Strength and microstructural characteristics of chemically activated fly ash-cement systems

The use of fly ash as a cement replacement material increases the long-term strength and durability of concrete. Despite these great benefits, the use of fly ash is limited due to the low early strength of fly ash concrete. To eliminate this problem, many studies have been conducted on accelerating the pozzolanic properties of fly ash. The study reported below investigated the strength and microstructural characteristics of fly ash-cement systems containing three kinds of activators—Na₂SO₄, K₂SO₄, and triethanolamine—to accelerate the early strength of fly ash mortars. Through the use of thermal gravity analysis, it was demonstrated that the activators not only decreased or maintained the amount of Ca(OH)₂ products, but also increased the production of ettringite at early ages. X-ray diffraction, scanning electron microcopy, and mercury intrusion porosimetry also confirmed that in the early curing stages of fly ash-cement pastes containing activators, large amounts of ettringite were formed, resulting in a reduction in the pore size ranging from 0.01 to 5 μm. The research results support the supposition that the addition of small amounts of activators is a viable solution for increasing the early-age compressive strength of fly ash concrete.     

Study on high-strength composite portland cement with a larger amount of industrial wastes

It is one of important measures for the sustainable development of cement industry to utilize industrial wastes. High-strength composite portland cement with a large amount of granulated blast furnace slag (GBFS), fly ash and limestone was prepared by separate grinding method, optimizing gypsum and using activators. The total amounts of blending materials are between 45% and 65% and the strength grades of cements reach 525 or even 625 according to Chinese national standard for composite portland cement. Besides setting time and strength, the hydration heat, drying shrinkage and sulfate resistance were also determined.     

Effect of fly ash and nanosilica on compressive strength of concrete at early age

The replacement of cement by mineral admixtures in concrete has been of increasing interest in the construction industry. Nevertheless, several of the potential replacements, such as fly ash class F, lower the compressive strength of concrete at early age. This project investigates the use of nanosilica to compensate for such loss of compressive strength. A statistical experimental design involving mixtures of Portland cement, fly ash and nanosilica, in addition to water/binder ratio as an external factor, is proposed to study their combined effect on the compressive strength of concrete. This design allows estimating a cubic regression model that properly accounts for the effects of the mixture components within a constrained experimental region. The range of each factor was selected according to levels normally used in the industry. Finally, an optimisation strategy permits to recommend the use of nanosilica when high percentages of cement replacement by fly ash are present.     

粉煤灰-矿渣微粉复合双掺对水泥砂浆抗酸雨侵蚀性能的影响

研究了粉煤灰、矿渣微粉复合双掺时对水泥砂浆的强度以及抗模拟酸雨侵蚀性能的影响。通过试验发现:随着粉煤灰、矿渣微粉总掺量的不断增加,砂浆强度逐渐下降;各不同配比的砂浆经pH值为4.0的模拟酸雨干湿交替循环腐蚀后的强度变化规律为先升高后下降;与纯水泥砂浆试件相比,如粉煤灰、矿渣微粉的掺入过高,则会降低试件的强度值,但是如以强度增长率来评价砂浆的抗酸雨侵蚀能力,则各不同比例的粉煤灰、矿渣微粉复合双掺等量取代水泥配制的砂浆的强度增长率均优于同等条件下纯水泥砂浆试件,即粉煤灰、矿渣微粉复合双掺对水泥砂浆试件在模拟酸雨条件下的强度发展有改善作用。     

Topological controls on the dissolution kinetics of glassy aluminosilicates

Fly ash which encompasses a mixture of glassy and crystalline aluminosilicates is an abundant supplementary cementitious material (SCM), valuable for replacing ordinary portland cement (OPC) in the binder fraction in concrete. Because higher OPC replacement levels are desired, it is critically important to better understand and quantify fly ash reactivity. By combining molecular dynamics (MD) simulations and vertical scanning interferometry (VSI), this study establishes that the reactivity of the glassy fractions in a fly ash with water (i.e., their aqueous dissolution rate) is controlled by the number of constraints placed on atoms within the disordered aluminosilicate network. More precisely, an Arrhenius‐like dependence of dissolution rates on the atomic network topology is observed. Such topological controls on fly ash reactivity are highlighted for a range of U.S. commercial fly ashes spanning CaO‐enriched and SiO₂‐enriched compositions. The structure‐property relationships reported herein establish an improved framework to control and estimate fly ash‐cement interactions in concrete.     

