Degree Of Pozzolanic Reaction Research Articles

Effect of nano-CaCO 3 slurry on the mechanical properties and micro-structure of concrete with and without fly ash

Abstract The effect of nano-CaCO3 slurry on the mechanical properties and microstructure of concrete with and without fly ash was studied in this paper. Experimental results showed that nano-CaCO3 slurry was helpful to increase the mechanical strength of concrete with and without fly ash at early ages due to the acceleration of cement hydration from seeding effect and the more compact microstructure of cement paste from nano-filling. However, the efficacy of nano-CaCO3 slurry on the concrete without fly ash was quite limited at the late ages just like other nano-materials. This may well affect the application of nano-materials in concrete. On the other hand, nano-CaCO3 slurry was still helpful to enhance the mechanical strength of concrete with fly ash at late ages. Furthermore, the enhancement degree of mechanical strength of concrete with fly ash at 56 days increased with the increasing content of fly ash when 2% fly ash was substituted by nano-CaCO3. The results of X-Ray diffraction analysis and SEM observation indicated that the pozzolanic reaction degree of fly ash could be accelerated by nano-CaCO3 slurry at late ages. Therefore, a feasible technique for the usage of composite nano-materials in concrete may well be provided by using nano-CaCO3 with fly ash due to the acceleration hydration of cement at early ages and that of fly ash at late ages.

Influence of nano-TiO2 on physical and hydration characteristics of fly ash–cement systems

This paper presents the effect of nano-TiO2 (NT) on physical and hydration characteristics of cement-based materials containing different fly ash (FA) contents ranging from 10% to 30% (by weight) as partial replacement of cement. The fluidity of mortars and the setting time of pastes were tested with different NT dosage. The compressive strength of mortars is measured at 3, 7, 28 and 90days. Results show that NT addition could accelerate initial and final setting and decrease the fluidity, while FA had the opposite effects. Introducing NT would lead to a considerable increase in compressive strength at early ages while it had an adverse effect on the later age strength. However, the positive functions of FA in late strength could offset the negative influence of NT and the optimum contents of NT and FA are 3% and 20%, respectively. Then the influence mechanisms of NT on properties of FA–cement materials are investigated by hydration heat, X-ray diffraction (XRD), differential scanning calorimetry–thermogravimetry (DSC–TG), scanning electron microscope (SEM) and nuclear magnetic resonance (NMR). The results of analyses indicate that incorporating NT promoted the early hydration of cement and accelerated the formation and precipitation of hydration products. Furthermore, in the strength development process, NT could raise the pozzolanic reaction degree of FA and lead to the formation of a C-S-H type with longer chain and higher Al:Si ratio. Also, the interfacial transition zone (ITZ) of FA mortars were improved by NT.

고강도 고로슬래그 혼합 시멘트 페이스트의 수화 및 포졸란 반응에 미치는 고로슬래그 미분말의 치환률과 분말도의 영향

This study investigated the fluidity, heat of hydration, setting time, strength development, and characteristics of hydration and pozzolanic reactions of high-strength high-volume ground granulated blast-furnace slag(GGBFS) blended cement pasts with the water-to-binder ratio of 20% by experiments, and analyzed the effects of the replacement ratio and fineness of GGBFS on the hydration and pozzolanic reaction. The results show that, in the high-strength mixtures with low water-to-binder ratio, the initial hydration is accelerated due to the “dilution effect” which means that the free water to react with cement increases by the replacement of cement by GGBFS, and thus, strengths at from 3 to 28 days were higher than those of plain mixtures with ordinary Portland cement only. Whereas it was found that the long term strength development is limited because the hydration reaction rates rapidly decreases with ages and the degree of pozzolanic reaction is lowered due to insufficient supply of calcium hydroxide according to large replacement of cement by GGBFS. Also, the GGBFS with higher fineness absorbs more free water, and thus it decreases the fluidity, the degree of hydration, and strength. These results are different with those of normal strength concrete, and therefore, should be verified for concrete mixtures. Also, to develop the high-strength concrete with high-volume of GGBFS, the future research to enhance the long-term strength development is needed.

