Pozzolanic Reaction Of Fly Ash Research Articles

Characterization of a fly ash-based hybrid well cement under different temperature curing conditions for natural gas hydrate drilling

This paper presents a study on the temperature effect (3°C, 10°C, and 50°C, in water bath) and activator effect on the characterization of a fly ash-based hybrid well cement, with a large fraction of fly ash (approximately 70 %), which has potential for natural gas hydrate drilling due to its low heat release. The mechanical strength, micro/nanostructure characteristics were investigated through various tests, including x-ray diffraction (XRD), thermogravimetry analysis (TGA-DTG), nuclear magnetic resonance (MAS NMR), and scanning electron microscope (SEM), to reveal the hydration mechanism of the fly ash-based hybrid well cement. The results indicate that the low curing temperatures (3°C, 10°C) inhibited both the hydration of the well cement and the pozzolanic reaction of fly ash in the cement system. However, the incorporation of solid Na2SO4 significantly contributed to the pozzolanic reaction during the hydration process by consuming more calcium hydroxide, promoting more ettringite formation and reducing the apparent activation energy of the blend cement (from 53.06 kJ·mol−1 to 41.23 kJ·mol−1), resulting in better compressive strength. MAS NMR results showed that the addition of solid Na2SO4 facilitated the substitution of Al with Si atoms, leading to the formation of more C-A-S-H and C-(N)-A-S-H gels in the system with a denser microstructure. Overall, the fly ash-based hybrid well cement can be developed as a cement system for natural gas hydrate drilling in the ocean. This cement not only exhibits low heat release but also shows improved strength development, with a 72-hour compressive strength of 8.05 MPa when cured at 10 °C.

Novel porous ambient temperature cured fly ash geopolymer for lead adsorption from wastewaters

ABSTRACT This paper discusses the ability of the fly ash (FA) and abmbient cured Alkali activated Fly ash geopolymer (FAAG) for sequestering Pb (II) ions from aqueous solution. Various physiochemical properties such as the influence of pH and contact time were investigated for achieving the conditions for maximum removal via batch mode using powdered samples. The adsorbents were characterized by employing powder X-Ray diffraction, Attenuated Total Reflection Fourier Transform Infra-Red spectroscopy, and Scanning Electron Microscopy/EDAX before and after the treatment. The adsorption capacity of Pb2+ ions by raw FA and FAAG were examined by the Inductively coupled plasma mass spectrometry (ICP-MS). The results represented that the maximum adsorption capacity of Pb2+ ions by FAAG is 75 mg/g with attained equilibrium time of 120 min whereas raw fly ash at 150 min at 21.5 mg/g capacity. The pozzolanic reactivity of fly ash in water glass hardening resulted mainly N-A-S-H gel and traces of lead silicate due to the immobilization of lead in the network. This was conformed from the binding energy lines at 139.7 eV in XPS survey lines and lead silicate at 25 and 34° 2θ as identified in XRD phase assemblages. ICP-MS test results of leachate of lead immobilized geopolymer sample are devoid of lead ions and thus sorption sites of fly ash geopolymer lead to potential safety disposal of lead. Thus, recycling of fly ash into geopolymer by less energy intensive process and subsequent use in heavy metal disposal could achieve the concept of disposal waste and recycling, which could contribute to achieve the sustainable goals.

Long-term Pull-out behavior of coir fiber from cementitious material with statistical and microstructural analysis

