Microconcrete with partial replacement of Portland cement by fly ash and hydrated lime addition

The reduction in Portland cement consumption means lower CO₂ emissions. Partial replacement of Portland cement by pozzolans such as fly ash has its limitations due to the quantity of calcium hydroxide generated in the mix. In this work we have studied the contribution of the addition of hydrated lime to Portland cement + fly ash systems. We have also studied several levels of cement replacement, ranging from 15% to 75%.The best mechanical results were obtained replacing 50% of Portland cement by the same amount of fly ash plus the addition of hydrated lime (20% respect to the amount of fly ash). In these systems, an acid-base self-neutralization of the matrix has occurred through a pozzolanic reaction of fly ash with portlandite liberated in the hydration of Portland cement and the added hydrated lime. It has been identified for these mixtures a significant amount of hydrated gehlenite, typical reaction product from rich-alumina pozzolans.     

Compressive strength and chloride resistance of self-compacting concrete containing high level fly ash and silica fume

The influence of high-calcium fly ash and silica fume as a binary and ternary blended cement on compressive strength and chloride resistance of self-compacting concrete (SCC) were investigated in this study. High-calcium fly ash (40–70%) and silica fume (0–10%) were used to replace part of cement at 50, 60 and 70 wt.%. Compressive strength, density, volume of permeable pore space (voids) and water absorption of SCC were investigated. The total charge passed in coulombs was assessed in order to determine chloride resistance of SCC. The results show that binary blended cement with high level fly ash generally reduced the compressive strength of SCC at all test ages (3, 7, 28 and 90 days). However, ternary blended cement with fly ash and silica fume gained higher compressive strength after 7 days when compared to binary blended fly ash cement at the same replacement level. The compressive strength more than 60 MPa (high strength concrete) can be obtained when using high-calcium fly ash and silica fume as ternary blended cement. Fly ash decreased the charge passed of SCC and tends to decrease with increasing fly ash content, although the volume of permeable pore space (voids) and water absorption of SCC were increased. In addition when compared to binary blended cement at the same replacement level, the charge passed of SCC that containing ternary blended cement was lower than binary blended cement with fly ash only. This indicated that fly ash and silica fume can improve chloride resistance of SCC at high volume content of Portland cement replacement.     

Shrinkage behavior of structural foam lightweight concrete containing glycol compounds and fly ash

Shrinkage behavior of the structural foam lightweight concrete with density of 1600 kg/m³ was investigated. Owing to high drying shrinkage of the lightweight concrete, glycol compounds were used in the concrete mixture to study their effect on shrinkage behavior. Propylene glycol (PG), triethylene glycol (TEG) and dipropylene glycol tert-butyl ether (DPTE) were selected for testing of drying shrinkage of the lightweight concrete. Partial replacement of cement and sand with fly ash was also used to reduce the shrinkage. Results indicated that PG, TEG and DPTE were effective in reducing the shrinkage of lightweight concrete through reduction of surface tension of water. However, DPTE significantly reduced the surface tension and caused the foam instability and early stiffening of mixture. The partial replacement of cement and sand with fly ash could also reduce the shrinkage of the lightweight concrete. In this case, the compressive strength was also enhanced owing to the additional pozzolanic reaction.     

Influence of activated fly ash on corrosion-resistance and strength of concrete

Various activation techniques, such as physical, thermal and chemical were adopted. By adopting these methods of activation, hydration of fly ash blended cement was accelerated and thereby improved the corrosion-resistance and strength of concrete. Concrete specimens prepared with 10%, 20%, 30% and 40% of activated fly ash replacement levels were evaluated for their compressive strengths at 7, 14, 28 and 90 days and the results were compared with ordinary Portland cement concrete (without fly ash). Corrosion-resistance of fly ash cement concrete was studied by using anodic polarization technique. Electrical resistivity and ultrasonic pulse velocity measurements were also carried out to understand the quality of concrete. The final evaluation was done by qualitative and quantitative estimation of corrosion for different systems. All the studies confirmed that upto a critical level of 20–30% replacement; activated fly ash cement improved both the corrosion-resistance and strength of concrete. Chemical activation of fly ash yielded better results than the other methods of activation investigated in this study.     

