The effect of fly ashes on the rheological behaviour of cement pastes

A study of the behaviour of fresh cement pastes under shear in a rotational viscometer of the Rheotest-2 type, especially designed for this purpose, is described. It is shown that an addition of fly ash to Portland clinker influences the change of flow behaviour of cement pastes from thixotropic flow to thixotropic-dilatant flow. Moreover, introduction of fly ashes without decreasing the amount of cement brings about an increase in the basic rheological characteristics of cement pastes, growth intensity being dependent on volume fraction of solid and percentage of addition.     

The effects of shearing on the rheological behaviour of thixotropic bentonite muds

The bentonite muds, widely used in civil engineering and in the oil industry, have a Binghamian and thixotropic behaviour that impedes their flow. It has been shown that vibrations, whatever may be their characteristics, are ineffective for improving this flow. Techniques aimed at applying large and slow shearing actions are no better: the thixotropy reduces considerably the size of the flow zone in the vicinity of the shearing surface, even with low concentrations of mud. It is now obvious that if one wishes to cause a large zone of bentonite mud to flow, it is necessary to reduce the thixotropy with the help of appropriate additives.     

Rheology of semi-solid fresh cement pastes and mortars in orifice extrusion

Short fiber-reinforced semi-solid fresh cement pastes and mortars, tailored for extrusion, have much lower water-to-binder ratio and higher viscosity than normal cement pastes or mortars. The rheology of these pastes or mortars cannot be characterized by traditional rheology test methods suitable for normal fresh cement pastes or mortars with much greater water-to-binder ratio and lower viscosity. In this paper, orifice extrusion is employed to calibrate rheology of the semi-solid fresh cement mortar. An analytical model is developed for orifice extrusion of semi-solid pastes and mortars obeying a rigid-viscoplastic constitutive relationship, von-Mises yield criterion and the associated flow rule. Orifice extrusion results are interpreted using the analytical model and the established experiment data interpretation method, and the associated rheological parameters are derived for the semi-solid fresh cement mortar. This study provides a simple analytical model, together with experiment and data interpretation methods, for characterizing the complex intrinsic rheological behavior of semi-solid fresh cement pastes or mortars.     

Influence of purified attapulgite clays on the adhesive properties of cement pastes as measured by the tack test

This study evaluates the influence of small additions of highly-purified attapulgite clays (0.2% and 0.5% addition by mass of cement) on the adhesive properties of cement pastes. Adhesive properties are measured by the tack test, a novel method of evaluating the rheological properties of granular materials. To better understand the results of the tack test as they pertain to cementitious materials, a highly concentrated material that is evolving due to thixotropic rebuilding and hydration, they are supplemented with a measure of the viscoelastic properties over time obtained through low-amplitude oscillatory shear rheometry. The influence of different preshear conditions and resting times (age of paste) on the adhesive properties are determined. Results show the tack test to be a suitable method for obtaining useful information about the adhesive properties and structural evolution of the material in the fresh state.     

Transient plane source measurements of the thermal properties of hydrating cement pastes

A transient plane source measurement technique is applied to assessing the heat capacity and thermal conductivity of hydrating cement pastes in their fresh state and during the course of 28 d of hydration at 20°C. Variables investigated include water-to-cement mass ratio (w/c – 0.3 or 0.4) and curing conditions (sealed or saturated curing). The heat capacity data for the fresh cement pastes are compared to a simple law of mixtures, and analytical expressions are developed to estimate the heat capacity as a function of degree of hydration for the two curing conditions. The measured thermal conductivities of the fresh pastes along with the known thermal conductivity of water are used to estimate the thermal conductivity of the original cement powder via application of the Hashin-Shtrikman (H-S) bounds. Hydration is seen to have only a minor influence on the measured thermal conductivity. Extension of the law of mixtures for heat capacity and the H-S bounds for thermal conductivity to predicting the corresponding properties of concretes are discussed.     

Flow Behavior of Fresh Very High Strength Portland Cement Pastes

Flow behavior of fresh very high strength portland cement pastes prepared at various water/cement ratios was studied by using a rate-controlled coaxial cylinder viscometer (Rotovisko-Haake 20, system M5-osc., measuring device MV2P with serrated surfaces). The tests were performed under continuous flow conditions. Experimental shear stress and shear rate data were fitted very satisfactorily with various models, one of which has already been proposed by the authors. Excellent results were achieved also by applying the Quemada equation. In addition, the influence of two different commercial superplasticizing agents (Concretan RX and Ergomix 1000, the former based on polycyclic copolymers with modified structures, carrying hydroxylated side chains, and the latter on a modified polyacrylic resin) was studied with the aim of determining their optimum dosage and verifying the effectiveness by comparing rheological results with those obtained in a previous work on an ordinary portland cement. The use of superplasticizers modified rheological behavior of the pastes; however, no value of optimum dosage was found for any additive, but only a superplasticizer concentration range within which pastes presented very low values of viscosity. Moreover, both superplasticizers showed a greater effectiveness when added to HSPC pastes rather then the OPC ones formulated with the same water/cement ratio.     

