Fly Ash Cement Paste Research Articles

Modification of water absorption and pore structure of high-volume fly ash cement pastes by incorporating nanosilica

High-volume (>50%) fly ash (HVFA) is used to partially replace cement in concrete to reduce the greenhouse gas emission, which is beneficial to both cement industry and environment. However, one drawback of using high-volume fly ash cement (HVFAC) is the increase of the water absorption and the reduction of the durability of cement-based materials. In this study, the effect of nanosilica (NS) on the compressive strength and water absorption of HVFAC pastes were investigated. In addition, the phase composition, microstructure and pore structure of the pastes were analyzed through X-ray diffraction (XRD), scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP). Results showed that the addition of NS increased the compressive strength and reduced the water absorption of HVFAC pastes. NS could react with Portlandite to produce additional C–S–H gel, and simultaneously a higher content of fly ash was hydrated in the presence of NS. Therefore, the incorporation of NS refined the pores and reduced the pore volume of HVFAC pastes. The pores with diameter larger than 10 nm were more pronounced for the water absorption ability of HVFAC composites. This study provided a possible way to reduce the negative effect brought by the increased water absorption when using HVFA in cementitious materials.

Reduction of heavy metals leaching and pore volume in high-volume fly ash cement pastes by adding nano-SiO2.

This study investigated the compressive strengths and leaching of heavy metals in high-volume fly ash cement (HVFAC) pastes with and without nano-SiO2 (NS). Further, scanning electron microscopy, X-ray diffraction, and mercury intrusion porosimetry were used to characterize the microstructure of the specimens. The results showed that the addition of NS increased the compressive strength of HVFAC specimens. NS accelerated the hydration of both cement and fly ash to form more hydration products, and it reacted with portlandite to simultaneously generate more C-S-H gel. Thus, the total pore volume, especially the capillary pores, in the NS-mixed HVFAC specimens was reduced, and their microstructure became denser because of the filling of more hydration products in the pores. This resulted in the reduction of the leaching of heavy metals (Cr, Pb, Cd, As, Zn, and Cu) in the NS-mixed HVFAC specimens compared with the HVFAC specimen containing no NS. According to the Chinese national standard (GB 3838-2002), the water quality of leachates from the HVFAC specimens mixed with both 3% and 5% NS reached grade I, thus causing nearly no harm to the water and soil environment.

Effect of Sodium Sulfate Activator on Compressive Strength and Hydration of Fly-Ash Cement Pastes

In the study, the effect of 4% sodium sulfate (Na2SO4) as an activator on cement pastes with 0%, 20%, and 40% fly-ash replacements and a low water-to-cementitious materials ratio of 0.30 was investigated. The investigation was conducted to evaluate the effectiveness of the technique for the utilization of fly ash in developing sustainable concrete. The use of Na2SO4 decreased setting times of the fresh pastes and increased compressive strength of the hardened pastes up to 28 days irrespective of fly-ash replacement. The use decreased Ca(OH)2 content in the hardened pastes irrespective of fly-ash replacement. Meanwhile, it increased Ca(OH)2 consumption by the pozzolanic reaction of fly ash and content of calcium silicate and aluminate hydrates in the hardened fly-ash–cement pastes. Consequently, the use of Na2SO4 negatively affected cement hydration in the hardened cement pastes without fly ash, while it accelerated ettringite formation and pozzolanic reaction of fly ash in the hardened pastes.

Effects of water content, water type and temperature on the rheological behaviour of slag-cement and fly ash-cement paste backfill

The pumping ability and placement performance of fresh cemented paste backfill (CPB) in underground mined cavities depend on its rheological properties. Hence, it is crucial to understand the rheology of fresh CPB slurry, which is related to CPB mixture design and the temperature underground. This paper presented an experimental study investigating the effects of binder type, content, water chemical properties and content, and temperature, on the rheological properties of CPB material prepared using the tailings of a copper mine in South Australia. Portland cement (PC), a newly released commercially manufactured cement called Minecem (MC) and fly ash (FA) were used as the binders added to the mine tailing materials. Various amounts of two different water types were added to the mixtures in the preparation of backfill material slurry. Six different temperatures ranging from 5 to 60 °C were to investigate the effect of temperature on CPB rheology. Overall, the increasing water content and decreasing temperature lead to lower yield stress. Based on the results obtained from the rheological properties of CPB slurry, it was found that at room temperature (25 °C), with regards to the unconfined compressive strength (UCS) performance, the replacement of 4% PC mixed CPB (28 days UCS 425 kPa) to 3% MC mixed CPB (28 days UCS 519 kPa), reduced the slurry yield stress from 210.7 to 178.5 Pa. The results also showed that the chemical composition of water affects the yield stress of CPB slurry and that MC mitigates the negative effect of mine-processed water (MW) and thus lead to improve the rheological properties of the slurry. However, the results suggested that the rheological properties of a mixture using MC is very sensitive to the water volume and temperature change. Therefore, using MC in backfill requires better quality control in slump mixing.