In this study, the long-term pullout behavior and microstructure of the embedded coir fibers (a bundle of multi-monofilament fibers) from cementitious matrix were investigated. The experimental results were analyzed through statistical tests (i.e., Shapiro-Wilk test, Kolmogorov-Smirnov test, and Analysis of variance with Tukey’s honestly significant difference test) to address the reliability of the analyses and conclusions. The duration of fiber embedment in the matrix (i.e., 1, 2, 4, 8, 12, and 24 weeks), and the addition of fly ash (i.e., weight replacement ratio to cement for 0%, 10%, and 20%) were considered as the test parameters. The results show that the fibers with larger diameters tended to have tensile failure rather than pullout failure during the test. When the fibers had larger embedment length in the cementitious material, the pullout load–displacement curve changed from slip-hardening to slip-softening pattern. The highest fiber pullout strength was recorded when tested at 24-week. However, the highest fiber pullout energy was not recorded at 24-week but at 2-week. It can be concluded that with a longer embedment duration, the interfacial bond between fiber and matrix will increase but the coir fiber itself will be embrittled. This is due to the penetration of mortar into fiber surface along with a longer embedded duration, which was observed under scanning electron microscope. The pozzolanic reaction of fly ash did not improve the fiber–matrix interfacial bond strength. To the contrary, the pullout energy of the samples with 20% fly ash at week 1 was significantly lower than that with 10% fly ash or without fly ash. This was due to the unreacted spherical fly ash particles on the fibers surface that acted as lubricant during pullout.

Synthesis and application of calcium silicate hydrate (C-S-H) nanoparticles for early strength enhancement by eco-friendly low carbon binders

Abstract C-S-H–PCE nanocomposites are known to perform as highly effective seeding materials in Portland cement. Here the effectiveness of a C-S-H–PCE nanocomposite admixture on early strength development of eco-friendly cements (clinker blended with up to 35% fly ash or calcined clay) was investigated. First, C-S-H–PCE nano-composites were synthesized from Ca(NO3)2/Na2SiO3 via a co-precipitation method and their particle size distribution was analyzed. Differently structured PCE comb polymers based on IPEG chemistry were used in the synthesis. Nanofoils of ∼50 nm length were identified as most promising seeding material. Furthermore, addition of such C-S-H–PCE nanofoils was found to substantially increase mortar strength at early ages, particularly after 6–24 h of curing. Isothermal heat flow calorimetry and in-situ XRD measurements indicate that silicate hydration in cement is significantly accelerated. Most remarkably, C-S-H–PCE also was confirmed to speed up the pozzolanic reaction of fly ash and calcined clay and to stimulate the formation of hemicarbo aluminate (Hc), especially in calcined clay cement. Thus, C-S-H–PCE can foster the more widespread use of eco-friendly CEM II/III cements.

Micro aggregate and pozzolanic reactivity of fly ash: effect on fracture properties

ABSTRACT The performance of fly ash blended concrete is influenced by the pozzolanic and/or micro-aggregate effect of fly ash particles based on the replacement levels. Dominance of either of this effect greatly influences the mechanical and fracture properties of fly ash blended concrete. In view of this, in the present study, 0%, 20%, 30%, 35%, 40%, 50%, 60% and 70% of fly ash is replaced for cement to investigate the dominance of pozzolanic or micro aggregate effect. The results obtained from the study indicate the reduced strength with increased replacement of fly ash. Similarly, the peak load at fracture decreases with the increased replacement of fly ash. Further, it is concluded that with the increased curing age the compressive strength increases and the fracture strength decreases. Furthermore, the decreased content of calcium hydroxide (CH) with increased fly ash content as obtained from thermo gravimetric analysis (TGA) confirms the presence of excessive fly ash that can contribute as filler material demonstrating the micro-aggregate effect.

INFLUENCE OF HIGH-TEMPERATURE CURING IN EARLY AGES ON COMPRESSIVE STRENGTH AND POZZOLANIC REACTION OF MORTAR MIXED WITH FINE MILLED FLY ASH

Compressive strength and pozzolanic reaction of specimens mixed with fine milled fly ash were examined. Mortar specimens mixed with three types of fine milled fly ash were cured under high-temperature condition in early ages. It was clarified that smaller particle size of the fly ash improved the early strength development of mortar. Moreover, both of finely milling fly ash particle and high-temperature curing in early ages remarkably accelerated the pozzolanic reaction of fly ash. The synergistic effect provided the acceleration of the strength development of mortar.

