Fly Ash Pastes Research Articles

Radon exhalation of cementitious materials made with coal fly ash: Part 2 – testing hardened cement–fly ash pastes

Increased interest in measuring radionuclides and radon concentrations in fly ash (FA), cement and other components of building products is due to the concern about health hazards of naturally occurring radioactive materials (NORM). The paper focuses on studying the influence of FA on radon exhalation rate (radon flux) from cementitious materials. In the previous part of the paper the state of the art was presented, and the experiments for testing raw materials, Portland cement and coal fly ash, were described. Since the cement and FA have the most critical role in the radon release process relative to other concrete constituents (sand and gravel), and their contribution is dominant in the overall radium content of concrete, tests were carried out on cement paste specimens with different FA contents, 0–60% by weight of the binder (cement+FA). It is found that the dosage of FA in cement paste has a limited influence on radon exhalation rate, if the hardened material is relatively dense. The radon flux of cement–FA pastes is lower than that of pure cement paste: it is about ∼3 mBq m −2 s −1 for cement–FA pastes with FA content as high as 960 kg m −3.

Radon exhalation of cementitious materials made with coal fly ash: Part 1 – scientific background and testing of the cement and fly ash emanation

Increased interest in measuring radionuclides and radon concentrations in fly ash, cement and other components of building products is due to the concern of health hazards of naturally occurring radioactive materials (NORM). The current work focuses on studying the influence of fly ash (FA) on radon-exhalation rate (radon flux) from cementitious materials. The tests were carried out on cement paste specimens with different FA contents. The first part of the paper presents the scientific background and describes the experiments, which we designed for testing the radon emanation of the raw materials used in the preparation of the cement–FA pastes. It is found that despite the higher 226Ra content in FA (more than 3 times, compared with Portland cement) the radon emanation is significantly lower in FA (7.65% for cement vs. 0.52% only for FA).

04/00730 Mineralogical and microstructural characterisation of alkali-activated fly ash/slag pastes: Puertas, F. and Fernandez-Jimenez, A. Cement & Concrete Composites, 2003, 25, (3), 287–292

This investigation reports on a comparative study of the residual compressive strength at different temperatures of alkali-activated fly ash (AAFA) pastes chemically activated using sodium silicate with three different concentrations named 20, 30 and 40 (wt.%). The behaviour of different mixtures in conditions of rapid temperature changes was studied. Water quench test was applied to determine thermal shock resistance. The traditional Portland cement pastes were used as a reference. The temperatures ranging from 200 °C to 1000 °C with an increment of 200 °C has been examined. Pore solution pH and compressive strengths before and after exposure to elevated temperatures were determined. The various decomposition phases formed were identified using X-ray diffraction (XRD), thermogravimetric analysis (TGA), differential thermal analysis (DTG) and scanning electron microscopy (SEM). The results indicated that fly ash activated by sodium silicate is more able to resist degradation caused by exposure to elevated temperature than Portland cement system as its relative strengths are superior. In the hardened AAFA pastes, as activator concentration dosage increased as the relative compressive strengths and thermal shock resistance decreased. The AAFA system is able to maintain a sufficient high pH to retain the passive protective layer on the surface of any reinforcing steel present.

Activation of Pozzolanic Reaction of Hydrated Portland Cement Fly Ash Pastes in Sulfate Solution

The activation of the pozzolanic reaction of fly ash in portland cement paste immersed in sulfate solution has been studied. Mixtures of two Spanish fly ashes (ASTM class F) with 0%, 15%, and 35% replacement of portland cement by fly ash were immersed in Na2SO4 solution, of 2880 ppm SO42− concentration, for a period of 90 days. The resistance of the different mixtures to the sulfate attack was evaluated using the Koch‐Steinegger test. The results showed that all of the mixtures were sulfate resistant, despite the high Al2O3 content of the fly ash. The diffusion of SO42− and Na+ ions through the pore solution activated the pozzolanic reactivity of the fly ashes, causing microstructural changes, which were characterized by X‐ray diffraction (XRD), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM). As a result, the flexural strength of the mixtures increased, principally for the fly ash of a lower particle size and 35% of addition.

