Fresh Mortar Research Articles

Study on Improvement of Vibration Compaction Performance of Fresh Mortar

Study on Improvement of Vibration Compaction Performance of Fresh Mortar

The effect of pozzolan addition on the physical and mechanical properties of lime mortar

Hydrated lime, due to its life cycle (return to the natural form as a result of binding and hardening) shows a positive ecological aspect. The binder is often used in building materials based on plant components due to the high alkaline reaction and low diffusion resistance, as well as in mortars and plasters. However, it is an air binder with low strength parameters and the process of its binding and hardening is long-lasting. One of the ways to accelerate the binding processes and also to increase the strength of the binder is the use of additives in the form of pozzolanic materials which react with calcium hydroxide and form compounds with hydraulic properties. These are natural materials or industrial waste, so they do not affect the environmental impact of lime binder. The article presents the influence of the addition of pozzolan (three types) in various weight amounts, on the strength properties of mortars, their absorbability and capillary rise (presented as water absorption coefficient). The influence of pozzolan presence on workability of fresh mortars - their consistency was also checked.

Distinguishing dynamic and static yield stress of fresh cement mortars through thixotropy

The dynamic and static yield stress of fresh cement mortar were measured in a rotational rheometer with a vane geometry using shear rate and shear stress-controlled protocols, respectively. Through a shear rate-controlled steady-state protocol, the equilibrium flow curve is measured and fitted with the Bingham model to obtain dynamic yield stress. A negative slope in the equilibrium flow curve, shear banding and stick-slip phenomena are observed and discussed. Through a stress-controlled creep-recovery protocol, viscosity bifurcation behavior is captured and static yield stress is marked as the creep stress when the bifurcation occurs. Finally, the discrepancy between dynamic and static yield stress is tied to thixotropy.

EFFECT OF VISCOSITY MODIFYING AGENTS ON DEWATERING UNDER PRESSURE AND THE PERFORMANCES OF MORTAR AND CONCRETE

This study emphasizes the effect of viscosity modifying agents (VMAs) on bleeding and dewatering of fresh mortars under high pressure. A detailed experimental dewatering device was designed and the analysis was carried out in order to evaluate the influence of two types of VMA on dewatering of freshly mixed mortars under high pressure. Additionally, the influence of the VMAs on mortar flow and air content of concrete was investigated. Two different types of VMA, which are a surfactant type and a starch ether type with various dosages were used in the tested mortar and concrete sample having the same water to binder ratio. Experimental results showed that by using dosages of surfactant type VMA equal to or higher than 3%, the pressurized dewatering of the mortars was significantly reduced. The use of starch in mortar mixtures was ineffective to reduce dewatering of mortars under high pressure. At same water to binder ratio, the addition of surfactant type VMA slightly increased the mortar flow and the v-funnel flow time due to enhancement of the air content in mixtures whereas starch ether decreased the mortar flow and increased the v-funnel time. The results of this study indicate that the surfactant type VMA is useful to control the dewatering of concrete under high pressure.

Spatial distribution of steel fibers and air bubbles in UHPC cylinder determined by X-ray CT method

This paper investigated the spatial distribution of steel fibers and air bubbles in UHPC cylinder specimen by using X-ray CT system. Four levels of superplasticizer dosage were adopted to obtain cement mortar mixtures with different rheological properties and then steel fibers were added by volume of 1%, 2% and 3% for every cement mortar mixture respectively. Cylindrical specimens with diameter of 100 mm and height of 200 mm were prepared under the same condition for every mixture and the content of steel fibers and air bubbles at different casting depth was determined by X-ray CT measurement. On the other hand, the cylinder specimen was cut into four pieces for evaluating the splitting tensile strength variation with the casting depth. The results reveal that steel fibers and air bubbles are uniformly distributed in UHPC specimen when the fresh mixture is prepared by adding a lower dosage of superplasticizer. This homogeneity is also proved by the splitting tensile strength for samples at different depths. When the yield stress of fresh mortar was reduced by the higher dosage of superplasticizer, the steel fiber sedimentation and air bubble rise occurred and the splitting tensile strength was significantly decreased at the top portion of cylinder specimen. The influence of steel fiber content on splitting tensile strength can be regressed into a good quadratic equation. Therefore, the balance between homogeneity and flowability should be fully considered for manufacturing UHPC elements with a high depth.

