could reduce CO2 emissions for 1 MPa strength. Third, a genetic-algorithm-based optimal design model is proposed to find the individual mass of cement and fly ash of low-CO2 concrete. The analysis results show that, as the water contents increased from 160 to 170 kg/m3, to obtain the same strength, cement mass and fly ash mass increased, while the water/binder ratio and fly ash/binder ratio did not change. This means that the reduction in mixed water is one feasible way to lower CO2 emissions. In summary, the proposed strength–emission integrated analysis method is useful for designing sustainable fly ash composite concrete with the desired strength and low levels of CO2 emissions.

Mechanical property and microstructure of cemented tailings backfill containing fly ash activated by calcium formate.

Cemented tailings backfill (CTB) is the most economical and environmental method to recycle tailings and fly ash (FA) for filling mining, but the high content of FA will weaken its strength property. This paper aims to use calcium formate (CF) as an activator to stimulate the activity of FA, thereby enhancing the mechanical property of CTB. The influence of FA and CF content on the stress-strain behavior, dilatancy deformation, and compressive strength of CTB was investigated using uniaxial compression test and scanning electron microscope. The coupling effect mechanism of FA and CF content on the compressive strength of CTB was revealed. The results show that increasing the content of FA and CF can enhance the bearing capacity of CTB during the dilatancy deformation stage, but the excessive content of FA and CF will lead to the attenuation of peak stress. The relations between FA content, CF content, and the compressive strength of CTB can be characterized by quadratic polynomial. Adding CF can stimulate the activity of insoluble FA, increasing the utilization of FA in CTB and producing rich hydration products to fill the internal defects of CTB. The microstructure of CTB is effectively improved by adding CF, including the size and distribution of microcracks and micropores, so that the strength property of CTB is optimized. However, too much CF will make the microstructure of CTB loose and porous, resulting in more microcracks and micropores. Microcracks propagate and connect with micropores to form defects, which deteriorate the microstructure of CTB, thus weakening the strength parameters of CTB. This study provides a method to increase the utilization of FA in CTB, which is of great significance for strengthening the mechanical properties of CTB and improving engineering economic benefits.

Durability Evaluation of Concrete in Cold and Arid Regions Based on Grey Relational Theory

Aiming at the outstanding problem of concrete durability in cold and arid regions, the Datonghe‐Qingwangchuan Diversion Project in Gansu Province, China, was used as a typical research area, and erosion products were determined by field sampling and laboratory XRD analysis. Three durability evaluation indices, namely, the mass loss rate, relative dynamic elastic modulus (RDEM), and compressive strength loss rate, were measured via indoor accelerated testing. Based on grey relational theory, a concrete durability evaluation model was constructed. The critical failure values of the three evaluation indices were introduced as a set of data to optimize the model and determine the grey relational degree of each group of concrete specimens under different working conditions. The results show that using the test results and the optimized grey relational analysis evaluation model, the concrete specimens mixed with an air‐entraining agent have strong resistance to composite erosion. The durability of concrete specimens with an air‐entraining agent content of 0.016% and fly ash content of 15% is better, and a high content (30%) of fly ash accelerates the damage of concrete specimens. The model calculations are consistent with the test results. This method can solve the problem of the durability of concrete specimens in the whole test cycle not being fully evaluated. The results of the study provide useful references for concrete mix proportion optimization and durability evaluation problems.

Effect of Curing Time on the Surface Permeability of Concrete with a Large Amount of Mineral Admixtures

In order to explore the influence of curing time on the surface permeability of concrete, 10 groups of high content fly ash and slag concrete with two kinds of water‐binder ratio were designed. There were cured under five curing methods, respectively. The influence of curing time on the surface permeability of concrete was studied. The mechanism was explained by hydration heat and mercury intrusion porosimetry. The results show that inadequate curing time leads to insufficient hydration of cementitious materials in concrete with large amount of mineral admixtures and significant increase in the diffusion of chloride ions on the surface of concrete. Standard curing for 14 days can basically ensure the chloride ion penetration resistance of concrete with large amount of mineral admixture. When the standard curing time is less than 7 days, the chloride ion penetration resistance deteriorates seriously. Under the condition of insufficient standard curing time, adding a small amount of limestone powder can effectively enhance the chloride ion permeability of concrete with a large amount of fly ash and slag.

A fractal-based approach for cracking characterization and whole process prediction exploration of PP fiber reinforced ECC containing sustainable ingredients

This study focuses on an eco-friendly engineered cementitious composite (SSPP-ECC) using high-content fly ash (FA) as a cementitious cleaner production alternative, waste superfine sand as silica sand replacement and economical polypropylene (PP) fiber as expensive polyvinyl alcohol (PVA) fiber replacement. The lack of quantitative sustainability assessment and prediction of high ductility SSPP-ECC has impeded the engineering application of SSPP-ECC. This study presents a fractal-based cracking whole process preliminary analysis for SSPP-ECC structural life cycle assessment in order to promote a wider application of SSPP-ECC as large-deformation structural materials, as well as develop the application of fractal geometry in ECC by novel evaluation modes. A new and SSPP-ECC with up to 60% cement replacement by fly ash was designed, followed by a four-point flexural test accompanying with surface crack monitoring. The typical deformation hardening SSPP-ECC has been proved to have engineering potential in improving the structural safety due to good deformation following and crack controlling capacities. It is recommended to combine the fractal dimension and average crack width to reflect the overall fracture energy in the four-point flexural failure process of SSPP-ECC, which would have reliable and effective evaluation effects. Based on the correlation analysis results, the quantitative assessment and prediction models of cracking whole process could be established for eco-friendly SSPP-ECC structural life cycle assessment, which can be benefit to the structural life cycle assessment of SSPP-ECC engineering application based on non-destructive parameters of load and deflection. The eco-friendly SSPP-ECC, as a potential cleaner production, not only is a technically reasonable strategy, but also would lead to significant sustainability benefits with a growing level of environmental awareness.