Influence of the composition of cement kiln dust on its interaction with fly ash and slag

Cement kiln dust (CKD), a by-product of the cement industry, contains significant amounts of alkali, free lime, chloride and sulfate. Wide variation reported in the chemical composition of CKDs limits their potential application as a sustainable binder component in concrete. In the current study, the performance of two different CKDs as components in a novel binder is evaluated. Several binders are developed by blending CKDs with fly ash or slag. Binders with 70% CKD were prepared at a water-to-binder ratio of 0.4, and heat-cured at 75 °C to accelerate the strength development. The hydration progress was monitored using X-ray diffraction, and morphological examination was performed using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Ettringite and calcium aluminosilicate hydrate (C-A-S-H) were identified as the main hydration products in the hardened binder system. Strength development of CKD-based binder was found to be significantly influenced by its free lime and sulfate contents.     

不同温度下矿渣对铝酸盐水泥早期水化行为的影响

通过凝结时间、抗压强度、电阻率、浆体内部温度测试和水化产物分析,研究了20 ℃、35 ℃和50 ℃下矿渣(GGBFS)对铝酸盐水泥(CAC)早期水化行为的影响。结果表明,掺入矿渣会逐渐减小CAC 72 h的化学收缩,降低化学收缩速率峰值。20 ℃时,电阻率变化曲线出现了明显的晶相转变期,化学收缩曲线存在明显的诱导期; 35 ℃时,凝结时间延长,掺入矿渣抑制了电阻率的发展;50 ℃时,电阻率在接近24 h时显著降低,凝结时间显著缩短,掺入矿渣缓解了24 h电阻率的减小。矿渣-铝酸盐水泥体系的水化产物和抗压强度受养护温度的影响较大。20 ℃时,掺入40%(质量分数)矿渣减少了CAH₁₀的生成量,降低了硬化浆体的强度;35 ℃和50 ℃时,1 d水化产物主要为C₂AH₈和少量C₃AH₆,掺入矿渣延缓了强度的倒缩。在28 d龄期时,不同养护温度下掺入矿渣均能促进C₂ASH₈的生成。     

粉煤灰对水泥浆体的电阻率与化学收缩的影响

研究了粉煤灰掺量分别为0、20%和40%,水胶比为0.4的水泥浆体的抗压强度、电阻率、化学收缩以及水化产物的变化规律.电阻率采用无电极电阻率法进行测试,化学收缩采用ASTM C1608-12规定的膨胀测定法进行测试.结果表明,在250 d龄期时,粉煤灰掺量为20%的硬化水泥浆体抗压强度仅比空白组低5%;当粉煤灰掺量增大时,水泥浆体在3 d龄期时的强度、电阻率和化学收缩均减小,抗压强度与电阻率之间具有很好的线性关系.XRD及热重分析表明,随着粉煤灰掺量增加,水化硅酸钙含量减少,在3 d龄期时水化产物中出现了钙矾石.     

水泥窑灰-粉煤灰体系的水化与强度发展(英文)

研究了配合比与养护温度对水泥窑灰(cement kiln dust,CKD)—粉煤灰(fly ash,FA)净浆的水化与强度发展的影响。净浆采用5种不同的水泥窑灰与粉煤灰比例配制,部分试件添加硅酸盐水泥作添加剂。试件在24、38℃及50℃的条件下进行养护。采用热重分析与X射线衍射测试试件的水化产物。结果表明在50℃的养护条件下,75%CKD+25%FA与45%CKD+45%FA+10%OPC试件的28 d与56 d强度分别达到了100%OPC水泥净浆强度的70%与80%以上。CKD-FA体系中的主要结晶水化产物是钙矾石。无论CKD与FA比例多大,CKD-FA浆体中钙矾石的含量显著高于水泥净浆。CKD-FA体系中钙矾石在90d的龄期仍可保持稳定。     

SAP内养护对碱激发矿渣胶凝材料性能的影响

碱激发矿渣(AAS)胶凝材料存在早期收缩大、开裂风险高的问题,限制了其工程应用。本文采用TAM、TGA、MIP等方法研究了高吸水性树脂(SAP)内养护对AAS胶凝材料水化热、水化产物及孔结构的影响,同时研究了SAP对AAS胶凝材料抗压强度及自收缩的影响规律。结果表明,SAP的加入会增加基体的孔隙率,降低AAS浆体的抗压强度,但是随着水化时间的延长,SAP的内养护作用可以促进矿渣水化,抗压强度的降低幅度逐渐减小。SAP的加入对AAS胶凝材料的水化放热过程有一定的延迟作用,表现为诱导期延长,第二放热峰滞后。SAP的加入使AAS胶凝材料水化产物总量增加,增加程度随着模数的增加而提高。此外,SAP抑制AAS浆体自收缩效果明显,添加SAP之后自收缩降低率最高可达81%。     