Prediction of compressive strength of concrete with fly ash as sand replacement material

Fly ash (FA) acts as a partial replacement material for both Portland cement and fine aggregate. The published information on FA as sand (fine aggregate) replacement material (SRM) is limited and rational guidelines to estimate the compressive strength of concrete are not available. This aspect was investigated and a formula to predict the compressive strength of concrete at 28 day is suggested in this paper. This formula, containing cementing efficiency factor, k, of FA, is useful also when the quantity of FA used is more than that of sand replaced. Application of the formula to the test data in published literature, indicate that it can estimate the compressive strength of concrete containing different levels of sand replacement by fly ash.     

Effect of fly ash fineness on compressive strength and pore size of blended cement paste

This paper presents an experimental investigation on the effect of fly ash fineness on compressive strength, porosity, and pore size distribution of hardened cement pastes. Class F fly ash with two fineness, an original fly ash and a classified fly ash, with median particle size of 19.1 and 6.4 μm respectively were used to partially replace portland cement at 0%, 20%, and 40% by weight. The water to binder ratio (w/b) of 0.35 was used for all the blended cement paste mixes.Test results indicated that the blended cement paste with classified fly ash produced paste with higher compressive strength than that with original fly ash. The porosity and pore size of blended cement paste was significantly affected by the replacement of fly ash and its fineness. The replacement of portland cement by original fly ash increased the porosity but decreased the average pore size of the paste. The measured gel porosity (5.7–10 nm) increased with an increase in the fly ash content. The incorporation of classified fly ash decreased the porosity and average pore size of the paste as compared to that with ordinary fly ash. The total porosity and capillary pores decreased while the gel pore increased as a result of the addition of finer fly ash at all replacement levels.     

Behaviour of concrete-filled steel tubular columns incorporating fly ash

The subject of this work is to investigate the effect of fly ash on the strength of concrete filled steel tubular columns from 28 to 365 days. A contrast study was carried out on concrete filled steel tubular columns incorporating 10–40 wt% fly ash, and for control Portland cement concrete filled steel tubular columns. The effect of pre-coating the inner surface of steel tubes with a thin layer of fly ash was also studied. Assessments of the concrete mixes were based on the compressive strength and the bond strength. The results show that a lower replacement with fly ash can improve both bond strength and compressive strength, while a higher replacement with fly ash requires a relatively longer time to achieve similar beneficial effects. Pre-coating the inner surface of steel tubes with a thin layer of fly ash can notably improve the bond strength. The microstructure of the interface between concrete and steel tube was also studied by using scanning electron microscopy analyzer.     

Influence of fly ash as a cement addition on the hardened properties of recycled aggregate concrete

The effects of the use of Class F fly ash as a cement addition on the hardened properties of recycled aggregate concrete were determined. In this study, four series of concrete mixtures were prepared with water-to-cement (w/c) ratios of 0.55, 0.50, 0.45 and 0.40. The recycled aggregate was used as 0%, 20%, 50% and 100% replacements of coarse natural aggregate. Furthermore, fly ash was employed as 0% and 25% addition of cement. Although the use of recycled aggregate had a negative effect on the mechanical properties of concrete, it was found that the addition of fly ash was able to mitigate this detrimental effect. Also, the addition of fly ash reduced the drying shrinkage and enhanced the resistance to chloride ion penetration of concrete prepared with recycled aggregate. Moreover, it was found that the drying shrinkage and chloride ion penetration decreased as the compressive strength increased. Compared with the results of our previous study, the present study has quantified the advantages of using fly ash as an additional cementitious material in recycled aggregate concrete over the use of fly use as a replacement of cement.     

Chloride transport in fly ash and glass powder modified concretes – Influence of test methods on microstructure

This paper discusses the influence of rapid chloride transport test methods on the microstructure and the measured chloride penetration resistance of concretes containing fly ash or fine glass powder as partial cement replacement materials. Rapid chloride permeability (RCP), non-steady-state migration (NSSM), and steady state conduction (SSC) tests are performed on plain and modified concretes. The glass powder modified concretes demonstrate similar or lower RCP values as compared to the fly ash modified concretes of the same cement replacement level whereas the steady state conductivities are lower for the fly ash modified mixtures. The NSSM coefficients are lower for the fly ash modified concretes even when the initial conductivities are similar to those of plain or glass powder modified concretes. Chloride binding under the conditions of NSSM test, that influence the microstructure and the transport parameter, is quantified using thermal analysis and XRD patterns as well as an electrical circuit model for the impedance response. The resistance of connected pores (R_c) extracted from the model adequately captures the changes in microstructure with time and with chloride transport. The changes in R_c between the start and finish of the NSSM test also indicate microstructural alteration in fly ash modified concretes.     

