High-volume Fly Ash Paste Research Articles

Enhancement of early-age properties of high-volume fly ash–cement paste with hydrated lime powder

Recycling and reusing of high-volume fly ash (HVFA) in the development of construction materials have received much attention from research scholars worldwide. However, the early-age properties of the materials with HVFA are not as good as the conventional cement-based materials. Therefore, the effect of hydrated lime powder (LP) addition on the enhancement of early-age properties of HVFA-cement pastes was investigated in the present study. Various paste mixtures were prepared by adding different LP amounts (5, 10, 15, 20, and 25% by mass) to the HVFA paste containing 30% Portland cement (PC) and 70% fly ash (FA). All of the HVFA paste mixtures were prepared with constant FA/powder (including PC and FA) and water/powder ratios (by mass) of 0.7 and 0.3, respectively. The setting time (ST) and workability of the fresh mixtures were first measured then the tests of compressive strength (CS), water absorption (WA), and drying shrinkage (DS) were performed on the hardened samples of up to 28 days to study the influence of LP addition on the early-age properties of the samples. It is found that adding LP greatly affected the early-age properties of the HVFA-cement pastes in both fresh and hardened stages. In detail, adding LP reduced the flowability and significantly shortened the ST of the HVFA paste mixtures. In addition, the CS values of the 28-day-old HVFA paste samples incorporating 5–15% LP were increased by 20.1–34.4% in comparison to that of the reference sample without LP addition. However, the strength values were found to be decreased remarkably when adding 20 and 25% LP to the paste mixtures. Similarly, the inclusion of 5–15% LP improved the WA and DS behavior of the hardened paste samples. As a result, a 15% LP addition was recognized as the optimum content for enhancing the short-term properties of the HVFA-cement pastes.

Accelerating effect of calcined hydrotalcite-Na2SO4 binary system on hydration of high volume fly ash cement

This work aims to propose a new method to overcome the shortcomings of slow setting and low early strength occurring in cement-based composites with high-volume fly ash (HVFA). The effects of binary system containing calcined hydrotalcite (CHT) and Na2SO4 (NS) on early strength, hydration process, pore structure and microstructural morphology of HVFA composites were investigated. Specifically, the setting time, the compressive strength and pore size distribution of HVFA cement doped with CHT and NS were measured. Isothermal calorimetry, quantitative analysis of Ca(OH)2 and chemically bound water contents, XRD, TGA and SEM were employed to characterize the hydration process, the assemblage of hydration products and microstructural morphology. Results indicated that higher contents of hydration products such as AFt and C-S-H gel in HVFA paste incorporating with CHT and NS were found in XRD and TGA analyses. Consequently, regarding improving mechanical properties and porosity of HVFA cement, the CHT and NS co-doped system illustrated a more outstanding performance than the CHT or NS single system. This improvement was attributed to the accelerated hydration process, increased hydration product content and refined pore structure. These findings are expected to provide guidance on enhancing the early strength of HVFA concrete in practical engineering.

Modification of high volume fly ash composites containing calcined stöber nano-SiO2 particles

The present study aims to explore the influence of 2 wt% calcined stöber nano-SiO2 (CSNS) on hydration properties of high volume fly ash (HVFA) system made with 50 wt% and 70 wt% of cement replacement up to 28 days. In the first part, the reactivity of lab-synthesized CSNS was evaluated based on a simulated pore solution method. The morphology and microstructure of the potential reaction products were analyzed. In the second part, the effect of SNS and CSNS on the preliminary cement hydration was compared by using isothermal calorimetry. Finally, the effects of CSNS on compressive strength, and water absorption were evaluated in HVFA mortars and the microstructural developments were studied in HVFA pastes by analyzing phase changes and pore structures. The experimental results show that both SNS and CSNS presented low pozzolanic activity within 3 days, but can be completely consumed up to 28 days, with the formation of lamelliform calcium silicate hydrate gel. CSNS effectively overcame the negative effects of SNS on cement hydration, and further accelerated cement hydration. This is possible because the hydroxyl group was removed after calcination of SNS, which made the well-dispersed CSNS better accelerate cement hydration by effective nucleation sites and filler effects. 2 wt% CSNS significantly contributed to generating more hydrates and refining pore structures in HVFA cement pastes, which increased the compressive strength of HVFA mortar up to 28 days and also significantly decreased its 28-day water absorption.

