Fly Ash Cement Paste Research Articles

Assessment of Hydration Degree of Cement in the Fly Ash-Cement Pastes Based on the Calcium Hydroxide Content

The hydration degree of fly ash and the calcium hydroxide (CH) content were measured. Combined with the equilibrium calculation of cement hydration, a new method for assessment of the hydration degree of cement in the fly ash-cement (FC) pastes based on the CH content was developed. The results reveal that as the fly ash content increase, the hydration degree of fly ash and the CH content decrease gradually; at the same time, the hydration degree of cement increase. The hydration degree of cement in the FC pastes containing a high content of fly ash (more than 35%) at 360 days is as high as 80%, even some of which hydrates nearly completely. The effect of water-cement ratio to the hydration degree of cement in the FC pastes is far less distinct than that of the content of fly ash.

温度履歴を与えたフライアッシュセメント硬化体の相組成と強度発現性状に関する基礎的検討

Recently, the demand for precast concrete have been increased with the object for construction, productivity, quality control and the shortening of a construction period. In addition, the study for compressive strength of fly ash concrete under the steam curing has been investigated. However the effect of various condition such as, temperature history and curing condition (water curing or air curing) after steam curing on concrete performance has not been clarified. With regard to the Fly ash concrete cured in temperature history, the study of compressive strength under long term curing age has not been enough. From the background, the relation between the phase composition and compressive strength of Fly ash concrete was investigated in this study. In order to clear the effect of the replacement of Fly ash for compressive strength under the period early age between long term ages, the phase composition of hardened Fly ash cement paste was measured. Furthermore, in order to evaluate the effect of the curing condition after the temperature history, the relation between phase composition and compressive strength cured at water curing or air curing were investigated by Gel/Space ratio. From these experimental results, the compressive strength of Fly ash cement paste cured at water curing after the temperature history was increased until the age of 365 days. The reaction ratio of Fly ash in cement paste cured at water curing was increased until the age of 365 days. On the other hand, the reaction ratio of Fly ash in cement paste cured at air curing was slightly increased until the age of 365 days. A high correlation is observed between the phase composition and the compressive strength independent of the temperature history and type of binder. Therefore, the compressive strength of hardened cement paste cured at water curing and air curing was evaluated by the relation between phase composition and compressive strength. From these results, the compressive strength of cement paste cured at air curing after the temperature history was not increased until the age of 365 days.本研究は、蒸気養生を模擬し温度履歴を与えたフライアッシュセメント硬化体の相組成と圧縮強度の関係を明らかにするため検討を行った。また、蒸気養生後の養生（以下、後養生という）の違いにも着目し、温度履歴後の水中または気中養生の違いがセメント硬化体の強度発現に及ぼす影響を把握することを目的とした。その結果、温度履歴を与えた後に水中養生を行うことでフライアッシュの反応は進行した。また、セメント硬化体の圧縮強度を相組成に基づきゲル空隙比によって整理した結果、両者は高い相関を示した。さらに、両者の関係から気中養生の硬化体の相組成に基づいて、気中養生のフライアッシュセメント硬化体の強度発現性状を評価した。

High temperature resistance of a very high volume fly ash cement paste

The physical and chemical changes taking place in a very high volume fly ash cement paste (FAN-4) following exposure to temperatures up to 1000°C are presented. Tests were repeated with commercially available CEM II/A-M Portland composite-cement (MS). FAN-4 pastes showed impressive residual strengths after heat exposure which increased dramatically when heated previously at 800 or 1000°C. This was in stark contrast to the MS paste, which showed a continual decrease in residual strength following heating and subsequent cooling. The increase in residual strength with FAN-4 paste coincided with a major shrinkage event, which was associated with sintering and the formation of the new mineral phases anorthite, gehlenite, wollastonite, diopside and albite. The MS cement formed generally non-hydraulic calcium silicate phases upon heating at ⩾800°C. The differences in phase formation were linked to different starting elemental compositions. The FAN-4 paste was considerably richer in Si, Al, Fe and alkalis but poorer in Ca. The elemental composition of the binder phase will be an important factor to take into account when determining the high-temperature performance of future cements and concretes, which are likely to contain significantly higher contents of supplementary cementitious materials than is presently the case.

Influence of triethanolamine on the hydration and the strength development of cementitious systems

The influence of triethanolamine (TEA) at varying dosages on the strength development of pure cement (PC) and fly ash cement (FAC) pastes was systematically investigated. Techniques including mercury intrusion porosimetry, X-ray diffraction, thermogravimetric analysis and scanning electron microscopy were employed to characterise the microstructure of hardened cement pastes and to monitor the cement hydration process with the aim of correlating strength variations with microstructural changes caused by TEA addition. The results showed that 0·03–0·10% addition of TEA visibly enhances the early strength but decreases the compressive strength after 3 d for PC pastes. A high dosage of TEA (1%) leads to a sharp decrease in 28-d strength for both PC and FAC pastes. Mechanistic explanations regarding why TEA affects the strength development differently at different dosages and ages are discussed by considering the impacts of TEA on the hydration degree, pore structure, composition and morphology of hydration products.