Effect of large amounts of natural pozzolan addition on properties of blended cements

In this study, the effects of 35, 45, and 55 wt.% natural pozzolan addition on the properties of blended cement pastes and mortars were investigated. Blended cements with 450 m²/kg Blaine fineness were produced from a Turkish volcanic tuff in a laboratory mill by intergrinding portland cement clinker, natural pozzolan, and gypsum. The cements were tested for particle size distribution, setting time, heat of hydration, compressive strength, alkali-silica activity, and sulfate resistance. Cement pastes were tested by TGA for Ca(OH)₂ content and by XRD for the crystalline hydration products. The compressive strength of the mortars made with blended cements containing large amounts of natural pozzolan was lower than that of the portland cement at all tested ages up to 91 days. Blended cements containing large amounts of pozzolan exhibited much less expansion with respect to portland cement in accelerated alkali-silica test and in a 36-week sulfate immersion test.     

Influence of air-cooled slag on physicochemical properties of autoclaved aerated concrete

Lime and sand in autoclaved aerated concrete (AAC) were replaced by air-cooled slag (AS). The compressive strength and the type and nature of the hydration products were studied for samples autoclaved at 8 bar for different periods of times: 2, 6, 12 and 24 h. The hydration reactions were monitored by determining free-lime contents and combined water. The types of the hydration products were investigated using XRD and SEM/EDX. Slag substitutions for sand and lime up to 50% enhance the compressive strength, especially at short curing times (2 and 6 h). The optimum strength is obtained by 50% AS substitution for low-lime mixes (10% CaO) and 30% AS substitution for high-lime mixes (25% CaO). In high-lime mixes containing up to 30% AS, the initially formed fibrous calcium-rich CSH was changed to needle-like and lath-like 1.1 nm tobermorite. In low-lime mixes with AS-substitution, tobermorite appears at 2 h processing time with grass-like silica-rich CSH around quartz particles.     

Mechanical properties in relation to the microstructure of fibre reinforced portland clinker pastes

Portland clinker was mixed with various water/clinker ratios ranging between 0.7 to 0.2 covering the range for both “normal” and “low” porosity pastes. These pastes were reinforced with five kinds of fibres, all with weight percentages of 0.5 and 3.0. All the samples were tested for compressive and tensile strengths, total porosity, microstructure and degree of hydration. Fibre reinforcement led to an increase in the total porosity and a decrease in the degree of hydration as compared with the neat pastes. Also, compressive strength has to be sacrificed to a certain extent in order to obtain better flexural strength. SEM furnished direct evidence that fibre reinforcement could affect the pore structure, the habit and shape of the hydration products, as well as their spatial distribution. The indication gained is that the mechanical behavior of the mix is due mainly to changes in the physicochemical properties induced by the presence of fibres.     

磷渣对水泥浆体水化性能和孔结构的影响

通过对水泥浆体凝结性能、水化放热、力学性能和孔结构的测定,以及扫描电镜分析和差热-热重分析,研究了不同掺量磷渣对水泥浆体水化性能和微观结构的影响.结果表明:随着磷渣掺量的增加,浆体的凝结时间延长,水化热减少,早期抗压强度下降.但掺磷渣水泥浆体的后期抗压强度已接近或超过了纯水泥浆体的,磷渣掺量的增加对水泥浆体的后期抗压强度影响不显著.浆体中的Ca(OH)2量随龄期的延长而增加并随磷渣掺量的增加而降低.磷渣的活性效应和填充效应的发挥有效地改善了浆体水化后期的微观结构和孔结构,从而使浆体的力学性能有所提高.     

Pore size distribution of portland cement slurries at very early stages of hydration (influence of curing temperature and pressure)

Pore size distribution of Portland cement pastes has been studied using helium picnometry and mercury porosimetry. Cement samples were hydrated under varying conditions of temperature and pressure and were investigated at very early stages of hydration : thickening, setting, and early hardening. The evolution of pore size distribution with time has been related to physical and chemical properties (compressive strength, shrinkage, and combined water). The interpretation has been based on the repartition between free pores of tubular shape and trapped pores of rounded shape, and a model is proposed for describing cement pore size distribution.     