The study on the activity of fly ash in Ca(OH)2 solution

The purpose of this study was to investigate the degree of pozzolanic reaction and active ion dissolution of fly ash in different concentrations of Ca(OH)2 solution dissolving in 48 hours. The active ion concentration and conductivity of filtrate were measured by inductively coupled plasma optical emission spectrometer and conductivity metre. The micromorphology of residue in different dissolving times was analysed by field emission transmission electron microscopy. The results indicate the following: (a) the fly ash has pozzolanic activity and it has similar variation of active Al3+, Fe3+, Si4+ ion concentration in 1 mol L−1 Ca(OH)2 solutions compared with 0·1 mol L−1 Ca(OH)2 solutions with an increase in dissolving time: the concentration of Al3+, Si4+ ions appears to have two peaks at 6 and 24 hours, whereas Fe3+ ion concentration appears declining. (b) Conductivity of 1 and 0·1 mol L−1 fly ash–Ca(OH)2 solution shows decreasing tendency, the degree of pozzolanic reaction in 1 mol L−1 Ca(OH)2 solution appears to be higher than in 0·1 mol L−1 Ca(OH)2 solution.

Properties and pozzolanic reaction degree of tuff in cement-based composite

In order to investigate the feasibility and advantage of tuff used as pozzolan in cement-based composite, the representative specimens of tuff were collected, and their chemical compositions, proportion of vitreous phase, mineral species, and rock structure were measured by chemical composition analysis, petrographic analysis, and XRD. Pozzolanic activity strength index of tuff was tested by the ratio of the compression strength of the tuff/cement mortar to that of a control cement mortar. Pozzolanic reaction degree, and the contents of CH and bond water in the tuff/cement paste were determined by selective hydrochloric acid dissolution, and DSC-TG, respectively. The tuffs were demonstrated to be qualified supplementary binding material in cement-based composite according to relevant standards. The tuffs possessed abundant <TEX>$SiO_2+Al_2O_3$</TEX> on chemical composition and plentiful content of amorphous phase on rock texture. The pozzolanic reaction degrees of the tuffs in the tuff/cement pastes were gradually increased with prolongation of curing time. The consistency of CH consumption and pozzolanic reaction degree was revealed. Variation of the pozzolanic reaction degree was enhanced with the bond water content and relationship between them appeared to satisfy an approximating linear law. The fitting linear regression equation can be applied to mutual conversion between pozzolanic reaction degree and bond water content.

Pozzolanic activity of volcanic rocks from Southern Jiangxi Province, China

The pozzolanic activity of volcanic rocks from southern Jiangxi Province, China, was studied in terms of material properties, pozzolanic reaction degree (PRD), and pozzolanic activity index. The pozzolanic activity component of the volcanic rocks calculated from chemical composition and amorphous phase content was appraised as a characteristic index of potential pozzolanic activity. The PRD of the volcanic rock was determined by hydrochloric acid dissolution. The pozzolanic activity index of the volcanic rocks was evaluated as the ratio of the compressive strength of the cement mortar blended with volcanic rock to that of a control cement mortar. The results showed that the volcanic rocks demonstrated great potential for pozzolanic activity in terms of pozzolanic activity component, PRD, and pozzolanic activity index, because they have high SiO2 + Al2O3 contents and plentiful amorphous phases. Finally, a consistent relationship between PRD and pozzolanic activity index was found.

Ultrafine grinding of fly ash with grinding aids: Impact on particle characteristics of ultrafine fly ash and properties of blended cement containing ultrafine fly ash

