Effects of the addition of nanosilica on the rheology,hydration and development of the compressive strength of cement mortars

This paper examines the rheology, hydration kinetics and development of the compressive strength of cement mortars including nanosilica and fly ash. The contents of these materials and the superplasticizer dosage are related to different rheological and strength parameters. Effects on rheology were analysed through yield stress and viscosity. Calorimetry tests were carried out to assess the variations in cement hydration kinetics, and the maximum and minimum heat release rates were analysed. Compressive strength was evaluated at different ages up to 56 days. The equations presented in this paper make it possible to optimize mortar proportionings that fulfil required performance levels in both fresh and hardened states.     

Effect of temperature on the rheology of flowable mortars

In addition to the characteristics of mixture constituents and mix design, the rheological behavior of concrete is influenced by material temperature and time after water–cement contact. The study presented herein evaluates the combined influence of time and temperature on the workability of micro mortars. The mixtures were proportioned with polymelamine (PMS), polynaphtalene (PNS), polycarboxylate (PCP) polymer, and made of different supplementary cementitious materials. Seven micro mortars proportioned with various binder compositions and water-to-binder ratio of 0.42 and 0.53 were prepared at 10–33 °C. Test results show that the yield stress and plastic viscosity vary linearly with the coupled effect of time and temperature for mixtures made with PNS or PMS superplasticizers. However, for mixtures made with PCP superplasticizer, both the material temperature and type of supplementary cementitious materials are shown to influence the evolution of rheology with time.     

Rheological test methods for the characterization of extrudable cement-based materials—A review

In order to develop cement extrusion technology, it is absolutely necessary to perform an adequate rheological characterization of cement-based formulations, by the use of non-traditional testing methods specifically developed for this purpose. Current rheological methods proposed for fresh concrete, mortars and pastes are applicable to highly flowable mixes, but it is not possible to use them for the characterization of dense, cohesive and very dry compositions (such as the cement mixes used for extrusion). Furthermore, the technological development of the extrusion process is quite complex, due to the chemio-rheological behaviour of the cement-based materials to be used. This paper describes the most promising test methods currently adopted for the rheological characterization of cement-based compositions to be used in the extrusion process. Some examples of experimental results will also be reported.     

The effect of viscosity modifying agents on mortar and concrete

The influence of different viscosity modifying agents (VMA) on the flow properties and the rheology of self-compacting mortars is studied. Additionally, their effect on the early hydration of cement pastes and on the development of concrete strength is determined. Beside the inorganic VMA microsilica (MS) and nanosilica slurry (NS) organic VMA based on high molecular ethylenoxide derivate (EO), natural polysaccharide (PS) and starch derivate (ST) are used. The different VMA are combined with a superplasticizer (SP).At constant water-to-binder ratio (w/b) the addition of VMA causes a decrease of mortar flow and an increase of flow time (V-funnel test). At the same time yield stress and plastic viscosity are increased. At a constant dosage of superplasticizer (SP) mixtures with VMA require a higher w/b to keep the same flow properties as the reference mixtures without VMA. In spite of the higher w/b flow time and plastic viscosity respectively are only slightly reduced. This property is especially beneficial for the production of stabilizer-type self-compacting concrete where the amount of fines can be reduced with the use of VMA. However, only the use of VMA PS and ST leads to smaller changes of flow when w/b is changed. The organic VMA show almost no influence on early cement hydration and the development of compressive strength. However, the inorganic VMA cause an acceleration of hydration and higher compressive strength at the age of 1 day.     

Rheology of alkali-activated slag pastes. Effect of the nature and concentration of the activating solution

An understanding of the rheological behaviour of OPC-based products has been widely studied, for it is essential to determining and predicting the fresh and hardened characteristics and properties of pastes, mortars and concretes. The rheology of alkali-activated material (AAM) systems has been much less intensely researched, however.The present study aimed to ascertain the effect of factors such as the nature and concentration of the alkaline activator on the rheological behaviour of alkali-activated slag (AAS) pastes, with a comparison between the rheological parameters and fluidity of these pastes to the same parameters in OPC. More specifically, the study explored how paste rheology was affected by the nature of the alkaline activator (NaOH, 50/50 wt% NaOH/Na₂CO₃ or waterglass – Wg), its concentration (3–5% Na₂CO₃ of slag weight) and, in the waterglass solution, the SiO₂/Na₂O ratio.The findings showed that AAS paste rheology is affected by the nature of the activator. The rheological behaviour in AAS pastes activated with NaOH alone or combined with Na₂CO₃ was similar to the rheology observed in OPC pastes, and fit the Bingham model. Conversely, the AAS pastes activated with waterglass fit the Herschel–Bulkley model and their rheology proved to depend on both the SiO₂/Na₂O ratio and the Na₂O concentration. Moreover, regardless of the activator used (NaOH, Na₂CO₃ or waterglass), an increase in Na₂O concentration implies a raise of shear stress.The formation of primary C–S–H gel in Wg–AAS and its effect on paste rheology were confirmed. Gel formation was likewise shown to be related to the SiO₂/Na₂O ratio and activator concentration.     