Matlab Image Processing Technique and Application in Pore Structure Characterization of Hardened Cement Pastes

Based on Matlab image processing technique and backscattered electron image analysis method, a characterization method is set up to make quantitative analysis on pore structure of hardened cement pastes. Adopt Matlab to acquire images, and carry out gradation and binarization processing for them; use the combination method of local threshold segmentation and histogram segmentation to obtain pore structure characteristics. The results showed that evolution law of pore structure of fly ash cement pastes via Matlab image analysis method is similar to the conclusion obtained through BET and DVS. Selecting different angle of backscattered electron images in the same sample, its statistic results are more representative.

Effect of Cuing Conditions on the Microstructural Characteristics of Complex Binders

The microstructure characteristics of complex binders under two curing conditions were studied by observing the micro-morphology, analyzing the pore character and means of spectrum test. The results showed that high temperature curing improved the pore structure formed in the early hydration period and reduced the porosity and the coarse pores in pastes. More hydration products were generated on the surface of fly ash particles that made the microstructure of pastes more denser than cured in room temperature. However, high temperature curing had insignificant influence on the CaO/SiO2 ratio of C-S-H gel in pastes and contributed little to the improvement of the later hydration degree and microstrcture of fly ash-cement pastes.

A Raman spectroscopic study of the evolution of sulfates and hydroxides in cement–fly ash pastes

Raman spectroscopy has been employed to study the evolution of sulfo-aluminate and hydroxyl phases in pastes made with ordinary Portland cement (OPC) and fly ash (FA). Three fly ashes with different CaO contents were used as a cement replacement at the level of 0 and 50% by weight. The pastes were analyzed at 0, 0.2, 2, 4, 8, 12, 16, 20, 24, 48, 72h, and 7, 14, 21, 28, and 56days after mixing. The wavenumber ranges used for Raman spectroscopic analysis are 950–1050cm−1 for evolution of sulfates and 3600–3700cm−1 for evolution of hydroxides. Gradual disappearances of gypsum in parallel with the formation of ettringite (AFt) are clearly observed in most paste mixes. Evolution of hydroxides showed the gradual spatial growth of portlandite. In addition to the potential benefits, the limitations of using Raman spectroscopy in study of cement-based materials are also noted in this paper.

Dispersion of CaCO3 nanoparticles by sonication and surfactant treatment for application in fly ash–cement systems

This research aims to offset the negative effects of fly ash on the early-age properties of cementitious materials with the use of calcium carbonate (CaCO3) nanoparticles. The main focus is to enhance the effect of the nanoparticles by improving dispersion through ultrasonication and use of surfactants. CaCO3 aqueous suspensions with various surfactant types and concentrations are prepared and subjected to different sonication protocols (varying duration and amplitude). Dispersion and stability are quantitatively measured by comparing their absorbance spectra through spectrophotometry and qualitatively evaluated through SEM imaging. The effectiveness of sonicated CaCO3 nanoparticle additions in accelerating setting and improving early-age compressive strength gain of fly ash–cement pastes is investigated. The sonication protocol is optimized and the most effective dispersion is achieved with polycarboxylate-based superplasticizer. Good agreement is found between the dispersion measurements and mechanical performance.

Durability of very high volume fly ash cement pastes and mortars in aggressive solutions

Abstract The resistance of very high volume fly ash cement pastes and mortars activated by Na 2 SO 4 has been monitored following immersion for up to 90 d in 0.1 M HCl, 4.4% Na 2 SO 4 and ASTM-compliant sea water. Changes in the compressive strengths of mortars and in crystalline phases, bond environments, and the microstructure of pastes following immersion were monitored. Experiments were repeated with a commercially available sulfate resistant cement. Both cements were found to present adequate resistance to both sea water and the Na 2 SO 4 solution. However, both were severely degraded by acid immersion. Differences in potential degradation mechanisms based on the chemistry of the fly ash binder and the reference cement are discussed.

Influence of Fly Ash on Alkalinity of Pore Solution and Microstructure Characteristics of Hardened Cement Pastes

Influence of fly ash on alkalinity of pore solution and microstructure characteristics of hardened cement pastes were investigated by means of ex-situ leaching (ESL), MIP and SEM. The results reveal that the decline of alkalinity of pore solution is not endless, even though the contents of fly ash in fly ash-cement(FC) pastes is very high(65%).The incorporation of fly ash in proper ratios could increase the ratio of harmless pores and improve the microstructure of the FC pastes. Ca(OH)2 was not consumed heavily or exhausted when cementitious material was hydrated due to the low reaction degree of fly ash.