改性高钙粉煤灰水泥的各项性能研究

以高钙粉煤灰为原料,机械粉磨高钙粉煤灰至比表面积388.14 m2/kg,配比为50%高钙粉煤灰、45%熟料、3%石膏、1%Ca(OH)2、1%J2,制备高钙粉煤灰水泥,并对比市售普通硅酸盐水泥,通过测量计算不同龄期条件下的强度损失率、长度变化率和干缩率等参数来表征水泥各项性能.结果表明,随着龄期的增长,硅酸盐水泥的抗折、抗压强度、抗冻性及抗硫酸盐侵蚀性强度损失率和干缩率分别为8.05 MPa、56.97 MPa、45.73%、36.2%、1.8× 10-4%,而高钙粉煤灰水泥分别为8.33 MPa、58.77 MPa、37.5%、26.7%、1.5×10-4%,均优于普通硅酸盐水泥,并随龄期增长越加明显.     

高掺量增钙粉煤灰水泥的研制

本文研制的激活安定剂，能解决高掺量增钙粉煤灰水泥体积安定性不良的问题，提高该水泥的7天、28天强度，缩短凝结时间，使增钙灰掺量高达65%可以制备275砌筑水泥、掺42%的高钙粉煤灰的水泥达32.5级水泥。工业怀实验生产出65%增钙灰掺量的275砌筑水泥。     

石灰石粉-粉煤灰对水泥浆体性能的影响

通过测定水泥浆体流动度和不同龄期强度、吸水率以及化学结合水量,研究了单掺石灰石粉、粉煤灰以及石灰石粉与粉煤灰双掺对水泥浆体性能的影响.结果表明:随着石灰石粉掺量增加,水泥浆体流动度减小.粉煤灰掺量增加,水泥浆体流动度增大.粉煤灰能改善石灰石粉水泥浆体流动性.水泥浆体早期强度随石灰石粉掺量增加出现先增大后减小变化,掺量为...     

Water consumption of the early-age paste and the determination of “time-zero” of self-desiccation shrinkage

Self-desiccation shrinkage (SDS) is closely related to the interior water consumption and the relative humidity (IRH) drop in the cement paste. Substantial self-desiccation shrinkage has been observed at very early-age for high performance concrete. However, it is difficult to investigate the IRH by conventional method of hygrometer at this time because the materials are still in the superhygroscopic range. In this paper, an automatically measuring system of meniscus depression is developed on the base of the mechanism of tensiometer and Laplace formula. The interior water consumption and the IRH changing within the paste could be automatically monitored at the very early-age (here specially refers to the stage from the beginning of casting till several hours after final setting). By using this system, the effects of water to binder ratio and replacement of cement by fly ash and ground granulated blast furnace slag on the self-desiccation were investigated for the very early-age cement paste. Experimental results could potentially explain the mechanism of the SDS at very early-age as well as determine the “time-zero” of SDS corresponding to its definition.     

Industrial wastes derived solid adsorbents for CO2 capture: A mini review

Coal combustion in thermal power plants throughout the world produces large amounts of fly ash. Disposal of fly ash is a serious threat to the environment and hence is a worldwide concern for conversion of these wastes into useful products. Synthesis of mesoporous silica materials from coal fly ash has already been proposed as an option which can be utilized as an adsorbent. Adsorption is considered to be one of the more promising technologies for capturing CO₂ from flue gases. This paper reviews the recent development of solid adsorbents from industrial waste materials with special reference to fly ash for post-combustion capture of CO₂.     

Improvement of the durability of glass fiber reinforced cement using blended cement matrix

Blended cements prepared with two fly ashes were used as matrices in glass fiber reinforced cement (GRC) composites in an attempt to improve their durability. The hydrated matrices from the two blended cements investigated here had similar strength and composition. Both fly ashes reduced the Ca(OH)₂ content to the same extent but in both cases the pH level was only slightly reduced compared to the portland cement matrix. In spite of these similarities, the GRC prepared with one fly ash showed considerable improvement in durability while the other one had only a small positive effect. SEM observations indicated that the improved durability in one case was associated with modification in the microstructure of the hydration products deposited in between the glass filaments, resulting in a much more open structure compared to that of portland cement matrix or the other blended cement. It is therefore suggested that the potential of the blended cement matrix to improve the durability of GRC is associated with its ability to modify the microstructure of the paste at the glass interface. This characteristic is not necessarily related to the overall composition of the blended cement matrix and to the reactivity of fly ash with Ca(OH)₂.     

