Pozzolanic Reaction Of Fly Ash Research Articles

Effectiveness of a calcium silicate hydrate – Polycarboxylate ether (C-S-H–PCE) nanocomposite on early strength development of fly ash cement

C-S-H–PCE nanocomposites are known seeding materials for Portland cement. Here, the effectiveness of a C-S-H–PCE admixture on early strength development of a fly ash blended cement (35 wt% fly ash) was investigated. It was found that addition of C-S-H–PCE drastically increases the compressive strength of mortar and concrete, particularly at 6–24 h of hydration. XRD and calorimetric measurements confirm that the hydration of C3S/C2S in cement is greatly accelerated. Most interestingly, however, the C-S-H–PCE nanocomposite also stimulates the pozzolanic reaction of fly ash, thus explaining the exceptionally high early strength values. Accordingly, C-S-H–PCE might also be useful in other CEM II cements holding pozzolanic materials.

Effect of sodium sulfate and nano-SiO2 on hydration and microstructure of cementitious materials containing high volume fly ash under steam curing

Generally, high volume fly ash would considerably reduce the early strength of cement-based materials, and because of this disadvantage, the added amount of fly ash in steam-cured precast concrete is very limited. In this study, an attempt to enhance the early strength of steam-cured cement-high volume fly ash (40 wt%, CHVFA) system has been done with addition of nano-SiO2 (NS) and sodium sulfate (SS). Effect of these two chemicals on hydration, pore structure, and microstructure of interfacial transition zone (ITZ) were investigated. The results show that both NS and SS can improve the compressive strength, and the reason is mainly because these two chemicals can accelerate the hydration of cement minerals and can also hasten the pozzolanic reaction of fly ash. By contrast, because of the high activity of pozzolanic reaction and the filling and nucleation effects, NS has much stronger effect on refining pore structure and promoting the hydration of cement; while SS can exert much stronger effect on accelerating the hydration of fly ash than that of NS. An interesting thing can also be seen that with co-doped NS-SS, the enhancement in compressive strength is much stronger than that of the sum of these single systems with the same dosage, indicating that NS and SS can exert the synergistic effect; this effect is involved in the accelerated hydration of CHVFA, the refined pore structure, and the improved microstructure of ITZ. Such results would be expected to provide guidance on enhancing the early strength of steam-cured precast concrete with high volume fly ash in practical engineering.

Effect of TIPA on Chloride Immobilization in Cement‐Fly Ash Paste

Utilization of sea sands and coral aggregate for concrete in ocean construction is increasingly attracting the attention all over the world. However, the potential risk of steel corrosion resulting from chloride in these raw materials was one of the most concerned problems. To take this risk into account, chloride transporting to the surface of steel should be hindered. The formation of Friedel’s salt in hydration process is widely accepted as an effective manner for this hindrance. In this study, an attempt to hasten the formation of Friedel’s salt by adding triisopropanolamine (TIPA) was done in the cement‐fly ash system, with intention to chemical bind chloride, and the chloride‐binding capacity at 60 d age was examined. The results show that TIPA can enhance the chloride‐binding capacity of cement‐fly ash paste at 60 d age, and the reason is that the formation of Friedel’s salt can be accelerated with addition of TIPA. The mechanism behind is revealed as follows: on the one hand, the accelerated cement hydration provides more amount of calcium hydroxide to induce the pozzolanic reaction of fly ash, which can hasten the dissolution of aluminum into liquid phase; on the other hand, TIPA can directly hasten the dissolution of aluminum in fly ash, offering more amounts of aluminum in liquid phase. In this case, the aluminum/sulfate (Al/S) ratio was obviously increased, benefiting the formation of Friedel’s salt in hydration products. Such results would expect to provide useful experience to promote the chloride‐binding capacity of cement‐fly ash system.

Study on Strength Behavior of Organic Soil Stabilized with Fly Ash.

