Performance of new viscosity modifying admixtures in enhancing the rheological properties of cement paste

The use of viscosity modifying admixtures (VMA) has proved to be very effective in stabilizing the rheological properties and consistency of self-compacting concrete (SCC). SCC is known for its excellent deformability, high resistance to segregation and use, without applying vibration, in congested reinforced concrete structures characterized by difficult casting conditions. Most of the commercial VMAs currently available in the market are costly and increase the price of such a concrete. Identification or production of new low-cost VMA is then essential. This paper presents the performance of four new polysaccharide-based VMAs in enhancing the rheological and consistency properties of cement paste. The study of the rheological properties and consistency of cement paste to screen the dosage and type of new VMA to be used in SCC is a promising approach. Investigation was carried out on cement pastes with combinations of various dosages of new VMAs and of a superplasticizer (SP) to study the influence on rheology, consistency and washout mass loss. A commercial VMA designated in this paper as “COM” was tested for comparison. The study on new VMAs is encouraging and confirms that pastes with satisfactory rheological and consistency properties comparable with or even better than commercial VMA can be developed. The combined use of proper dosages of VMA and SP is shown to clearly contribute to securing high-performance cement pastes that is highly fluid yet cohesive enough to reduce water dilution and enhance water retention. Attempt has also been made to correlate rheological properties (yield stress) to consistency (slump) of pastes.     

Shifting factor—A new paradigm for studying the rheology of cementitious suspensions

One of the main challenges when studying the rheology of cementitious systems is that cement particles are large and susceptible to gravity, in particular in dispersed systems. This implies that the range of volume fractions over which rheological properties of cement pastes can be measured is limited, as too high water content causes segregation and too low water content results in pastes that are too cohesive to measure. Consequently, the impact of the volume fraction of solids on yield stress is still not reliably established. A similar situation is encountered when studying cementitious systems with superplasticizers, whose impact is highly dependent on dosage. An important issue here is that admixture effects cannot be reliably established with respect to nonadmixed references as the latter ones can most often not be measured at the volume fractions for which admixed pastes do not segregate. The “Shifting factor” approach presented in this paper overcomes this long-standing issue establishing exponential dependences of yield stress both with respect to volume fraction and admixture adsorption.     

Rheology and conduction calorimetry of cement modified with calcined paper sludge

This paper considers calcined paper sludge as an alternative source of metakaolin, an established supplementary cementitious material. Calcination of the sludge generated in the recycling of newsprint paper at 700 °C yields a product with pozzolanic properties. The effects of this recycled metakaolin on the rheology and conduction calorimetry of cement pastes have been studied and compared to the effects of commercial metakaolin. The effects are similar and the results show that calcined paper sludge has the potential to be used as a supplementary cementitious material. This offers a route for utilising this waste material, as an alternative to the increased environmental burden associated with the production of metakaolin from natural kaolinite resources.     

Effects of sucrose on the rheological behavior of wheat starch pastes

The rheological properties of starch–water–sucrose pastes have been determined under steady and oscillatory shear conditions. The results show that the effect of sucrose at concentrations of less than about 20% w/w is to increase the apparent viscosity, yield stress, dynamic viscosity, and dynamic rigidity. At higher surose concentrations the yield stress and dynamic rigidity tend to zero while values of the dynamic viscosity and apparent viscosity are reduced compared with controls. Sucrose causes the dispersed gel particles in pastes to change volume, and it is suggested that this is one of the factors responsible for the observed effects of sucrose on rheological behavior.     

Hydration in high-performance cementitious systems containing vitreous calcium aluminosilicate or silica fume

The influence of a relatively new high-performance cement replacement material—vitreous calcium aluminosilicate (VCAS)—on the hydration behavior in cementitious systems, and its comparison to silica fume (SF) are presented in this paper. VCAS is shown to have no cementitious qualities, but exhibits significant pozzolanicity, which has been quantified using strength activity index and electrical conductivity change. VCAS modified pastes are found to consume more water during hydration than the corresponding SF modified pastes. Based on a normalized calcium hydroxide content defined in this paper, it is seen that the pozzolanic reaction of VCAS does not happen until 7 days while that of SF occurs as early as the first day. The degrees of hydration of the modified pastes are predicted using a model that employs the change in non-evaporable water resulting from the use of these replacement materials. VCAS modified pastes show lower later age porosities as compared to the plain and SF modified pastes. However, at equal degrees of hydration, SF modified pastes show the lowest porosity.     