Investigating the physical and chemical contribution of ground low-quality fly ash particles to cementitious composites

This work investigates grinding as a potential solution to improve the utilisation of low-quality fly ashes in concrete. Factors contributing to the performance of ground fly ash in cementitious composites were investigated. It was found that grinding significantly crushed large cenosphere particles and increased the specific gravity and surface area of fly ash, which led to a higher filler effect and improved the rate of cement hydration, contributing to strength development. The comparison of strength of quartz-blended pastes and fly ash-blended pastes showed that the pozzolanic reaction of fly ash marginally contributes to strength development. The major influence of ground particles was seen in the region near aggregates. Ground fly ash significantly increased the microhardness of the interfacial transition zone near aggregates. Since this zone has a direct influence on the failure of mortars and concrete, its improvement resulted in a comparable strength of fly ash-blended mortars to the reference ordinary Portland cement mortar.

Effect of mechanical activation on reaction mechanism of one-part preparation fly ash/slag-based geopolymer

One-part geopolymers are a greener alternative to Portland cement and are more suitable in engineering applications compared with two-part geopolymers. The effects of mechanical activation on the properties and the mechanism of the pozzolanic reaction of fly ash (FA)/slag-based geopolymer paste prepared using the one-part method were studied. Simple mixing was used for the control group and the effect of mechanical activation on the macroscopic properties of geopolymers was studied using tests for compressive strength, fluidity and setting times. The effect of mechanical activation on the pozzolanic reaction of geopolymers was assessed using isothermal calorimetry, X-ray diffraction, Fourier transform infrared spectroscopy and thermogravimetric analysis. The results showed that the 28-day compressive strength of the geopolymer formed by mechanical activation was 26% higher than that of the geopolymer prepared using simple mixing. Workability and fluidity were also enhanced. The reactivity of the precursor was improved by mechanical activation, particularly for the FA. The mechanically activated FA experienced a pozzolanic reaction within 7 days, while the undisturbed FA produced pozzolanic activity after 14–28 days. In addition, mechanical activation lessened the carbonisation of the one-part FA/slag-based geopolymer The implications of these results in terms of the influence of mechanical activation on pozzolanic activity are discussed.

Contribution of fly ash to mortar strength development under steam and internal curing

The purpose of this study is to quantitatively evaluate the effects of steam curing and internal curing on contribution of fly ash to strength development of mortar by using cementing efficiency factor (k-value) that represents strength development performance as a binder of fly ash. In addition, the pozzolanic reaction of fly ash was evaluated from the viewpoint of calcium hydroxide consumption by using thermogravimetry and differential thermal analysis as well as the degree of fly ash reaction by using selective dissolution method. The result indicated that steam curing improved early compressive strength and internal curing improved compressive strength and k-value at all ages. Also, a linearrelationship between the degree of fly ash reaction and the k-value was shown regardless of the age and the replacement ratio of fly ash.

Strength, permeability and microstructure of self-compacting concrete with the dual use of ferronickel slag as fine aggregate and supplementary binder

The study herein addresses the scope to maximize the combined use of by-product ferronickel slag (FNS) as a fine aggregate and ground FNS (GFNS) as a supplementary binder to enhance the sustainability of self-compacting concrete (SCC). The development of strength, permeability and microstructures of SCC containing up to 50% supplementary binder and 50% FNS fine aggregate were investigated. It was found that the fresh properties of concrete incorporating FNS and GFNS remained well within the recommended limits of the European standard for SCC. The 28 days compressive strength of high strength SCC using FNS and GFNS varied within 10% of that using natural sand and fly ash as an SCM. Splitting tensile strength and elastic modulus could be predicted by the standard equations used for conventional vibrated concrete. Tests such as volume of permeable voids, water absorption, sorptivity and water penetration showed good physical properties of SCC incorporating FNS and GFNS. Investigation by SEM and EDS showed a compact microstructure by the reaction product of GFNS which is similar to that produced by pozzolanic reaction of fly ash. Overall, up to 30% cement replacement by GFNS and 50% sand replacement by FNS was found feasible for the production of SCC of high strength and good physical properties indicating high durability.