Combination of precipitated-calcium carbonate substitution and dilute-alkali fly ash treatment in a very high-volume fly ash cement paste

Poorer reactivity of fly ash (FA) than Portland cement brings to the problems of low strength and setting time delays of very high-volume fly ash cement paste. This study combines precipitated-calcium carbonate (PCC) substitution and dilute-alkali fly ash treatment for overcoming those drawbacks. The strength was examined at 3-, 7-, 28- and 56-day. Scanning electron microscopy and Fourier-transform infrared spectroscopy were applied for characterisations. PCC substitution improved the strength and shortened the setting time; then additional dilute-alkali fly ash treatment results in better properties. Combination of PCC substitution and dilute-alkali treatment enhances cement hydration and FA pozzolanic reaction simultaneously.

Kinetic study on elemental mercury release from fly ashes and hydrated fly ash cement pastes.

The kinetics of elemental mercury (Hg0) release from fly ashes and hydrated fly ash cement pastes was investigated using a homemade Hg measurement system. Three types of fly ash (FA) and ordinary Portland cement (OPC) were used to prepare cement pastes. After standard curing for 28 days, the hydrated cement paste (HCP) was ground into a fine powder for Hg emission measurements. Detectable Hg0 was found released from both fly ashes and hydrated fly ash cement pastes. The results show that elevated temperatures and evaporation of the capillary pore water in wet HCP samples accelerate Hg0 release. Both desorption of Hg0 from the particle surface of HCP powder and migration of Hg0 from the inner pores contribute to Hg0 release. The kinetic calculation indicates that the hydration products of hydrated fly ash cement have little immobilization effect on Hg0, which is mainly physically encapsulated in the HCP particles by hydration products.

Influence of Nano-SiO2, Nano-CaCO3 and Nano-Al2O3 on Rheological Properties of Cement-Fly Ash Paste.

Rheological curves of cement–fly ash (C–FA) paste incorporating nanomaterials including nano-SiO2 (NS), nano-CaCO3 (NC) and nano-Al2O3 (NA) at different resting times (hydration time of 5 min, 60 min, and 120 min) were tested with a rheometer. The rheological behaviors were described by the Herschel–Bulkley (H–B) model, and the influences of these nanomaterials on rheological properties of C–FA paste were compared. Results show that the types, content of nanomaterials and resting time have great influences on the rheological properties of C–FA paste. Incorporating NS and NA increases yield stress and plastic viscosity, and decreases the rheological index of C–FA paste. When the content of NS and NA were 2 wt%, the rheological index of C–FA paste was less than 1, indicating rheological behavior changes from shear thickening to shear thinning. Meanwhile, with rising resting time, yield stress and plastic viscosity increased significantly, but the rheological index decreased evidently, showing paste takes on shear thinning due to the rise of resting time. However, incorporating 3 wt% NC and the rising of resting time did not change the rheological properties of C–FA paste. These differences are mainly that the specific surface area (SSA) of NS (150 m2/g) and NA (120 m2/g) are much larger than that of NC (40 m2/g). The huge SSA of NS and NA consume lots of free water and these tiny particles accelerate the hydration process during resting time.

Influences of coal fly ash containing ammonium salts on properties of cement paste

This paper aims to investigate the influence of coal fly ash containing ammonium salts on properties of cement paste. Fly ash was incorporated at percentage of 20% by weight of the total binder to replace Portland cement. Ammonium hydrogen sulfate (NH3HSO4) or ammonium sulfate ((NH3)2SO4) were introduced at percentages of 3.0%–6.0% or 1.5%–3.0% by fly ash weight. Compressive strength, setting time and hydration heat were evaluated on variable blend mixtures. Adsorption behaviors of polycarboxylate-based superplasticizer and air entraining agent on fly ash particles were also evaluated using total organic carbon (TOC) method. Semi-adiabatic calorimetry, X-ray diffraction, Fourier transform infrared spectroscopy, thermogravimetry-differential thermal analysis, mercury intrusion porosity and scanning electron microscope measurements were carried out on typical samples. Experimental results showed that the chemical admixtures adsorbed by coal fly ash were increased by the introduction of NH3HSO4 or (NH3)2SO4. The addition of 3.0%–6.0% NH3HSO4 and 1.5%–3.0% (NH3)2SO4 decreased the 28d compressive strength of fly ash-cement pastes by 4.3%–10.4% and 6.3%–8.9%, respectively. The initial and final setting times were delayed and the early age hydration of Portland cement was also retarded. Moreover, the pore structure was coarsened and porosity was increased for the hardened cement specimens due to the release of ammonia and lower hydration degree. Therefore, more attention should be paid to the application of denitration fly ash to the cement and concrete industry.