高掺量粉煤灰水泥胶凝材料的水化性能研究

用ＴＭＳ－ＧＣ,ＸＲＤ,ＤＴＡ,ＳＥＭ等方法研究了高掺量粉灰水泥胶凝材料的水化性能;分析了粉煤灰掺量、激发剂等对高掺量粉煤灰水泥胶凝材料水化性能的影响,并与硅酸盐水泥的水化性能进行了对比。结果认为：高掺量粉煤灰水泥的水化速度低于不掺灰的硅酸水泥的水化速度,但后期增长较快;激发剂能加快高掺量粉煤灰水泥的水化速度。     

Characteristics of polyester polymer concrete using spherical aggregates from industrial by‐products

Despite its excellent physical and mechanical properties, polymer concrete has not been widely used owing to its much higher unit price than conventional portland cement concrete. To ensure the economic efficiency of polymer concrete, it is utmost important to reduce the use of polymer binder, which occupies most of the production cost of polymer concrete. Based on the experimental investigations, replacing filler (calcium carbonate) and fine aggregate (river sand) with fly ash and rapid‐cooled steel slag (RCSS), which are spherical materials obtainable from industrial by‐products, was found to be effective for improving the strength characteristics and durability as well as the cost efficiency of polymer concrete. The product developed in this study successfully reduced the demand for polymer binder by 21.3% compared to the conventional product, which in turn saved the total material costs by 18.5%. Although the use of RCSS showed performance degradation at an elevated temperature condition, considering typical temperature ranges that actual concrete infrastructures experience, it is expected that the polymer concrete using fly ash and RCSS will provide high‐level performances as construction and repair materials. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

粉煤灰对铝酸盐水泥和硅酸盐水泥耐热性能的影响

将粉煤灰作为矿物掺合料分别掺人到铝酸盐水泥和硅酸盐水泥中,通过耐热性能测试、X射线衍射和扫描电子显微镜研究粉煤灰对水泥石在750℃条件下的耐热性能的影响.结果表明:粉煤灰在铝酸盐水泥中没有发生反应,对铝酸盐水泥的耐热性能没有改善作用;粉煤灰对硅酸盐水泥的耐热性能有明显的改善作用,在掺量40％以下,随着粉煤灰掺量的增加,水泥石煅烧后的残余强度增加;综合比较,粉煤灰-硅酸盐水泥体系的耐热性能优于粉煤灰-铝酸盐水泥体系.     

碱-磷渣-粉煤灰胶凝材料的性能与硬化浆体结构

为充分利用磷渣和粉煤灰两种工业废渣生产高性能胶凝材料,研究了不同磷渣/粉煤灰配合比的碱-磷渣-粉煤灰胶凝材料性能,并用扫描电子显微镜和压汞仪分析了硬化浆体的细观结构和孔结构.结果表明:碱-磷渣-粉煤灰胶凝材料的凝结时间正常,在粉煤灰掺量为0～30 %(质量分数)范围内,随粉煤灰的掺量的增加,碱-磷渣-粉煤灰胶凝材料的凝结时间略有延长.与普通硅酸盐水泥相比,碱-磷渣胶凝材料的抗压强度较高,其3d和28d抗压强度分别可达到30.9MPa和98.8MPa,但其抗折强度相对较低.掺加粉煤灰后碱胶凝材料的抗压强度降低,而抗折强度提高.碱-磷渣-粉煤灰胶凝材料的耐蚀性和抗冻性能均显著优于硅酸盐水泥,其干缩比硅酸盐水泥的大.用部分粉煤灰取代磷渣粉可一定程度减小干缩.碱-磷渣-粉煤灰胶凝材料硬化浆体的结构非常致密,其孔隙率和平均孔径均小于普通硅酸盐水泥硬化浆体.     