Effect of tidal zone and seawater attack on high-volume fly ash pastes enhanced with metakaolin and quartz powder in the marine environment

The behavior of high-volume fly ash (HVFA) cement pastes modified with metakaolin (MK), quartz powder (Q-P) and their combination in the marine environment was studied. HVFA pastes were manufactured by partially replacing Portland cement (PC) with 70% fly ash (FA). To develop the mechanical performance of this mixture, HVFA was modified by partially replacing FA with 20% MK, 10% MK coupled with 10% Q-P and 20% Q-P. After initial curing, the specimens were transferred to different treatment media, namely air, tap water, seawater and wet/dry cycles for 9 M. In the current procedure, the wet/dry cycle was used to simulate the condition of the tidal zone, of which the specimens were immersed in seawater for 18 h followed by drying for another 6 h per day. The phase composition and microscopic structure of the blended cement composites were studied using X-ray diffraction (XRD), thermogravimetric analysis (TGA/DTG) and scanning electron microscopy (SEM). The results indicated that the deterioration in the specimens exposed to the tidal zone is more severe than their counterparts exposed to seawater attack. Whereas the inclusion of 20% Q-P in the blends resulted in the lowest deterioration, followed by that of 10% MK coupled with 10% Q-P and 20% MK, respectively. The results showed that HVFA specimens blended with 20% Q-P demonstrated superior stability in terms of compressive strength and microstructure among all examined specimens. Accordingly, HVFA blended cement improved with Q-P can be applied as coverings in building structures close to coastal areas to cope with harsh conditions.

Effects of calcium formate on early-age strength and microstructure of high-volume fly ash cement systems

Replacement of Portland cement (PC) by fly ash (FA) is currently limited to 15–30% by mass, mainly due to low early age strength development of concrete. This research uses calcium formate (Ca(HCO2)2; CF) as an admixture to high-volume FA (HVFA) composites to improve its strength properties. HVFA represents 60–70% of cement replaced by FA and dosage of CF varies from 0.5% up to high dosage of 9% of cement content. Compressive strength, isothermal calorimetry, thermogravimetric analysis, X-ray diffraction and scanning electron microscopy were conducted to investigate the effects of CF on hydration and microstructural aspects. The results show that both HVFA pastes with 60% and 70% FA achieved the highest strength at the CF dosage of 3%. At the age of 28 days, adding 3% CF to HVFA mixes led to higher consumption of FA as well as higher formation of calcium hydroxide (Ca(OH)2), calcium silicate hydrates, calcium carbonate (CaCO3) and ettringite, which contribute to the increase of strength. The addition of very high dosages of CF at 9% increased the hydration of tricalcium aluminate but could hinder the hydration of tricalcium silicate in both PC and HVFA pastes with 60% and 70% FA.

Investigation on high-volume fly ash pastes modified with micro-size metakaolin subjected to high temperatures

Portland cement (PC) containing high-volume fly ash (HVFA) is usually used to obtain economical and more sustainable merits, but these merits suffer from dramatically low compressive strength especially at early ages. In this work, the possibility of using micro-size metakaolin (MSK) particles to improve the compressive strength of HVFA paste before and after subjecting to high temperatures was studied. To produce HVFA paste, cement was partially substituted with 70% fly ash (FA), by weight. After that, FA was partially substituted with MSK at ratios fluctuating from 5% to 20% with an interval of 5%, by weight. The effect of MSK on the workability of HVFA mixture was measured. After curing, specimens were subjected to different high temperatures fluctuating from 400 to 1000 °C with an interval of 200 °C for 2 h. The results were analyzed by different techniques named X-ray diffraction (XRD), thermogravimetry (TGA) and scanning electron microscopy (SEM). The results showed that the incorporation of MSK particles into HVFA mixture exhibited a negative effect on the workability and a positive effect on the compressive strength before and after firing.

Synergistic Effects of Nano-Montmorillonite and Polyethylene Microfiber in Foamed Paste with High Volume Fly Ash Binder.

Foamed cement-based materials have attracted much attention as a new type of thermal insulation materials (TIMs) that may offer a sustainable solution to the built environments. This laboratory study explores the combined use of nano-montmorillonite and polyethylene microfiber in foamed paste with high volume fly ash (HVFA) binder. A total of 16 foamed HVFA paste mixtures were fabricated which consisted of 70% Class F fly ash, 30% Portland cement, 2% sodium alpha-olefin sulfonate, 0.38% Na₃PO₄, and 2% nano-montmorillonite. The dosage and type of polyethylene microfibers (90 μm in diameter) were explored in the present study, with six dosages (0, 0.1%, 0.2%, 0.3%, 0.4%, 0.5% by volume) and three lengths (3 mm, 6 mm, and 9 mm) tested. Based on the experimental results, the highest 28-day rupture strength (1.51 MPa) was achieved with the use of 3-mm long PE microfibers at 0.4 vol.%. Synergistic utilization of nMMT and microfibers exhibited a great influence on the dry density and water absorption of the foamed paste. The SEM micrographs illustrated the multiple mechanisms by which the microfibers serve to reduce shrinkage-induced cracking of the foamed paste. Energy-dispersive X-ray spectroscopy was employed to obtain the contents of Ca, Si, Al, S and mole ratios of Ca/Si, Ca/(Si + Al), S/Ca, and Al/Si in the hardened pastes, which reveal the difference in hydration products near or away from the nMMT-pretreated polyethylene microfibers. The results of microhardness test were also used to elucidate such nano-/micro-synergistic effects, which improved the bonding between microfibers and foamed paste matrix. A mechanism was proposed to explain the role of various admixtures and the balanced performance of such inorganic TIMs.