纤维素醚-膨胀珍珠岩耦合作用下水泥浆体早期水化规律

利用水化放热速率、XRD、FT-IR、DTG和Ca(OH)2含量测试手段,研究了羟丙基甲基纤维素醚和膨胀珍珠岩两种保水因子单掺和耦合作用条件下水泥浆体的早期水化规律及二者的作用机理。结果表明羟丙基甲基纤维素醚能够显著降低水泥浆体早期水化放热速率峰值,且能延缓诱导期和加速期出现的时间,对水泥浆体中后期水化没有明显的延缓效应。膨胀珍珠岩可以降低早期水化放热速率峰值,效果较羟丙基甲基纤维素醚差,但对水泥水化无延缓效应。当二者耦合作用时,具有显著的叠加效应。水化12h时,羟丙基甲基纤维素醚和膨胀珍珠岩能够显著降低水泥浆体中Ca(OH)2含量,较空白样分别降低了43.6%和9.1%。水化1和3d时,水泥浆体中Ca(OH)2含量随二者掺加而降低的趋势变缓。     

Évolution du comportement rhéologique d'une pâte de ciment fraîche suivant la distance à une source vibrante

Résumé L'efficacité d'une vibration dans une pate de ciment fra?che est avant tout liée à l'amplitude communiquée aux grains par la source. Il faut en effet qu'elle soit suffisamment grande pour que les forces de liaisons existant entre les grains puissent être diminuées notablement, entra?nant ainsi une meilleure fluidité. Par contre, la fréquence semble être le paramètre essentiel régissant la propagation de la vibration; plus elle est élevée, plus l'amortissement est faible. Pour un vibrateur de puissance donnée, l'augmentation de l'amplitude entra?ne la diminution de la fréquence; l'amélioration de l'efficacité se fait donc au détriment du rayon d'action.

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Summary During placing, fresh concrete should retain flowability whilst remaining homogeneous. To ensure this, casting is usually carried out under vibration. In order to determine the parameters which improve the efficiency of a poker vibrator, the authors have undertaken investigations on the rheological behaviour of cement pastes and following their development not only as affected by the distance from the vibrating source but also the type of vibration. Guided by practical requirements, they sought essentially to define the areas where the behaviour is made exclusively viscous by vibrating (yield value suppressed) and those where vibration is ineffective since the material remains viscoplastic (with a yield value). After describing the test apparatus and method used, the paper shows that there is never really a sudden change in the rheological behaviour of cement pastes, so the existence of any shear threshold linked to measuring equipment sensitivity becomes an arbitrary concept. The efficiency of a vibrator is therefore defined conventionally starting from the moment when the yield stress can be considered no longer negligible in the range of normal practical applications. Vibration efficiency in fresh cement paste is known to be above all related to the amplitude the source exerts on the grains. It must be sufficient especially to lower the forces of bonding between the grains significantly, in this way ensuring improved flowability. On the other hand, frequency seems to be the essential parameter governing the propagation of vibration; the higher the frequency, the lower the damping. For a vibrator of given output, increase in amplitude implies lowering of frequency. Any improvement in efficiency is therefore achieved to the detriment of the radius of action. This paper helps to define areas of efficiency but the development of material rheological properties within within these areas is not dealt with. This will be the subject of a forthcoming article.

     

In situ observation of cement particle growth during setting

The evolution of the cement paste microstructure is a complex phenomenon, which governs the setting of concrete. During setting, cement particles tend to flocculate and agglomerate due to their surface charges, attraction forces and variety of other reasons. However, it is still unclear how these developments in cement paste microstructure influence the setting of concrete. In order to better understanding the correlation between cement paste microstructure development and corresponding concrete behavior during setting, in situ observations on cement particles behaviors are essential. In this study, in situ observations on microstructure development of fresh pastes were made on three different cement pastes by using a newly developed Quantomix WETSEM? capsuling system in a conventional scanning electron microscope. Further, by employing image analysis techniques on the captured images, microstructure changes of these cement pastes were investigated quantitatively. During the study single and multiple particle growth, hydration rate of different cement particles, and total solids growth in association with various solid concentrations and corresponding heat of hydration were studied quantitatively and as well as qualitatively. The purpose of this quantitative study is to investigate the feasibility of using such a new technology to evaluate the factors influencing the cement paste microstructure evolutions during setting.     