The aim of this study is to investigate the effect of fly ash on the consistency, compactness, acidic properties, and strength of organic soil. The presence of organic content in the soil has detrimental impacts on the physical and strength behavior of soil. To investigate the effectiveness of fly ash in the stabilization of organic soil, two types of fly ashes (Type I and Type II) at different percentages were used. It is found that fly ash significantly reduces the plasticity index of the organic soil, whereas the liquid and plastic limits increase. The dry density of the fly ash-soil mixture increases significantly, while the water requirement reduces due to the addition of fly ash. The increase of dry density compromises higher strength. The increase of qu with the increase of fly ash content is mainly due to the pozzolanic reaction of fly ash, although the reduction in water content results from the addition of dry fly ash solid. Moreover, Type I fly ash contributes a higher value of qu compared to Type II fly ash. This is attributed to the characteristics of fly ash including CaO and CaO/SiO2 ratio.

Effect of fly ash on the pore structure of cement paste under a curing period of 3 years

The service life and durability of concrete structures strongly depend on the transport properties of the concrete. These transport properties are highly related to their microstructure, especially the pore structure of the hydrated cement paste. In this paper the microstructure development of cement paste and cement paste blended with fly ash is investigated, from solid phase to pore phase, at a long-term curing period up to 3years. The solid phases are observed by environmental scanning electron microscope. The pore structure of blended cement paste is determined by mercury intrusion porosimetry. The results indicate that the addition of fly ash increases the total porosity of cement paste, not only at early ages, even at curing age of 3years. The voids in hollow fly ash particles act as ink-bottle pores with progress of the pozzolanic reaction of fly ash, resulting in the increase of the total porosity of blended cement paste.

Pozzolanic reaction of fly ash modified by fluidized bed reactor-vapor deposition

This study aimed to introduce fluidized bed reactor-vapor deposition (FBR-VD) into the modification of fly ash (FA) using the activators of NaOH, Na2SO4, and NaCl. The compressive strength, non-evaporable water and the hydration degree fly ash (HDFA) were investigated. The scanning electron microscopy (SEM), and the X-ray powder diffraction (XRD) Rietveld analysis were also conducted. The results demonstrate that FBR-VD can benefit the compressive strength when the dosage rates of NaOH, Na2SO4 and NaCl are 0.1%, 0.5%, and 0.5%, respectively. The non-evaporable water and HDFA also increased at certain activator dosage rates. A modification layer consisting of nanometer-scale crystals forms on the surface of the FA beads, with NaOH or Na2SO4 applied. NaOH can react with FA and promote its pozzolanic activity. Na2SO4 and NaCl can benefit the HDFA by remaining undissolved, to increase their concentration around FA. The enhanced HDFA is also proved by the consumption of Ca(OH)2.

Effect of Internal Activation Using Porous Ceramic Aggregate on Hardness and Pore Structure of Fly Ash Cement Paste

The utilization of fly ash not only reduces the environmental impact but also improves some mechanical properties and durability of concrete. However, the early-age strength of fly ash concrete is sometimes lower than that of normal concrete due to the slow pozzolanic reaction of fly ash. In recent years, some researchers have suggested alkali or sulfate activation to accelerate the pozzolanic reaction. Some studies have used sodium hydroxide (NaOH) solution, while others have applied potassium sulfate (K2SO4) or sodium sulfate (Na2SO4) as activators which are effective in accelerating the pozzolanic reaction and increasing the strength at early age. On the other hand, the early-age strength of fly ash concrete is also improved by using porous ceramic aggregate (PCA) as an internal curing agent. Therefore, the present study aims at investigating the effect of an internal activating agent using PCA on hardness and pore structure of fly ash cement paste. In the experimental program, PCA immersed in two kinds of solution (K2SO4 and Na2SO4) was placed in the center of specimen with dimension of 21x21x20 mm. In addition, normal aggregate (NS) was used for reference. As a result, internal sulfate activation using PCA improved the hardness of interfacial transition zone (ITZ) between paste and PCA, and reduced the Ca(OH)2 content in cement paste with 40% replacement with fly ash significantly at the age of 1 day, but negligibly at the ages of 7 and 28 days when compared with reference specimen. K2SO4 was more effective in improving hardness of ITZ as an internal activating agent than Na2SO4. Although the total pore volumes of the fly ash cement pastes using PCA imbibing sulfate activators were not reduced at the age of 28 days, their pore volumes with diameters less than 0.05 µm were increased.