基于不同粒径分布的低熟料矿渣水泥水化特征和孔结构特性

低熟料矿渣水泥(LSC)是一种水泥熟料用量低,主要由粒化高炉矿渣和石膏组成的水硬性胶凝材料.本文研究水泥不同粒径分布(对应比表面积分别为358 m2/kg、450 m2/kg和516 m2/kg)对低熟料矿渣水泥的抗压强度、电阻率和水化热、水化产物、孔结构的影响.结果表明,当比表面积从358 m2/kg增加到450 m2/kg可以提高低熟料矿渣水泥浆体的抗压强度,当从450 m2/kg增加至516 m2/kg时,强度提高甚微.低熟料矿渣水泥主要的水化产物是钙矾石和水化硅酸钙,增加水泥细度导致放热速率明显加快,电阻率变化曲线的下降段持续时间明显缩短,因而会产生更多的钙矾石.水泥细度增加,浆体的凝胶孔的体积分数增大,大孔减少,进一步提高浆体的密实度.     

Novel high-resistance clinkers with 1.10 < CaO/SiO2 < 1.25: production route and preliminary hydration characterization

New amorphous calcium silicate binders, hydraulically active, were produced by a process consisting in fully melting and rapid cooling of a mixture of typical raw materials (limestone, sand, fly-ash and electric furnace slag) with overall CaO/SiO₂ molar ratios (C/S) comprised between 1.1 and 1.25. Pastes were produced from these materials by mixing them with water in a water/binder ratio of 0.375. Compressive strength was determined at the ages of 7, 28 and 90 days and the hydration of these pastes was followed during this period by XRD, FTIR and ²⁹Si MAS-NMR. Tobermorite-like structures with low C/S and semi-crystalline character were observed to develop upon hydration of these new amorphous calcium silicate hydraulic binders. Moreover, no Portlandite was formed during hydration of these materials. The maximum compressive strength after 90 days is above 40 MPa. TGA was performed in order to determine the amount of structural water present in the pastes and their content related to the amount of hydrated products obtained. The relation between compressive strength and the amount of hydration products was investigated and some considerations about the mechanical properties of the hydration products and paste microstructure were inferred.     

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.     

Replacement of “coarse” cement particles by inert fillers in low w/c ratio concretes: II. Experimental validation

It has been suggested that in low water-to-cement ratio (w/c) concretes, the “coarser” cement particles could be replaced by an “inert” filler with little loss in performance in terms of hydration and strength development. This communication presents the results of an experimental study conducted to validate this hypothesis, using a coarse limestone filler and a classified cement. The cement and limestone powders were both classified with a cutoff diameter of about 30 μm. The coarse limestone was then blended with the fine cement, and water-to-solids ratio=0.3 pastes and mortars were prepared to compare to reference (original cement powder) systems. The results for chemical shrinkage for the pastes were consistent with a simple dilution of the cement by the limestone and also with the results predicted by the CEMHYD3D hydration model. In mortars, the predicted compressive strength loss in the filled system at 7 days was consistent with model predictions, and furthermore, at 56 days, no detectable difference in strength was measured. Thus, this study further supports the idea that coarse limestones could be used to replace equivalent size cement particles in low w/c concretes with little loss in hydration and strength performance.     

Influence of expansion admixtures on the properties of water‐glass‐based refractory pastes

Water‐glass‐based pastes show good prospect for high‐temperature applications, but they are subject to substantial dry‐shrinkage. Silicon and aluminum powders were added to prevent the dry‐shrinkage. The influence of them on compressive strength, bonding strength and volume of the pastes was discussed. SEM, XRD, and TG/DSC were employed to study the influencing mechanisms. The results show that Si and Al effectively prevented the dry‐shrinkage; Al improved compressive strength and bonding strength, while Si was bad for bonding strength. When Al dosage was 6%, the pastes presented mild expansion of 1.05%, the compressive strength increased by 8.9%, and the bonding strength by 33.4%.     