Properties and performance of silane: blended cement systems

The paper presents the results of a study dealing with the performance of water repellents on hardened blended cement pastes. Since on the European market Portland cement does not play the dominant role anymore and due to the new national and European policies concerning Greenhouse Gases and sustainability, cement manufacturers produce more and more blended cements (CEM II–CEM V). Nevertheless, the majority of experience concerning the efficacy of water repellents is gained from Portland cement; therefore knowledge in regard to the interactions of blended cement with water repellent agent is minimal. Two silane-based products were applied on ‘fresh’ and carbonated cement substrates containing limestone, fly ash, slag and trass, and were investigated in terms of their functionality. The evaluation of the treatments’ performance and effectiveness were assessed using various laboratory measurements. Hydrophobicity, water absorption, colour changes and the penetration depth of silanes into the substrate were evaluated before and after artificial aging experiments. Moreover, the outdoor weathering test was performed to shed light on treated surface appearance in a ‘real’ outdoor environment. The results showed that surface wettability was independent on water ingress or colour variations, especially for cement specimens artificially aged by accelerated carbonation. Cement pastes containing slag and trass seemed to more distinctly affect the water repellents’ surface performance.     

The effects of CaCl<Subscript>2</Subscript>-blended acrylic polymer emulsion on the properties of cement mortar

Although acrylic polymer emulsions have been reported to impart many desirable attributes to cement mortar; delayed hydration, excessive air entrapment and moisture induced loss of strength have been highlighted as constraints. This paper explores the utilization of hydrated calcium chloride blended-acrylic polymer emulsion (CP) as a mitigation measure to these aforementioned drawbacks. First, the effects of 0, 0.5, 1.0 and 1.5% of CP by mass of cement on the early-age cement paste hydration and mortar flow were investigated. Thereafter, the influence of CP on the hardened porosity, moist-cured compressive strength, initial rate of capillary water absorption and rapid chloride permeability (RCPT) were evaluated. Test results indicate that the addition of CP to pastes sped up the cement hydration process, accelerating the final setting time of pastes by approximately 0.5–1.5 h as the CP content of pastes increased. Moreover, CP slightly increased the flow of fresh mortar, the hardened porosity of mortar mixtures containing 0.5 and 1.0% CP were also comparable to those of the plain reference mortar. With the exception of the 1.5% CP blended mortar, the 14 days moist-cured compressive strength of 0.5–1.0% CP blended mortar mixtures were also comparable to that of the plain reference mixture. Relative to the reference mixture, the addition of CP to mortar reduced the initial rate of capillary water absorption of mortar, with the mixture containing 1.5% CP giving a maximum reduction of 23%. Conversely, RCPT results indicate that above 0.5% CP addition level, CP generally increased the electrical conductivity of mixtures.     

The marsh cone as a viscometer: Theoretical analysis and practical limits

In this study, we have investigated the possibility of using the Marsh cone as a viscometer. Rheological measurements along with digital image recording of Marsh cone flow on glycerol-water mixes were carried out. The equations needed to solve the flow problem are written in the case of a purely Newtonian viscous fluid. We show that flow time can be directly linked to the Newtonian viscosity. Flow time is proportional to viscosity. The Marsh cone is then used to test several cement pastes and measured flow time is compared to predicted flow time. The correlation between flow time and cement pastes apparent viscosity stays valid only for no yield stress cement pastes and for flow time higher than about 15 s.     

硬化水泥净浆基于纳米压痕的相态识别与水化程度计算

采用纳米原位压痕手段测量硬化水泥净浆中单一相态的代表性微观力学性能,并采用纳米点阵压痕研究各相态的含量。研究对象囊括水灰比为0.3、0.4、0.5的纯水泥净浆和水灰比为0.3情况下含50%、70%矿渣掺量复合体系,共5种配比,以表征它们的相态分布和微观力学性质的异同点。掺矿渣的试件中含有明显多的复合相,因此提出三相模型测算复合相中未水化物的体积分数。此外,提出基于纳米压痕技术计算纯水泥和掺矿渣水泥试件水化程度的方法,结果吻合于热重分析的结果,其中纯水泥净浆中复合相较少,计算得到的水化程度优于对掺矿渣水泥试件的计算。     

Hydrothermal Characteristics of Blended Cement Pastes Containing Silica Sand Using Cement Kiln Dust as an Activator

The hydrothermal reactivity of silica sand was studied using cement kiln dust (CKD) as an activator in addition to the portland cement fraction of El-Karnak cement (a blend of ordinary Portland cement and ground sand).Autoclaved El-Karnak cement pastes were studied at pressures of 0.507,1.013 and 01.520 MPa of saturated steam with respect to their compressive strength,kinetics of hydrothermal reaction and the phase composition of the formed hydrates.The role of CKD in affecting the physicochemical and mechanical properties of El-Karnak cement pastes was studied by autoclaving of several pastes containing 5,7.5,10 and 20% CKD at a pressure of 1.013 MPa of saturated steam.CKD was added either as a raw CKD (unwashed) or after washing with water (washed CKD).The results of these physicochemical studies obtained could be related as much as possible to the role of CKD (raw or washed) in affecting the hydrothermal reactivity of silica sand in El-Karnak cement pastes.     