Creep of cement paste containing fly ash - An investigation using microindentation technique

Grid-microindentation tests were conducted to study the effect of fly ash content, water-binder ratio and capillary water on creep of cement paste. Cement paste specimens containing 0%, 20% and 40% fly ash (by weight of binder) were tested at 1, 3, 7, 28 and 90 days. It is found that the effect of fly ash content on creep depends on the age at loading. It is also noted that for a given loading age, the creep deformations increase with increase in water-binder ratio. Presence of capillary water is found to have an influence on creep of cement-fly ash paste system. Logarithmic fit for creep deformation, while overestimating the experimental values in the early stages, is in good agreement at later stages.

Effective Medium Method for Chloride Diffusion Coefficient of Mature Fly Ash Cement Paste.

The chloride diffusion coefficient of concrete plays an essential role in the durability assessment and design of concrete structures built in chloride-laden environments. The purpose of this paper is to present an effective medium method (EMM) for evaluating the chloride diffusion coefficient of mature fly ash cement paste. In this method, a numerical method is used to estimate the degrees of hydration of cement and fly ash. Fly ash cement paste is then modeled as a two-phase composite material, composed of a solid phase and a pore space. By introducing the percolation theory, the EMM is modified to derive the chloride diffusion coefficient of fly ash cement paste in an analytical manner. To verify the EMM, a chloride diffusion test of fly ash cement paste at a curing age of up to 540 days is conducted. It is shown that, within a reasonable fly ash content, a larger fly ash content and/or curing age results in a smaller chloride diffusion coefficient. The chloride diffusion coefficient decreases with a decreasing water/binder ratio. Finally, the validity of the EMM is verified with experimental results.

Advancements in Properties of Cements Containing Pulverised Fly Ash and Nanomaterials by Blending and Ultrasonication Method (Review- Part II)

The second part of this review paper will explain Hydration properties of Portland pulverised fly ash cement section, Effect of nanomaterial on setting–time section, Effect of nanomaterial on heat of hydration section, Strength gain mechanisms for hardened Portland pulverised fly ash cement paste and mortar section, Effect of nanomaterial on compressive strength section, Effect of nanomaterial on flexural strength (Bending) section, and Conclusion section. Experiments reported include the setting–times, the heat of hydration, the compressive strength gain, and the flexural strength gain in the current article. According to the result, nanoparticles, especially the GNP, increase the heat of hydration of cement, and accelerate the time of setting evidently, both initial and final setting-time. The most effective nanoparticle on early compressive strength gain and flexural strength gain is the GNP. The article also points the effects of the nanoparticles on the strength gain of cement comprehensively. Consequently, the prominent cement technology can use the nanoparticles dispersed in liquid by ultrasonication method to increase the properties of cement based materials effectively.

Effect of curing temperature and fly ash content on the hydration and microstructure of fly ash–cement pastes

The effect of FA and curing temperature on the hydration and microstructure of FC pastes were studied by means of XRD, 29Si NMR and SEM. The results showed that with increasing FA content, the hydration degree of FA decreases, whereas the hydration of cement was accelerated. The incorporation of FA leads to an increase in the substitution fraction of Si4+ by Al3+ in aluminosilicate chains (Al[4]/Si) and also an increase in the mean aluminosilicate chain length of C-S-H gel (MCL). In the FC pastes with 30% FA cured for 28 d, with the increase of curing temperature, there is an overall increase in the MCL , the Al[4]/Si, the hydration degree of cement and the hydration degree of FA. This validated that the higher curing temperature favor more to enhance the hydration of cement, the pozzolanic reaction of FA and the polymerization of C-S-H gel for accelerated ion diffusion rate.