Investigations on the performance of silica fume-incorporated cement pastes and mortars

Studies on the performance of cementitious products with silica fume (SF) are very important, as it is one of the inevitable additives to produce high-performance concrete (HPC). In this study, some experimental investigations on the influence of SF on various preliminary properties of cement pastes and mortars are reported. The properties included specific gravity and normal consistency (NC) of cement and air content and workability of mortar with different SF contents. Pozzolanic and chemical reactions of SF have been studied on setting times, soundness and shrinkage of cement pastes. Further, strength developments in compression and tension in cement mortars have also been studied at various SF contents. SF was varied from 0% to 30% at a constant increment 2.5/5% by weight of cement. Test results show that the SF changes the behavior of cement pastes and mortars significantly. It has been observed that the water-binder (w/b) (cement+SF) ratio seemed to play an important role for the performance of the products with higher SF contents. NC, soundness and drying shrinkage of cement pastes and the strength of mortar increase as the SF content increases, while the initial setting times of cement pastes and the air content and workability of mortar decrease as the SF content increases. However, hardly any influence has been observed on the final setting times of cement pastes. The early age hydration reactions of C₃A and C₃S increase with the addition of SF. The optimum SF content ranges between 15% and 22%.     

Rheology of cementitious paste with silica fume or limestone

The rheological behaviour of cementitious pastes where cement has increasingly been replaced by densified silica fume (SF), untreated SF or limestone has been studied. The effect of SF on the flow resistance, taken as the area under the flow curve, was found to depend on the dispersing ability of the plasticizer as illustrated by pastes with naphtalene sulphonate-formaldehyde condensate (SNF) and polyether grafted polyacrylate (PA).The gel strengths increased with increasing SF replacement of cement independently of plasticizer type. The cementitious gel strength was, however, depending on the type of SF since pastes with densified SF developed lower gel strengths than pastes with untreated SF. This phenomenon was attributed to agglomerates in the densified SF which remained unbroken by the mixing and measurement sequence.Both flow resistance and gel strength decreased with increasing limestone replacement. Thus, silica fume may have an advantage over limestone filler as stabilizing agent for self-compacting concrete preventing segregation upon standing and reduced form pressure due to a more rapid gel formation.     

铁尾矿粉、砂泥粉对水泥-减水剂体系工作性能的影响

为揭示铁尾矿粉和天然河砂中砂泥粉对水泥净浆体系的影响规律,分别测试了聚羧酸减水剂（ PCA）和萘系减水剂（ FDN）与两类粉体双掺后水泥浆动电电位和流变参数。结果表明：铁尾矿粉与砂泥粉对水泥-减水剂体系工作性能的影响存在显著差异。在PCA-水泥浆与铁尾矿粉或砂泥粉的双掺体系中,含铁尾矿粉水泥浆动电电位及塑性粘度变化率明显低于含砂泥粉水泥浆体系,铁尾矿粉对水泥浆流动性影响较小;采用FDN时,含砂泥粉的水泥浆体系流动性能优于掺铁尾矿粉的水泥浆体系;当水泥浆体系中粉体外掺量达到7％时,浆体塑性粘度显著增加,浆体体系流动性较差。     

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%。     

Influence of magnesium and silver ions on rheological properties of hydroxyapatite/chitosan/calcium sulphate based bone cements

Rheology of bioceramic bone cements is usually described as properties of ceramic slurries, neglecting the self-setting character of these materials. In our studies calcium sulphate based bone cements with Ag⁺, Mg²⁺ and Mg²⁺/CO₃^2- modified hydroxyapatite were investigated. Despite of expectations, it has been proven that the presence of magnesium ions significantly influence the rheological properties of cement pastes. Changes in rheological properties were connected with (I) chemical interactions between Mg²⁺ and sulphate ions (II) chemical interaction between Mg²⁺ and chitosan. These effects were not observed for silver additive. Most of the developed calcium sulphate based pastes, except material containing MgHA and chitosan, have been categorized as thick pastes applicable with the spatula. It has been found that the chitosan present around and at the calcium sulphate grains acted as a lubricant and prolong the period of quasi-constant viscosity of the pastes.     