Influence of filler type on the properties of foam concrete

Most of the investigations on foam concrete in the past have been confined to neat cement paste, cement paste with partial replacement with admixtures and to cement–sand mixes. This paper reports the results of a systematic study to ascertain the influence of filler type (i.e., sand and fly ash) and the particle size of sand on the properties of moist cured foam concrete. This study shows that the consistency of mixture, for achieving pre-formed foam concrete of design density, mainly depends on the filler type. The flow behaviour of foam concrete is mainly influenced by the foam volume. A reduction in particle size of sand caused an improvement in strength of foam concrete. For a given density, replacement of sand with fly ash resulted in higher strength. Finer filler resulted in a higher ratio of strength to density.     

Effect of specimen size and shape on compressive strength of concrete containing fly ash: Application of genetic programming for design

The use of fly ash as a mineral admixture in the manufacture of concrete has received considerable attention in recent years. For this reason, several experimental studies are carried out by using fly ash at different proportions replacement of cement in concrete. In the present study, the models are developed in genetic programming for predicting the compressive strength values of cube (100 and 150 mm) and cylinder (100 × 200 and 150 × 300 mm) concrete containing fly ash at different proportions. The experimental data of different mixtures are obtained by searching 36 different literatures to predict these models. In the set of the models, the age of specimen, cement, water, sand, aggregate, superplasticizers, fly ash and CaO are entered as input parameters, while the compressive strength values of concrete containing fly ash are used as output parameter. The training, testing and validation set results of the explicit formulations obtained by the genetic programming models show that artificial intelligent methods have strong potential and can be applied for the prediction of the compressive strength of concrete containing fly ash with different specimen size and shape.     

Heat deformations of fly ash concrete

When dealing with concrete resistance to high temperatures it is important for design purposes to know the elastic parameters, such as the temperature–strain curves and the modulus of elasticity.Concretes containing a high volume of fly ash differ from conventional mixes in the cementitious phase. This results in a different behaviour under heating compared to plain Portland cement concretes. To find the elastic response of fly ash concrete four series of concrete mixtures were manufactured: one with cement only, another with 30% by mass partial replacement of cement by fly ash, and two with 30% and 40% by mass replacement of cement by ground fly ash. Tests were carried out on cylinders (150 × 300 mm). A high-calcium fly ash was used.The conditions were selected so that the applied level of stress corresponded to 25% or to 40% of the ultimate compressive strength of concrete, and a transient type of temperature regime was followed. Based on the experiments the critical temperature, the residual deformation and the modulus of elasticity were determined.The results indicate that concretes containing a high volume of fly ash are more sensitive to high temperatures, since they developed greater deformations. The fineness of the fly ash used also seems to influence the degree of deformation in an adverse way.     

Freeze–thaw resistance and transport properties of high-volume fly ash roller compacted concrete designed by maximum density method

The aim of this study is to evaluate the effect of high-volume fly ash on some durability characteristics of roller compacted concrete (RCC). In addition to a control mixture without fly ash, two different series of mixtures were prepared by partial replacement of either cement or aggregate with fly ash. The mixtures were designed by a maximum density method. A total of 28 mixtures having four different water/binder ratios (0.30, 0.35, 0.40 and 0.45 by mass) were prepared to determine the optimum water/binder ratio. Among these, seven mixtures containing the optimum water content were selected for further experimental study. It was observed that in the mixtures where cement was substituted with fly ash, increasing the fly ash content adversely affected the durability performance up to 90 days. However, fly ash substitution for a part of the aggregate improved the durability characteristics of the mixture as the amount of fly ash increased.     