This paper presents the ultrafine grinding performance of fly ash with grinding aids (GA) and effect of GA on the particle characteristics, pozzolanic reaction degree of ultrafine fly ash (UFFA) and properties of blended cement containing UFFA. The experimental results indicate that GA can improve the particle characteristics of ground fly ash. The specific surface area of UFFA with 0.05wt% GA is higher by 104m2/kg than control sample. The particle size distribution of UFFA with GA becomes narrow and particle content of less than 16μm is obviously increased. Additionally, the fluidity and loose bulk density of UFFA have also been improved. For pozzolanic reaction degree, 3-day, 7-day and 28-day hydration degree of UFFA with GA is increased by 1.2%, 2.3% and 4.6%, respectively; and 3-day, 7-day and 28-day activity index of UFFA with GA is also increased by 6%, 8% and 9%, respectively. For properties of blended cement containing UFFA, enhancement effect of UFFA with GA on the strength of blended cement containing 20wt%, 30wt% and 40wt% fly ash are significant, and 3-day strength is increased by 12.6%, 6.6% and 6.6%, respectively, 28-day strength is increased by 6.5%, 9.3% and 10.5%, respectively. In this study, the hydration degree and microstructure of blended cement containing UFFA paste are also analyzed by SEM, XRD, IR and TG-DTA. The studies show that improvements such as hydration degree, products quantity, uniformity and compactness of products structure of blended cement containing UFFA are attributed to finer particle size and better particle size distribution caused by the addition of GA.

Determination of Reaction Degree of Silica Fume and Fly Ash in a Cement - silica fume - fly ash Ternary Cementitious System

The reaction processes of mineral admixtures in a cement-silica fume-fly ash ternary cementitious system aged for different time were quantitatively studied by means of selective dissolution and non-evaporative water content methods. To determine separately the degree of pozzolanic reaction for silica fume in the ternary system, fly ash was replaced by the inert quartz powder with the same fineness and same dosage in order to eliminate the dilution effect and heterogeneous nucleation effect. Then the reaction degree of fly ash can be calculated from the total reaction degree of all the admixtures in the system. The test results demonstrated that only 1 d after the beginning of the hydration, the pozzolanic reaction for silica fume already started, and the reaction exhibited a fast rate in the early stage but a slow one in the later stage. However, the same reaction with a rapidly increasing rate for fly ash just began only 7 d after the beginning of the hydration. In the ternary cementitious system, the hydration degrees for both silica fume and fly ash were reduced with the increasing fly ash content.

Effect of Heat-Curing Procedure on Strength and Microstructure of Reactive Powder Concrete Having High Volume of Mineral Admixtures

The effect of heat-curing procedure on strength of reactive powder concrete (RPC) having high volume of mineral admixtures, such as ultra-fine fly ash (UFFA), steel slag powder (SS) and silica fume (SF) was studied in this paper. Moreover, the effect of the cuing temperature (20°C, 60°C and 90°C) and the duration (1 day and 3 days) of heat-curing both on microstructure of RPCs’ samples were investigated by SEM-EDXA. The results indicate that the heat-curing procedure has a great influence on strength of this RPC; the compressive strength of specimens cured in a appropriate condition could achieve more than 190 MPa. Moreover, a high curing temperature or a long duration of heat-curing will cause not only a high degree of pozzolanic reaction but also a low n(Ca)/n(Si) ratio of hydration product. Thus, the microstructure of the paste becomes more compact, which would definitely improve the mechanic properties of the hardened paste.

Hydration properties of cement pastes containing high-volume mineral admixtures

This research aimed to investigate the influence of high-volume mineral admixtures (MAs), i.e., fly ash and slag, on the hydration characteristics and microstructures of cement pastes. Degree of cement hydration was quantified by the loss-on-ignition technique and degree of pozzolanic reaction was determined by a selective dissolution method. The influence of MAs on the pore structure of paste was measured by mercury intrusion porosimetry. The results showed that the hydration properties of the blended pastes were a function of water to binder ratio, cement replacement level by MAs, and curing age. Pastes containing fly ash exhibited strongly reduced early strength, especially for mix with 45% fly ash. Moreover, at a similar cement replacement level, slag incorporated cement paste showed higher degrees of cement hydration and pozzolanic reaction than that of fly ash incorporated cement paste. Thus, the present study demonstrates that high substitution rates of slag for cement result in better effects on the short- and long-term hydration properties of cement pastes.

