Fly Ash Replacement Level Research Articles

Effect of High Loss on Ignition-Fly Ash on Properties of Concrete Fully Immersed in Sulfate Solution

This paper presents the experimental results of an investigation on the engineering properties and durability of high loss on ignition-fly ash concrete fully immersed in 5% sodium sulfate solution. The fly ash used in this study is a raw material with the loss on ignition of 15.8%, which is much higher than the upper limitation of 6% as stipulated by ASTM C618. Eight concrete mixtures were designed with different water-to-binder ratios of 0.35 and 0.45. For each water-to-binder ratio, the fly ash was used to replace 0%, 10%, 20%, and 30% amount of cement. Test results show that the workability of fresh concrete increased, while its unit weight decreased with increasing the fly ash replacement level. In addition, the concrete mixtures with 10% and 20% fly ash content showed the higher compressive strength, especially at long-term ages, than that of the fly ash-free concrete mixture. Further, all of the fly ash concrete mixtures exhibited the good quality with ultrasonic pulse velocity values of higher than 4100 m/s.

Micromechanical Behavior and Micromorphology of Single PVA Fiber Pullout

To evaluate the suitability of domestic polyvinyl alcohol (PVA) fiber for preparing high ductility cementitious composites (HDCC), single PVA fiber pullout tests were conducted. The tests were carried out with four matrices having a water-to-cement ratio of 0.25 or 0.30, and fly ash replacement level 60% or 80%. The micromechanical behavior of single PVA fiber pullout from the matrix was monitored using a fiber tensile testing machine. Micromorphology of the pulled fiber was subsequently investigated using scanning electron microscopy (SEM). The results indicated that the slip-hardening coefficient is greater than zero and that the fiber therefore meets the micromechanical requirements for preparing HDCC. The chemical bonding energy and frictional bond strength were determined to decrease with the increasing fly ash content and/or water-to--cement ratio. SEM images indicate a significant number hydration products attached to the fiber surface, associated with an increase friction force during fiber pullout. After the pullout test, a complete tunnel containing fiber fragments was observed in the matrix, indicative of matrix cutting.

Effect of fly ash on freeze–thaw durability of concrete in marine environment

Deterioration of structural concrete exposed to freeze–thaw cycles is one of the most important durability problems under subzero temperature conditions. It becomes more devastating when associated with marine environment. Supplementary cementitious material such as fly ash may be used as partial replacement of cement for making more dense, less absorptive and less permeable concrete which improve freeze–thaw durability. This paper presents a part of an experimental study on the freeze–thaw effect of concrete specimens exposed to artificial seawater simulating marine environment and plain water over 360 cycles. Three different grades of concrete M38, M33 and M28, each with four different fly ash replacement level, 20, 30, 40 and 60% were used for the experimental programme. The deteriorative effects were measured by studying weight and volume change, compressive strength, permeability characteristics and rapid chloride penetration resistance of the deteriorated test specimens. the optimum amount of cement replacement is reported to be around 30 to 40%. The study reveals that fly ash concrete has better resistance against freeze–thaw deterioration due to pozzolanic activity of fly ashes that creates more calcium silicate hydrate gel and fills pore spaces, which can effectively reduce the corrosion of the embedded steel reinforcement.

Damage in cement pastes exposed to NaCl solutions

Friedel’s salt forms in cementitious materials exposed to NaCl solutions. This paper quantifies the amount of Friedel’s salt that forms and relates the formation of Friedel’s salt to the damage. Thermogravimetric analysis (TGA) is used to quantify the amount of Friedel’s salt. The volume change associated with the formation of Friedel’s salt is measured. Flexural strength is measured for cement pastes exposed to NaCl solutions using the ball-on-three-ball test. While Friedel’s salt was observed, no calcium oxychloride was detected in cement pastes exposed to NaCl solutions. As the NaCl concentration increases, the amount of Friedel’s salt formed in the cement pastes increases before remaining constant at high concentrations. For the same NaCl concentration, similar amounts of Friedel’s salt are obtained in cement pastes with varying water-to-cementitious materials ratios (w/cm), while a greater amount of Friedel’s salt exists in cement pastes with a higher fly ash replacement level. As the NaCl solution concentration increases, the flexural strength of the cement paste decreases. There is a greater flexural strength reduction in the cement pastes with higher w/cm. For cement pastes with a higher fly ash replacement, the flexural strength shows a smaller decrease when exposed to NaCl. Since TGA confirmed little leaching of Ca(OH)2 in the cement paste, the strength reduction appears to be mainly due to the formation of Friedel’s salt. Little volume change is associated with the formation of Friedel’s salt, which indicates that the damage may primarily be attributed to the crystallization pressure associated with the formation of Friedel’s salt in pores.