Hydration Performance and Pore Structure of Fly Ash-Cement Pastes

The hydration performance and pore structure of fly ash-cement (FC) pastes were investigated by means of XRD, TG, ex-situ leaching (ESL) and MIP. The results reveal that the influence of curing age to the kinds of hydration products of FC pastes is unconspicuous. As the content of fly ash increase, the Ca(OH)2 content decrease gradually. However, the content of Ca(OH)2 left in every paste is almost constant after 28d, regardless of the replacement ratio of fly ash and water-binder ratio. The decline of alkalinity of pore solution is not endless, even though the contents of fly ash in FC pastes is very high(65%).The incorporation of fly ash in proper ratios could increase the ratio of harmless pores and improve the microstructure of the FC pastes. Ca(OH)2 was not consumed heavily or exhausted when cementitious material was hydrated due to the low reaction degree of fly ash.

Experimental Investigation on Reaction Rate and Self-healing Ability in Fly Ash Blended Cement Mixtures

This paper investigates the hydration rate in fly ash blended cement paste and self-healing ability in mortar incorporating fly ash for long-term period. The hydration rate of fly ash and consumption of calcium hydroxide in fly ash paste containing calcium hydroxide reagents were examined at different ages and curing temperatures. Five types of fly ash blended cement paste, each of which with 10% by mass fly ash replacement ratio, were prepared for the acceleration test at 80&degC. Four fly ash cement pastes, with two types of fly ash and with fly ash replacement ratios by mass 10% and 30%, were tested to measure the rate of reaction of fly ash in the mixtures. Ten mixtures were tested to evaluate the self-healing ability of mortar incorporating fly ash, considering different types of cement and fly ash. Compressive strength, bending strength, accelerated carbonation depth, after applying freeze/thaw cycling until 60% and 80% relative dynamic modulus of elasticity, and porosity were examined. The experimental results revealed that incorporating fly ash in cement paste would affect the hydration rate of fly ash and consumption of calcium hydroxide. Curing temperature can affect hydration rate, acceleration rate and velocity of reaction rate in fly ash cement paste. Moreover, it is confirmed that self-healing ability, carbonation rate coefficient and the pore volume modification in mortar incorporating fly ash rely on the curing conditions. Finally, it is suggested that the practical fly ash replacement ratio in concrete could be 10% to 15% and 11% to 20% for water to cement ratios 0.50 to 0.55 and 0.55 to 0.60, respectively.

Leaching Characteristics of Cement-Fly Ash Pastes

The leaching characteristics of cement-fly ash pastes were investigated by MIP, XRD and TG. The results show that the microstructure of hardened cement-fly ash paste deteriorates partly after leaching of soft water. At the leaching duration of 180 days, the hardened paste containing fly ash deteriorated a little more serious than the plain cement paste, but the incorporation of fly ash in a proper ratio was helpful to defer the trend of deterioration of the hardened paste microstructure. The microstructural stability of hardened cement-fly ash paste wasn’t damaged severely during the 180 days leaching duration.

Effects of colloidal nanoSiO2 on fly ash hydration

The influences of colloidal nanoSiO2 (CNS) addition on fly ash hydration and microstructure development of cement–fly ash pastes were investigated. The results revealed that fly ash hydration is accelerated by CNS at early age thus enhancing the early age strength of the materials. However, the pozzolanic reaction of fly ash at later age is significantly hindered due to the reduced CH content resulting from CNS hydration and the hindered cement hydration, as well as due to a layer of dense, low Ca/Si hydrate coating around fly ash particles. The results and discussions explain why the cementitious materials containing nanoSiO2 had a lower strength gain at later ages. Methods of mitigating the adverse effect of nanoSiO2 on cement/FA hydration at later ages were proposed.

Analysis of pore structure, contact angle and pore entrapment of blended cement pastes from mercury porosimetry data

Abstract The mercury intrusion porosimetry (MIP) was used to investigate the pore structure characteristics of fly-ash cement pastes with two water to binder ratios ( w / b = 0.3, 0.5). The total porosity, pore fraction distribution, contact angle hysteresis and the pore entrapment are quantified from the intrusion/extrusion data. The tortuosity of pore structure is further calculated from pore entrapment. The results show: (1) the w / b ratio is determinant for the formation of pore structure patterns, high w / b ratio providing high porosity, high connectivity but low pore surface roughness to pore structure; (2) the contact angle hysteresis can be related to pore surface roughness, the hysteresis factor α θ attaining respectively 0.42, 0.75 for Paste I ( w / b = 0.5) and Paste II ( w / b = 0.3) samples at 180 d; (3) the tortuosity and the porosity obey just roughly the power law and more pore characteristics are needed to describe the tortuosity of blended cement pastes.

