Siliceous Fly Ash Research Articles

Statistical assessment of the factors influencing fly ash reactivity

Siliceous fly ash is a pozzolanic supplementary cementitious material often used in cement and concrete production. The influence of the chemical, mineralogical and physical properties of 75 siliceous fly ashes on their reactivity was examined in this study. The reactivity was analyzed by relative mortar compressive strength and a rapid screening test (a version of the R³ test). A statistical data analysis was carried out to evaluate the factors influencing the reactivity of the ash. The screening test was found to be able to predict the reactivity within short periods of time, while the relative strength is not a good measure for reactivity. It was also found that the TiO2 and chloride content of the fly ash can serve as an indicator for its reactivity.

Influence of the availability of calcium hydroxide, sulphates, and alkalis on hydration of supplementary cementitious materials

The hydration of siliceous fly ash (FA), slag (S) and metakaolin (MK) with calcium hydroxide (CH) was studied at 27 °C to isolate the roles of CH, alkali and sulphate. The influence on rate of hydration was studied along with phase assemblage and strength development. While the FA was the most slowly reacting SCM, it was also the least influenced by the addition of alkalis and sulphates. For MK and S, not only the rate of hydration is influenced, but the phases formed are also affected by the amount of alkalis and sulphates. The addition of sulphates delays the hydration of alumina in S and MK. In more reactive SCMs such as MK, the availability of CH can be a limiting factor in the reaction of the SCM. The results show that to achieve maximum possible hydration and contribution to strength by SCMs, an optimisation of the alkali and sulphate content is required.

Interaction Mechanism of Cementitious Composites Containing Different Twisted PVA Fiber Bundles

Fiber-reinforced cementitious composites (FRCCs) have made significant progress in improving the tensile and flexural properties of concrete members. Studies have shown that polyvinyl alcohol (PVA) fibers can effectively enhance the toughness of FRCCs, but the haphazard distribution of short-cut fibers makes it difficult to give full play to the high tensile strength of the fibers, and it is difficult for bundled fibers to effectively bond with the concrete substrate, which has become a challenge in the study of changing materials. Twisting the fibers by a physical process to improve the bonding performance of PVA fibers with FRCC substrate is a safe and feasible solution. However, research on silicate cementitious composites reinforced with twisted PVA fibers is limited. In this study, the interaction mechanism of through-length twisted PVA FRCCs with different twist factors and fiber bundle numbers was investigated. A concrete matrix material configured from silicate cement, fly ash, silica fume, and medium sand, in which PVA fibers with different twist factors were pre-incorporated, was used for the tests. Three-point bending tests were carried out on specimens with different twist factors (0, 50, 100, and 150 twists per meter) and fiber bundle numbers 1, 2, and 3. Compared to the untwisted PVA fiber specimens, the twist factor of 100 and the single fiber bundle specimens showed significant improvements in the bending properties, including a 36% increase in deflection, a 68% increase in the equivalent bending stress, and a 119% increase in energy consumption. Micro-X-ray computed tomography scans showed improved bending properties and energy consumption capabilities due to enhanced bonding properties as a result of the increased fiber–matrix interaction area and surface toughness.

Concrete Composites Based on Quaternary Blended Cements with a Reduced Width of Initial Microcracks

This article is devoted to the study of the combined effect of siliceous fly ash (FA), silica fume (SF), and nanosilica (nS) on the cement matrix morphology and size of microcracks occurring in the Interfacial Transition Zone (ITZ) between the coarse aggregate and the cement paste of concrete composites based on ordinary Portland cement (OPC). The manuscript contains analyses of width of microcracks (Wc) occurring in the ITZ area of concretes based on quaternary blended cements and changes in ITZ morphology in the concretes in question. Experiments were planned for four types of concrete. Three of them were composites based on quaternary blended cements (QBC), while the fourth was reference concrete (REF). Based on the observations of the matrices of individual composites, it was found that the REF concrete was characterized by the most heterogeneous structure. However, substitution of part of the cement binder with active pozzolanic additives resulted in a more compact and homogenous structure of the cement matrix in each of the QBC series concretes. Moreover, when analyzing the average Wc values, it should be stated that the modification of the basic structure of the cement matrix present in the REF concrete resulted in a significant reduction of the analyzed parameter in all concretes of the QBC series. For QBC-1, QBC-2, and QBC-3, the Wc values were 0.70 μm, 0.59 μm, and 0.79 μm, respectively, indicating a decrease of 38%, almost 48%, and 30%, respectively, compared with the working condition of concrete without additives. On the basis of the above results, it can therefore be concluded that the proposed modification of the binder composition in the analyzed materials clearly leads to homogenization of the composite structure and limitation of initial internal damages in concrete.