Influence of a fine glass powder on the durability characteristics of concrete and its comparison to fly ash

A detailed investigation carried out to ascertain the durability characteristics of fine glass powder modified concretes is reported in this paper. Tests were designed to facilitate comparisons between concretes modified with either glass powder or fly ash at the same cement replacement level. The optimal replacement level of cement by glass powder is determined from strength and hydration tests as 10%. The later age compressive strengths of glass powder and fly ash modified concretes are seen to differ by only 5%. The durability characteristics are ascertained using tests for rapid chloride permeability, alkali–silica reactivity, and moisture transport parameters. The chloride penetrability values indicate some amount of pore refinement. The potential of glass powder to reduce the expansion due to alkali–silica reaction is established from tests conducted in accordance with ASTM C 1260, but fly ash is found to perform better at similar replacement levels. Glass powder–fly ash blends that make up a 20% cement replacement level are found to be as efficient as 20% fly ash in reducing expansion. The control concrete is seen to exhibit the lowest overall moisture intake after 14 days of curing, and fly ash concrete the highest, with the glass powder concrete in between. The trend is reversed at later ages, demonstrating that both the replacement materials contribute to improved durability characteristics. The sorptivity and moisture diffusion coefficient values calculated from the moisture intake-time data also demonstrate a similar trend. These studies show that fine glass powder has the potential to improve the durability of concretes.     

Development of recycled strain-hardening cement-based composite (SHCC) for sustainable infrastructures

This paper presents the experimental results of an attempt to develop sustainable strain-hardening cement-based composite (SHCC) using recycled materials. SHCC exhibits desirable mechanical properties, including strain hardening and ductility. However, SHCC is composed of silica sand and a high volume of cement, which makes it more energy intensive than conventional concrete. The aim of this study is to promote SHCC sustainability in infrastructure design through the use of recycled materials. Alternative recycled materials – sand, fly ash, and polyethylene terephthalate (PET) fibers – are used to partially replace silica sand, cement, and polyvinyl alcohol (PVA) fibers, respectively, in SHCC specimens. The effects of the recycled materials on the mechanical behavior of the SHCC specimens are examined by conducting compressive tests, four-point bending (flexural) tests, and uniaxial tensile tests. Fundamental information is then used in the constitutive model to analyze and design infrastructures using SHCC with recycled materials. Test results indicate that fly ash improves both the bending and uniaxial tensile performance of SHCC due to an increase in chemical bond strength at the interface between the PVA fibers and cement matrices. However, SHCC that contains PET fibers does not perform well in the bending and uniaxial tensile tests due to the inferior material properties of the PET fibers, although its compressive behavior is similar to that of the PVA2.0 specimen. Also, it is noted that recycled sand increases the elastic modulus value of SHCC due to its larger grain size compared to that of silica sand. Based on the desire to maintain well-performing SHCC, a replacement ratio below 20% for fly ash or below 50% for recycled sand is deemed appropriate for creating sustainable SHCC, as concluded from this study.     

Effect of fly ash on the microstructure of cement mortar

A microstructural study of mortars prepared with a low-alkali, low-C₃A cement and a Class F fly ash, both of Swedish origin, was carried out using the scanning electron microscopy-energy-dispersive X-ray analytical technique. Supplementary phase analyses were made by X-ray diffraction and thermogravimetry-differential thermal analysis. Normally, CH crystals in the transition zone grow with their c axis parallel (or the (0 0 1) cleavage plane perpendicular) to the aggregate surface. The encapsulation of the fly ash particles by the growing CH reduces the amount of orientated CH at the aggregate-paste interface. The growth mechanism of these crystals is discussed. The reduction of CH, most significant after 28 days of hydration, is mainly due to the reaction of CH with the fly ash glass phase. Initially, the replacement of cement by fly ash weakens the paste-aggregate interfacial zone due to reduction of contact points, and increases the local water-to-cement ratio. This, however, improves significantly when the fly ash has reacted. In order to enhance the reaction of fly ash, extra gypsum was added. The results show that gypsum can accelerate the fly ash reaction, but the products formed, and the beneficial effects of gypsum, are mainly determined by the total amount of gypsum in the paste.     

