Effect of EVA on the fresh properties of cement paste

The improved workability effect of latex in dry mortars has not been fully clarified. The purpose of this research was to investigate the influence of the EVA copolymer on the cement hydration and on the rheological properties of cement pastes. The results pointed to a minor influence of EVA on cement hydration and to a ball-bearing effect. In fact, the shear thinning behavior of reference paste at 15 min after mixing changed to shear thickening owing to the EVA addition. This behavior could be explained by the decrease in the interparticle separation distance as consequence of the solid content increase in case of shearing detachment of weakly adhered EVA particles from the cement particles surfaces. The expected EVA plasticizing effect was observed at 60 min. Such behavior points to the stabilization of EVA on the cement particles surfaces, thus resulting in a steric barrier effect.     

Rheology of cement paste under high pressure

The objective of this study was to investigate whether cementitious materials undergo changes during pumping processes due to pressure variation. The influence of pressure on the rheological properties of cement pastes, which assumably represented the lubricating layer that forms along the profile of concrete during pumping, was evaluated using a rotational rheometer with a high-pressure cell. Cement pastes with water-to-cement ratios ranging from 0.35 to 0.6 were tested according to a protocol designed to simulate the conditions of an actual pumping process based on field tests. The shear rates, shearing durations, and pressure levels from 0 to 30 MPa were experimentally simulated. The test results indicated that below a certain water-to-cement ratio (0.40) elevated pressures lead to changes in the rheological properties, while changes were negligible when the ratio was above this threshold. Further, at low water-to-cement ratios the thixotropy of the cement paste can reverse into rheopexy after pressurization.     

Influence of crushed aggregate fines with micro-proportioned particle size distributions on rheology of cement paste

This paper presents a study on how crushed concrete aggregate fines affect rheological properties of cement paste. The fines (≤250 μm) were produced by high-speed vertical shaft impact (VSI) crushing of rock types from 10 different quarries representing a wide range of local Norwegian geological variety with respect to rocks of different mineralogy and mechanical properties (mono- and multimineralic igneous (intrusive and extrusive), metamorphic and sedimentary rocks). The results show that the rheological properties of cement paste are governed mainly by the specific surface of the fines calculated from SediGraph measurements, and their surface properties causing different interaction with the superplasticiser (SP) molecules, as detected by zeta potential measurements. The rheology of cement paste is proportional to the specific surface and can be controlled by both altering particle size distribution (PSD) and volume fraction of crushed fines. The effect depends on the SP dosage and total surface of the fines present in the mix. The effect of the shape of the VSI crushed fine particles is of less importance at constant PSD for the materials studied here, because VSI crushing effectively normalised the equi-dimensionality of the grains, i.e. created similar particle shapes.     

Superfine cement for improving packing density, rheology and strength of cement paste

Superfine cement is a cement ground to a much higher fineness than ordinary cement. The addition of a small quantity of superfine cement to fill into the voids of ordinary cement can improve the packing density of the cement and thereby reduce the amount of mixing water needed to fill the voids. In this study, the effects of superfine cement on the packing density of cement (directly measured by a wet packing test), the water film thickness of cement paste (taken as the excess water to solid surface area ratio), and the flowability, rheology and strength of cement paste were investigated. The results showed that the addition of 10% to 20% superfine cement can significantly increase the packing density of the cement and the water film thickness of the cement paste. Such increases in packing density and water film thickness would then improve the flowability, rheology and strength of the cement paste. Hence, superfine cement is an effective cementitious filler for improving cement performance.     

