Investigation of blended cement hydration by isothermal calorimetry and thermal analysis

Hydration of portland cement pastes containing three types of mineral additive; fly ash, ground-granulated slag, and silica fume was investigated using differential thermal analysis, thermogravimetric analysis (DTA/TGA) and isothermal calorimetry. It was shown that the chemically bound water obtained using DTA/TGA was proportional to heat of hydration and could be used as a measure of hydration. The weight loss due to Ca(OH)₂ decomposition of hydration products by DTA/TGA could be used to quantify the pozzolan reaction. A new method based on the composition of a hydrating cement was proposed and used to determine the degree of hydration of blended cements and the degree of pozzolan reaction. The results obtained suggested that the reactions of blended cements were slower than portland cement, and that silica fume reacted earlier than fly ash and slag.     

高掺量混合材复合水泥的水化性能

通过水化微量热、化学结合水测定和X射线衍射、热重-差热分析、扫描电镜等测试方法研究了3种高掺量矿渣、粉煤灰、石灰石复合水泥的水化性能,并与硅酸盐水泥的水化进行了对比。结果表明：高掺混合材复合水泥的水化放热特征与硅酸盐水泥有明显不同,早期水化反应速度低于硅酸盐水泥,但后期由于矿渣、粉煤灰的二次水化反应使其水化速度增长较快。主要的水化产物亦为水化硅酸钙凝胶、钙钒石和Ca(OH)2晶体,但Ca(OH)2含量明显低于硅酸盐水泥浆体中的Ca(OH)2含量。     

Hydration of fly ash-portland cements

Hydration products of fly ash-portland cements were studied with x-ray diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscopy (SEM) as part of a continuing research effort to understand the pozzolanic activity of fly ashes. It was found that the amount of calcium hydroxide crystals in the cement pastes is diminished due to the addition of fly ash to the cement. Ettringite was produced in the early age, and the consumption of sulfate by the formation of ettringite was accelerated by the addition of fly ash. A partial conversion of ettringite to monosulfate within the first 7 days of hydration in the fly ash-portland cement pastes, but the formation of ettringite continued to form up to at least 28 days of hydration in the pastes without fly ash. Examination of the fly ash bearing pastes showed, in all cases, varying amounts of calcium hydroxide and unreacted portland cement, with minor quartz and gehlenite hydrate. It appears that hydration reactions actually occur in the fly ash cement pastes more or less on a particle-by-particle basis.     

Effects of three Turkish fly ashes on the heat of hydration of PC-FA pastes

Effects of the type and amount of fly ash substitution on the heat of hydration of portland cement-fly ash pastes were investigated. Three Turkish fly ashes were used. One of them was a high-calcium and the other two were low-calcium fly ashes. The specimens contained 0, 10, 20, and 40% fly ash by weight of portland cement. The tests were carried out as described in ASTM C 186 however one separate set of specimens were first subjected to an early external temperature of 67±2°C for six hours followed by the standard temperature until time of test. The results revealed that the low-calcium fly ashes, regardless of their type, reduce the heat evolution when used for partial cement replacement. The high-calcium fly ash, on the other hand, does not produce significant changes in the heat of hydration.     

Effects of Al2O3 on the hydration activity of municipal solid waste incinerator fly ash slag

This study investigates the effects of slag composition on the hydration activity of slag-blended cement (SBC) pastes. Synthetic slag samples were prepared by melting Al₂O₃-modified, municipal solid-waste incinerator (MSWI) fly ash. In addition to the original slag (containing 25.0% CaO and 17% Al₂O₃), two other synthetic slag types, A1 and A2 slag, were prepared, having a 15% or 5% Al₂O₃ content, respectively. These synthetic slags were blended with ordinary Portland cement (OPC) at weight ratios ranging from 10% to 40%. The results indicate that the incorporation of 10% A1 slag tended to enhance the degree of hydration in SBC pastes during the early ages (3-28 days), but at later ages, significant difference in the degree of hydration between the OPC and SBC pastes with 10% A1 slag was not observed. The tendency of the 10% A2 slag case was similar, but with a limited enhancement during the early ages (3-28 days). However, samples that incorporated the Al₂O₃-modified slag (AMS) showed decreased degrees of hydration. The degree of hydration of the 40% blend ratio sample decreased significantly.     

