Water desorption and shrinkage in mortars and cement pastes: Experimental study and poromechanical model

The aim of the present research was to study moisture changes and strains induced by smooth water desorption of several cement based materials. The main advantage of this small-steps drying is to dramatically limit the structural effect within tested samples, by lowering moisture gradients and therefore cracking due to differential shrinkage. Resulting data are of importance as they allow water retention curves, porosity distribution and desiccation shrinkage to be determined versus a large range of relative humidity. Experiments were conducted on ordinary mortars and cement pastes with water-to-cement ratio of 0.5 and 0.8. The role of the cementitious matrix and of aggregates over water-related behaviour of these materials can also be studied. Finally, a simple numerical model, based on experimental poromechanical results, was proposed to predict the shrinkage when the material is submitted to drying.     

Impact of superplasticizer concentration and of ultra-fine particles on the rheological behaviour of dense mortar suspensions

This work aims at investigating the impact of the addition of superplasticizer and of ultra-fine particles, namely of silica fume and of precipitated titania, on the rheological behaviour of water-lean mortar pastes. The pastes are characterised in terms of their spread, their flowing behaviour and by means of performing a shear test, giving access to viscosity/shear gradient correlations. Adding superplasticizer is shown to shift the onset of shear thickening of the referring pastes to higher shear rates and to attenuate its otherwise rapid evolution, possibly by means of favouring steric particle-particle interactions. The workability of these mortars, which is characterised in terms of spread values and draining, is also improved. For the case of fly ash based mortars, adding ultra-fine particles is another way of (slightly) “retarding” shear thickening and of attenuating its evolution, possibly because of resulting in - on the average - lower hydrodynamic forces and reduced attractive Van der Waals interactions between particles. However, at the same time these mortars are characterised by a worsening in workability which is attributed to the huge amount of surface area provided by the ultra-fines.     

Backlash: a feature of the cyclic deformation of ceramic pastes

This paper offers a mathematical model to explain the rather peculiar results obtained in the cyclic tension–compression tests on various stiff ceramic pastes. The experimental results indicate a material with low strength in the middle of the cycle, rapidly strengthening at the extremes. Additionally there is a pronounced Bauschinger effect on strain reversal. The mathematical model is based on a previously published model for dry granular materials. This was specifically designed to simulate cyclic deformation, but a modification to include capillary tension allows excellent agreement between the simulation and experimental results on pastes. This demonstrates that the mechanics of granular materials is the appropriate basis for modelling these and similar materials rather than modified polymer rheology.     

The squeezing test: a tool to identify firm cement-based material's rheological behaviour and evaluate their extrusion ability

The squeezing test is used in this paper as a rheological behaviour identification tool of cement-based materials in order to evaluate the extrusion ability. Experimental flow curves are obtained for firm cement pastes and fine sand firm mortars using two different compression speeds. These tested products may be considered as frictional plastic materials. Using the proposed testing method, two important aspects are discussed. On one hand, it is shown in this paper that the mortars bulk and apparent friction factors increase with sand addition proportion. On the other hand, cement paste's apparent yield shear stress increases at low compression speed because of fluid drainage through the granular skeleton and sample hardening. Some pictures obtained after squeezing test are used to illustrate this drainage phenomenon. Finally, some mortar mix design indications are proposed according to extrusion criteria based on rheological conditions.     

Influence of bentonite clay on the rheological behaviour of fresh mortars

Fine mineral additives are often used in the formulation of ready-mix mortars as thickeners and thixotropic agents. Yet, these attributed fresh state properties are not clearly defined from the rheological point of view. In the present study, we consider the influence of bentonite (montmorillonite-based clay mineral) on the rheological behaviour of mortars, including in particular creep and thixotropy. The mortar pastes are subjected to different shear-rates and then allowed to creep under fixed shear stresses until reaching steady state, which corresponds to either rest if the applied stress is smaller than the yield stress or permanent flow otherwise. The evolution of the creep strain is investigated depending on shear history for different contents of bentonite. The microstructure rebuilding kinetics after shear (thixotropy) is considered by analysing the temporal evolution of the creep strain for different applied shear stresses (lower than the yield stress). As expected, bentonite is found to enhance the mortar creep (or sag) resistance. This enhancement consists of both an increase of the yield stress recovered after shear, and a diminution of the characteristic time for yield stress recovery (related to microstructure rebuilding).     