Bleeding and sedimentation of cement paste measured by hydrostatic pressure and Turbiscan

Sedimentation and bleeding of cement pastes with lignosulfonate were studied by visual observation, HYdroStatic Pressure Test (HYSPT) and Turbiscan measurements showing two bleeding stages: a fast initial phase followed by a phase with diminishing sedimentation rate. A turbid bleeding zone establishes during the fast bleeding phase and the top layer gradually becomes transparent in the diminishing phase within minutes or hours depending on admixture and solid fraction. The bleeding rates measured visually and by HYSPT in the first 2 h are higher than the ones calculated by Kozeny-Carman Equation, whereas turbiscan shows lower rates. Both HYSPT and Turbiscan monitor the particle and fluid fluxes and thus describe bleeding from turbid to clear zone respectively by observing the density variation of bulk paste or the change in optical density of the surface region. Lignosulfonate reduces bleeding by improving particle dispersion to various degrees depending on types and dosages.     

Rheological behaviour of fresh cement paste as measured by squeeze flow

A method is proposed for measuring the rheology of cement paste under conditions that suppress shear flow, i.e. squeezing. This method is based on squeezing samples in a servohydraulic compression-tension testing machine, and is different from the commonly used shear flow experiments. Possible artefacts such as the buoyancy of the piston that penetrates the paste, sedimentation of cement paste, geometry of the container, and friction at the interface between the top plate (or piston) and sample are investigated. Plots of stress versus apparent strain were obtained and compared with results from standard shear flow experiments. Because cement paste has both viscoelastic and viscoplastic characteristics, results are analysed in terms of both solid-like deformation and liquid-like flow behaviour. A first-approximation theoretical analysis is developed, based on the assumption that cement paste behaves as a non-Newtonian liquid, and results are compared with the experimental results.Nomenclature Shear strain rate in power law fluid model - _zr Shear strain converted from _zr - Shear strain rate - Normal strain rate - _zr Component of shear strain - _zr Component of shear strain rate - _zz Component of normal strain - Viscosity - Density of cement paste (3.2 g cm^–3) - _Cav Calculated average normal stress of cement paste - _Nav Calculated average normal stress of power law fluid - _m Measured normal stress of cement paste - _zz Normal stress in z direction - _eq Equivalent shear stress converted from normal stress - _rz Shear stress in momentum equation - a _i Coefficients in polynomial function of geometric factor for cement paste - B Buoyancy force - CGF Geometric factor for cement paste - d _o Amplitude of squeeze motion - F _N Load in normal direction - g Gravitational constant - h Sample height - h _o Initial sample height - Velocity of platen - k Order of polynomial function of geometric factor for cement paste - m Consistency in power-law fluid model - n Power index in power-law fluid model - P Pressure - P _a Atmospheric pressure - PGF Geometric factor for power-law fluid model - r Radial direction in cylindrical coordinates - R Radius of sample - s 1/n - V Volume of the top platen submerged into cement paste - v _r Velocity inr direction - v _z Velocity in z direction - z Vertical direction in cylindrical coordinates     

Water depercolation of setting cement paste evaluated by diffuse ultrasound

Microstructural evolution during setting of cement-based materials explains its solidification. While various studies have attempted to reveal the microstructural evolution, many questions still remain. The setting process refers to the phase change from a cement suspension to a poroelastic solid. This study proposes a diffuse ultrasound method to investigate microstructural evolution and to determine the setting of cement paste. The diffuse ultrasound refers the propagation of an incoherent component on P-waves through-transmission. Its velocity and attenuation reflect the tortuous paths in early-age cement paste. Closing the tortuous paths by cement hydration and coagulation causes the diffuse ultrasound to combine with the propagated P-wave. The combined time is then an indicator of the setting point. The diffusivity, dissipation, and permeability evaluated by the diffuse ultrasound also shed light on the microstructural evolution of fresh cement paste.     

Analysis of damage development in cement paste due to ice nucleation at different temperatures

From the thermodynamic considerations and from a virtual microstructure of cement paste, a quantitative assessment of the internal damage of cement paste due to hydraulic pressure is investigated by 3D lattice fracture model. Two issues are solved in this study: (1) the capillary pores related to the penetration of ice crystals are characterized by a multi-step digitalization algorithm. (2) The hydraulic pressure P_L is achieved through an iterative fracture process simulation where the creation of micro-cracks is taken into account. The iterative fracture process simulation then is verified by a benchmark test. Simulations indicate that randomly distributed micro-cracks are formed at a low ice crystal saturation degree S_C (defined as the molar percentage of water transformed into ice crystals), while local propagation of micro-cracks occurs and transverse cracks can be formed at high S_C. Comparison of S_C and cracks pattern by simulation and by experiments are finally discussed.     