Rate of pozzolanic reaction of two kinds of activated coal gangue

Two kinds of activated ways are used to prepare activated coal gangue fine powder, one is calcining coal gangue at 800 °C (gangue A), and the other is calcining coal gangue with a certain calcite at 800 °C (gangue B). The experiment shows that strengths of blended cement mortar with coal gangue B are higher than that of blended cement with coal gangue A. Hydration of cements with the two kinds of activated coal gangue is investigated through a differential thermal analysis. The weight loss due to Ca(OH)2 decomposition of hydration products by differential thermal analysis/thermo gravimetric (DTA/TG) can be used to quantify the pozzolanic reaction. A new method based on the composition of hydration cement is proposed to determine the degree of pozzolanic reaction. The results obtained suggest that the degree of pozzolanic reaction of gangue B is faster than that of gangue A.

INFLUENCE OF RELATIVE HUMIDITY ON COMPRESSIVE STRENGTH OF FLY ASH CEMENT PASTE

This study is aimed to elucidate the influence of ambient relative humidity on the compressive strength of fly ash cement paste. The ambient relative humidity has an influence on both the internal relative humidity and the hydration reaction, and further influences the compressive strength development. The experiment results revealed that the compressive strength of paste decreases with a reduction in ambient relative humidity. The same tendency can be found on the internal relative humidity and the hydration reaction as well. The detriment of strength development at low ambient relative humidity is affected by the retardation of the hydration reaction of C2S and C4AF. Moreover, the internal relative humidity at the time of test also has an influence on the compressive strength. The compressive strength decreased with the reduction in the relative humidity below the relative humidity lower than 95%. In addition, the compressive strength can be expressed as a function of the internal relative humidity and the hydrated gel volume derived from the degree of hydration of cement and the degree of pozzolanic reaction of fly ash.

HYDRATION OF CEMENT PASTE AND ESTIMATION OF STRENGTH DEVELOPMENT OF MORTAR USING FLY ASH

The reaction rates of C3S, C2S and fly ash were examined by proposed XRD method to investigate the strength developing mechanism of fly ash concrete. The strength development of fly ash mortar was examined by specimens and estimated with the reaction rates of C3S, C2S and fly ash by proposed method in this paper. The following observations are made: (1) the larger of surface area of cement, the more of hydration degrees; (2) the pozzolanic reaction degree of fly ash correlates with the calcium hydroxide, is approximately in proportion to the account of calcium hydroxide, and influenced by the liquation of calcium hydroxide; (3) when adding fly ash, the hydration of C2S appears to be retarded, though it has little apparently effect on the hydration of C3S.

Simulation of silica fume blended cement hydration

A model is proposed in this paper to simulate silica fume (SF) blended cement hydration based on the kinetics, stoichiometry and physical chemistry of cement hydration and pozzolanic reaction. The pozzolanic reaction degree, volume fraction of hydration products, capillary porosity and gel porosity can be obtained from model simulation. By using proper amount of silica fume replacement, the microstructure of silica fume blended cement paste is improved since the volume fraction of C-S-H gel is increased, Ca(OH)2 content and capillary porosity are decreased due to pozzolanic reaction compared with ordinary Portland cement (OPC) paste. The effects of silica fume particle size, glass phase content and the percentage of silica fume replacement on pozzolanic reaction degree, volume fraction of hydration products, and capillary porosity are simulated. The simulation results show that finer silica fume particles with higher glass phase content (GP) are of higher reactivity. There is an optimum silica fume replacement; extra silica fume only acts as inert filler because there is no enough Ca(OH)2 from cement hydration to react with it pozzolanically.

Fly ash effects: II. The active effect of fly ash

This paper examines the method for determining the hydration degree of cement clinker and the pozzolanic reaction degree of fly ash in the system of cement and fly ash. In the base, the active effect of fly ash is studied. The studied results show that the active effect includes two aspects: (1) Fly ash has stronger pozzolanic activity and can react with Ca(OH) 2, and (2) it can promote the hydration of cement. When the content of fly ash is less, its pozzolanic activity can exert well, but its promoting role to the hydration of cement is weaker. When the content of fly ash is more, it is less than its pozzolanic activity can be used, but its promoting role to the hydration of cement is stronger.