ANN Modeling to study strength loss of Fly Ash Concrete against Long term Sulphate Attack

This paper presents the findings from experiments to investigate the effect of fly ash addition in concrete as partial cement replacement on compressive strength under prolonged sulphate exposure. Further, an artificial neural network (ANN) model has been developed to predict the compressive strength of the concrete at varied fly ash replacement level, water curing days and sulphate exposure periods. Cement in concrete has been replaced by fly ash in two levels of replacements such as 25% (Low) and 40% (High). The 28, 90 and 180 days water cured samples of control concrete (CC), Low Fly ash Concrete (LFC) and High Fly ash concrete (HFC) have been subjected to 5% sodium sulphate solution for chemical exposure periods of 6, 12, 15, 18 and 24 months respectively. A total number of 162 concrete cubes of CC, LFC and HFC have been cast towards the comparative assessment of the effect. The experimental data have been used to train the neural network. The predicted data and experimental data display a close match

Strength Durability-Based Design Mix of Self-Compacting Concrete with Cementitious Blend using Hybrid Neural Network-Genetic Algorithm

Sustainable development in self-compacting concrete (SCC) has been studied extensively for the recent years for the purpose to address its growing demand in construction projects. Sustainable SCC can be defined as concrete mix with partially replaced cement content that varies from low to high level using different mineral admixtures. Silica fume and fly ash which is considered as the most common sustainable mineral admixtures for binary and ternary cementitious blends show good effect to the compressive strength and chloride penetration resistivity of hardened SCC. In this paper, this effect was further investigated by using two widely used biological inspired computing models namely the artificial neural network (ANN) and genetic algorithm (GA). The test results of compressive strength and chloride ion penetration resistance from thirty-six concrete samples with varying replacement level of binary and ternary cementitious blends were utilized as inputs for model development. ANN was used to obtain models that describe analytically the relationship of material components to the compressive strength and chloride penetration resistivity. The derived models were further explored through optimization using GA. Results shows that ANN was able to establish the relationship of strength-durability parameters to the material components while GA is able to derived optimal mix proportion for best strength-durability performance. The present study also validates the sensitivity of the replacement level of silica fume and fly ash as a ternary cementitious blend to the strength-durability performance of SCC. This indicates that high volume content of ternary blended cement can improve chloride penetration resistivity and exhibited high compressive strength.

Hydration and physical characteristics of ultrahigh-volume fly ash-cement systems with low water/binder ratio

Replacing Portland cement by fly ash in concrete has attracted extensive attention, as this approach is effective in controlling heat release rate, reducing material cost and enhancing greenness. However, only limited studies have been reported on the hydration and physical characteristics of ultrahigh-volume fly ash (UHVFA, fly ash/binder > 60 wt%) concrete. This study aims to explore these characteristics of fly ash–cement systems with low water/binder ratios and a wide range of fly ash replacement levels (from 20% to almost 100%). Even if 80% of the cement was replaced by fly ash, the 28-day compressive strength of the mortar reached over 65 MPa under normal curing conditions, and the total hydration heat was 70% less than that of conventional cement mortar. Moreover, the morphological and micro-aggregate effects of the fly ash were found to play important roles in maintaining adequate strength of the systems, especially for those cases with UHVFA. The findings of this study can support the future designs and applications of sustainable UHVFA concrete.