Consequences of competitive adsorption between polymers on the rheological behaviour of cement pastes

Self Compacting Concretes (SCC) are characterized by their high fluidity so they can be placed in sections with congested reinforcements and in restricted areas without vibration. Furthermore, SCC cement paste has to be viscous enough to avoid segregation and to maintain the stability of the suspension until the onset of hardening. To fulfil these rheological requirements, mix design engineers combine use of superplasticizers and viscosity agents. The mechanism of action of these chemical admixtures is very sensitive to their adsorption. The blending of these polymers generates a competitive adsorption on surface sites of cement particles, which influences their performances. For a better understanding of competitive adsorption, we measure here both the amount of adsorbed polymers and its consequences on the rheological behaviour of the system in terms of yield stress and plastic viscosity. Our results suggest that the competitive adsorption prevents some of the polymer molecules from adsorbing, thereby moderating the performances of adsorbing polymers and enhancing the effects of polymers potentially in solution on the rheological properties of cement paste.     

Effects of technological parameters on permeability estimation of partially saturated cement paste by a DEM approach

In this study, a numerical approach developed in our group is used to assess the permeability of partially saturated cement paste based on a discrete element modelling (DEM) technique. The relationship between saturation degree and permeability is found to be in good agreement with experimental observations. Also, outcomes of a systematic study of the effects of technological parameters (i.e., hydration period, water/cement ratio and cement particle size range) on the permeability of partially saturated specimens follow expected trends. Moreover, permeability is found to be correlated to effective porosity. This is a sound basis for investigating the impact of the interfacial transition zone on water and gas permeability. Effects of partial saturation are demonstrated different for water and gas transport. Possible mechanisms underlying these permeability characteristics are discussed in this paper.     

Characterization of adhesive interphase between epoxy and cement paste via Raman spectroscopy and mercury intrusion porosimetry

When exposed to harsh environmental conditions, the adhesive bond between epoxy and concrete was found to depend almost entirely on the effectiveness of the mechanical interlock which is contingent upon the ability of the adhesive to properly wet and penetrate the substrate. The current study is an investigation of the existence of a distinct interphase region within cement paste through the combined use of mercury intrusion porosimetry and Raman spectroscopy. A positive correlation between median pore size and depth of epoxy permeation into the cement paste substrate was observed. The Lucas-Washburn equation was found to accurately represent the driving mechanisms behind permeation of curing epoxy into the porous cement paste matrix. Interconnectivity of the capillary and gel pore structures was determined to play an important role in the permeation mechanisms. Analysis of relative conversion of epoxy via Raman spectroscopy in the interfacial region showed evidence of epoxy-cement paste interactions.     

Investigation of strength and hydration characteristics in nano-silica incorporated cement paste

A hydration model for Portland cement pastes modified with nano-silica in partial substitution is formulated based on the nucleation growth process from microstructural investigations over time. The model is calibrated against thermogravimetry, X-ray diffraction and calorimetry data for four different substitution rates from 0 to 12 wt% and is validated by backscattered electron microscopy. Finite element based compressive strength predictions using representative volume element analysis of the nano modified cement pastes agreed with the experimental values. The model predictions indicate that a rate of 8 wt% is the optimum replacement level of cement by nano-silica leading to a high density matrix promoting a maximum mechanical strength.     