含硫铝酸钙硅酸盐水泥中粉煤灰活化机理

用差示扫描-热重分析扫描电镜和化学分析法研究了含硫铝酸钙粉煤灰硅酸盐水泥和含硫铝酸钙矿渣硅酸盐水泥水化过程中水化产物的种类、形状、数量和孔溶液离子浓度等方面的变化规律.借助固体高分辨核磁共振波谱技术对水泥中硅氧四面体不同聚合状态的分布,Al的不同配位状态进行分析.结果表明:含硫铝酸钙硅酸盐水泥对粉煤灰激发作用大于对矿渣的激发作用,含硫铝酸钙粉煤灰水泥水化放热量多,促进粉煤灰玻璃体中AlO2-的溶出,在较高温度粉煤灰玻璃体网络结构激活,解聚速率加快.     

低品位粉煤灰的改性及其对水泥浆体强度和自收缩的影响

采用石灰石粉对低品位粉煤灰进行煅烧改性,利用X射线衍射、扫描电镜和能谱分析等方法对改性粉煤灰的矿物组成和化学组成进行表征.同时测定了掺改性粉煤灰的水泥浆体的抗压强度和自收缩,并采用背散射扫描电镜和压汞测孔仪研究了掺改性粉煤灰水泥浆体的微观结构.结果表明,粉煤灰经煅烧改性生成了水硬性矿物β-C2S,水化可生成CSH凝胶,改善了等外粉煤灰颗粒与水泥基体的界面粘接,降低了复合水泥浆体的孔隙率和自收缩,提高了复合水泥浆体的强度.     

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。     

含C_4A_3矿物硅酸盐水泥中混合材反应程度的研究

采用非蒸发水量法测定含硫铝酸钙矿物水泥的水化程度 ,萃取法测定该水泥中混合材的反应程度 ,通过SEM/EDS观察分析水化产物形貌和种类。结果表明 :粉煤灰水泥的水化程度高于同龄期的矿渣水泥的水化程度 ,粉煤灰的反应程度高于矿渣的反应程度。同龄期粉煤灰水泥中的水化产物多于矿渣水泥的水化产物 ,且水化产物发育更良好     

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.     

-3 ℃养护下考虑矿物掺合料影响的水泥水化程度计算模型

针对多年冻土地区工程施工时混凝土养护的问题,采用10%、20%、30%的矿粉和粉煤灰替代量等量替代水泥,测试了-3 ℃恒温养护条件下0.38水胶比水泥浆体在各个龄期的水泥水化热,计算了水泥水化程度;分析了龄期及矿物掺合料对水泥水化程度的影响规律,建立了综合考虑龄期和矿物掺合料替代量的水泥水化程度计算模型.结果表明:-3 ℃恒温养护下,矿物掺合料等量替代水泥,水泥浆体的水化程度会降低,粉煤灰降低水化程度的值要比矿粉高;在相同矿物掺合料替代量下,随着龄期的增长,矿物掺合料对水泥水化程度的影响逐渐减弱;同一龄期时,随着矿物掺合料的增加,矿物掺合料对水泥水化程度的影响逐渐增强;利用建立的模型计算了分别掺入15%矿粉和粉煤灰的水泥水化程度,与实测值相比,计算值偏离值较少,预测精度较高.     

粉煤灰矿渣复合水泥强度协同效应的研究

研究了由粉煤灰、矿渣、钢渣与一定量熟料组成的粉煤灰矿渣复合水泥中各组分对水泥的强度协同效应以及影响协同效应的主要因素。并利用SEM和MIP等技术研究了粉煤灰矿渣复合水泥的水化硬化过程、水化产物相组成及硬化浆体结构,以此来论证各组分间协同效应的作用机理。研究结果表明:当各组分比例适当时,通过石膏和添加剂的有效作用,采取合理的粉磨工艺和制度,粉煤灰矿渣复合水泥各组分可以产生强度协同效应。     

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.     

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.     