Role carbon nanomaterials in reinforcement of concrete and cement; A new perspective in civil engineering

Nanotechnology has resulted in many advantages in different majors of science and created new approaches to them. nanotechnology has more effect on civil engineering such as utilizing nanomaterials as cement reinforcement. The improvement in quality of materials used in civil engineering such as self-cleaning, self-structural health monitoring, self-sensing, self-vibration damping, self- rehabilitation, cement reinforcement, and self-healing created a new revolutionary civil engineering. Concrete is one of the important parts of civil engineering and especially in the discussion of building and bridge retrofitting. Nanotechnology plays an important role in the concrete process such as alkali silicate reactions (ASR), hydration reactions, and fly ash reactivity. Much of the research showed that nanomaterials improved compressive and flexural strength in concrete and created the best situation for building resistant structures such as bridges, towers, and tall buildings. Due to the mechanical behavior and tensile strength of carbon nanotubes (CNTs), these compounds play a special role in improving concrete properties. Prevention of cracking is one of the positive effects of using carbon nanotubes in improving concrete properties, which has increased significant growth in the application of it in the industry. The present review paper focused on the role of nanomaterials in improving and reinforcement of concrete and the advantages of them in civil engineering.

Volumetric stability and elastic properties of concrete subjected to simulated service exposure conditions on road pavements

Expressway and highway pavements are exposed to intensive impacts of exploitation, including heavy vehicle traffic and environmental factors such as temperature and moisture along with the aggression of de-icing agents. Proper selection of mineral aggregates and design of the concrete mixture composition are important for the durability of the concrete pavement. Experimental tests were conducted on concrete under simulated service exposure conditions with external exposure to a 3% sodium chloride solution. The subject of the research was the expansion and elastic properties of air-entrained concrete made with siliceous aggregates from rocks containing moderate amounts of reactive minerals. The category of reactivity of coarse aggregates was uncertain, and in the fine aggregate was moderately reactive quartz sand. Under simulated service exposure conditions on road pavements, the significant expansion of concrete specimens and a significant decrease in the resonance elastic modulus up to 12.5% were observed. The tendency of concrete to expand was related to the presence of reaction products of reactive silica in the aggregate grains with sodium and potassium hydroxides in the cement paste [ASR], confirmed by microscopic observations. Replacement of Portland cement with CEM II/A-V 42.5 N cement and CEM II/B-S 42.5 N cement significantly reduced concrete expansion and improved the stability of elastic properties. In two of the nine concrete mixtures, the selected content of siliceous fly ash and granulated blast furnace slag, 18% and 30%, respectively, turned out to be sufficient to counteract the deleterious effects of the ASR under conditions of external contact with sodium chloride solution. A practical method for predicting the durability of concrete in the conditions in which the reactivity category of the aggregate from local natural resources is uncertain was established.

Modifying Water-Frost Resistance and Mechanical Properties of Lime Mortar Using Siliceous and Fluidised Bed Fly Combusted Ashes Activated with Cement.

The research focuses on pozzolanic additives, which are compatible with traditional lime mortars, and enable the modification of the rheological, physical and mechanical properties of tested composites. It was noted that lime mortars with fluidised bed fly ash require sand without impurities to avoid possible ettringite crystallisation. The work presents siliceous fly ash and fluidised bed combustion fly ash to modify the frost resistance and mechanical properties of traditional lime mortars with and without the addition of cement. The results show better effects using fluidised bed ash. Traditional Portland cement CEM I 42.5R was used to activate ash and increase the results. The possibility of a significant improvement of properties is indicated with a hybrid addition to the lime binder of 15-30% ash (siliceous or fluidised bed ash) and 15-30% cement. Changing the class and type of cement provides an additional opportunity to alter the properties of the composites. For architectural reasons relating to colour, the suitability of lighter fluidised bed ash instead of darker siliceous ash and of white Portland cement instead of the traditional grey cement can be used. The proposed mortars can be the basis for future modifications with admixtures and additives, e.g., metakaolin, polymers, fibres, slag, glass powder and impregnating agents.