Rheological behaviour of ultra-high performance cementitious composites containing high amounts of silica fume

This study investigates the rheological behaviour of ultra-high performance cementitious composite mortars containing 15–25 % of silica fume. The utilization of two Portland cements with different mineralogical compositions and their influence on yield stress of mortar was monitored. The coaxial rheometer was used for determination of flow curves of tested samples. It was found that besides the relation between flow and water-to-binder ratio, there is also a substantial relationship with the mortar composition, in particular with the content of silica fume. The yield stress can be described by an exponential function of volume content of solids in the mortar. Such a function can describe not only the influence of granulometry but also the impact of structure formation on early age Portland cement hydration. It was found that the estimation of yield stress can be done even by a simple modular in-field technique such as a spread flow test.     

Characteristics of cement-soil mortars

Cement-soil mortars are commonly used for the construction of soil-cement block masonry. The paper focuses on an experimental study in understanding the various characteristics of cement soil mortars in fresh and hardened state. Workability, strength, water retentivity, shrinkage and stress-strain characteristics of cement soil mortars and bond strength of soil-cement block couplets using such mortars are examined. Characteristics of 1:6 cement mortar and 1:1:6 cement lime mortar are also examined for the purposes of comparison. Workability of mortars has been quantified by conducting flow table tests. Results of flow values obtained for mortars from various construction sites are reported. There is a linear relationship between flow and water cement ratio of the mortars. Flow increases with increase in water-cement ratio. Very high flow value of 130% can be achieved for cement soil mortars and cement lime mortars. Reduction in flow value from 100% to 80% leads to increase in strength and modulus of mortars. Clay fraction of the mortar mix controls the flow, strength, density, shrinkage value and modulus of cement soil mortars. Cement-soil mortars lead to better tensile bond strength for soilcement block couplets when compared to the cement mortar and cement lime mortar.     

Alkali-activated cements and mortars based on blast furnace slag and red clay brick waste

This study investigates the effects of adding various concentrations, sources and compositions of ground red clay brick waste (RCBW) on the properties of fresh and hardened pastes and mortars of alkali-activated slag. The method used to grind the granulated blast furnace slag (GBFS) and RCBW (separate and conjoint) is also assessed, along with the fineness (300–900 m²/kg) of the blended alkali-activated GBFS-RCBW cement, the alkali activator (sodium carbonate or sodium silicate) and the curing conditions (normal conditions or steam curing). The water requirement and setting time for the fresh pastes are also considered; and in the case of the hardened paste and mortar, the water absorption, density and compressive/flexural strength are measured after 1, 3, 7 and 28 days of aging. From the results obtained, it is demonstrated that the addition of 40% RCBW improves the 7- and 28-day strength of blended alkali-activated slag pastes and mortars, and can replace up to 60% of the slag without losing strength.     

Effects of the molecular architecture of comb-shaped superplasticizers on their performance in cementitious systems

The objective of this study is to link the molecular structure of polycarboxylate-ether-type superplasticizers with the performance of cementitious systems in order to develop new products with enhanced properties, e.g. improved water reduction with a wide range of cements or a reduced retardation of cement hydration. Different experimental superplasticizers have been synthesized varying length and density of the polyether chains as well as the molecular weight of the polymer. The influence of these polymers on the properties of cement pastes and mortars was determined using various characterization methods like mortar flow, rheological and calorimetric measurements, adsorption measurements and mortar compressive strength.Characteristic connections between molecular structure of the polycarboxylate-type water reducers, adsorption behaviour, workability and retarding effect have been determined allowing the synthesis of new superplasticizers with improved performance.     

Energy Dissipation and Apparent Viscosity of Semi-solid Metal during Rheological Processes Part Ⅱ: Apparent Viscosity

This study investigated the rheological properties of semi-solid metal. An analytical model of apparent viscosity was built up based on analysis of energy dissipation during rheological processes such as slurry preparing,delivering and model filling. The rheological properties of SSM (semi-solid metal) slurry was described by an analytical model in terms of microstructural parameters, which consist of effective solid fraction, particle size and shape, and flow parameters such as mean velocity, fluctuant velocity and relative velocity between liquid and solid phase. The model was verified in the experiment of A356 alloys with a coaxial double-bucket rheometer. And the maximum relative error between the theoretical value and measured one is less than 10%.The results of experiment and theoretical calculation also indicate that the microstructural parameters and flow parameters are two major factors that affect the apparent viscosity of semi-solid alloys, and fluctuant velocity and relative velocity between liquid and solid phase are the key factors to distinguish between steady and transient rheological properties.     