Mix Design Aspects of High Performance Concrete Comprised of Silica Fume and Fly Ash

With massive developments taking place in the construction industry, the demand for high performance concrete (HPC) is steadily increasing. It is possible to use industrial by-products such as silica fume (SF) and fly ash (FA) as supplementary cementitious materials (SCMs), to enhance the attributes of HPC. Although numerous investigations have been carried out to identify the optimum replacement levels of these SCMs, some inconsistences are noticed in the results. In this context, a study was conducted to look into the combined effect of SF and FA on the strength and workability of HPC. Four SF replacement levels: 5%, 10%, 12.5%, and 15%; and five FA replacement levels: 0%, 5%, 10%, 15% and 20% were proposed. A total of nineteen mix proportions were used including a control mix. Water-binder ratio was kept constant at 0.35. The mixes were tested for 7 and 28-day compressive strengths and for their workability. Results obtained revealed that the maximum 7-day compressive strength was in the mix with 10% of SF with no FA and the maximum 28-day compressive strength was in the mix with 12.5% of SF and 5% of FA. The workability increased with the addition of fly ash while the SF content kept below 10%. In terms of economy, the best strength to cost ratio was found in the control mix. The extension of the k-value concept for the water/binder ratio found in EN 206 for SF-FA combinations revealed that the existing parameters kSF = 2.0 and kFA = 0.4 show a good correlation with the experimental results.

Shear behavior of full-scale high volume fly ash-self consolidating concrete (HVFA-SCC) beams

An experimental test was carried out to investigate the shear behavior of full-scale beams constructed with high volume fly ash self-consolidating concrete (HVFA-SCC). HVFA-SCC is a new concrete grade of HVFA concrete with the rheology of self-consolidating concrete that satisfies the quality of construction work, environment aspects, and concrete sustainability. Mixes with different cement replacement levels of fly ash and hydrated lime [50%, 60%, and 70% (by weight)] were used. Twelve full-scale reinforced concrete beams were cast and tested using a four-point load test setup. This study focused on observing the effect of factors such as cement replacement level, longitudinal reinforcement ratio, and shear reinforcement ratio on the beam shear behavior. All beams were 4000mm (13ft) in length, 457mm (18in.) in thickness, and 305mm (12in.) in width. Rheological and mechanical properties of the mixes were monitored. During testing, cracking and ultimate shear, deflection, crack pattern, and mode of failure were recorded. Furthermore, test results were compared to conventional concrete study, finite element modeling, and database of conventional concrete and self-consolidating concrete.

Early age shrinkage and heat of hydration of cement-fly ash-slag ternary blends

This study investigated the early age autogenous and chemical shrinkage of mortars with slag and fly ash obtained by sieving the freshly mixed concrete after mixing. The hydration heat and contents of air voids formed within mortars during cement hydration were also tested using isothermal calorimetry and 1H NMR. The results showed that the decrease of w/b ratio significantly increased the autogenous shrinkage of mortars, which was more significant for the mortars containing fly ash and slag. With the increase of slag and fly ash replacement level, the autogenous shrinkage of cement mortars decreased linearly. At the same w/b ratio the cumulative heat of hydration and autogenous shrinkage of cement pastes have a good linear correlation. The fraction of voids formed during hydration increased significantly with increasing w/b ratio. For samples with high w/b ratio (e.g. 0.4), the volume of air voids was higher than that of autogenous shrinkage, which was lower for sample with w/b ratio 0.3. The chemical shrinkage calculated from air voids and measured autogenous shrinkage can be well correlated to the hydration heat.

Synergistic effects of ASR and fly ash on the corrosion characteristics of RC systems

This research investigated the synergistic effects of fly ash and aggregate reactivity on chloride transport and time to corrosion. Specimens containing fly ash replacement levels of 0, 20, and 40% by weight with and without reactive aggregate were exposed to wetting/drying cycles. Expansion, corrosion potential, and macrocell current were measured monthly until corrosion of the embedded reinforcement initiated. The results indicate that inclusion of fly ash in specimens containing non-reactive aggregate results in lower apparent diffusion coefficient values (Da) and lower critical chloride threshold values (CT) in concrete. The benefits from the lower Da values are more significant than the disbenefits from the reduction in CT when assessing time to corrosion initiation. Specimens containing reactive aggregate and 20% fly ash exhibited longer times to corrosion initiation than specimens containing 40% fly ash. Results indicate that the ASR gel resists the transport of chlorides and the formation of small amounts of ASR gel in the specimens containing 20% fly ash reactive aggregate results in slower transport rates and longer times to corrosion initiation.

Drying shrinkage and cracking resistance of concrete made with ternary cementitious components

The compressive strength, free and restrained drying shrinkage and cracking resistance of concrete under drying condition were investigated for ternary cementitious systems containing Portland cement, slag and fly ash. The restrained shrinkage test (ring test) was carried out following ASTM C1581. The results showed that the increased replacement level of slag or fly ash from 0 to 50% led to a gradual decrease in 28d compressive strength of concrete. The free drying shrinkage increased with the increase of slag content, but reduced with the fly ash content. The cracking resistance was well related to the free drying shrinkage of concrete, other factors such as strength also showed certain effects on cracking resistance of concrete. The cracking resistance of concrete was enhanced with the addition of fly ash while weakened with the slag replacement level up to 50%.