Carbonation Curing of Precast Fly Ash Concrete

AbstractThe feasibility of carbonation curing of precast fly ash concrete is studied. If fly ash concrete can be produced by carbonation curing, the carbon footprint of the products can be significantly reduced. In this paper, the relationship between carbonation reaction and pozzolanic reaction was examined. After carbonation curing with different duration and fly ash content, the cement reaction degree was estimated through the equivalent nonevaporable water content, and the fly ash reaction degree was analyzed through a selective acid dissolution test. It was found that the pozzolanic reaction of fly ash in a fly ash–ordinary portland cement (OPC) system was hindered by early carbonation reaction. The higher the early carbonation degree, the lower the pozzolanic reaction of fly ash. In addition, fly ash–OPC paste was more reactive with carbon dioxide than plain cement paste. Therefore, controlled carbonation at an early age is necessary to trade off the carbon emission reduction with performance gain. ...

Fundamental properties of industrial hybrid cement: utilization in ready-mixed concretes and shrinkage-reducing applications

Utility properties of novel hybrid cement (H-Cement) are influenced by pozzolanic reaction of fly ash, latent hydraulic reaction of metallurgical slag together with the alkali activation of inorganic geopolymer based on precipitated waste water coming from bauxite residues. Content of Portland cement clinker is at maximum of 20 mass %, the remaining portion consists of inorganic geopolymer. Up to 80% of CO2 emissions are saved by H-Cement manufacture compared to ordinary Portland cement (OPC). No heat treatment or autoclaving is needed at H-Cement production. The field application of H-Cement is performed by the same way than that of common cements listed in EN 197-1, and is also connected with highly efficient recovery and safe disposal of red mud waste. H-Cement is suitable for ready-mixed concretes up to C30/37 strength class and is specified by beneficial shrinkage-reducing property of the concrete kept in long dry-air cure opposite to common cements.

フライアッシュを混合したコンクリートの中性化進行に関する定量的検討

We simultaneously studied increase in carbonation inhibition effect by densification of the concrete structure due to pozzolanic reaction and decrease in the same due to consumption of Ca(OH)2, and we attempted to shed light on the effect of mixing fly ash on the durability of concrete with the help of accelerated carbonation test. As a result, In the concrete having the same water-cement ratio, we studied carbonation inhibition effect by combining the effect of consumption of Ca(OH)2 due to pozzolanic reaction of fly ash and increase in compressive strength. Based on the compressive strength of concrete and Ca(OH)2 amount, we could demonstrate the carbonation inhibition effect of fly ash mixed concrete.

Numerical simulation of fly ash concrete under sulfate attack

Portland cement concrete (PCC) suffers sulfate attack in sulfate-rich environments. On the other hand, industrial by-products such as fly ash and slag are routinely added to PCC to improve its properties. However, the impact of fly ash on concrete durability is not always conclusive and it is time- and energy-consuming to test concrete durability in the laboratory or through field observation. This study presents a numerical simulation of the durability of Portland cement concrete made with fly ash. The simulation of sulfate attack on fly ash concrete was performed based on ion transport mechanism. The chemical reactions among chemical compounds of hydrated cement paste and sulfate ions were considered. Additionally, the pozzolanic and hydration reactions of fly ash were incorporated as well to incorporate the impact of fly ash. The pozzolanic and hydration reaction were simulated through the extended version of an existing numerical procedure. Field observation data from the United States Bureau of Reclamation (USBR) was employed to validate the extended simulation program. After pozzolanic and hydration reactions of fly ash were incorporated, the prediction agreed well with the field measurements. The simulation shows that pozzolanic reaction of fly ash can significantly slow down sulfate attack on concrete, thus increasing the sulfate resistance of PCC and extending the service life of concrete infrastructures. The major factors that affect the sulfate resistance of concrete were explored through the numerical simulation as well.