Rate of pozzolanic reaction of metakaolin in high-performance cement pastes

This paper assesses the hydration progress in metakaolin (MK)-blended high-performance cement pastes with age from the measurements of compressive strength, porosity, and pore size distribution, the degree of pozzolanic reaction, and the Ca(OH) 2(CH) content of the MK-blended cement pastes at a water-to-binder ratio (w/b) of 0.3. Comparisons are also made with pastes containing silica fume (SF), fly ash (FA), and control Portland cement (PC). It is found that at early ages, the rates of pozzolanic reaction and CH consumption in the MK-blended cement pastes are higher than in the SF- or FA-blended cement pastes. The higher pozzolanic activity of MK results in a higher rate of strength development and pore structure refinement for the cement pastes at early ages. Although the rate of pozzolanic reaction of MK becomes slower after 28 days of curing, the pozzolanic reaction in the blended cement pastes with a w/b of 0.3 still continues at the age of 90 days. At this age, about half of the MK still are unreacted.

A study of ground coarse fly ashes with different finenesses from various sources as pozzolanic materials

The aim of this study is to evaluate the properties of ground coarse fly ashes, from five sources in Thailand, the shapes, sizes, and chemical compositions of which are completely different. Coarse fly ash was fractionated by an air classifier and ground into three different finenesses ranging from median particle sizes of 1.9–17.2 μm. Physical and chemical properties of the Portland cement and the fly ashes were investigated. Mortar cubes of 5 cm were cast with 20% replacement by weight of Portland cement with ground coarse fly ash. The compressive strengths of the fly ash–cement mortars were determined and compared with the control mortar.The results revealed that the degree of pozzolanic reaction, as determined using compressive strength, of coarse fly ash increased when its fineness was increased by grinding. The strength activity indices of the original fly ash–cement mortars at the curing ages of 7 and 28 days were in the range of 69–82% and 76–90%, respectively. When the particle size smaller than 9 μm of ground coarse fly ash was used, the strength activity index achieved was over 100% of that of the control within 28 days. The results also showed that the fineness of fly ash, not the chemical composition, was the major factor affecting the strength activity index of ground coarse fly ash–cement mortar.

A study on the hydration rate of natural zeolite blended cement pastes

Natural zeolite is a type of mineralogical material containing large quantities of reactive SiO 2 and Al 2O 3. It is widely used in the cement industry in China as a cement blending material. Like other pozzolanic materials such as silica fume and fly ash, zeolite contributes to concrete strength mainly through the pozzolanic reaction with Ca(OH) 2, Thus, the pozzolanic reactivity of this type of material in comparison with other pozzolans is of much interest. This paper presents experimental results on the compressive strength, degree of pozzolanic reaction, and porosity of zeolite modified cement pastes. These results are compared with those obtained from similar blended cement pastes prepared with silica fume and fly ash replacements. Based on the experimental results, it can be concluded that natural zeolite is a pozzolanic material, with a reactivity between that of silica fume and fly ash. Generally, in blended cement pastes with a lower water-to-cementitious materials ratio, the natural zeolite contributes more to the strength of the pastes. But in the pastes with a higher water to cementitious ratio and a lower cement replacement level it undergoes a higher degree of reaction.

Pozzolanic reactivity of lightweight aggregates

Over recent years more attention has been given to the influence of the aggregate-cement paste interfacial zone on the various properties of concrete. For the lightweight aggregate - cement paste interface, it is believed that the interface is characterized by a mechanical interlocking in combination with a chemical interaction in the form of pozzolanic reaction. In the literature, however, not too much information on the effect of the pozzolanic reactivity of lightweight aggregates is available. Therefore, an investigation on the pozzolanic reactivity of some lightweight aggregates based on expanded clay and sintered fly ash was carried out. A certain degree of pozzolanic reaction between cement paste and lightweight aggregate was observed, but the effect was not very pronounced. The low degree of pozzolanic reactivity may be the result of a recrystallization of the mineral compounds during the manufacturing process of the aggregate.