Characterization of pore solutions expressed from high-calcium fly-ash–water pastes

A pore solution study was undertaken to provide additional understanding of the hydration behavior of high-calcium fly ash in fly-ash–water pastes. Three sources of fly ash were selected from a previous study on the basis of their hydration behavior, particularly their ability to form ettringite. Pastes were made from each fly ash and water, and pore solutions were expressed at varying times and were analyzed for the following ions: SO 4 2−, Ca 2+, Al 3+, Mg +, K +, Na +, Si 4+, Fe 3+, and OH −. The mineralogical compositions of the paste solids after expression of pore solutions were examined by X-ray diffraction analysis. The principal hydrated phases formed in the fly ash pastes were typical of those seen previously in hydrated high-calcium Class C fly ash, namely, the calcium aluminosilicate hydrate minerals, ettringite and monosulfate, and the calcium aluminosilicate hydrate mineral, strätlingite. Variations in the chemical composition of the pore solutions helped to explain differences in the relative amounts of ettringite, monosulfate and strätlingite formed in the pastes. Two of the fly ash samples had very similar bulk chemical compositions but contained slightly different amounts of soluble crystalline sulfur-bearing minerals. The fly ash that had the smaller amount of soluble sulfur-bearing minerals formed less ettringite in its paste and more monosulfate indicating that sulfur was more limited in this fly ash. The fly ash paste that formed the least ettringite and the most strätlingite had the lowest solution concentration of sulfur and the highest of aluminum. Strätlingite formation continued to increase with curing in all three fly-ash–water pastes suggesting that dissolution of the fly glass during pozzolanic reactions was releasing calcium, silicon, and aluminum ions. Sulfate concentrations in pore solutions from all three of the fly ash pastes rose significantly between 7 and 90 days, but data on pore solutions only extended to 90 days. At that time, the sulfate concentration in the pore solutions of one fly ash paste had decreased; concentrations in the other two fly ash pastes had not. However, it is likely that this increase in sulfate concentrations was a short-term phenomenon. Fly ash pastes studied previously have not been observed to form significant amounts of ettringite or any other crystalline sulfur-containing phase, after 28 days. The likely source of the sulfur between 7 and 90 days is the fly ash glass.

Stabilization/solidification of MSW incineration residues from facilities with different air pollution control systems. Durability of matrices versus carbonation

This paper discusses the stabilisation/solidification process with Portland cement applied to municipal solid waste incineration residues. Two types of residues were considered: fly ash (FA) produced in an electrostatic precipitator, and air pollution control (APC) residues from a semi-dry scrubber process. Cement pastes with different percentages of FA and APC residues were characterised according to their physical properties, the effect of the hydration products and their leaching behaviour. Portland pastes prepared with APC residues showed a rapid setting velocity in comparison with setting time for those pastes substituted with FA residues. Portland cement hydration was retarded in FA pastes. Leaching test results showed that heavy metals (such as Zn, Pb and Cd) and sulphates are immobilised within the paste, whereas chlorides are only partially retained. The carbonation process increases the leachability of SO 4 2− and heavy metals such as Zn and Cr.

Stabilisation and solidification of sewage sludges with Portland cement

The objective of this work is to create a binding and stabilising matrix with sludge–cement and sludge–cement–coal fly ash. With this purpose, studies have been made on the effects of various percentages of waste and binder and the behaviour and evolution of sludge in the system of cement and cement–coal fly ash pastes. In these studies, evaluations were made on the setting speed, the hydration products formed and the chemical stability by means of the pH values attained and of the types and concentrations of microorganisms and organic compounds in the system created.

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.

Self-compensating autogenous shrinkage in Portland cement—metakaolin—fly ash pastes

For concrete containing high amounts of cement, particularly for (but not restricted to) hot climatic conditions, the rate of hydration may be high enough to cause shrinkage problems. Use of some admixtures such as silica fume and pulverized fuel ash (PFA) may further increase shrinkage. Blending Portland cement (PC) with metakaolin (MK) has shown expansive tendencies which may be applicable in suppressing early shrinkage. By blending MK with PFA partially to replace PC, shrinkage may be regulated. The current work is particularly directed towards regulation of autogenous shrinkage. Paste bars in which PC was replaced by up to 20% MK, 30% PFA and 30% MK plus PFA blends were monitored for autogenous shrinkage for up to 200 days during sealed moist curing at room temperature and 100% relative humidity. The results reported show reduced autogenous shrinkage, although cement type was also found to be an important factor in influencing shrinkage. The use of PC-MK-PFA blends should provide technological, economic and environmental advantages by reducing PC production and its associated greenhouse gas emissions, and improving concrete durability

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.