Development of self-compacting strain-hardening cementitious composites by varying fly ash content

This study investigated the role of the matrix in the production of self-compacting strain-hardening cementitious composites (SHCC). High volume fly ash pastes with desirable flowability were first designed, followed by adding micro polyvinyl alcohol (PVA) fibres to produce composite materials. The results showed that the composites were self-compacting in the fresh state, and exhibited stable multiple-cracking and strain-hardening behaviour in the hardened state. Fly ash is used not only to modify the workability of fresh mixtures, but also to adjust matrix strength to the values favourable for producing SHCC materials.

High-volume fly ash paste for developing ultra-high performance concrete (UHPC)

Ultra-high performance concrete is a kind of high-tech composite material which shows superb characteristics such as self- compactness, compressive strength higher than 150MPa, and exceptional durability performances compared to other kinds of concrete. In this research, compared to known commercially available UHPCs, a type of UHPC paste with greener pozzolans was developed. In this regard, cement and silica fume, as two main constituents of the prevalent UHPC compositions and particularly with high cost and environmental impacts, were replaced by fly ash as a waste material. It was found that the highest fluidity and strength could be achieved with 13% and 16% of fly ash substitution, respectively. Furthermore, ultra-fine fly ash with mean particle size of 4.48μm showed its applicability to be used in UHPC with 20wt.% cement substitution resulting in a paste with 153MPa compressive strength and 37.5cm flow diameter. Moreover, addition of at least 5% silica fume seems to be a prerequisite regarding strength gain of UHPC paste. Metakaolin as another pozzolanic material was studied. Although it improved the paste strength, it demonstrated lower fluidity and showed its inability to be applied in UHPC with required high workability.

Characterization of morphology and hydration products of high-volume fly ash paste by monochromatic scanning x-ray micro-diffraction (μ-SXRD)

The present study focuses on identification and micro-structural characterization of the hydration products formed in high-volume fly ash (HVFA)/portland cement (PC) systems using monochromatic scanning x-ray micro-diffraction (μ-SXRD) and SEM-EDS. Pastes with up to 80% fly ash replacement were studied. Phase maps for HVFA samples using μ-SXRD patterns prove that μ-SXRD is an effective method to identify and visualize the distribution of phases in the matrix. μ-SXRD and SEM-EDS analysis shows that the C-S-H formed in HVFA system containing 50% or more of fly ash has a similar structure as C-S-H(I) with comparatively lower Ca/Si ratio than the one produced in PC system. Moreover, coexistence of C-S-H(I) and strätlingite is observed in the system containing 80% of fly ash, confirming that the amount of alumina and silicate phases provided by the fly ash is a major factor for the formation of C-S-H(I) and strätlingite in HVFA system.

Advanced Nanoscale Characterization of Cement Based Materials Using X-Ray Synchrotron Radiation: A Review

We report various synchrotron radiation laboratory based techniques used to characterize cement based materials in nanometer scale. High resolution X-ray transmission imaging combined with a rotational axis allows for rendering of samples in three dimensions revealing volumetric details. Scanning transmission X-ray microscope combines high spatial resolution imaging with high spectral resolution of the incident beam to reveal X-ray absorption near edge structure variations in the material nanostructure. Microdiffraction scans the surface of a sample to map its high order reflection or crystallographic variations with a micron-sized incident beam. High pressure X-ray diffraction measures compressibility of pure phase materials. Unique results of studies using the above tools are discussed—a study of pores, connectivity, and morphology of a 2,000 year old concrete using nanotomography; detection of localized and varying silicate chain depolymerization in Al-substituted tobermorite, and quantification of monosulfate distribution in tricalcium aluminate hydration using scanning transmission X-ray microscopy; detection and mapping of hydration products in high volume fly ash paste using microdiffraction; and determination of mechanical properties of various AFm phases using high pressure X-ray diffraction.