Energy Dissipation and Apparent Viscosity of Semi-solid Metal during Rheological Processes Part Ⅱ： Apparent Viscosity

This study investigated the rheological properties of semi-solid metal. An analytical model of apparent viscosity was built up based on analysis of energy dissipation during rheological processes such as slurry preparing, delivering and model filling. The rheological properties of SSM （semi-solid metal） slurry was described by an analytical model in terms of microstructural parameters, which consist of effective solid fraction, particle size and shape, and flow parameters such as mean velocity, fluctuant velocity and relative velocity between liquid and solid phase. The model was verified in the experiment of A356 alloys with a coaxial double-bucket rheometer. And the maximum relative error between the theoretical value and measured one is less than 20%. The results of experiment and theoretical calculation also indicate that the microstructural parameters and flow parameters are two major factors that affect the apparent viscosity of semi-solid alloys, and fluctuant velocity and relative velocity between liquid and solid phase are the key factors to distinguish between steady and transient rheological properties.     

Mix design method for self compacting metakaolin concrete with different properties of coarse aggregate

This study deals with a proposed mix design method for SCC utilizing different properties of coarse aggregate. The work was conducted in three phases, i.e. paste, mortar and concrete to facilitate the mix design process. Initial investigation on cement paste determined the basis for water cement ratio and superplasticizer dosage for the concrete. For the study on mortar, metakaolin (MK) as pozzolan was used at replacement levels of 5%, 10%, 15%, and 20% by weight of cement. Self compactability of mortars was obtained by adding suitable materials such as mineral admixtures and superplasticizer which provided a sufficient balance between flowability and viscosity of the mix. The optimum MK replacement level for cement was 10% from the viewpoint of workability and strength. Flowability of mortar decreased with the use of metakaolin. Moreover, strength of mortar increased when the optimum replacement level of pozzolan was used. Different fresh concrete tests were adopted. The results obtained for fresh concrete properties showed that flowability of concrete increased with increase flowability of mortar. The mixes which contained coarse aggregate with lower volume, small size, and continuous grading affected positively the fresh properties of SCC. Finally, the mix design method used was successful in producing SCC with different coarse aggregate properties.     

Properties of self-compacting mortars with binary and ternary cementitious blends of fly ash and metakaolin

The link between flow properties and the formulation is actually one of the key-issues for the design of self-compacting concretes (SCC). As an integral part of a SCC, self-compacting mortars (SCMs) may serve as a basis for the design of concrete since the measurement of the rheological properties of SCCs is often impractical due to the need for complex equipment. This paper discusses the properties of SCMs with mineral admixtures. Portland cement (PC), metakaolin (MK), and fly ash (FA) were used in binary (two-component) and ternary (three-component) cementititios blends. Within the frame work of this experimental study, a total of 16 SCMs were prepared having a constant water-binder (w/b) ratio of 0.40 and total cementitious materials content of 550 kg/m³. Then, the fresh properties of the mortars were tested for mini-slump flow diameter, mini-V-funnel flow time, setting time, and viscosity. Moreover, development in the compressive strength and ultrasonic pulse velocity (UPV) of the hardened mortars were determined at 1, 3, 7, 14, and 28 days. Test results have shown that using of FA and MK in the ternary blends improved the fresh properties and rheology of the mixtures when compared to those containing binary blends of FA or MK.     

水泥砂浆流变学性能的评价方法

提出Ｕ型流动法评价水泥砂浆塑性粘度,运用流变学对Ｕ型流动法作了理论分析。结果表明,当浆体的剪应力屈服值较小时,该方法的测量参数与浆体的塑性粘度之间有线性关系。该方法可用来评价预填骨料混凝土用砂浆及其它剪应力屈服值较小的Ｂｉｎｇｈａｍ体的塑性粘度。     

Composition of lime-cement and air-entrained cement mortar as a function of distance to the brick-mortar interface: consequences for masonry

The composition of a lime-cement mortar and an air-entrained cement mortar was studied as a function of distance to the brick-mortar interface. Both mortars had the same cement-to-sand ratio and the same water-to-cement ratio; in the lime-cement mortar the binder-to-sand ratio was highest. The measurements indicate that the mortar composition (i.e. the contents of sand, cured binder and voids) and the contents of chemical substances of the cured binder (i.e. the contents of calcite, portlandite and ettringite) change with distance to the brick-mortar interface. For the mortar composition the tendency of these changes is the same, but for the contents of the chemical substances of cured binder for the two mortar types the tendency of these changes is opposite and also the extent of the changes is significantly different. For the air-entrained cement mortar, the observations are explained by the enrichment of binder towards the brick-mortar interface, resulting from the compaction of fresh mortar. In the lime-cement mortar such an enrichment of binder hardly occurs and the observations are explained by the intense carbonation that takes place. As a result, the contents of the chemical substances in the mortars is very much different. In the air-entrained cement mortar, near the brick-mortar interface the enrichment of cement and the low water content (resulting from the low water retentivity of this mortar), lower the water-to-cement ratio and as a consequence the cement is not fully hydrated. In the lime-cement mortar, as the Ca(OH)₂ content and the water content is higher, near the brick-mortar interface, a carbonated zone is formed which is hardly permeable for CO₂ (and probably water). This does not occur in the air-entrained cement mortar, it remains permeable.     