Effect of triisopropanolamine on compressive strength and hydration of steaming-cured cement-fly ash paste

Fly ash (FA) has been accepted as one of the most popular supplementary material in blends with cement. However, high content of FA added would result in very slow strength development at the early age, and this problem limits the use of FA in precast concrete. In this study, an attempt was made to enhance the early strength of steam-cured high content cement-fly ash paste (SCHF, 50% FA) by incorporating triisopropanolamine (TIPA), and the underlying mechanism was systematically investigated in terms of hydration products, pore structure, and ions dissolution of FA. The early hydration process was assessed with isothermal calorimetry, thermogravimetric with differential thermo-gravimetric, and scanning electron microscopy with energy dispersive spectrometer. The pore structure of hardened paste was characterized with mercury intrusion porosimetry. The solubility of FA was analyzed with inductive coupled plasma emission spectrometer, nuclear magnetic resonance, and scanning electron microscopy. The chelating ability of TIPA with Fe3+ and Al3+ was evaluated with the conductivity. These results show that with the dosage of TIPA less than 0.10%, the compressive strength will increase with increasing dosage of TIPA, mainly because of the acceleration of both hydration of cement and pozzolanic reaction of FA; while with the dosage more than 0.10%, the compressive strength was sharply decreased owing to its air-entraining effect to increase the harmful pores, with negative effect on mechanical performance. The reason for the accelerated pozzolanic reaction of FA is that TIPA can accelerate the dissolution of aluminum, ferric and silicate into solution to participate in hydration. The findings are expected to provide guidance on promoting the use of FA in precast concrete.

Microstructure and composition of fly ash and ground granulated blast furnace slag cement pastes in 42-month cured samples

Microstructure and composition of 42-month cured pastes using Portland cement (OPC), fly-ash cement (FAC), and ground-granulated-blast-furnace-slag cement (GGBFSC) were investigated. The granular structure of C-A-S-H in outer products of GGBFSC and FAC pastes was finer than in OPC paste. Fly ash offered higher capacity in increasing the Si/Ca ratio of C-A-S-H than slag in the pastes. However, their effects on the chain length were opposite. The incorporation of aluminum into silicate anions in GGBFSC paste was greater compared to others. AlO45− played an essential role in bridging the shorter silicate anions together, forming a longer chain of aluminosilicate anions in the pastes. Mg-Al layered-double hydroxide (LDH) was abundant in the inner products of slag particles with the interbedding of TAH, leading to a low ratio of Mg/Al ratio on hydrotalcite-like phase.

Deterioration Characteristics of Cement-Fly Ash Paste under Strong Ultraviolet Radiation and Low Temperature Conditions

The pastes containing different dosages of fly ash were taken into ultraviolet radiation and low temperature freeze condition simultaneously(URL) for 30 days and only ultraviolet radiation condition(UR) for 30 days after standard curing, so as to investigate the influences of the conditions on the deterioration characteristics of the pastes. Microscopic test methods, such as XRD, TG-DTA and SEM, were used to study the UR effect on the deterioration process of hardened paste. The results show that the deterioration tests, such as URL and UR, inhibit the common development of paste strength, especially after the standard curing age of 360 days. With the increase of fly ash dosage, from 0 to 50%, the reference value decreases, especially at early age. While at the later age, i e, 180 and 360 days, the paste strength cured for 30 days under URL and UR conditions all increase to different extent and the strength is slightly affected very low, especially for the paste containing 25% fly ash. From XRD results, URL and UR dispositions do not influence the hydration product kinds but the amount, especially Ca(OH)2 and CaCO3. Deterioration experiments can decrease the diffraction peak of Ca(OH)2 sharply, and increase that of CaCO3 rapidly, especially under only ultraviolet radiation. From TG-DTA and SEM results, with the increase of curing age, the content of Ca(OH)2 decreases and that of CaCO3 increases. The Ca(OH)2 content of paste under continuous UR curing for 30 days is less than that under URL curing for 30 days, which indicates that UR has more negative effects on the pastes than URL.

Effect of triisopropanolamine on compressive strength and hydration of cement-fly ash paste

This paper aims to investigate the effect of triisopropanolamine (TIPA) on compressive strength and hydration of cement-fly ash paste. The samples with various dosages of TIPA were prepared with 30% fly ash (FA) and 70% cement (water/binder ratio by weight = 0.38), and cured under the standard condition. The compressive strength, pore structure, hydration process, and hydration products were investigated. The results show that TIPA can obviously increase the compressive strength of cement-FA system at the age of 7 d and 60 d, and the reasons are involved in pore structure and hydration of cement-FA system. Pore structure was characterized with mercury intrusion porosimetry, and the results show that TIPA can reduce total porosity but increase the amount of pore with size more than 50 nm, implying the air-entraining effect with negative effect on compressive strength. The result suggests that TIPA and defoaming agent should be used together to minimize the negative effect in real concrete. Furthermore, analysis of hydration products shows that TIPA can accelerate the hydration of both cement and FA, and this can also be illustrated from solid-state nuclear magnetic resonance. It is noticed that TIPA can hasten the conversion of AFt to AFm, which can be indicated from hydration heat. Additionally, the acceleration of pozzolanic reaction of FA is because TIPA can accelerate the dissolution of aluminate, silicate, and ferric into liquid paste which was demonstrated from morphology characterization and the change of ions in pore solution. Such results would be expected to provide experience for the use of alkanolamine in promoting the performance of cement-based materials.