Numerical homogenization of hardened cement paste

Based upon a three-dimensional computer-tomography of hardened cement paste, a finite-element mesh at micrometer length scale is introduced. Effective material properties are obtained through numerical homogenization techniques using representative volume elements. Statistical tests, two- and three-dimensional computations and a comparison with experimental data are shown. For the hydration products of hardened cement paste a visco-plastic constitutive equation of Perzyna type including isotropic damage is introduced. The inelastic material parameters are identified solving an optimization problem through a combination of a stochastic genetic algorithm and the deterministic Levenberg-Marquardt method. The time-consuming evaluations of the corresponding objective function are distributed within a network environment automatically.     

氧化石墨烯对水泥净浆流动度及水泥石结构和性能的影响

采用Hummers法及超声破碎分散法制备了氧化石墨烯(GO)纳米相分散液,研究了GO对水泥净浆流动度和水泥石微观结构的影响,用FT-IR、AFM、XRD及SEM对GO及水泥石结构进行了表征。结果表明GO的掺入降低了净浆流动性,每增加0.01%的GO需要增加0.02%的聚羧酸系减水剂(PCs)以保持水泥净浆流动度在3 h内在200 mm以上,GO的掺入能够使水泥石的微观结构发生明显的变化,当GO/PCs掺量为0.01%/0.24%~0.03%/0.28%时,水化龄期7 d的水泥石出现了大量分散均匀的花状微晶体,当GO/PCs掺量为0.05%/0.32%~0.07%/0.36%时,同龄期水泥石中出现大量的片状晶体,在水化龄期延长到28 d时有转化为密实结构的趋势,结果说明GO具有调控水泥水化产物形貌的能力及增强增韧的作用,此研究结果对于提高水泥基材料的力学性能具有重要意义。     

Release of internal curing water from lightweight aggregates in cement paste investigated by neutron and X-ray tomography

A sealed sample of cement paste containing a pre-wetted and a dry lightweight aggregate (LWA) particle was investigated in the period between 0.5 and 20.3 h after mixing. Changes in the local water distribution in the sample during hydration were evaluated using the subtraction of 3D images obtained by subsequent neutron tomographies (NT). As both water retention in the LWA and its release to the cement paste are influenced by the pore structure of the aggregate, a high-resolution image of the sample was subsequently captured by X-ray tomography. The internal curing water released from the LWA traveled at least 3 mm from the LWA into the cement paste in the first day. Hardly any gradient in the water content of the cement paste against the distance from the LWA was observed. This suggests that the release of water for internal curing (IC) is relatively fast and the water is distributed fairly homogeneously from the LWA for at least 3 mm within the hydrating cement paste.     

Rheological changes associated with setting of cement paste

A method is described in which the yield stress of portland cement paste is determined from its creep/recovery behavior measured using a constant stress rheometer. As long as the applied stress does not exceed the yield stress, yield stress can be determined as a function of time on a single specimen without breaking down the microstructure. The early hydration is seen to strengthen the interparticle bonding within the flocculated structure, hence increasing the yield stress. Time evolution of the measured yield stress shows two regions, the first in which the yield stress increases slowly, and the second in which the yield stress increases rapidly. These correlate with the induction period and the acceleratory period, and the transition to a rapid increase in yield stress corresponds to the initial setting time. Reducing the water:cement ratio caused this initial setting time to be reduced.     

Effects of water and temperature variations on deformation of limestone aggregates,cement paste,mortar and High Performance Concrete (HPC)

The present paper deals with the dimensional variations of limestone aggregates, cement paste, mortar, and High Performance Concrete (HPC) made with these constituents, when subjected to temperature changes in drying and sealed conditions. Variable water contents are studied for each material. The overall experimental results were obtained using strain gauges and Digital Image Correlation technique (DIC) is also applied for one configuration test. A significant difference between coefficients of Thermal Expansion (CTE) of cement paste and aggregate is observed, which leads to a large differential thermal deformation, able to explain thermal damage for temperatures between 50 °C and 100 °C. Water content has a great influence on material thermal dilation, causing delayed deformations especially for temperatures above 60 °C and for initially saturated samples. These experimental data may allow Thermo-Hydro-Mechanical models to be improved so that the in-situ long term behaviour of concrete can finally be predicted. The results confirm the dilation coefficient contrast between paste and aggregate, and also show that this contrast is markedly affected by the drying conditions and the initial state of saturation.     