Predicting the chloride diffusion in concrete incorporating fly ash by a multi-scale model

A prediction model was developed in this paper to quantify the effect of fly ash on the chloride diffusion process in concrete samples. To capture the hierarchical characteristics of constituents at multiple scales, the model adopted the general self-consistent (GSC) method to calculate the effective chloride diffusivity of fly ash concrete. Moreover, the proposed model adopted the Papadakis model to estimate the amount of hydration products of fly ash pozzolanic reaction as well as cement hydration. In addition, the chloride diffusivity of fly ash concrete was corrected by introducing the filling effect of unreacted fly ash particles and the aggregate tortuosity factor. Beside the theoretical modelling, this study also carried out RCM and RCPT tests on concrete samples with different replace ratio of fly ash, whose measurements were used to validate the proposed model. A good agreement was obtained between model calculations and experimental measurements demonstrates the proposed model can successfully characterize the effect of fly ash on chloride diffusion in concrete samples, which can be an effective means to assessing the durability of fly ash concrete materials and structures in practical engineering.

Influence of triethanolamine on mechanical strength and hydration performance of blended cement containing fly ash, limestone and slag

Triethanolamine (TEA) is known as a widely-used cement additive for regulating setting time, hydration process and mechanical strength. However, the working mechanism of TEA on blended cement with various kinds of minerals, has not yet thoroughly explored. This invesitigation focused on effects of TEA on the strength enhancement and hydration properties of the blended cementitious materials, containing fly ash, slag, limestone. Hydration heat evolution of the composite binder was studied by isothermal calorimetry. Pore size distribution was analyzed through mercury injection porosimetry (MIP), and the evolution of hydration products in the function of TEA was investigated by X-ray diffractometry (XRD), differential thermal analysis (DTA) and thermogravimetric analysis (TG). Microstructure and composition characteristics of hydration products were studied through scanning electron microscopy (SEM) equiped with energy dispersive X-ray spectroscopy (EDX). Results showed that strength enhancement effect of TEA varied when the component and proportion of cementitious materials altered. Containing 30% fly ash and 10% limestone, the composite binder got the optimal strength-enhancing effect to nearly 15%. When containing 30% slag and 10% fly ash, the composite binder obtained the worst strength-enhancing effect with about 5%. The addition of TEA prolonged the induction period, and enhanced the formation of AFt. With high aluminate glass phases, fly ash was vulnerable to be influenced by the complexation of TEA, and hence the pozzolanic reaction of fly ash was accelerated significantly, resulting in reduced content of Ca(OH)2 (CH) as well as increased production of hydrates. Limestone participated to accelerate the hydration of C3A, and provided microcrystals for the growth of hydrates. Slag was mainly composed of high calcium glass phases and was rarely impacted by the complexation of TEA. The findings would gain a comprehensive insight of TEA in affecting the hydration process of blended materials, and would provide expedient information on the technique to control the hydration progress of composite binder.

Effect of Super absorbent polymer on workability, strength and durability of Self consolidating concrete

This study presents the effect of super absorbent polymer (SAP) as internal curing agent on workability, durability and compressive strength of self-consolidating concrete (SCC). In order to estimate the internal curing efficiency of SAP in different curing conditions and curing ages, compressive strength and electrical resistivity tests have been performed. Homogenous and denser microstructure was formed by gradual release of water from SAP into pores created by SAP. Further pozzolanic reaction of fly ash has enhanced the strength and durability properties. High desorption rate of water from SAP in air curing condition resulted in an increased electrical resistivity and compressive strength. Compressive strength of internal cured SCC mixtures increased to 15-25% at 7 days and 10-19% at 28 days. Electrical resistivity values were increased 11-30% in water curing condition and 16-53% in air curing condition. The costs for 0.35w/b and 0.40 w/b at optimum internal cured SCC mixtures compared to control SCC mixtures were reduced to 9.39 and 9.70%, respectively.

Mechanical Transformation of Fly Ash toward Its Utilization in Cemented Paste Backfill

Fly ash (FA) showed low reactivity when being used to prepare the binder for cemented paste backfill (CPB). In the present work, wet‐grinding treatment was used to increase the pozzolanic reactivity of FA and promote its sustainable utilization. The results showed that wet‐grinding could be a suitable and efficient technology for FA pretreatment. Wet‐grinding strongly modified the structure of FA by decreasing the crystalline phase content and the binding energy of Si 2p and Al 2p, contributing to the increase in pozzolanic reactivity of FA. The performance of CPB samples prepared by wet‐ground FA was then optimized. This was reflected by the acceleration in the sample setting and increase in the strength development. The compressive strength of the CPB samples prepared by wet‐ground FA for 120 min was increased by around 40% after curing for 28 d compared with the control samples.