Effect of superabsorbent polymer and polypropylene fiber on mechanical performances of alkali-activated high-calcium fly ash mortar under ambient and elevated temperatures

This study investigated the mechanical properties of alkali-activated fly ash (AAF) mortars containing superabsorbent polymers (SAPs) and polypropylene (PP) fibers before and after exposure to elevated temperatures. AAF mortars were synthesized using sodium silicate and high-calcium fly ash as the solid activator and aluminosilicate source, respectively. This procedure classified the binder as a one-part AAF mortar. The mechanical properties of the AAF mortar, including compressive and flexural strengths, were investigated considering eight mixes with varying SAP contents (0%, 0.5%, 1%, and 2% by weight of binder) and fiber contents (0 and 0.2% by volume). Moreover, physical properties including the permeability and sorptivity were examined, and physiochemical analysis was performed to evaluate the behavior of AAF mortar at room temperature, 300, 500, and 700 °C. The results show that the compressive strength of specimens at room temperature slightly decreased owing to the increased macroporosity caused by SAPs through pre-swelling, while the flexural strength was maintained via an internal curing effect. The PP fibers significantly enhanced the flexural capacity of the cement mortar, while the compressive strength initially decreased at 28 d and later exhibited a negligible effect at 90 d. The specimens produced using SAPs were effective in preserving the residual compressive and flexural strengths compared with those without SAPs after exposure to 300 °C. Meanwhile, the residual strength of PP fiber specimens diminished after elevated temperature exposure because the melted fiber led to the excessive formation of cavities.

Microstructure, deformation and durability of high-strength non-steam-cured concrete with C-S-H seed

The effect of the early strength agent named calcium silicate hydrate polycarboxylate ether nanocomposite (C-S-H seed), fly ash and slag on the development of microstructure, autogenous deformation and durability, as well as the mechanism of interaction between microscopic (e.g., pore structure) and macroscopic properties of concrete, were investigated. Results showed that C-S-H seed increased the compressive strength of concrete before 7 d. The addition of C-S-H seed decreased both relative humidity and capillary pore diameter due to rapid hydration, which resulted in high negative capillary pressure and autogenous deformation of up to 40%. The higher fly ash and slag contents decreased early-age autogenous deformation. Further analysis showed that the addition of C-S-H seed decreased the percentage of pores>1000 nm and the most probable pore diameter of concrete, thus increasing resistance to chloride migration into concrete. The chloride diffusion coefficient was linearly related to the amount of cementitious material together with the water-to-binder ratio. The C-S-H seed decreased the availability of pores of critical diameter (14 nm) of concrete exposed to water freezing, leading to an increase in the freeze–thaw resistance of concrete. In addition, the C-S-H seed also reduced the overall porosity of concrete, leading to an increase in resistance to sulfate attack. Similar trends were observed for increased fly ash and slag content. Besides, the concentrations of AP and Ca2+ significantly affected sulfate resistance.

Calcined marl as a potential main component of cement

The cement industry is facing the challenge of a lack of supplementary cementitious materials substituting clinker. This applies primarily to granulated blast furnace slag, the most commonly used as the main component of cements, due to changes in the metallurgical industry, and to siliceous fly ash due to the abandonment of fossil fuels in the world. A potential new main component of common cements may be calcined marls, which are available in large quantities for every producer of Portland clinker. In this article, research was carried out on calcined marls from the Folwark deposit, near Opole, Poland. The conducted experiments showed that marls after calcination show pozzolanic-hydraulic properties and can replace both granulated blast furnace slag and silica fly ash in the composition of cement. The best pozzolanic-hydraulic properties are obtained by calcination of Folwark marls at 900°C

Mechanical properties and brittleness of concrete made by combined fly ash, silica fume and nanosilica with ordinary Portland cement