The viscoelastic response of cement paste to three-dimensional loading

The majority of the viscoelastic constitutive data for cement paste or concrete found in the literature deal exclusively with uniaxial loading. To predict the isotropic response of concrete or cement paste under multiaxial loading or multiaxial prescribed deformation, it is necessary to have knowledge of at least two viscoelastic constitutive properties. In the past, the typical treatment of three-dimensional modeling of concrete viscoelasticity has involved the assumption of a time-independent viscoelastic Poisson ratio. However, the experimental evidence supporting this simplification is inconclusive. In this study, experiments were performed on hardened cement paste that allowed the simultaneous measurement of both the dilatational and shear compliances, allowing the full three-dimensional characterization of the constitutive response. It was found that the dilatational compliance leveled off after several days for three of four mixtures tested. In these three materials, the Poisson’s ratio was found to be an increasing function of time. Prediction of the measured uniaxial compliance using the measured bulk and shear compliances indicated that the confined compressive test used in this research may cause changes in the material which affect the measured dilatational compliance, and therefore the calculated viscoelastic Poisson ratio.     

Characterization of chemical accelerators for sustainable recycling of fresh electric-arc furnace dust in cement pastes

A comprehensive investigation focusing on the effect of fresh EAFD on the setting of cement pastes was conducted. A new approach of using accelerators to counteract the extended retardation associated with using fresh EAFD was investigated. Two chloride-free chemical accelerators (CFCA), namely, calcium nitrite (CN) and calcium formate (CF) were used and their performance was benchmarked against calcium chloride (CC). Temperature profile was used for validating setting times by Vicat needle apparatus. Homogenization of fresh EAFD from different furnaces resulting in an average ZnO content of 15.1% was adopted to ensure consistent retardation in setting time. The use of an accelerator/homogenized EAFD ratio of 0.25 for CN and CF has shown significant reduction in setting time more than CC. Additional reduction was achieved with an increased ratio to 0.5. However, limited effect was noted for CN and CF with a further increase in the ratio compared to CC. Scanning electron microscope (SEM) was used to monitor the microstructure development of selected mixtures at early age. Finally, Prediction models were developed to predict the EAFD and CFCA effects on setting time. The results of this study are important in the quest to develop EAFD from waste byproducts into viable construction material.     

Influence of colloidal silica sol on fresh properties of cement paste as compared to nano-silica powder with agglomerates in micron-scale

Influence of colloidal silica sol (SS) with mono-dispersed nano-particles on fresh properties of cement paste was investigated as compared to nano-silica powder (NS) with agglomerates in micron-scale. The SS addition showed a much greater influence on sedimentation and rheological behavior of the paste than the NS incorporation, because the nano-particles in SS coagulate immediately once cement is mixed into water containing SS, forming loose floc and coating layer around cement particles. The loose floc cannot function as fillers to release free water, but possesses a more open microstructure, leading to a higher free water retention capacity than the agglomerates in NS. However, addition of SS presented an obviously better accelerating effect on cement hydration than that of NS, though the nano-particles in SS are nearly the same as those in NS in primary particle size and the flocs in the paste with SS addition are typically larger than the agglomerates in NS, implying that the acceleration may have nothing to do with the seeding effect. Through detecting calcium-absorbing properties of NS and SS, it is found that the accelerating effect is highly dependent on the rapid depletion of calcium ions in the paste. Finally, it was interestingly found that the CH crystals are even more prone to grow along (0001) plane with larger size in the paste with SS addition, because the coagulated gel network in the paste slows down the diffusion rate of the released ions and eliminate the convection in the system, thus the 3D nucleation and growth of the CH crystals were suppressed.