Influence of polymer fibers on rheological properties of cement mortars

AbstractThe reinforcing effect of fibers in cement composites often results in the improvement of the brittle nature of cementitious materials. But the decrease in the workability of fresh concrete is often the disadvantage of fibers addition. Conventional single-point workability tests cannot characterize workability of concrete in terms of fundamental rheological parameters. To this end, this paper describes an investigation of the influence of synthetic fiber additions (fiber length in the range 12–50 mm and volume fraction in the range 0–4%) on the rheological properties of fiber reinforced fresh mortar (FRFM) and development of these properties over time. The rheometer Viskomat XL was used in this study. Within the limitations of the instrument and testing procedure it is shown that FRFMs conform to the Bingham model. Natural postglacial sand 0/4 mm was used as a fine aggregate and cement CEMI 42.5 R was used as a binder. Three commercial synthetic fibers were selected for these examinations. Rheological properties were expressed in terms of Bingham model parameters g (yield value ) and h (plastic viscosity). Based on the test results it was found out that the fiber type and volume fraction affected both the yield stress and plastic viscosity.

Effect of chitosan ethers on fresh state properties of lime mortars

The fresh state properties of mortars are eminently important since determine the material workability and also have a great influence on its hardened state characteristics. In this paper, the behaviour of fresh lime mortars modified by etherified derivatives of chitosan (hydroxypropylchitosan (HPCH) and carboxymethylchitosan (CMCH)) is assessed with the purpose of exploring a new application of such derivatives as lime mortar admixtures. The rheological parameters (relative yield stress, consistency coefficient and fluidity index) and viscoelastic properties were correlated with flow table tests, relative density measurements, water retention abilities of mortars and air content in mortars. Results were seen to be strongly dependent on substituents of the chitosan. Non-ionic derivative (HPCH) had a plasticizing influence on the mortars; the ionic CMCH showed the thickening effect. The effect of chitosan ethers was found to be dosage-dependent. CMCH had low impact on water retention, while HPCH displayed high water retention capability. It was concluded, that the ionic derivative (CMCH) is very similar by its viscosity enhancing effect to starch ether.

An assessment on volume stabilization of mortar with stainless steel slag sand

This study aims at employing stainless steel oxidizing slag (SSOS) as fine aggregate, replacing river sand in mortar mixtures with various ratios (e.g. 0%, 25%, 50%, 75%, and 100%). The received slag was left outdoor from 1- to 8months as a treatment before be used as aggregate. The content of slag expansion source (free lime) was first determined after the end each month of stabilization. The X-ray diffraction (XRD) technique was offered to identify the mineralogical of the treated slag. Next, fresh and hardened properties of the stabilized-slag mortar including flow table, expansion, compressive strength, ultrasonic pulse velocity were examined in laboratory, accordingly. The expansion in term of length change of the slag mortar bars (25×25×285mm) was investigated after the autoclave and heating catalytic (100°C) curing conditions. The results indicated that free lime content is gradually decreased over times when SSOS exposed to air environment and effectively restricted the expansion capacity of the slag mortar. In addition, a higher SSOS replacement ratio a larger expansion is measured on the mortar bars. Meanwhile, a lesser period of slag treatment could make the specimens fracture at a lower level of slag substitution. The expansion of mortar bar under the autoclave is similar to that of about 72–96h heating catalysis. Moreover, compressive strength of mortar is improved when containing as much as slag in mixtures, although this induces less workability of fresh mortar.