<abstract> <p>This paper introduced a new concrete composites made by quaternary binder by partially replacing ordinary Portland cement (OPC) with different percentages of supplementary cementitious materials (SCMs). The motivation is to reduce our dependency on OPC to reduce CO<sub>2</sub> emission and carbon foot print. As the main substitute for the OPC, siliceous fly ash was used (FA). Moreover, silica fume (SF) and nanosilica (nS) were also used. This study utilized the following contents of SCMs used: 5% of nS; 10% of SF; 0, 15, and 25% of FA. During examinations the main mechanical properties of concrete composites, i.e. compressive strength (<italic>f</italic><sub>cm</sub>) and splitting tensile strength (<italic>f</italic><sub>ctm</sub>) were assed. The brittleness of these materials was also analysed. Based on the conducted studies, it was found that concrete composite based on quaternary blended cements, of series Mix3, has shown the best results in terms of good strength parameters, whereas the worst mechanical parameters were characterized by concrete of series Mix4. On the other hand, concrete including only SF and nS (Mix2 series) were characterized by the greatest brittleness. It was observed that <italic>f</italic><sub>cm</sub> of concrete composites for series Mix2, Mix3, and Mix4 increase of 41%, 48%, and 31% respectively compared with the concrete without additives, i.e. series Mix1. In addition, <italic>f</italic><sub>ctm</sub> also increase of 39%, 47%, and 30%, respectively, for the three series mentioned above, compared with the control concrete. Concrete of series Mix3, with high mechanical properties and demonstrating the features of quasi-plastic material, i.e. having lower brittleness, can be used in concrete and reinforced concrete structures subjected mainly to dynamic and cyclic loads. Therefore, it can be used, in the construction of foundation structures for machines and other types of structures in which the above-mentioned loads are dominant.</p> </abstract>

The Role of Pozzolanic Activity of Siliceous Fly Ash in the Formation of the Structure of Sustainable Cementitious Composites

The following article introduces, in a thorough manner, how the chemical pozzolanic reaction takes place in cement composites containing the fly ash (FA) additive. In the research part, however, the development of phases in the structure of the cement paste in the initial period of its curing and after 28 days from its preparation was traced. For this purpose, a Scanning Electron Microscope (SEM) was used. In order to accurately highlight all the characteristic stages of the formation of the structure of the composite containing FA, an analysis of the cement matrix was carried out between 0.5 and 28 days of their curing. Microstructural studies were complemented by tests of pozzolanic activity of FAs used. In order to conduct a full analysis of this feature, experiments were carried out using two types of research methods, i.e., chemical and physical. On the basis on the conducted studies it was found that: in cement composites with the addition of FA, in the period until the third day of curing, the development of the material structure is mainly the result of the hydration reaction, and between the seventh and fourteenth day after sample preparation, the first signs of the pozzolanic reaction on FA grains are visible; however, in the period between 14 and 28 days, there is a clear homogenization of the structure of the cement composite with the addition of FA, resulting from the change of disordered phases into compact and homogeneous forms and filling in the composite of porous places with pozzolanic reaction products. The use of cement composites based on materials whose application makes it possible to reduce GHG emissions to the atmosphere, reduce energy consumption, and reduce industrial waste landfills leads towards the development of ecological and sustainable building engineering.

Correlation between Compressive Strength and Heat of Hydration of Cement Mortars with Siliceous Fly Ash

This paper presents the results of calorimetric and strength tests of mortars with ordinary Portland cement and two substitution rates (10 and 20%) of cement by siliceous fly ash. The prepared samples were cured under isothermal conditions at four different temperatures: 23, 33, 43 and 53 °C. Heat of hydration was measured using an isothermal calorimeter dedicated to monitor the hydration process of cementitious composites such as paste, mortar and concrete. Thermal data were recorded continuously for 7 days, while compression tests were performed at 6 h and 1, 2, 5, 7 and 14 days of hardening. The studies proved that the amount of heat released correlates with the early-age strength of 50 mm mortar cubes. In the case of samples cured at 23 and 53 °C, a linear relationship between the heat of hydration and compressive strength was observed. The high value of the coefficient of determination (R2) allows one to estimate the mortar compressive strength after 7 and 14 days based on hydration heat tests conducted at standard temperature (23 °C), regardless of the percentage of siliceous fly ash. The increased curing temperatures do not guarantee a high correlation between compressive strength and the amount of heat released.