Hydration kinetics of composite binder containing slag at different temperatures

Fly ash has been widely used as supplementary cementitious material in concrete industry. Hydration mechanism of composite binder containing fly ash is much more complicated due to the mutual effect of the hydration of cement and the pozzolanic reaction of fly ash. This paper involves the hydration kinetics of composite binder containing up to 65 % of fly ash and comparison of the results with data on composite binder containing slag that are previously published. The hydration heat evolution rate and cumulative hydration heat of composite binder containing fly ash were measured at 298, 318 and 333 K with an isothermal calorimeter. Based on the hydration kinetics model, three hydration processes, namely nucleation and crystal growth (NG), interactions at phase boundaries (I) and diffusion (D) were characterized, the relationship between the hydration rate and hydration degree was discussed at different stages, and kinetics parameters, n, K and Ea, were calculated and analyzed. Results show that the hydration heat evolution rate and cumulative hydration heat of composite binder obviously decrease with increasing the replacement ratio of fly ash. Elevated temperatures promote the hydration process, especially for composite binder containing high amount of fly ash. The kinetics model could simulate the hydration process of composite binder containing no more than 65 % of fly ash, whose hydration process sequence is NG → I → D at 298 and 318 K, but it becomes NG → D at 333 K. Fly ash has relatively smaller effect on the overall reaction of composite binder than slag. The reaction rates of composite binder containing fly ash at different stages are higher than those of composite binder containing slag at the same replacement ratio. The value of Ea for the overall reaction of composite binder decreases first and then increases with increasing the content of fly ash, and it is lower than that of composite binder containing slag at the same replacement ratio.

Development of fly ash-based composite binder and its application in construction materials

The effect of addition of metakaolin on hydration and physico-chemical properties of fly ash composite binder prepared by blending 50–60% fly ash with hydrated lime sludge, Portland cement and gypsum plaster were investigated. The metakaolin has a beneficial effect on the properties of composite binder by reducing its porosity and pore diameter with curing period which are responsible for the attainment of high strength and better stability towards water over the composite binder without metakaolin. The strength development in composite binder takes place due to the hydration of gypsum plaster to gypsum and pozzolanic reaction of fly ash, metakaolin with lime and alkali in cement thereby forming ettringite, tobermorite and C4AH13. The reaction products formed during the hydration were confirmed by X-ray diffraction, differential thermal analysis and scanning electron microscopy. The composite binder has low heat of hydration and is found suitable for use in masonry works, concrete and bricks.

Mechanical Properties of Pervious Concrete

In recent times, major cities around the world have experienced frequent flooding due to the combination of increased rainfall and reduced permeable surface areas. As a consequence, the drainage system gets overloaded and flash flooding becomes inevitable, thus causing disruption to the road transport. In order to overcome such issues, an engineered solution is needed. The solution is Pervious Concrete, a special type of concrete having a high void content which ranges from 15% to 35% and is instrumental in recharging groundwater and reducing storm water run-off. Also, considering the environmental effects of Portland cement, it is essential in the concrete industry to minimize the use of cement in the production of both structural and non-structural concrete. One way to reduce cement consumption is to use fly ash as a cement replacement material because of the pozzolanic reactivity of fly ash. This paper reports the results of an experimental investigation into the various mechanical properties of pervious concrete. The various mixes containing cement with varying percentages of fly ash as binding material were used. Although it may seem tough to balance the demand for sustainable development with the need to preserve our natural resources, but with the results obtained so far, this balance can be easily achieved.