Deformation and flow of stiff pastes: review of rheology of some soft solids

Abstract

This paper reviews modelling and supporting experimental work that provides insight into the nature of stiff ceramic pastes. These pastes consist of discrete powder particles in a matrix of viscous fluid. At sufficiently high particle volume fractions, such pastes are able to withstand, without excessive deformation, the forces usual during handling of green ceramics. First, the industrial motivation for the study of pastes is examined, paying special attention to difficulties which are often glossed over in the open literature but which the present authors know from first hand experience. Common test methods are then discussed, and theories are outlined that have been developed to explain or generalise the ex perimental results. The penultimate section reviews some of the progress that has been made in applying theory to industrially based problems. The approach pioneered in the discipline of soil mechanics appears particularly promising as a basis for developing models of pastes that give some physical insight into their mechanical behaviour, and so might give a link to mix design. In conclusion a look into the future of the study of pastes is presented.     

The application of the Mackenzie model to the mechanical properties of cements: A reply to the Discussion by R. G. Wilson and S. Chatterji

This reply sets out to justify the derivation of the equation relating strength to porosity originally proposed in the first paper. The approximation used is reasonable and a more rigorous approach is not necessary. It is also shown that the Mackenzie equation for the bulk modulus requires no amendment when applied to liquid filled pores provided the liquid can escape from the pores when the system is stressed. Further refinement of the model is possible but the simple hydrostatic stress model is an adequate fit to much experimental data on the strength of cement pastes and mortars.     

Effect of superplasticizer and shrinkage-reducing admixtures on alkali-activated slag pastes and mortars

This paper shows how several superplasticizers (polycarboxylates, vinyl copolymers, melamine and naphthalene-based) and shrinkage-reducing (polypropylenglycol derivatives) admixtures affect the mechanical and rheological properties and setting times of alkali-activated slag pastes and mortars. Two activator solutions, waterglass and NaOH, were used, along with two concentrations—4% and 5% of Na₂O by mass of slag. All admixtures, with the exception of the naphthalene-based product, lost their fluidifying properties in mortars activated with NaOH as a result of the changes in their chemical structures in high alkaline media. The difference in the behaviour of these admixtures when ordinary Portland cement is used as a binder is also discussed in this paper.     

Potential use of natural perlite powder as a pozzolanic mineral admixture in Portland cement

This paper investigates the effect of incorporating natural perlite powder (NPP) as a cement replacement on cementitious materials properties. For this purpose, cement pastes and mortars were prepared by replacing 5, 10, 15, and 20% of NPP by mass of the Portland cement. Physicomechanical performances of pastes and mortars based NPP were inspected using normal consistency, setting times, heat of hydration, and compressive strength testing. Resistance against sodium sulfate attack and sulfuric acid attack were also assessed to investigate the durability characteristics of different mortar mixes. Experimental results show that cement pastes and mortars incorporating up to 20% of NPP demonstrate satisfactory physical and mechanical properties with very comparable results to cementitious materials without NPP. In addition, improved sulfate and acid attacks resistance with increased NPP content were demonstrated. The X-ray diffraction analyses confirm that NPP can be considered as a good pozzolanic material that can be used satisfactorily as a mineral admixture in cement production.     

Steady and transient flow behaviour of fresh cement pastes

Fresh cement pastes behave as non-Newtonian viscous fluids. During steady flow, their apparent viscosity depends on the applied strain rate. During transient flow, the apparent viscosity is a function of time. In this work, a thixotropic model is presented. Its four parameters are identified experimentally for a tested cement paste using coaxial viscometer test. Viscometer flow simulations are then carried out. The model proves to be able to predict the trends of the fresh behaviour of cement pastes in various flow situations.     

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。     

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.     

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%.     

Squeeze film testing of ceramic pastes

Abstract

Equipment is described in which pastes are squeezed out from the region between two parallel circular discs. The force on the upper disc is measured for different fixed approach speeds of the discs. Two disc diameters are used, in conjunction with two ceramic pastes and one model paste. Different surface finishes are used on the discs and the effects of adding glass fibre and alumina fibre to the pastes are examined. Industrial squeezing flows are found in such processes as filtration, pressing, rolling, and injection moulding. A theory for squeezing flow is given which shows that, under certain conditions, a plot of squeezing force against the reciprocal of gap will be a straight line. The conditions are that the shear stress is constant and that the flow is dominated by shear forces. This is shown to occur in many real cases, although the biaxial extension of the bulk material also affects the load. It is shown that, for ceramic pastes, plate velocity has little effect on the force. For all pastes, rougher surfaces increase, and lubrication reduces, the force required to squeeze the paste. Wall slip is shown to occur in some non-lubricated situations. Fibre addition usually increases the stiffness of the pastes, especially for longer, thinner, fibres. In one case, a paste is found to flow more easily when thick, short fibres are used. Rings of coloured paste are used to show that several deformation profiles are possible: bell shaped, triangular, and near parabolic. Tests are shown to be simple to perform and the shear stresses obtained in this geometry found to be higher than those for flow in dies.     