The influence of granite powder waste and the addition of fly ash on concrete bleeding

AbstractBleeding is a common problem in concrete slabs, and may lead to serious damage. The goal of this article is to understand the impact of alternative binders and their properties on the bleeding of concrete. Therefore, the impact of the type of binder on the bleeding process is investigated. The results show that the addition of granite powder or fly ash allows for the increased control of the bleeding process. It was found that a finer particle size distribution (PSD), an increased specific surface area (SSA), and a higher bulk density may reduce the amount of dispensed water in the concrete mix. Furthermore, the use of additives with an increased SSA leads to even a 30% reduction of the bleeding rate of mixes. The utilization of additives with a finer PSD than cement enables a 37% reduction of the bleeding rate of mixes. The influence of bleeding on compressive strength was assessed using destructive and non‐destructive tests: Replacing 30% of cement with granite powder leads to a 30% reduction of concrete strength after 28 days of curing; on the other hand, replacing 30% of cement with siliceous fly ash leads to an 18% reduction of strength. Importantly, bleeding was also found to lead to the heterogeneity of the physical and mechanical properties across the concrete section. Consequently, the proper control of the bleeding process leads to more homogeneous properties of concrete across its cross‐section.

Effect of Foaming Agent, Binder and Density on the Compressive Strength and Thermal Conductivity of Ultra-Light Foam Concrete

The study is focused on ultra-light foam concrete (FC) aimed as a thermal insulation material. Two important properties of such material were investigated: compressive strength and thermal conductivity. In the conducted tests, the influence of the air-dry density (200–500 kg/m3), type of foaming agent (synthetic and protein) and binder type (ordinary Portland cement—OPC; calcium sulphoaluminate cement—CSA; metakaolin; siliceous fly ash—SFA; calcareous fly ash—CFA) on the mentioned properties were examined. The results confirmed the dependence of compressive strength and thermal conductivity on the FC density but also indicated the important effect of the nature of the foaming agent and the binder type. The best thermo-mechanical properties were obtained for the foam concrete made of protein-based foaming agent, OPC and metakaolin. Simultaneously, the use of CSA mixed with metakaolin and foam based on the synthetic foaming agent also shows satisfactory properties.

Assessment of blended coal source fly ashes and blended fly ashes

The availability of traditional siliceous Class F fly ash, produced when burning bituminous coals, is decreasing around the world as many countries begin to switch to renewable energy sources. Existing coal burning power plants are also starting to convert from bituminous to subbituminous coals, which produce calcareous Class C fly ash, or to a blend of the two to reduce emissions and cost. To prolong the availability of siliceous fly ash, suppliers have started distributing blended coal ashes and have also begun to blend siliceous fly ash with other coal combustion products (CCPs), such as calcareous fly ash or bottom ash, or natural pozzolans. A variety of these blended fly ashes were examined in this study and compared to a traditional siliceous fly ash. Although these fly ashes have similar oxide compositions and physical characteristics to a traditional siliceous fly ash, the crystalline and glassy phases present in the fly ash may differ. This can alter cement hydration and result in failure to mitigate expansion due to sulfate attack. In all other performance and durability tests conducted in this study, all fly ashes performed well in cement-based mixtures.

Hydration Mechanisms of Alkali-Activated Cementitious Materials with Ternary Solid Waste Composition.

Considering the recent eco-friendly and efficient utilization of three kinds of solid waste, including calcium silicate slag (CSS), fly ash (FA), and blast-furnace slag (BFS), alkali-activated cementitious composite materials using these three waste products were prepared with varying content of sodium silicate solution. The hydration mechanisms of the cementitious materials were analyzed by X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, and energy dispersive spectroscopy. The results show that the composite is a binary cementitious system composed of C(N)-A-S-H and C-S-H. Si and Al minerals in FA and BFS are depolymerized to form the Q0 structure of SiO4 and AlO4. Meanwhile, β-dicalcium silicate in CSS hydrates to form C-S-H and Ca(OH)2. Part of Ca(OH)2 reacts with the Q0 structure of AlO4 and SiO4 to produce lawsonite and wairakite with a low polymerization degree of the Si-O and Al-O bonds. With the participation of Na+, part of Ca(OH)2 reacts with the Q0 structure of AlO4 and the Q3 structure of SiO4, which comes from the sodium silicate solution. When the sodium silicate content is 9.2%, the macro properties of the composites effectively reach saturation. The compressive strength for composites with 9.2% sodium silicate was 23.7 and 35.9 MPa after curing for 7 and 28 days, respectively.