Rheological characteristics of wheat starch pastes measured under steady shear conditions

Wheat starch pastes consist basically of swollen gel particles of various sizes which are dispersed in a continuous phase containing dissolved polysaccharides. The rheological properties of such pastes have been measured in the steady shear mode. At high shear rates the pastes behave as shear thinning liquids, while at low shear rates they exhibit a yield stress. The rheological properties vary with wheat starch variety and paste preparation conditions; this variation is of considerable economic importance to the starch industry. The present investigation shows that the flow behavior of pastes depends largely on two factors, namely, the volume which the starch gel particles would occupy when close packed if excess solvent were present, and the size distribution of the particles. Starch variety and paste preparation conditions influence these two factors and hence steady shear properties.     

Capillary porosity depercolation in cement-based materials: Measurement techniques and factors which influence their interpretation

The connectivity of the capillary porosity in cement-based materials impacts fluid-and-ion transport and thus material durability, the interpretation of experimental measurements such as chemical shrinkage, and the timing and duration of curing operations. While several methods have been used to assess the connectivity of the capillary pores, the interpretation of some experimental procedures can be complicated by the addition of certain chemical admixtures. This paper assesses capillary porosity depercolation in cement pastes using measurements of chemical shrinkage, low temperature calorimetry (LTC), and electrical impedance spectroscopy. The experimental results are analyzed to identify the time of capillary porosity depercolation. In addition, the factors that influence the interpretation of each technique are discussed. Experimental evidence suggests that capillary porosity depercolation, as defined by Powers, occurs after hydration has reduced the capillary porosity to around 20% in cement paste systems. The influence of capillary porosity depercolation on the transport properties is demonstrated in terms of a reduction in the electrical conductivity of the cementitious material. Special attention is paid to understand and interpret the influence of shrinkage-reducing admixtures (SRAs) on the freezing behavior of cementitious systems, particularly in regard to the inapplicability of using LTC to detect porosity depercolation in cement pastes containing such organic admixtures.     

Estimating rheological properties of cement pastes using various rheological models for different test geometry, gap and surface friction

Shear stress-shear rate flow tests were carried out on various cement pastes incorporating different mineral additions and chemical admixtures using various test geometries. Different gaps and friction capacity of shearing surfaces of the test geometries were employed in the flow tests. Rheological properties of cement pastes were calculated from the resulting flow curves using various rheological models. The Bingham, Modified Bingham, Herchel-Bulkley and Casson models were used to estimate yield stress. Plastic viscosity was estimated by the Bingham, Modified Bingham and Casson models, while the Williamson and Sisko models were used to estimate the theoretical viscosity at zero and infinite shear rates. It was observed that the rheological properties of cement pastes varied with the change of the test geometries and rheological models used for their calculation. The performance of rheological models in estimating the rheological properties of cement pastes, as expressed by a standard error, varied with the test geometries as well as with the composition of cement pastes. The paper highlights the difficulty in reconciling rheological results from different sources and the need for standardizing rheological test methods for rheological interlaboratory results to be critically analyzed and compared.     

Influence of hydration on the fluidity of normal Portland cement pastes

The rheological properties of cement paste strongly influence the workability of concrete. It is known that early hydration processes alter phase composition and microstructure of cement pastes. These processes affect fluidity and setting behaviour of cement paste. While many studies tried to measure and model rheological properties of cement pastes, only a few studies assessed the influence of the hydrate morphology on the fluidity of cement pastes.Results of the present study compare the influence of long prismatic hydrates (i.e. syngenite, secondary gypsum) on the fluidity of cement pastes with the effect of other hydrates (AFm).To induce the formation of certain hydration products the cement composition was modified by addition of set regulators and alkali sulphates. Furthermore a combination of various analytical methods such as fluidity (viscometric) testing and microstructural analysis (phase quantification by XRD-Rietveld analysis, investigation by Environmental SEM, BET analysis etc.) was performed. Results are implemented into a fundamental discussion on the influence of various hydration products on the fluidity of the paste.     