An improved basis for characterizing the suitability of fly ash as a cement replacement agent

Fly ash is a critical material for partial replacement of ordinary portland cement (OPC) in the binder fraction of a concrete mixture. However, significant compositional variability currently limits fly ash use. For example, the performance of OPC‐fly ash blends cannot be estimated a priori using current characterization standards (eg, ASTM C618). In this study, fly ashes spanning a wide compositional range are characterized in terms of glassy and crystalline phases using a combination of X‐ray fluorescence (XRF), X‐ray diffraction (XRD), and scanning electron microscopy with X‐ray energy‐dispersive spectroscopy (SEM‐EDS) techniques. The compositional data are distilled to a unitless parameter, the network ratio (N_r), which represents the network behavior of atoms that form alkali/alkaline earth‐aluminosilicate glasses that make up fly ashes. N_r is correlated with known composition‐dependent features, including the glass transition temperature and amorphous XRD peak (“hump”) position. Analysis of heat release data and compressive strengths are used to evaluate the impact of fly ash compositions on reaction kinetics and on the engineering properties of cement‐fly ash blends. It is shown that fly ashes hosting glasses with a high network ratio (ie, having a less stable glass structure) are more reactive than others.     

Influence of steam curing on the compressive strength of concrete containing supplementary cementing materials

Heat treatment is widely used to accelerate the strength-gaining rate of concrete. In general, the ultimate strengths of the heated-treated concrete are lower than those of the standard cured specimens. When ultrafine fly ash (UFA) is included in concrete, the pozzolanic reaction is accelerated through the heat treatment. Sometimes, various chemical activators were used to activate the reactivity of fly ash. In the current study, UFA and slag were used as a replacement for cement, steam curing and chemical activators were used to accelerate hydration of cement and fly ash, and then compared with moist curing. This paper presents the influence of steam curing on the compressive strength of concrete containing UFA with or without slag. The experimental results indicated that the concrete containing UFA has low early strength after 13-h steam curing and that the difference between the 28-day compressive strength of concrete through 13-h steam curing and that of moist-cured concrete is large, but the concrete with UFA and CaSO₄ or Ca(OH)₂ has a high early strength, thus, the reactivity of fly ash must be accelerated. Concrete containing UFA and ground slag was prepared, whose compressive strengths were improved.     

Compressive strength studies on portland cement mortars containing fly ash and superplasticizer

The effects of two super water-reducing admixtures on the compressive strengths and elastic moduli of portland cement mortars containing fly ash were determined. The results show that the effectiveness of the chemicals for improving the strengths of mixtures in which portland cement is partially replaced by fly ash decreases with an increase in the percentage of ash. Increased strength can be nil or too low to justify the additional expense associated with the chemicals. The mixtures did exhibit normal stress-strain relationships: the elastic moduli were related to the ultimate compressive strengths in the usual manner. The results are based on mixes with equal workabilities.     

外加剂和粉煤灰对碱式硫酸镁水泥混凝土性能影响研究

利用碱式硫酸镁水泥制备了不同外加剂和粉煤灰掺量的碱式硫酸镁水泥(BMSC)混凝土.研究了外加剂和粉煤灰对BMSC混凝土抗压强度以及抗硫酸盐腐蚀性能的影响,并对BMSC混凝土物相组成和微观形貌进行了分析.结果表明:掺加外加剂后混凝土的强度有大幅度地提高.当外加剂掺量为水泥质量的0.5%时,混凝土的强度达到最大值;继续增加外加剂掺量,对混凝土的强度影响不大.掺加粉煤灰后,混凝土的强度有所下降.且水灰比一定时,粉煤灰掺量越多,对混凝土的强度越不利.掺加外加剂和粉煤灰后,混凝土的抗硫酸盐腐蚀性能得到了明显的改善;且同等条件下,碱式硫酸镁水泥混凝土抗硫酸盐腐蚀性能优于普通硅酸盐水泥混凝土.     

Effect of fly ash–gypsum blend on porosity and pore size distribution of cement pastes

Abstract

Abstract

This paper reports results on the porosity and pore size distribution (PSD) of cement paste containing simulated desulphurised waste (SDW). The SDW was chosen due to the variability in chemical composition of real desulphurised waste. The SDW is a combination of fly ash and gypsum. The content of fly ash in the SDW changed from 0 to 100% by weight. The water to binder ratio was 0·5. The binder consists of cement and SDW. Cement in the pastes was partially replaced with 25 wt-% SDW. The porosity and PSD of cement pastes at 28 days of curing is reported. Increasing amount of gypsum does not seem to greatly change the pore volume; however, there is tendency of obtaining coarser pore structure in the presence of gypsum. The compressive strength increases with increasing amounts of gypsum. Correlation between strength and PSD is conducted.