The effect of silica fume on the washout resistance of environmentally friendly underwater concrete with a high-volume of siliceous fly ash

Concrete with a high-volume of siliceous fly ash has recently been widely applied as underwater concrete (UWC) in marine structures. This study investigated the effect of the content of silica fume (SF) and the source of fly ash (FA) on the washout resistance of high-volume fly ash concrete (HVFAC) mixes. Class F fly ash sourced from two different locations in India was used to prepare eight HVFAC mixes, which were stabilized using increasing SF contents of 0, 5, 10, and 15 wt%, respectively. The washout loss of these UWC mixes was determined using a special plunge test with a long acrylic washout tube and a perforated steel container. Moreover, other properties such as the slump, air content, strength, void content, water permeability and sorptivity, and drying shrinkage of the prepared mixes were also validated. An enhancement in slump and washout resistance of up to 19% and 11% were noticed when the UWC mixes were prepared using FA with a greater surface area. With an increase in the content of SF, there was also an increase in the loss of the workability and strength of the UWC mixes. Moreover, the washout resistance and durability of these mixes increased. This confirms that the inclusion of SF stabilized the HVFAC mixes against water environments by adsorbing more free water on their surfaces. The pozzolanic reaction and microstructure densification mechanisms also increased the stabilization of the HVFAC. Designing UWC using HVFAC mixes stabilized with SF is environmentally friendly, and the application of such concretes in underwater environments is simple and cost-effective.

Characterisation of the Hydration Products of a Chemically and Mechanically Activated High Coal Fly Ash Hybrid Cement

Cement companies are significant contributors of the planet’s anthropogenic CO2 emissions. With increased awareness of the substantial volume of CO2 emissions from cement production, a variety of mitigation strategies are being considered and pursued globally. Hybrid cements are deemed to be technologically viable materials for contemporary construction. They require less clinker than that for ordinary Portland cement, leading to a decrease in CO2 emissions per tonne of hybrid cement manufactured. The hybrids produced in this study consist of 70% siliceous coal fly ash and 30% Portland cement, and combines chemical (sodium sulphate) and mechanical (milling) activation. The aim of this work was to develop a better understanding of the hydration products formed and the resulting effect of activation on these hydration products, of hybrid coal fly ash cement pastes over an extended curing period of up to one year. The results indicated that chemical activation increases the formation of stable, well crystallised ettringite. Chemical activation as well as mechanical activation increased the rate of the pozzolanic reaction between portlandite contained in cement and coal fly ash. The application of combined chemical and mechanical activation definitely resulted in the fastest rate of portlandite consumption, hence an increased rate of the pozzolanic reaction.

Engineering of green cementitious composites modified with siliceous fly ash: Understanding the importance of curing conditions

Siliceous fly ash can be applied as a cement replacement material for mortars used in cement-based floor overlays. At present, there is no knowledge of the appropriate curing conditions that need to be applied on concrete floors. Moreover, previous research related to curing conditions has left research gaps related to the conditions in which such floors are cured. This study focuses on understanding the importance of the curing conditions that are typical for concrete overlays on the physical, chemical, mechanical, and microstructural properties of siliceous fly ash-modified mortars. The results indicate that curing conditions have a significant impact on these properties (particularly long-term effects). Differences were observed between siliceous fly ash-modified mortars cured in different conditions. In this research, we investigated the volume porosity, water absorption, microstructure, hydration process, pore segmentation, and strength of composites that have had cement partially replaced with the addition of fly ash. It should be noted that the use of siliceous fly ash in properly cured mortars that are applied in floor overlays could lead to a better use of green cementitious composites.