Impact of chemical variability of ground granulated blast-furnace slag on the phase formation in alkali-activated slag pastes

The influence of ground granulated blast-furnace slag (GGBS) chemical variability on phase formation in sodium hydroxide-activated GGBS pastes has been investigated using X-ray total scattering and subsequent pair distribution function (PDF) analysis. Crystalline phase identification based on reciprocal space analysis reveals that despite large chemical variations in the neat GGBSs the secondary reaction products are quite similar, with the majority of pastes containing a hydrotalcite-like phase. However, PDF analysis reveals considerable differences in short range atomic ordering of the main calcium-sodium aluminosilicate hydrate (C-(N)-A-S-H) gel phase in the pastes. Quantitative analysis of these local structural differences in conjunction with published PDF data identifies the important role calcium plays in dictating the atomic structure of disordered silicate-rich phases in cementitious materials. This study serves as a crucial step forward in linking GGBS chemistry with phase formation in alkali-activated GGBS pastes, revealing key information on the local structure of highly-disordered cementitious materials.     

水泥复合胶凝材料体系密实填充性能研究

利用密实模型研究了掺超细火山灰质的水泥基三元固体颗粒混合料体系的密实填充性能,并以浆体相对密度为指标,分析了低水胶比条件下火山灰质超细颗粒填充作用对水泥基复合浆体密实性、流动性及胶砂强度的影响,试验结果表明：就提高水泥基颗粒体系的堆积密实度而言,双掺平均粒径相差较大的超细颗粒材料比单掺更效;采用浆体相对密度指标评价超细火山灰质材料对水泥基复全合浆体的密度填充作用是合理且有效的;浆体的流动性及胶砂强度随浆体相对密度的提高而增大;在低水胶比条件下,超细火山灰质材料的密实填充效应更为显著。     

The effects of superabsorbent polymers on the microstructure of cementitious materials studied by means of sorption experiments

Superabsorbent polymers (SAPs) are a promising additive to be used in the building industry but may induce microstructural changes. Water vapour sorption may be used to characterize the change in pore structure of cementitious materials, but the technique is difficult to interpret. In the present paper, static and dynamic vapour sorption (DVS) measurements were performed and compared to nitrogen adsorption experiments. The models of Dubinin-Radushkevich and Barrett-Joyner-Halenda were hereby applied to study pores in the micro- and mesopore range. The results show that cement pastes with SAPs and without additional water show a slight decrease in porosity in the micro- and mesopore range. Cement pastes with SAPs and with additional water show no significant change of porosity in the micropore range and a slight increase in the larger mesopore range. These new findings give insight into the effects of SAPs on the microstructure and strength of cementitious materials.     

硅微粉对水泥浆体流变性能的影响

运用水化热测定仪、流变仪、以及Dinger-Funk紧密堆积等方法,研究了硅微粉掺入水泥中对复合浆体的流变性能的影响,比较了不同硅微粉掺量对复合浆体的早期水化放热、紧密堆积程度、屈服应力和塑性粘度的作用.结果显示:硅微粉取代水泥后,降低浆体水化热放热量,提高了体系紧密堆积程度;Bingham流体仍适用于硅微粉-水泥复合浆体,取代5%、10%、15%水泥的复合浆体,屈服应力和塑性粘度在0、60 min时都小于纯水泥浆体,并且取代10%水泥的复合浆体,其屈服应力和塑性粘度在0、60 min时都最小,流变性能最好.     

Rheological behaviour of fresh cement pastes formulated from a Self Compacting Concrete (SCC)

Self Compacting Concrete (SCC) has a high flowability and can be placed without vibration. It is defined as a concrete that exhibits a high deformability and a good resistance to segregation. This kind of concrete is of great interest and has gained wide use especially in the case of difficult casting conditions such as heavily reinforced sections. From a rheological point of view, the use of a Viscosity Enhancing Admixture (VEA) along with an adequate superplasticizer content enables to ensure high deformability and stability. However, little is known about the interactions between superplasticizer and viscosity agent. Hence, we propose to study several cement pastes formulated from the original paste of a typical SCC mix. Depending on their rheological behaviour, these pastes will be used later to study the stability of coarse aggregates. The major aim of this paper is to show that empirical tests such as spread and flow time are suitable to characterise the rheological behaviour of cement pastes instead of more complex ones. Rheological properties, i.e. viscosity and shear yield stress, are well correlated with empirical test results in the range of flowable mixes. Moreover, the experimental program leads to emphasize the effects of the mixing procedure on the rhelogical properties of cement pastes. Finally, test results enable to underline the interactions between superplasticizer and Viscosity Enhancing Admixture used in designing Self Compacting Concrete.