Microwave absorption and mechanical properties of double-layer cement-based composites containing different replacement levels of fly ash

Abstract Double-layer absorbing cement-based composites with the thickness of 10 mm were prepared, including different replacement levels of fly ash (FA) in the absorbing layer as well as the matching layer for impedance matching. Waste polyethylene terephthalate bottle fragment was introduced as electromagnetic transparent reinforcement aggregate. Carbon black was used to be original absorbent in the absorbing layer. The microstructure and electromagnetic parameters of FA were closely looked at through scanning electron microscope, X-ray diffraction, and analyzer of vector network. The absorption and mechanical properties of cement-based composites were tested. It turned out that when the optimal replacement ratio of FA in the absorbing layer and matching layer gets to 50%:30%, the minimum value of reflection loss achieves −22.3 dB at 13.2 GHz; also, the value of absorption bandwidth that is effective (<−8 dB) is 6.4 GHz. Ni-Zn ferrite proves to be a feasible absorbent that is additional for the matching layer compared to what is added to the absorbing layer. The compressive strength of all the mixtures decreased, while the flexural strength decreased first and then increased with the rise of the FA replacement level.

Influence of temperature history on chloride diffusion in high volume fly ash concrete

This paper presents the influence of temperature history on chloride diffusion of high volume fly ash (HVFA) concrete. Four different curing temperature histories, referenced as SDC (standard curing), SMC (steam curing), OC (outdoor curing) and TMC (massive concrete temperature matched curing), were set. Concretes with different replacement levels of fly ash were cast and cured under the four temperature histories, respectively. The variations of compressive strength, porosity and chloride diffusion coefficient with different ages were determined by experiment. SEM and XRD were used to analyze the microstructure and compositions of hydrated pastes. The results show that the performances of fly ash concrete were influenced by temperature histories. And the influence was more obvious with the addition of replacement level of fly ash. For ordinary concrete, higher accumulated temperature at early age got higher compressive strength, better pore structure and lower chloride diffusion coefficient, but had negative effects at later age compared to SDC. For HVFA concrete, it was beneficial for the development of performances. Although the chloride diffusion coefficient was higher than ordinary concrete at early age, it would become small even lower at later age. And higher accumulated temperature promoted this process. In addition, by utilizing the concept of maturity, good power function relations between chloride diffusion coefficient of fly ash concrete and Maturity were obtained at early age.

Micro-environment temperature and relative humidity response of fly ash concrete under natural climatic conditions

The internal micro-environment temperature and humidity, which depend on external climate and concrete material properties are important factors affecting the durability. In a natural environment, the diurnal and seasonal weather cycles lead to the fluctuations in internal climate in concrete. This paper is focused on the internal temperature and relative humidity of fly ash concrete under natural climatic conditions. Three fly ash replacement levels, namely, 15%, 30% and 45% are considered. The temperature and humidity inside and outside concrete specimens are recorded chronically by temperature and humidity sensors. The test shows that the internal temperature varies similarly to the external ambient temperature, and the content of fly ash has a little influence on temperature response. The internal humidity fluctuates more slowly than the temperature; in particular, it may remain almost constant for a long time during winter. The higher fly ash content (such as 30% or 45% replacement ratio) increases the fluctuation in internal humidity, especially during summer. Therefore, in the concrete durability studies involving impact of temperature and relative humidity, while the external environment temperature can be directly considered instead of internal temperature, the ambient humidity and the micro-environment humidity must be clearly distinguished.

Use of municipal solid waste incinerator (MSWI) bottom ash in high calcium fly ash geopolymer matrix

In order to reduce municipal solid waste incinerator (MSWI) ash, the effect of replacement level of high calcium fly ash by MSWI bottom ash on fly ash based geopolymer properties were studied. The results were compared to cement matrix containing MSWI bottom ash. It showed that the 20% MSWI bottom ash replacement level produced the highest compressive strength at 7 d of 52.8 MPa. The MSWI bottom ash could replace fly ash at high dosage level of 40% and the compressive strength was still higher than that of the control geopolymer mortar. The porosity, pore size distribution, and SEM indicated that the use of MSWI bottom ash could increase the denseness and homogeneity of geopolymer and cement matrix. In addition, the increase of MSWI bottom ash level significantly increased the crystalline phases of calcite and quartz as shown in the XRD pattern and C-O stretching in the FTIR images of both matrices.