Study on the physical and mechanical property of ordinary portland cement and fly ash paste

An experimental investigation has been carried out to study the physical and mechanical property of high volume fly ash cement paste. Ordinary portland cement was replaced by 0, 20, 30, 40, 50, 60 and 70% class F fly ash (by weight). Water- binder ratio in all mixture was kept constant at 0.3. Cube specimens were compacted in table vibrator. As expected bulk density decreases with fly ash increment in the mixture. Apparent porosity and water absorption value increases with replacement of cement by fly ash. Results confirm the decrease in compressive strength at 3, 7 and 28 day with fly ash addition and it is more prominent in case of more than 30% fly ash content mixes. Ultrasonic pulse velocity test results indicate that the quality of the paste deteriorate with increase of fly ash content in the mixture.

Effects of MgO-based expansive additive on compensating the shrinkage of cement paste under non-wet curing conditions

Abstract Expansive additives are widely used to compensate the drying shrinkage of cement-based materials to avoid cracking. However, the expansion of conventional ettringite-bearing expansive additive depends strongly on wet curing and is mainly generated at early age, and hence it may not work well in concretes without sufficient water supply or exhibit long-term shrinkage. MgO-based expansive additive, for which less water is needed for the formation of Mg(OH) 2 in comparison to ettringite, was prepared and its compensating effect on the autogenous shrinkage and late age thermal shrinkage of Portland and fly ash cement pastes at low water-to-cement ratio was investigated. The tests were conducted under sealed condition, so that the moisture exchange with the environment was prevented. Results show that, even under the non-wet curing condition, the shrinkages of cement pastes can be compensated effectively. Microstructure analysis by scanning electron microscope indicates that the macro-expansion of cement pastes is probably caused by the locally restrained expansion of MEA due to the hydration of MgO.

Influence of particle size distributions on yield stress and viscosity of cement–fly ash pastes

The rheological properties of blended cement-based materials depend strongly on mixture proportions and the characteristics of the components. In this study, design of experiments is used to investigate the influence of three variables (cement particle size distribution (PSD), fly ash PSD, and ratio of fly ash to cement) at each of four levels on the yield stress and viscosity of blended pastes. Both rheological parameters are seen to vary over several orders of magnitude for the evaluated design space. Physical characteristics of the powders, such as cement and total particle densities and total particle surface area, are computed for each mixture. A percolation-type relationship is observed between yield stress and cement particle (number) density. While neither apparent nor plastic viscosities were particularly well described by the commonly employed Kreiger–Dougherty equation, plastic viscosities were found to be linear functions of either total (cement+fly ash) particle surface area or total particle density.

The microstructure of 4-year-old hardened cement-fly ash paste

In this study, the microstructure of 4-year-old hardened cement-fly ash paste containing 50% fly ash has been investigated. 65.44% of Ca(OH)2 produced by cement hydration is consumed by the pozzolanic reaction of fly ash in 4years and the Ca(OH)2 content of the hardened cement-fly ash paste is only 3.15% at the age of 4years. Most of the small fly ash particles react heavily but many large fly ash particles remain almost unreacted. 72.7% of fly ash remains after 4years, which indicates that most of the fly ash takes physical effect during the hardening process of paste. Compared with cement paste, there is a relative more significant improvement of the pore structure of cement-fly ash paste due to the pozzolanic reaction of fly ash from 90days to 4years. The mean Ca/Si, Al/Si, and Ca/(Si+Al) atom ratios of the gel in the 4-year-old hardened cement-fly ash paste are 1.52, 0.33 and 1.15 respectively, which indicate that the gel has a lower Ca/Si atom ratio and is richer in Al content than that of Portland cement paste.

Influence of Lime Dust on Chloride Binding Capacity of Fly Ash-Cement Paste

The influence of the way and the volume of adding lime dust to fly ash-cement pastes on chloride physical adsorption capacity, chemical combination capacity and binding capacity of fly ash-cement pastes is investigated by adsorption equilibrium method. The results show that with the volume of lime dust as cement replacement raising, the amount of chloride physically adsorbed by fly ash-cement pastes reduces, while the amount of chloride chemical combining is firstly increase, and then decrease, and it reaches the maximum when the lime dust content is 10% , that of chloride binding is firstly increase, and then decrease, which reaches the maximum when the lime dust content is 5%. In addition, with the volume of lime dust as fly ash replacement increasing , the volume of chloride physically adsorbed by fly ash-cement pastes reduces firstly, and then raises, reaching the minimum when the lime dust content is 6%. Whereas, volume of chloride chemical combining and binding both decrease gradually with the increase of lime dust contents. When the population of lime dust as cement or fly ash replacement is low(less than 15% for cement and 9% for fly ash), effect of lime dust content on chloride binding capacity of fly ash-cement pastes is not obvious (under 7.5%).