Rheological and thermal properties of aerated sprayed mortar

This paper studies the rheological properties in aerated cement pastes in order to develop an aerated sprayed mortar with low thermal conductivity. For this purpose, different mixtures of cement with additions of metakaolin (MK) and sepiolite (SP) were tested. In addition, influence of yield stress on build-up thickness, as well as expansion speed and pore network were studied using cement pastes aerated with aluminum powder. At the same time, the volume increase during expansion, water retention and yield stress of the fresh foamed cement pastes were studied to characterize the fresh mortar. Results obtained show that the incorporation of siliceous fly ashes (SFA) in ordinary Portland cement together with additions of MK or/and SP provided the largest expansion speed increase and the lowest density of the aerated mortar. Pastes with greater yield stress show higher thermal conductivity but are better to be pneumatically gunited due to its bigger build up thickness. The most suitable paste from all them was selected, sprayed and aerated over ceramic bricks, checking the results on site. Results obtained by means of these tests confirm that is possible to spray an aerated mortar over vertical surfaces. In addition, this mortar has acceptable thermal insulation properties to be used in chambers of building façades and for external wall thermal insulation systems (EWIS).

Numerical flow simulation of fresh concrete with viscous granular material model and smoothed particle hydrodynamics

The goal of this paper is to develop a numerical approach for fresh concrete's flow, by using the mesh free method, called Smoothed Particle Hydrodynamics (SPH), and a new rheological model named VGM (viscous granular material) model, which can describe the nonlinear, shearing time-dependent, and pressure-dependent characteristics of the flow behaviors of fresh concrete. Moreover, the visco-plastic model proposed by Murata was adopted to treat the slippage resistance of fresh concrete at its flow boundary. Next, the flow behaviors of fresh mortar in the L-flow test were observed experimentally and simulated numerically. As a result, it was found that VGM model is more accurate than Bingham model for simulating the flow of low fluidity mixture, since the former can reflect the change of internal structure of fresh concrete during the flow process.

Comparative characterization of the durability behaviour of textile-reinforced concrete (TRC) under tension and bending

This paper presents the results of an experimental investigation into the durability behaviour of textile-reinforced concrete (TRC) subjected to both tensile and bending loads to compare its macroscale characterization. Composites are produced as a laminate material using 2 and 6 layers of glass fibre mat as reinforcements. The instantaneous and long-term (durability) properties of composite TRC are identified by tensile and flexural testing with specimens made from the same fresh mortar. The long-term tests are conducted in two ageing environments: natural ageing, which was conducted under the laboratory atmosphere (20°C and 50% RH), and accelerated ageing, which consisted of immersion of the specimens in hot water at 50°C. The durability performance of TRC is evaluated by the reduced macroscale performance and damage mechanisms. The effects of the reinforcement ratio, testing conditions, and thermal conditions on the long-term response of TRC are investigated, and the microstructure of both matrix and fibre is observed and discussed after the tests with observations via scanning electron microscopy (SEM). The addition of layers of glass fibre (reinforcement ratio) increased the strength capacity and the first macro-crack strength of TRC in both tensile and bending tests. The ultimate strength value of the TRC composites obtained from the tensile test is lower than that from the bending test over the same tested days as a result of the different redistribution mechanisms; however, the difference between tensile and flexural stress remains approximately equal with similar deviations. An increase in the tensile and flexural strength capacity over the ageing time is observed before 90days of ageing. However, after 90days, the test results showed a loss of ductility of the accelerated aged specimens.

Silicon carbide waste as a source of mixture materials for cement mortar

This paper presents an investigation of the feasibility of recycling silicon carbide waste (SCW) as a source of mixture materials in the production of cement mortar. Mortars with SCW were prepared by replacing different amounts of cement with SCW, and the properties of the resulting mortars, such as the fluidity, strength and shrinkage, were studied in this work. Thermogravimetry-differential scanning calorimetry and scanning electron microscopy were employed to understand the reasons for the property changes of the mortars. The results indicate that SCW decreases the initial and 1-h fluidity of fresh mortar but improves the loss of fluidity. The mortar with SCWexhibits a lower strength at 3 d and 7 d but a higher strength at 28 d and 56 d compared to the control. The shrinkage rate of cement mortar with SCW shows an obvious decrease as the replacement ratio increases. In addition, the content of calcium hydroxide in hardened paste also shows that SCW has some impact on the hydration of the cement-SCW system. The microstructures of the hardened paste also show evidence for a later strength change of mortar containing SCW. This work provides a strategic reference for possibly applying SCW as a mixture material in the production of cement mortar.