Effect of superplasticizers on the rheological properties of cements

In this paper, the rheological properties of cement pastes made with different types of cement and superplasticizers are discussed. As a tool for the discussion, experiments involving dispersion of cement particles, fluidity, viscosity, yield stress and zeta potential have been conducted. The results obtained show that the chemical compositions of the cements such as C₃A and sulfate content, alkali and ground lime content are important features controlling the rheology of cement pastes. Three types of sulfonated superplasticizers (lignosulonate-based, melamine formaldehyde sulfonic acid, naphthalene formaldehyde sulfonic acid) and one type of polycarboxylic acid polymer were used. The results revealed that the mechanism by which these polymers disperse cement particles differs fundamentally. Sulfonated superplasticizers induced a negative charge on cement particles dispersing them by electrostatic repulsion, whereas with the polycarboxylate-based polymer the dispersion mechanism is mainly controlled by steric hindrance. A model for the adsorption of superplasticizer on a cement particle is proposed.     

丁苯乳液改性水泥砂浆的物理性能

本文研究了丁苯乳液SD622S改性水泥砂浆的物理性能,包括新拌砂浆的减水率、保水率、体积密度、凝结时间,硬化砂浆的体积密度、孔隙率、毛细孔吸水率等.实验结果表明:丁苯乳液可提高砂浆的减水率和保水率,使砂浆的凝结时间延长.随着丁苯乳液掺量的增加,硬化砂浆的毛细孔吸水率降低.丁苯乳液使新拌砂浆和硬化砂浆的体积密度减小,但随聚合物掺量的增加有所回升.硬化砂浆的孔隙率随聚合物掺量的变化趋势正好相反.     

The influence of powder packing on the rheology of fibre-loaded pastes

The effects of fibre addition on the extrusion rheology of ceramic particulate pastes has been investigated using model pastes containing chopped glass fibre and alumina powders. In these pastes where the fibre diameter was much larger than the alumina particle diameter, there was a decrease in the pressure required to extrude the paste as the powder component was replaced by fibre, up to a ratio of 0.4 powder to 0.6 fibre by volume. When the pastes were characterized using physically based models this behaviour was reflected in lower values of the derived rheological parameters. It is shown that this behaviour can largely be attributed to the packing behaviour of the fibre and powder-phase components. The results also suggest that the presence of fibres increases the die entry pressure drop relative to the die land pressure drop. It is shown that while the models proposed by Milewski in combination with the rheological models can be used to illustrate the expected behaviour of composite pastes, the observed behaviour was better predicted by using modified models proposed by Karlsson and Spring. At high fibre loadings (>60 vol%), extrudable materials could not be formed; this was attributed to packing and mixing problems leading to increased fibre-fibre interaction preventing flow. This is highlighted by materials in which the ratio of powder to fibre was 1 to 4 by volume, where the paste-like body was compressible and exhibited some elastic springback. For pastes where the solid phase contains <50 vol% fibre the rheological behaviour is predictable if the packing behaviour of the fibre and the rheological behaviour of the pastes containing only the powder are known. This can be used as a tool to aid the design of composite pastes and for the development of suitable process equipment.     

Rheological research of partially-solidified dendritic Sn–14.8wt%Pb alloy by two backward extrusion methods

Two different backward extrusion methods were compared to study the rheological behaviour of partially-solidified Sn–14.8wt%Pb alloy for strip casting. For this work, the partially-solidified alloys were dendritic, as opposed to globular semi-solid slurries which have been studied more intensively. The first method was a constant-velocity experiment operating at constant extrusion-velocity of 3.5?mm?s^?1 and four different extrusion ratios. The second method was a varying-velocity experiment with unchanged extrusion ratio and four different extrusion velocities.

Both backward extrusion experiments were carried at different temperatures ranging from 183 to 208°C. The two experiments obtained quite different results, which are discussed accordingly. Moreover, an unreported abnormal shear thinning behaviour was the main characteristic of the rheological behaviour of the partially-solidified alloys. Finally, the applicability of these two different methods of measuring the viscosity of partially-solidified alloys at different solid fractions is compared and discussed.