Revisiting the microstructure of hydrated tricalcium silicate––a comparison to Portland cement

The microstructures of 0.40 w/s-ratio pastes of tricalcium silicate (C₃S), alite and Portland cement have been studied at the hydration times 24 h and 1 month. A field-emission SEM (FE-SEM) was used to obtain high-resolution backscattered electron images. Comparison revealed no microstructural differences between C₃S and alite, but there were considerable differences in microstructure between C₃S and Portland cement pastes. The microstructure of the C₃S paste was simpler than that of the Portland cement, and could be described by a few characteristic features. Distribution of the reaction products differed substantially in the two systems. While hollow shells (Hadley grains) were a prominent feature of the Portland cement paste, their occurrence was more limited in the C₃S and alite pastes. Hollow shells were restricted to grains smaller than about 5 μm in the C₃S and alite pastes, and gapped hollow shells were not a common feature.     

Interactions between superabsorbent polymers and cement-based composites incorporating colloidal silica nanoparticles

The relatively new applications of superabsorbent polymers (SAPs) in cement based materials call for investigations regarding their behaviors in relation to other constituents in the system. Colloidal silica nanoparticles (CS) are becoming increasingly important for the improvement of strength and durability of cement based materials. In this study, a poly (AA-co-AM) SAP was synthesized by free radical polymerization, and its behaviors in cement based composites incorporating CS were investigated. These included swelling behavior, setting time, mechanical performance in different curing conditions, and rheological properties of fresh pastes. The observation of an unusual reduction in swelling, revealed the role of SAP in precipitation of calcium carbonate from the cement paste filtrate, and provided evidence for the less than expected reduction in workability and setting times. Combinations of the SAP and CS increased the compressive and decreased the flexural strengths, respectively, which is supported by changes in the microstructure as observed by SEM.     

Carboxylates and sulfates of polysaccharides for controlled internal water release during cement hydration

Calcium and aluminium carboxylates and sulfates of polysaccharides form hydrogels. Four Al- and three Ca-salt hydrogels of such polysaccharides, which were found to be stable enough to be mixed into fresh Portland cement pastes of low initial water-to-cement ratio of 0.275, were compared with respect to their water retardation potential using non-destructively operating low-field ¹H NMR relaxometry. All of the investigated hydrogels release their water to the cement mainly during the accelerated period of cement hydration. At a degree of hydration of about 0.7 the water in the hydrogels is consumed completely by the hydration reactions. Based on Powers’ hydration model the development of the volumetric phase distribution of the investigated hydrating cement with internal curing by the hydrogels was quantified. Within the hardening cement the Ca-salt hydrogels are more stable than the Al-hydrogels which is concluded from the changes of the T₂ relaxation times during the cement hydration.     

Properties of polyvinyl alcohol cement pastes

Properties of cement pastes containing varying amounts of each of polyvinyl alcohol (PVAL), mixtures of polyvinyl alcohol and phenol formaldehyde (PF) and mixtures of poly vinyl alcohol and borax were studied in this paper. Though the strength parameters of the PVAL-cement pastes are comparable to virgin cement paste their resistance to acid is far superior. Soxhlet extraction with water, done to determine leachability of the polymer from the polymer cement paste, revealed that the percentage of polyvinyl alcohol leached out was less when borax or PF resin was added to the PVAL cement paste. The compressive strength of the poly vinyl alcohol–phenol formaldehyde cement paste was found to be inferior to the other two cases but the retention of compressive strength after immersing in each of acid, base and kerosene was much better. In general, polyvinyl alcohol when added to cement pastes improves the chemical resistance properties in terms of retention of compressive strength after exposure to chemicals.     

Micro-structural development of gap-graded blended cement pastes containing a high amount of supplementary cementitious materials

To clarify the strength improvement mechanism of gap-graded blended cements with a high amount of supplementary cementitious materials, phase composition of hardened gap-graded blended cement pastes was quantified, and compared with those of Portland cement paste and reference blended cement (prepared by co-grinding) paste. The results show that the gap-graded blended cement pastes containing only 25% cement clinker by mass have comparable amount of gel products and porosity with Portland cement paste at all tested ages. For gap-graded blended cement pastes, about 40% of the total gel products can be attributed to the hydration of fine blast furnace slag, and the main un-hydrated component is coarse fly ash, corresponding to un-hydrated cement clinker in Portland cement paste. Further, pore size refinement is much more pronounced in gap-graded blended cement pastes, attributing to high initial packing density of cement paste (grain size refinement) and significant hydration of BFS.