New approach to calculate water film thickness and the correlation to the rheology of mortar and concrete containing reactive MgO

This study proposes a new method to calculate the water film thickness of the finest particles in a blend, which is based on the close-packing of equal spheres. A series of fluidity and rheological measurements, including flow test, slump test and rheometer test, is conducted on fresh mortar and concrete blends containing different replacement levels of reactive MgO. The results reveal that: (1) water film thicknesses obtained from the new method are better correlated with fluidity/rheological properties of mortar and concrete, compared to the results calculated based on the particle packing density approach; (2) there are three regressions of correlation between fluidity/rheological properties and the water film thickness of finest particles in blends according to the levels of fine particle replacement (0%–10%, 10%–30% and 30%–100%); and (3) the microstructural change of the fresh blends as the incorporation of MgO is the main cause impacting the fluidity/rheological properties.

Rheological Model of Fresh Concrete in Vibrated State

Vibration is always used in the concrete construction. The rheological behaviors of vibrated fresh concrete are no longer represented by Bingham model. In this paper, the rheological behaviors of fresh mortar and fresh concrete subjected to vibration were investigated by the RSNS rheometer. For making a comparison, the rheological tests of fresh mortar in static state were also performed. The relationship of shear stress and shear strain rate was examined. Then, a model was presented for describing the rheological behaviors of fresh concrete in vibrated state. The effects of water-cement ratio and water content on the parameters of rheological model were discussed.

Mathematical model relating dynamic dewatering process of fresh mortar and its composition

The composition and properties of raw materials have significant influences on water retention capacity of mortar, but the basic origin remains unclear. To clarify the movement of water inside mortar and the mechanism of water retention, the dynamic dewatering experiment was designed and the mathematical model describing the dewatering process of mortar was set up in this work. The model proved to be effective according to the results from dynamic dewatering experiment. The study demonstrated that capillary gradual contraction model is suitable for describing early dewatering process of fresh mortar and the parameters in the dewatering equation are relevant to the properties of raw materials, which certify that the model is of physical significance. Moreover, influencing factors such as water properties and binding materials properties were obtained and their relationship with water retention of mortar were discussed theoretically based on capillary gradual contraction model.

Combined use of waste glass powder and cullet in architectural mortar

The use of 100% waste glass cullet (WGC) as fine aggregates in architectural cement-based mortar had been proven to be feasible in previous works. This paper reports a further study on investigating the influence of using waste glass powder (WGP) as a supplementary cementitious material on the properties of glass-based architectural cement mortars. The experimental results showed a good linear relationship between the particle size of WGP and the flow values of the fresh mortar, revealing that the particle size of WGP played an important role in controlling the workability. For the hydration of white cement, the inclusion of WGP not only affected the second exothermic peak of hydration but also changed the third peak. In particular, the result indicated that the use of finer WGP had an advantage in increasing the flexural strength of the cement mortar when compared with the corresponding compressive strength, which was attributed to the morphological and pozzolanic effect of WGP. In addition, the very fine WGP could act as micro-fibers and micro-aggregates in filling the microstructure of the mortar. At 90 days of curing, the mortar prepared with finer WGP showed a distinct improvement in strength due to the improved interfacial transition zone and the pore-size refinement.