Hydration of high-volume fly ash cement pastes

The hydration processes of high-volume fly ash cement paste were investigated by examining the non-evaporable water content, the CH content, the pH of pore solution and the fraction of reacted fly ash, curing at either 20°C or elevated temperatures after an initial curing at 20°C. The replacement percentage levels of fly ash were 40%, 50% and 60% by weight, respectively. The results revealed that the non-evaporable water content in high-volume fly ash cement pastes does not develop as plain cement pastes does, so it may be improper to apply the non-evaporable water content to evaluate the hydration process in high-volume fly ash cement matrix. The reduction in CH content increases with the progressing of hydration process and varies linearly with the logarithm of curing age. The addition of 3.0% of Na₂SO₄ could accelerate the pozzolanic reaction of fly ash at early ages. At 20°C, the pH of pore solution of high-volume fly ash cement paste was reduced to a great extent at early ages and it continued to decline at later ages due to the inclusion of large amount of fly ashes. At elevated temperatures, however, this trend was not found. The fraction of reacted fly ash directly reflects the pozzolanic reactivity of fly ash both at normal and elevated temperatures. There is some inherent correlation between the reduction in CH content, the pH of pore solution and the fraction of reacted fly ash. For specified matrix, the consumption of CH and the pH of pore solutions change linearly with the increase of the fraction of reacted fly ash.     

Interactive Effects of Fly Ash and CNI on Corrosion of Reinforced High-Performance Concrete

The interactive effects of fly ash and CNI in corrosion of reinforced concrete were investigated. A 3⁴ full factorial design was developed considering water to cement ratio, fly ash percent, CNI and cracked condition as factors. The response factors were the weight loss calculated from Linear Polarization Resistance measurements and the pit depth of the steel bars embedded in concrete. Small-scale concrete slabs containing steel reinforcement with a cover depth of 20 mm were cast for this purpose. The slabs were subjected to a simulated marine environment with two cycles of wetting and drying per day during one year; after the exposure, the slabs were broken, the bars were cleaned and the pith depth measured by using SEM. Under the studied conditions, it was found that CNI alone does not provide corrosion protection of the steel reinforcement even for uncracked silica fume concrete in a 0.45 w/c ratio; however, the combination of CNI and fly ash can be useful to overcome this problem. The results indicate that low w/c ratio concrete in its crack state creates conditions suitable for the development of pitting corrosion.     

Abrasion resistance and mechanical properties of high-volume fly ash concrete

The abrasion resistance and mechanical properties of concrete containing high-volume fly ash (HVFA) were investigated. Sand (fine aggregate) was replaced with 35, 45, and 55% of Class F fly ash by mass. The water to cement ratio and the workability of mixtures were maintained constant at 0.46 and 55 ± 5 mm respectively. Properties examined were compressive strength, splitting tensile strength, flexural strength, modulus of elasticity and abrasion resistance expressed as depth of wear. Test results indicated that replacement of sand with fly ash enhanced the 28-day compressive strength by 25–41%, splitting tensile strength by 12–21%, flexural strength by 14–17%, and modulus of elasticity by 18–23% depending upon the fly ash content, and showed continuous improvement in mechanical properties up to the ages of 365 days. Replacing fly ash with sand significantly improved the abrasion resistance of concrete at all ages. Strong correlation exists between the abrasion resistance and each of the mechanical properties investigated.     

Prediction of chloride permeability of concretes containing ground pozzolans by artificial neural networks

This research was to study the chloride penetration resistance of normal (W/B of 0.80, 0.62, 0.48) and high (W/B of 0.41, 0.35, 0.30) strength concretes containing ground pozzolans such as fly ash, bottom ash and rice husk ash using the rapid chloride penetration test and the immersion test methods. Furthermore, on the basis of this experimental data, an artificial neural network technique is carried out to derive an explicit artificial neural network formulation for the prediction of chloride permeability as a function of six input parameters: water to binder ratio, percent replacement, testing ages, pozzolans types, aggregate to cement ratio and the actual compressive strength. To verify the model, linear and non-linear regression equations are carried out and compared with the proposed artificial neural network prediction model. The results indicate that the incorporation of ground fly ash, ground bottom ash and ground rice husk ash substantially improve the workability and chloride permeability. The artificial neural network models have more accurate and precise prediction than linear and non-linear regression technique. Moreover, it is concluded that the artificial neural network models have a strong prediction capability of chloride penetration of concrete and can be easily expanded for the new additional database to re-train the network.