粉煤灰水泥石碳化性能的化学分析

开发了一种自制测量装置。利用该装置,沿碳化深度方向,分层测定了碳化粉煤灰水泥石中CaCO3的含量。绘制了＂CaCO3含量–深度＂曲线。根据曲线特点,定义了完全碳化区、部分碳化区、未碳化区等概念和碳化程度、碳化速度等特征指标。研究了粉煤灰掺量、水胶比、龄期、养护条件、碳化前烘干处理、碳化时间、碳化湿度等对粉煤灰水泥石碳化性能的影响。结果表明：粉煤灰掺量越大,水胶比越高,龄期越短,养护环境越干燥,碳化前试件越干燥,粉煤灰水泥石的抗碳化性能越差;碳化时间越长,碳化湿度越低,粉煤灰水泥石的碳化程度越高;但碳化速度随碳化时间的增长而减慢。这些结论与文献报道的用其他方法得到的公认的结论一致。本方法还得到了更加深入的研究结果,即：粉煤灰掺量越大、粉煤灰水泥石中可碳化物质的相对含量越低;水胶比、龄期、养护条件、碳化时间、碳化湿度基本不改变粉煤灰水泥石中可碳化物质的相对含量;龄期、养护条件可改变未碳化粉煤灰水泥石中碳酸盐的相对含量。     

矿物掺合料对水泥耐酸性的影响及配比设计

以硅粉、粉煤灰和矿渣为矿物掺和料,采用XRD和SEM研究了矿物掺和料对水泥水化产物、水泥石结构和形貌的影响,探讨了酸溶液对水泥的腐蚀机理,并对矿物掺和料配比进行优化设计,以提高水泥的耐酸性.研究结果表明:掺加矿物掺和料有利于改善水泥石孔结构,降低水泥碱度,当硅粉、粉煤灰和矿渣以占水泥质量5%,10%和15%的配比掺入时,水泥的耐酸性最好.     

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。     

Application of Powers’ model to modern portland and portland limestone cement pastes

Powers’ model is a simple approach for estimating the relative volumes of hydration products, porosity, and chemical shrinkage present in portland cement paste as a function of its starting water‐to‐cement ratio (w/c) and current degree of hydration. It forms an important link between cement composition, microstructure, and performance, necessary for modeling cement‐based systems. Previous researchers have adapted Powers’ model for inert fillers to illustrate their effects on the hydration, porosity, and chemical shrinkage of blended cements; however, it is well‐documented that limestone is not, in fact, an inert filler, but rather participates in cement hydration through both chemical and physical processes. This research experimentally investigates the applicability of Powers’ model to modern portland cements containing up to 15% by mass finely divided limestone. The results demonstrate that the modified Powers’ model is insufficient for predicting the influence of finely divided limestone additions on the chemical shrinkage of both ordinary portland cement pastes and portland limestone cement pastes. Possible explanations for the discrepancy are discussed and a plausible source is proposed.     

Use of recycled copper slag for blended cements

Copper slag is a by‐product generated during smelting to extract copper metal from the ore. The copper slag obtained may exhibit pozzolanic activity and may therefore be used in the manufacture of addition‐containing cements. In this paper the effect of the incorporation of the copper slag in cement is measured. Blends of copper slag with Portland cement generally possess properties equivalent to Portland cement containing fly ash, but very different to the silica fume incorporation. Copper slag and fly ash reduce the heat of hydration more effectively than silica fume in mortars. The replacement of 30% cement by copper slag reduces the flexural and compressive strength in a similar way to fly ash; however, after 28 days, the reduction is less than the percentage of substitution. Hydrated calcium aluminate phases were analysed using scanning electron microscopy (SEM) and X‐ray diffraction (XRD) techniques. The pozzolanic activity of copper slag is similar to that of fly ash and higher than silica fume. In the presence of low water/cement ratios, certain pozzolanic materials produce a very compact cement paste that limits the space available for hydration products, a determining factor in the formation of hydrated calcium aluminates. SEM was found to be a useful analytical technique when aluminates are formed and can be clearly detected by XRD. Copyright © 2008 Society of Chemical Industry     

Fire resistance of fired clay bricks-fly ash composite cement pastes

This work aims to study the effect of substitution of fly ash for homra on the hydration properties of composite cement pastes. The composite cements are composed of constant proportion of OPC (80%) with variable amounts of fly ash and homra. The addition of fly ash accelerates the initial and final sitting time, whereas the free lime and combined water contents decrease with fly ash content. The fly ash acts as nucleation sites which may accelerate the rate of formation of hydration products which fill some of the pores of the cement pastes. The fire resistance of composite cement pastes was evaluated after firing at 250, 450, 600, 800 °C with rate of firing 5 °C/min with soaking time for 2 h. The physico-mechanical properties such as bulk density and compressive strength were determined at each firing temperature. Moreover, the phase composition, free lime and microstructure for some selected samples were investigated. It can be concluded that the pozzolanic cement with 20 wt% fly ash can be used as fire resisting cement.