Improving the stability of entrained air in self-compacting concrete by optimizing the mix viscosity and air entraining agent dosage

The purpose of this study is to clarify the role of mixing procedure and air entraining agent (AE) on the entrained volume of fine and coarse air bubbles with the aim of improving the stability of entrained air in self-compacting concrete (SCC). Experiments were conducted in which the air bubbles size distribution of fresh mortars was measured with an air-void analyzer (AVA). Critical size of air bubble was defined as the size below which the bubble volume remains stable as time pass. This critical size, defined in terms of chord length, was found to be 500μm from the correlation between the volume of larger bubbles and the reduction in volume two hours after mixing. A lower mortar viscosity, obtained using a mixing procedure in which water additions were divided, reduced the total volume of both fine and coarse air bubbles. With a higher dosage of AE, a higher volume of fine air bubbles and a lower volume of coarse air bubbles were entrained. An upper limit volume of fine air was defined as the maximum volume of fine air bubbles entrained with a longer mixing time. This upper limit is proportional to the AE dosage multiplied by the funnel speed of the mortar as an index of viscosity.

Numerical and experimental studies of aggregate blocking in mortar extrusion

Numerical and experimental study of aggregate blocking in extrusion of mortar is presented. The studied system consists of a pipe with a nozzle at the tip. Fresh mortar is modeled by composite particles consisting of hard-core representing a sand particle surrounded by soft-shell layer representing interstitial cement paste. For the experimental study, two kinds of mortars are studied. The first kind is a mortar composed of monosized glass beads. This mortar is used for comparison with numerical results. The second kind of mortar is more realistic, polydispersed particles are used by replacing glass beads by standard sand. The effects of the dimension of the bottom nozzle opening, the rheological properties of the cement paste and the paste volume proportion on the blocking of mortar are studied both experimentally and numerically. For mortars composed of monosized beads, experiments and simulations both show that the main parameter governing aggregate blocking is the ratio ‘a’ between the bottom opening diameter of the nozzle and the beads diameter. Rheological properties of the paste and paste volume proportion are less influential. Experimental results obtained on both kinds of mortar confirm the limit ratio ‘a’ identified numerically in all the cases, except for the lower cement paste volume proportion (0.54 in our study) for which ‘a’ has to be slightly increased to avoid blocking.

Effect of Delaying Addition of Heat Stimulated Superplasticizer on Fresh Properties of Mortar

To assure the quality and required workability of mortar or concrete, various types of superplasticizers are used. There are many factors affecting the performance of superplasticizers explicitly, type and dosage of the superplasticizers, type of cement, temperature and mixing procedure, as well as the addition times of the superplasticizers. Some researchers investigated the effect of delaying the addition time of superplasticizer to mortar or concrete, but there is not enough data about the effect of external temperature on performance of superplasticizers and consequent influences on fresh properties of mortar or concrete. In this research the effect of delaying the addition time of superplasticizers and influence of external temperature, namely, heat stimulation of superplasticizers, on fluidity, fresh density and air content of fresh cement mortar was investigated. Two types of Precast and Ready-Mix of Polycarboxylic acid-based ether superplasticizers with Ordinary Portland Cement was used. Delaying the addition time of superplasticizers enhanced the fluidity, slightly decreased the fresh density and increased the air content of mortar in comparison with simultaneous addition time with both heated and non-heated Superplasticizers.

High temperature resistance of self-compacting lightweight mortar incorporating expanded perlite and pumice

This paper presents the effect of aggregate type on high temperature resistance of self-compacting mortars (SCM) produced with normal and lightweight aggregates like expanded perlite and pumice. Silica fume (SF) and fly ash (FA) were used as mineral additives. Totally 13 different mixtures were designed according to the aggregate rates. Mini slump flow, mini V-funnel and viscometer tests were carried out on the fresh mortar. On the other hand, bulk density, porosity, water absorption and high temperature tests were made on the hardened SCM. After being heated to temperatures of 300, 600 and 900oC, respectively, the tensile strength in bending and compressive strength of mortars determined. As a result of the experiments, the increase in the use of lightweight aggregate increased total water absorption and porosity of mortars. It is observed that, the increment in the usage of lightweight aggregate decreased tensile strength in bending and compressive strengths of mortar specimens exposed to high temperatures but the usage of up to 10% expanded perlite in mortar increased the compressive strength of specimens exposed to 300oC.