Calcium Hydroxide Saturation Factors in the Pore Solution of Hydrated Portland Cement Fly Ash Pastes

Journal of the American Ceramic SocietyVolume 79, Issue 4 p. 1041-1046 Calcium Hydroxide Saturation Factors in the Pore Solution of Hydrated Portland Cement Fly Ash Pastes S. Goñi, S. Goñi Institute of Construction Science Eduardo Torroja (CSIC), Madrid, Spain Member, American Ceramic Society. Author to whom correspondence should be directed.Search for more papers by this authorM. P. Lorenzo, M. P. Lorenzo Institute of Construction Science Eduardo Torroja (CSIC), Madrid, Spain Centre of Experimental and Technique Studies, University of S. Pablo (CEU), Madrid, Spain.Search for more papers by this authorA. Guerrero, A. Guerrero Institute of Construction Science Eduardo Torroja (CSIC), Madrid, SpainSearch for more papers by this authorM. S. Hernández, M. S. Hernández Institute of Construction Science Eduardo Torroja (CSIC), Madrid, SpainSearch for more papers by this author S. Goñi, S. Goñi Institute of Construction Science Eduardo Torroja (CSIC), Madrid, Spain Member, American Ceramic Society. Author to whom correspondence should be directed.Search for more papers by this authorM. P. Lorenzo, M. P. Lorenzo Institute of Construction Science Eduardo Torroja (CSIC), Madrid, Spain Centre of Experimental and Technique Studies, University of S. Pablo (CEU), Madrid, Spain.Search for more papers by this authorA. Guerrero, A. Guerrero Institute of Construction Science Eduardo Torroja (CSIC), Madrid, SpainSearch for more papers by this authorM. S. Hernández, M. S. Hernández Institute of Construction Science Eduardo Torroja (CSIC), Madrid, SpainSearch for more papers by this author First published: April 1996 https://doi.org/10.1111/j.1151-2916.1996.tb08545.xCitations: 20 P. W. Brown—contributing editor Member, American Ceramic Society. Author to whom correspondence should be directed. AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Citing Literature Volume79, Issue4April 1996Pages 1041-1046 RelatedInformation

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.

Modified water/cement ratio law for compressive strength of fly ash concretes

Experimental data are presented which suggest that the development of compressive strength of fly ash concretes can be explained by superposition of two independent mechanical pore-filling mechanisms in the cement—fly ash paste. It is also suggested that the traditional water/cement ratio law for ordinary Portland cement concretes can be applied to fly ash concretes, provided that a slight modification is introduced. This will be of assistance in the design of fly ash concrete mixes for compressive strength.

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.

Investigation of reactions in high-alumina fly ash and lime pastes

By-product fly ash from coal combustion generally contains a high percentage of active silica and smaller amounts of alumina, although some coals can have higher alumina contents (30 to 40%). During our experimental tests on the utilization f fly ash in road and highway foundation sheets, we have found a fly ash poor in silica, having an exceptionally high alumina content. This material has shown a good reactivity towards lime paste, giving hardened mixtures with high mechanical properties.

Evidence for Minimal Pozzolanic Reaction in a Fly Ash Cement during the Period of Major Strength Development

AbstractStrengths developed in fly ash concretes usually equal or exceed that of similar plain concrete after a few months, with much of the response usually attributed to “pozzolanic” reaction between ash and secondary calcium hydroxide (CH). The CH contents of pastes made with five different fly ashes were determined by DTA for periods up to six months. The CH contents found did not decrease notably over the period, and were substantially identical to that expected for plain cement pastes diluted with the same amount of inert material as the amount of fly ash used. Scanning electron microscope examination of the pastes showed only minimal evidence of reaction even up to 1 year of age, although many fly ash grains were in intimate contact with CH. Non-evaporable water contents of the fly ash pastes were substantially higher than expected at each age, suggesting that the fly ash promoted more complete cement hydration or that the hydration products formed bound substantially greater amounts of water than plain cement paste ordinarily does.