Degree of hydration and gel/space ratio of high-volume fly ash/cement systems

Although fly ash has been widely used in concrete as a cement replacement, little work has been done on determining the degree of hydration of high-volume fly ash/cement (FC) systems. In the present study, the degree of hydration of the cement in Portland cement (PC) paste was obtained by determining the non-evaporable water (Wn) content. The degree of reaction of the fly ash in FC pastes was determined using a selective dissolution method. Based on the relation between the degree of cement hydration and effective water-to-cement (w/c) ratio, the degree of hydration of the cement in FC pastes was also estimated. It was found that high-volume fly ash pastes underwent a lower degree of fly ash reaction, and in the pastes with 45% to 55% fly ash, more than 80% of the fly ash still remained unreacted after 90 days of curing while the hydration of the cement in high-volume fly ash pastes was enhanced because of the higher effective w/c ratio for the paste. This effect was more significant for the pastes with lower water-to-binder (w/b) ratios. Thus, preparing high-volume fly ash concrete at lower w/b ratios can result in less strength losses. This paper also introduces a model to describe the relationship between the w/c ratio and the degree of cement hydration and gel/space ratio. The gel/space ratios of the FC pastes, evaluated based on the proposed model, were found to be consistent with the gel/space ratio of PC pastes in terms of the relationship with compressive strength. The gel/space ratio data correlated (inversely) linearly with mercury intruded porosity, but the former correlated more with compressive strength than the latter.

Pore structure and its effect on strength of high-volume fly ash paste

Pastes made with different fly ash contents and water/binder ratios were tested at different ages. The total porosity and pore size distribution were measured by mercury intrusion. The fractal dimension of pore-solid interface was first measured by small angle X-ray scattering (SAXS). The relationship between porosity and strength for high-volume fly ash (HVFA) paste was determined. It is obtained that the pore size distribution affected both compressive strength and flexural strength, but the effects on compressive strength and on flexural strength are different. Preliminary experimental results indicated that strength was closely related to the pore surface fractal dimension.

Microstructure, crack propagation, and mechanical properties of cement pastes containing high volumes of fly ashes

This paper presents information on the effect of fly ashes on the microstructure of high volume fly ash (HVFA) pastes and how this affect the crack pattern and the stress-strain relationship. The HVFA pastes were prepared incorporating 58% of fly ashes. Fly ash particles appear to act as microaggregates in the pastes. Crack propagation generally deviates around the fly ash particles. Compared with that for the Portland cement paste of similar strength, the stress-strain curves for the HVFA pastes are less linear and begin to deviate at about 60–70% of the ultimate strength at 28 days. Based on these information, it was proposed that the HVFA paste may be considered as a composite material microscopically with fly ash particles as reactive microaggregates embedded in a matrix of hydration and reaction products. The effect of high volumes of fly ash on various properties of the concrete such as the modulus of elasticity, shrinkage and creep, as well as permeability were also discussed.

Microstructural Development in High‐Volume Fly‐Ash Cement System

The writers present the results of research findings on the micro‐structural development in a high‐volume fly‐ash (HVFA) cement system containing 60% fly ash by weight of binder. Although the one‐day strength of HVFA mortar is low compared to plain cement mortar, from three‐day onward the strength starts to increase. Corresponding to the development of microstructure, the strength increases significantly at later ages and reaches 78% of the control specimen at 180 days. The literature survey reveals that hydration of fly‐ash aluminate phases produces hydrates similar to those in cement. To reveal the fly‐ash hydration process more vividly, a special type of cement containing very low C3A and alkalies was used with a class F fly ash. In order to precisely determine the role of high‐volume replacement in a cementitious system, a comparative study of plain cement and HVFA paste was carried out. The mechanism of fly‐ash hydration in this system is discussed and a correlation between its microstructure/strength development established. Better dispersion of cement grains, smaller Ca(OH)2 crystals, and formation of more gel phases largely account for the strengthening effect. The effect of fly‐ash replacement on neat water‐to‐cement ratio, initial pore content, Ca(OH)2 crystallization, and dissolution of fly‐ash glass phase are discussed. Comparison of experimental results determined by x‐ray diffraction (XRDA) analysis, strength measurement, and thermogravimetry/differential thermal analysis (TG/DTA) is also presented.

Mechanisms of hydration reactions in high volume fly ash pastes and mortars

This paper describes investigations of high-volume fly ash (HVFA)-Portland cement (PC) binders, the physical and chemical properties of which have been characterized up to 365 days of curing. Physical investigations were made of compressive strength development, pore structure by porosimetry, and morphology by scanning electron microscopy. Chemical examination was conducted for solid phase composition and degree of hydration by X-ray diffraction and thermal analysis, and for pore-fluid composition by high pressure extraction and analysis. Up to 365d the cement in the HVFA pastes is not fully hydrated. However, the ash participates in both early (sulpho-pozzolanic) and late (alumino-silicate) hydration reactions. In addition to the usual products of cement hydration, ettringite (AFt) has been identified as a product of the early hydration of the fly ash. It has not been possible to identify long term hydration products of fly ash which appear to be non-crystalline. A two-step mechanism for pozzolanic reaction between fly ash and Portland cement has been proposed involving: (a) depolymerization/silanolation of the glassy constituents of the ash by the highly alkaline pore fluids, followed by (b) reaction between solubilized silicate and calcium ions in solution to form CSH.