Effects of the incorporation of Municipal Solid Waste Incineration fly ash in cement pastes and mortars: II: Modeling

This work falls within the scope of the general problem of the assessment of concretes manufactured from waste materials. The main objective is to study the long-term evolution of these materials during leaching using the cellular automata-based hydration model developed at the National Institute of Standards and Technology (NIST). The work is based on the analysis of mortars and cement pastes containing experimental waste: Municipal Solid Waste Incineration fly ash (MSWI fly ash). After having determined the mineralogical composition of the MSWI fly ash and its interactions with cement during hydration, presented previously as Part I, the phases comprising the fly ash have been incorporated into the hydration model. The increase in porosity of cement pastes containing MSWI fly ash during leaching has then been simulated. Finally, a simplified leaching model has been developed to study the influence of the changes in microstructure on the release of calcium and sodium.     

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.     

The origin of the pozzolanic activity of calcined clay minerals: A comparison between kaolinite, illite and montmorillonite

This paper investigates the decomposition of three clayey structures (kaolinite, illite and montmorillonite) when thermally treated at 600 °C and 800 °C and the effect of this treatment on their pozzolanic activity in cementitious materials. Raw and calcined clay minerals were characterized by the XRF, XRD, ²⁷Al NMR, DTG and BET techniques. Cement pastes and mortars were produced with a 30% substitution by calcined clay minerals. The pozzolanic activity and the degree of hydration of the clinker component were monitored on pastes using DTG and BSE-IA, respectively. Compressive strength and sorptivity properties were assessed on standard mortars. It was shown that kaolinite, due to the amount and location of OH groups in its structure, has a different decomposition process than illite or montmorillonite, which results in an important loss of crystallinity. This explains its enhanced pozzolanic activity compared to other calcined clay–cement blends.     

石灰石粉对水泥基材料抗硫酸盐侵蚀性的影响及其机理

用天然石灰石粉等质量取代水泥20%和30%,将制备的水泥净浆和砂浆试件常温浸泡在0.35 mol/L Na2SO4溶液中,测量试件的线长度和抗折强度随浸泡时间的变化.结果表明:石灰石粉对水泥基材料的抗硫酸盐性有严重的影响,它们使水泥基材料在硫酸盐环境中的强度急剧下降并导致水泥基材料产生较大体积膨胀,引起开裂.掺石灰石粉的水泥基材料主要因形成大量较大尺寸的石膏晶体而膨胀开裂.石膏的形成导致硫酸盐侵蚀水泥基材料产生膨胀开裂.因此,在硫酸盐侵蚀环境下,不宜采用含石灰石粉的复合水泥或将石灰石粉作为矿物掺合料制备的混凝土.     

Maldistribution of fluids in extrudates

Solid–liquid pastes featuring high volume fractions of particulates are frequently used in ceramic forming operations. When pastes are used it is important that the particulate distribution remains uniform throughout the body. The stresses imposed during extrusion processing can, however, promote differential flow between the solid and liquid phases giving rise to product and processing problems. Reliable models for predicting phase distribution changes in these multi-phase systems are in their infancy.This paper reports progress towards developing simulation techniques and practical systems to verify the numerical approaches. Pastes containing glass spheres suspended in a highly viscous Newtonian fluid have been extruded at various speeds and solids loadings. Load and liquid content data are presented which form the basis for model verification. Soil mechanics approaches are used here to encapsulate the inherently multi-phase nature of these systems. The modified Cam–Clay model has been implemented in a finite element analysis simulation of ram extrusion using the ABAQUS platform. The simulation requires regular and extensive remeshing and monitoring of the conservation of mass. Predictions of extrusion pressures and deformation behaviour are compared with the experimental data for a series of square-ended and conical dies.     

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.