Evidence for Minimal Pozzolanic Reaction in a Fly Ash Cement During the Period of Major Strength Development

ABSTRACTStrengths developed in fly ash concretes usually equal or exceed that of similar plain concrete after a few months, with much of the response usually attributed to “pozzolanic” reaction between ash and secondary calcium hydroxide (CH). The CH contents of pastes made with five different fly ashes were determined by DTA for periods up to six months. The CH contents found did not decrease notably over the period, and were substantially identical to that expected for plain cement pastes diluted with the same amount of inert material as the amount of fly ash used. Scanning electron microscope examination of the pastes showed only minimal evidence of reaction even up to 1 year of age, although many fly ash grains were in intimate contact with CH. Non-evaporable water contents of the fly ash pastes were substantially higher than expected at each age, suggesting that the fly ash promoted more complete cement hydration or that the hydration products formed bound substantially greater amounts of water than plain cement paste ordinarily does.

Monitoring of Fluctuations in the Physical and Chemical Properties of a High-Calcium Fly Ash

ABSTRACTThe physical and chemical properties of fly ash produced at Ottumwa Generating Station have been monitored since April, 1985. The fly ash is produced from burning a low sulfur, sub-bituminous coal obtained from the Powder River Basin near Gillette, Wyoming. One-hundred and sixty samples of fly ash were obtained during the two year period. All of the samples were subjected to physical testing as specified by ASTM C 311. About one-hundred of the samples were also subjected to a series of tests designed to monitor the self-cementing properties of the fly ash. Many of the fly ash samples were subjected to x-ray diffraction and fluorescence analysis to define the mineralogical and chemical composition of the bulk fly ash as a function of sampling date. Hydration products in selected hardened fly ash pastes, were studied by x-ray diffraction and scanning electron microscopy. The studies indicated that power plant operating conditions influenced the compressive strength of the fly ash paste specimens. Mineralogical and morphological studies of the fly ash pastes indicated that stratlingite formation occurred in the highstrength specimens, while ettringite was the major hydration product evident in the low-strength specimens.

A Method for Measuring the Self-Cementing Characteristics of Fly Ash Pastes

SUMMARYThe development of an improved method of trimethylsilylation [1] made possible the assessment of different polysilicate fractions in hydrated cement pastes, as a means of structural characterization. Several laboratories have presented various gas-liquid chromatographic data on the polysilicate derivatives of C-S-H [2–5]. Identification of the different molecular weight fractions has been made, in most laboratories, by matching the gasliquid chromatographic peaks with the gel permeation chromatographic peaks. The success of this matching process is always dependent on type of instruments, length of columns and temperature range.In the present investigation, a mass spectrometer, in direct contact with the gas-liquid chromatograph, was used to identify the molecular weight of each of the fractions separated from the chromatograph. The technique was applied to determine the molecular weights and abundances of different polysilicate species in hydrated fly ash pastes.

Strength gain and calcium hydroxide depletion in hardened cement pastes containing fly ash

Synopsis Hardened cement paste specimens, with and without partial replacement of the cement by fly ash, were cured f o r various ages up to one year at temperatures 20-80°C. Compressive strength, calcium hydroxide content and porosity, by mercury intrusion porosimetry, were measured as curing proceeded. Results indicate that elevated curing temperature accelerates the pozzolanic reaction to a greater extent than the normal hydration reactions of Portland cement. When elevated temperatures are used, the strength of fly ash pastes may surpass that of plain pastes at early ages. Strength degradation was observed in pastes cured at the higher temperatures; this degradation was more pronounced in the plain pastes. Consumption of calcium hydroxide with no corresponding gain in strength, or with a reduction in strength, was observed for ash-pastes cured at 50°C and above. Changes in mercury porosity, on the other hand, correlate well with changes in strength during the early strength-gain periods and also during the later strength-degradation periods.