Effect of copper slag fine aggregate on corrosion processes and behavior in reinforced concrete prism specimen

This paper reports the result of experimental on the resistance to chloride diffusion and steel corrosion of concrete mixtures. A series of tests were performed to investigate the effect of copper slag as partial replacement of fine aggregate on corrosion processes and behavior in prism specimens. Chloride migration tests in steady-state were executed at the age of 28 and 91 days in accordance with Japan Society of Civil Engineers (JSCE) G571-2003 to determine the diffusion coefficient of chloride ions permeated through concrete specimens. In addition, some electrochemical tests were carried out through the repeated wet and dry (NaCl 10%) cycles up to 1 year to examines the probability of corrosion of steel bars in concrete, corrosion current density, and the rate of oxygen permeability. Based on the chloride migration test, which was obtained from the cumulative increase of chloride ion concentration, the results indicate that concrete with a longer curing period achieved lower chloride-ion concentration, thus suggesting that there is an increase in the resistance of concrete against chloride attack. The chloride penetration depth and distribution of chloride content in CUS prism specimens after exposure to wet and dry cycles for one year are almost similar to those of the OPC specimens. Thus, the durability performance of CUS concretes with respect to the resistance against chloride ingress was comparable to that of OPC concrete. Besides that, it is generally recognized that the incorporation of fly ash in blended cement by its the pozzolanic reaction of fly ash improves concrete protection against chloride-induced corrosion of steel bars by reducing its diffusivity and the rate of oxygen permeability.

Utilization of untreated-phosphogypsum as filling and binding material in preparing grouting materials

The industrial by-product phosphogypsum (PG) is one of the bulk solid wastes with the risk of environmental pollution. Recycling PG as filling and binding material in preparing grouting materials is not only an effective solution to the PG disposal, but also a new approach to replacing the conventional cement-based grouts that conforms to the sustainable development. The flowability, setting time and early strength are vital characteristics for the grouts. Water-resistance should also be considered in underground construction. This paper works on the workability of the PG-based grouting slurry and the mechanical strength of the hardened paste. The water-resistance and permeability of the grouts are investigated. XRD analysis and SEM observation are used to study the hydration products and the microstructure. Experimental results show that the increase of water/binder ratio and PG dosage leads to a longer setting time and a higher flowability. The grouts with superplasticizer dosage of <0.8% and accelerated agent dosage of 0.3 ~ 0.5% obtain an ideal final setting time and flowability for grouting construction. The addition of OPC and silica fume improves both the compressive strength and the water-resistance of grouting materials. Microstructure observation shows that the formation of the ettringite and C-S-H gel is responsible for the early strength. The filling effect and pozzolanic reaction of fly ash and silica fume contribute to the compactness of the structure.

Research on properties evolution of ultrafine fly ash and cement composite

Compared with fly ash, addition of ultrafine fly ash manufactured by mechanical grinding will significantly improve the performances of the cement-based materials. In this paper, firstly the cement hydration is simulated based on the modified shrinking core model with the concept of multi components. Then it is combined with the fly ash pozzolanic reaction via the contents of capillary water and calcium hydroxide (CH) in the blended cement paste. The effects of the heterogeneous nucleation, fineness of fly ash and local w/c augmentation are quantitatively analyzed and incorporated into the hydration models. The integrated models are validated by the contents of CH and non-evaporable water (NEW) in the (ultrafine) fly ash and cement composite The coupled evolution of cement hydration, pozzolanic reaction and porosity is presented using the integrated hydration models. Compared with the fly ash and cement composite (FAC), it is found that the extent of the pozzolanic reaction significantly increases in the ultrafine fly ash and cement composite (UFAC) while the increment of cement hydration extent due to nucleation effect is relatively small. Furthermore, the contributions of gel content by different effects are quantitatively calculated and compared between UFAC and FAC. Finally, an empirical formula is established to predict the compressive strength of the hardening UFAC. The filling effect of the ultrafine fly ash, gel content and cumulative pore volume are adopted to evaluate the development of the compressive strength. The results indicate that the physical filling effect of ultrafine grains reflected on the compressive strength of UFAC will gradually increase during the first seven days of curing and then keeps constant. With the increase of cement replacement ratio (0 ~ 0.5), the physical filling effect brought by UFA will be greatly enhanced. The analyses presented in this paper will give a better understanding on the hydration process and strength development for the FAC and UFAC.