Use of Fly Ash to Minimize Deicing Salt Damage in Concrete Pavements

Premature damage has been observed at the joints in numerous concrete pavements where calcium chloride and magnesium chloride deicing salts have been used. This damage results from a reaction between the deicing salt and the calcium hydroxide (CH) in the hydrated cement paste. This reaction leads to the formation of an expansive product known as calcium oxychloride (CAOXY). The use of supplementary cementitious materials as a replacement for cement has been proposed to reduce the CH that is available in the mixture to react with the deicing salts. Reducing the CH can reduce the amount of CAOXY that forms. In this study, mixtures representative of paving concrete were made with cements and fly ashes from across the country. CH amounts were determined by using thermogravimetric analysis, and CAOXY amounts were determined by using low-temperature differential scanning calorimetry. Various replacement levels of fly ash were used to demonstrate that the main parameter that influences the amounts of CH and CAOXY that form is the replacement level of fly ash. This paper proposes that a prescriptive specification requiring 35% cement replacement by volume with fly ash would reduce the damage caused by CAOXY formation and further proposes a performance specification to limit the CAOXY formation to below 15 g/100 g paste.

From Hydration to Strength Properties of Fly Ash Based Mortar

Fly ash (FA) is important alternative or supplement to cement to reduce the environmental impact of concrete industry. However, early age strength development of FA is a concern due to the slower reaction rate of FA compared to cement. This paper examines the early age hydration properties of cement paste containing typical Australian FA and establishes correlations between the degree of hydration and the early-age strength development properties of mortar mixes. All mixes have the same mixture proportion of water to binder (w:b) ratio. FA sourced from different power plants are used for the tests. Cement replacement levels of 0%, 10%, 30% and 40% by mass are considered. The degree of hydration was established from the heat production of the mixes by using isothermal calorimetry. The hydration properties were characterized by hydration curve parameters obtained from curve fitting. The results show that both hydration rate and strength of the binder materials (FA and cement) were reduced with higher replacement levels of FA contributing to a reduced hydration rate at early ages. Linear relationship could be obtained between degree of hydration and strength at early ages for all the fly ash binders.

Effects of pore water saturation on the mechanical properties of fly ash concrete

This paper investigates the effects of pore water saturation on the ultrasonic pulse velocity, rebound value, compressive strength, and elastic modulus of fly ash concrete. Fly ash concrete was placed in an artificial climate chamber to achieve different degrees of pore water saturation. Effects of the level of fly ash replacement and the degree of pore water saturation were examined on the concrete rebound value, ultrasonic pulse velocity, compressive strength and elastic modulus. The results showed that the influence of fly ash replacement on rebound values, ultrasonic velocity and elastic modulus was basically the same as on compressive strength. For the range of 0–45% for the level of fly ash replacement, the rebound value, the ultrasonic velocity, the elastic modulus and the compressive strength all reached their maximum value corresponding to the fly ash replacement level of 15%. As the degree of pore water saturation increased, the rebound value of fly ash concrete was increased first and then decreased, the compressive strength was decreased, but the ultrasonic velocity and the elastic modulus were increased.

Properties of pervious concrete containing high-calcium fly ash

This paper presents the properties of pervious concrete containing high-calcium fly ash. The water to binder ratios of 0.19, 0.22, and 0.25, designed void ratios of 15, 20, and 25%, and fly ash replacements of 10, 20, and 30% were used. The results showed that the use of fly ash as partial replacement of Portland cement enhanced the mixing of paste resulting in a uniform mix and reduced amount of superplasticizer used in the mixture. The compressive strength and flexural strength of pervious concrete were slightly reduced with an increase in fly ash replacement level, while the abrasion resistance increased due mainly to the pozzolanic and filler effects. The compressive strength and flexural strengths at 28 days were still higher than 85% of the control concrete. The aggregate size also had a significant effect on the strength of pervious concrete. The compressive strength and flexural strength of pervious concrete with large aggregate were higher than that with small aggregate.