Development of an eco-friendly Ultra-High Performance Concrete (UHPC) with efficient cement and mineral admixtures uses

This paper addresses the development of an eco-friendly Ultra-High Performance Concrete (UHPC) with efficient cement and mineral admixtures uses are investigated. The modified Andreasen & Andersen particle packing model is utilized to achieve a densely compacted cementitious matrix. Fly ash (FA), ground granulated blast-furnace slag (GGBS) and limestone powder (LP) are used to replace cement, and their effects on the properties of the designed UHPC are analyzed. The results show that the influence of FA, GGBS or LP on the early hydration kinetics of the UHPC is very similar during the initial five days, while the hydration rate of the blends with GGBS is mostly accelerated afterwards. Moreover, the mechanical properties of the mixture with GGBS are superior, compared to that with FA or LP at both 28 and 91days. Due to the very low water amount and relatively large superplasticizer dosage in UHPC, the pozzolanic reaction of FA is significantly retarded. Additionally, the calculations of the embedded CO2 emission demonstrate that the cement and mineral admixtures are efficiently used in the developed UHPC, which reduce its environmental impact compared to other UHPCs found in the literature.

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.

Permeation Properties and Pore Structure of Surface Layer of Fly Ash Concrete.

This paper presents an experimental study on the nature of permeation properties and pore structure of concrete surface layers containing fly ash. Concretes containing different dosages of fly ash as a replacement for cement (15% and 30% by weight of total cement materials, respectively) were investigated. Concrete without any fly ash added was also employed as the reference specimen. Laboratory tests were conducted to determine the surface layer properties of concrete including chloride transport, apparent water permeability and pore structure. The results demonstrate that incorporation of fly ash, for the early test period, promotes the chloride ingress at the surface layer of concrete but substituting proportions of fly ash may have little impact on it. With the process of chloride immersion, the chloride concentration at the surface layer of concrete with or without fly ash was found to be nearly the same. In addition, it is suggested that the water permeability at the concrete surface area is closely related to the fly ash contents as well as the chloride exposure time. Pore structure was characterized by means of mercury intrusion porosimetry (MIP) test and the scanning electron microscopy (SEM) images. The modification of pore structure of concrete submersed in distilled water is determined by the pozzolanic reaction of fly ash and the calcium leaching effect. The pozzolanic reaction was more dominant at the immersion time of 180 days while the calcium leaching effect became more evident after 270 days.

Effect of Curing Temperature on Pozzolanic Reaction of Fly Ash in Blended Cement Paste

Effect of Curing Temperature on Pozzolanic Reaction of Fly Ash in Blended Cement Paste

低水結合材比におけるセメント硬化体の力学特性に及ぼす高温履歴の影響

高温負荷が低水結合材比のセメント硬化体の力学特性に及ぼす影響を検討するため、普通ポルトランドセメント、高炉セメントB種、フライアッシュセメント用いたセメントペースト及びモルタルを、3種類の異なる温度履歴で養生した。混和材添加のセメントペーストは、高温負荷で水和反応が促進され常温養生に比べ材齢28日強度が増加し、水銀圧入試験からも組織の緻密化が確認された。モルタルに高温負荷を継続的に与えると、いずれも常温養生に比べ材齢28日強度が低下し、自己乾燥による骨材界面の損傷が理由に考えられた。初期高温履歴を与えると、モルタルの材齢91日強度は材齢28日より小さく、材齢経過により強度が低下する可能性が示唆された。

The effect of Superabsorbent Polymers (SAP) on microstructure and mechanical properties of fly ash cementitious mortars

The paper is focused on microstructure and strength development in fly ash cementitious mortars containing superabsorbent polymers. In order to evaluate the effect of different water absorption/desorption kinetics of SAPs, comprehensive MIP and SEM analysis supplemented by the mechanical properties assessments have been performed. The vast majority of pores created by SAP are ‘ink-bottle’ and/or closed. Gradual release of water stored by SAP facilitates formation of denser, more homogenous and predictable cementitious matrix, further enhanced by the pozzolanic reaction of fly ash with portlandite. Addition of SAP with high water absorption capacity results in an increased compressive strength.