Evolution of aluminate hydrate phases in fly ash-cement system under the sulfate conditions

Abstract The aluminate hydrate phases play an important role in the sulfate attack of concrete. In this paper, the aluminum hydrate phases evolution of fly ash blended cement paste powders immersed in different sodium sulfate solution is studied. In order to accelerate the reaction, the fly ash blended cement paste powders are immersed in different sulfate solution at ambient temperature up to 28 d. The results show that the hydration of fly ash and the distribution of aluminum hydrates phases in the hydration products of fly ash-cement blends are significantly influenced by the sulfate. The addition of fly ash into the cements introduces the reactive Al3+ into the systems, leading to an increase of Al(IV)/Si and mean chain length of C-A-S-H. Furthermore, although the pozzolanic reaction of fly ash consumes the calcium hydroxide, where the calcium hydroxide is required for the formation of ettringite, the addition of fly ash is favorable for the formation of ettringite.

Strengthening mechanism of lightweight cellular concrete filled with fly ash

Fly ash has been used to fill the lightweight cellular concrete (LCC) as the primary admixture to promote comprehensive engineering applications. However, the strengthening mechanism of LCC resulted from fly ash is not well understood. In this paper, the strengthening mechanism of LCC filled with fly ash (LCCFFA) was investigated with respect to the microstructures including porosity and skeleton characteristics. The variation rules of average pore size (PS), pore roundness value (PRV), and fractal dimension of pore distribution (FDPD) with different fly ash contents were explained. The relationship between microscopic parameters of PS, PRV, and FDPD and the overall compressive strength, along with the wet bulk density was established. Furthermore, the difference of skeleton structure evolution mechanism between LCC and LCCFFA was investigated with the results showing that the difference comes from the effects of ball and particle filling of fly ash when before 28 days. While after 28 days, the pozzolanic reaction of fly ash and hydration reaction of cement mutually promote the densification of skeleton structure. Finally, the SEM energy spectrum diagram was analyzed to further reveal the strengthening mechanism. From the perspective of characteristic pore parameters and skeleton hydration products, the recommended content of fly ash is approximately 25%. The ratio between Si and Ca (Si/Ca) can determine the degree of reaction of fly ash, and can therefore be used as a judging criterion to determine whether the content of fly ash is appropriate.

Lowering carbon foot print of Portland cement by class F fly ash substitution in mortar mixture in terms of workability and strength properties

In the experimental work, recycling by inclusion of class F fly ash in mortar was investigated in terms of workability and strength of mortar to fabricate an environment friendly construction material. For this aim, class F fly ash were used as cement replacement at 10%, 30%, 50% and 70% ratios in mortar. Water-binder ratios were 0.40, 0.45 and 0.50. A total of 15 different mortar mixtures including control Portland cement and fly ash mortar were produced. Workability of fresh mortar was measured using mini flow testing. Flexural and compressive strength of hardened standard sized samples were measured after completion of their specified curing time at 1d, 3d, 7d 28d, 3m and 6m. Experimental results showed that inclusion of fly ash in mortar improved workability with respect to control Portland cement mortar. Recycling low amount of fly ash in mortar did not show detrimental but beneficial influence on strength properties of mortar. However, employing high amount of fly ash in mortar reduced flexural and compressive strength at early ages, however, the reduction in flexural and compressive strengths were remedied due to pozzolanic reaction of fly ash at longer curing time compared to control Portland cement mortar. It was concluded that current fly ash was a suitable pozzolanic material to be recycled by replacement with Portland cement in mortar to manufacture clean construction material in terms of workability and strength concern.