水泥-硅灰/粉煤灰体系强度、收缩性能与微观结构研究

为研究硅灰及粉煤灰对不同养护龄期的水泥浆体强度及收缩性能的影响,以水胶比为0.29的水泥浆体为基体,设计制备了五种硅灰及粉煤灰掺量的复合水泥浆体,借助量热仪和压汞仪测试表征了不同复合水泥浆体的水化放热特性以及孔结构组成,分析了水化放热量、孔隙率等参数随硅灰和粉煤灰掺量增加的变化规律,建立了复合浆体抗压强度与孔结构以及水化特性与收缩应变之间的量化关系。结果表明,掺入粉煤灰会大幅降低水泥净浆早期抗压强度,但对减小自收缩应变和干缩应变极为有利。掺入硅灰能明显提高净浆3 d抗压强度,但当硅灰掺量超过10%(质量分数)后,净浆3 d自收缩应变及28 d干缩应变增加极为明显。掺入硅灰会使水泥水化诱导期开始和结束的时间提前,还会增加水化反应级数和各阶段的反应速率常数值,导致水泥-硅灰复合浆体的水化放热总量和放热速率相较于水泥-粉煤灰体系大幅增加。粉煤灰和硅灰的掺入均能有效细化水泥浆体内部孔结构,提高凝胶孔比例,大幅降低大孔比例。复合浆体的72 h水化放热总量和3 d自收缩应变呈现正相关关系,而孔隙率和抗压强度呈现明显的负相关关系。     

含有活性或惰性掺合料的复合胶凝材料硬化浆体的微观结构特征

用压汞法测定了不同温度条件下养护的含有粉煤灰或石英粉的复合胶凝材料硬化浆体的孔隙率.用扫描电镜观察了硬化浆体的微观形貌,同时测定了各种组成的复合胶凝材料的净浆强度.常温水化初期,活性与惰性矿物掺合料都只具有物理填充的作用,硬化浆体的孔隙率和强度由矿物掺合料的掺量所决定.高温水化条件下粉煤灰的火山灰反应提前发生.随水化龄期延长,粉煤灰逐渐发生火山灰反应,使硬化浆体结构密实,其强度逐步提高.活性与惰性矿物掺合料对复合胶凝材料浆体结构与性能的影响的差异在水化后期逐渐显现.     

Characterization by Multinuclear High-Resolution NMR of Hydration Products in Activated Blast-Furnace Slag Pastes

The effect of activators on the hydration of granulated blast-furnace slag (gbfs) was studied through compressive strength measurements, ²⁹Si, ²⁷Al, and ²³Na high-resolution nuclear magnetic resonance, and X-ray diffraction. Four different activations containing sodium hydroxide, sodium silicate, and/or calcium hydroxide (CH) were considered, at fixed amounts of alkali: 5% Na₂O, 5% Na₂O-2.5% CH, 5% Na₂O-7.5% SiO₂, and 5% Na₂O-2.5% CH-7.5% SiO₂. Silicate-activated gbfs cements have greater compressive strength than Portland cements over the whole period of study (1 yr). Also, silicate-free activated gbfs cements have poorer mechanical strength than silicate-activated cements. In fact, substantial structural differences were observed between hydration products in both kinds of activations. In silicate-activated pastes there exists an intimate mixture of C-S-H layers and AFm-like arrangements containing Al in octahedral sites bonded to the silicate layers, originated either from phase intergrowths or from a high density of Ca-Al incorporation in the interlayer spaces of C-S-H. In pastes obtained from silicate-free activation of gbfs there is a better chemical and structural definition among C-S-H and calcium aluminate hydrate domains (AFm and hydrogarnet).     

Hydration and properties of novel blended cements based on cement kiln dust and blast furnace slag

The aim of the present paper is to address the key technical issues pertaining to the utilization of cement kiln dust (CKD) as an activator for ground granulated blast furnace slag (GGBFS) to create nonconventional cementitious binders for concrete. The relatively high alkaline content of CKD is the predominant factor preventing its recycling in cement manufacture. However, it was observed that depending on the water-soluble alkali and sulfate compounds, CKD could provide the environment necessary to activate latent hydraulic materials such as GGBFS. Binary blends containing slag and CKDs from different sources were characterized and compared in terms of the rates of heat evolution and strength development, hydration products, and time of initial setting. A study of the effects of the influencing factors in terms of soluble alkali content, particle size, and free lime content was undertaken. The results confirm the dependence of the dissolution rate of slag on the alkalinity of the reacting system, and the importance of the optimum lime content on the rate of strength gain.     

The effects of Cr2O3, Cu(OH)2, ZnO and PbO on the compressive strength and the hydrates of the hardened C3A paste

The purpose of this paper is to investigate the effects of Cr₂O₃, Cu(OH)₂, ZnO or PbO on the hydration of C₃A and characterization of its hydrates. C₃A pastes adding the above compound were examined on the basis of the hydration products and their structure, compressive strength and rate of early hydration.