Polished sample preparing and backscattered electron imaging and of fly ash-cement paste

In recent decades, the technology of backscattered electron imaging and image analysis was applied in more and more study of mixed cement paste because of its special advantages. Test accuracy of this technology is affected by polished sample preparation and image acquisition. In our work, effects of two factors in polished sample preparing and backscattered electron imaging were investigated. The results showed that increasing smoothing pressure could improve the flatness of polished surface and then help to eliminate interference of morphology on grey level distribution of backscattered electron images; increasing accelerating voltage was beneficial to increase gray difference among different phases in backscattered electron images.

Advancements in Properties of Cement Containing Pulverised Fly Ash and Nanomaterials by Blending and Ultrasonication Method (Review - Part I)

This review research aims to discuss the results obtained researches on cement containing pure cement, pulverised fly ash, and nanoparticles, in order for eliminating negative side effects underlie the substitution of by–products for pure Portland cement. Nanoparticles (NP) used in these researches are nanoTiO2, nanoSiO2, nanoCaCO3, fibers of carbon nano tube (CNT), nanolimestone (nanoCaCO3), nanoZrO2, nanoclays, and nanometakaolin (nMK) for improving properties of cement systems. Published manuscripts explains two methods regarding on the usage of nanoparticles for cement system: blending and ultrasonication for dispersion of nanoparticles. However, differences between blending and ultrasonication methods suggested by various researchers are also discussed. Experiments reported these papers include the water demand, the density, the setting–times, the heat of hydration, the fluidity, the compressive strength and the flexural strength. According to these results, nanoparticles increase the water demand and heat of hydration of cement; it decreases the density and fluidity for cement mortars, evidently. The most effective nanoparticles on early compressive and flexural strengths are fibers of carbon nano tube and nanoCaCO3. These papers also point effects of these nanoparticles on the strength gain of cement. This review paper inform us until Effect of nanomaterial on water demand and density section in this Part I. Second part of this review paper will explain Hydration properties of Portland pulverised fly ash cement section, Effect of nanomaterial on setting–time section, Effect of nanomaterial on heat of hydration section, Strength gain mechanisms for hardened Portland pulverised fly ash cement paste and mortar section, Effect of nanomaterial on compressive strength section, Effect of nanomaterial on flexural strength (Bending) section, and Conclusion section.

Mercury release from fly ashes and hydrated fly ash cement pastes

The large-scale usage of fly ash in cement and concrete introduces mercury (Hg) into concrete structures and a risk of secondary emission of Hg from the structures during long-term service was evaluated. Three fly ashes were collected from coal-fired power plants and three blend cements were prepared by mixing Ordinary Portland cement (OPC) with the same amount of fly ash. The releasing behaviors of Hg0 from the fly ash and the powdered hydrated cement pastes (HCP) were measured by a self-developed Hg measurement system, where an air-blowing part and Hg collection part were involved. The Hg release of fly ashes at room temperature varied from 25.84 to 39.69 ng/g fly ash during 90-days period of air-blowing experiment. In contrast, the Hg release of the HCPs were in a range of 8.51–18.48 ng/g HCP. It is found that the Hg release ratios of HCPs were almost the same as those of the pure fly ashes, suggesting that the hydration products of the HCP have little immobilization effect on Hg0. Increasing temperature and moisture content markedly promote the Hg release.

Effects of calcium leaching from high volume fly ash cement paste and mortar

Calcium leaching is a deterioration problem of concrete that has long been in contact with water such as dam or reservoir. This paper investigated the effects of calcium leach out on high volume fly ash mixtures of paste and mortar. The samples, composed of fly ash to cement ratio, of 100:90 (by weight), were immersed in 6 M ammonium nitrate solution for 1- 28 days. Then, the leaching depths of samples were investigated and characterized by several techniques for porosity and microstructural studies namely; scanning electron microscopy/energy dispersive x-ray spectroscopy (SEM/EDS), mercury intrusion porosimetry (MIP) and Vickers hardness test. The results from SEM/EDS showed that the calcium ion was leached out as high as 42% and 36% for paste and mortar, respectively. The hardness of leaching zone was reduced by 27% for paste and 22% for mortar. It was found that pore diameter and pore volume of both paste and mortar were significantly increased. The results indicated significant changes in porosity and microstructure.