Plastic Viscosity Of Cement Paste Research Articles

Influence of SiO2@CaAl-LDH core-shell nanocomposite on rheology and hydration of cement

Nano-SiO2@ Layered double hydroxide (LDH) core-shell nanocomposite shows a promising application as high-efficiency chloride adsorbent in cement-based materials. However, its effect on rheology and hydration of cement is not yet known. In this study, the SiO2@CaAl- LDH core-shell nanocomposite by growing the LDH on nano-SiO2 surface was firstly created using in-situ co-precipitation method. The influence of as-synthesized nanocomposite on the rheology and hydration of cement was systematacially researched through contrasting experiments. Besides, microstructures of SiO2@LDH and its cement paste were characterized. The results show that the LDH with petal-like shape on the nano-SiO2 surface exhibits a higher specific area than pure nano-SiO2 and LDH alone. The addition of SiO2@CaAl-LDH core-shell material significantly shortens the initial and final setting time of cement and increases cement hydration heat generation rate and the total exothermic amount for 72 h. Contrary to the mixture of LDH, the additions of nano-SiO2 and SiO2@LDH increases the yield stress and plastic viscosity of cement paste. Compared to the additions of LDH and SiO2, the addition of SiO2@LDH more significantly enhances the strength of cement, of which the highest increasing level reaches 66 % at the early age of 1 d. In addition, SiO2@LDH exhibits the best refinement effect on the pore structure of cement because of the synergetic effect between the LDH and nano-SiO2.

Influences of CO2 Absorption under Ultrasonic Vibration and Water-Reducer Addition on the Rheological Properties of Cement Paste

Research shows that ultrasonic vibratory stirring can effectively increase CO2 absorption by cement slurry. However, with the increase in CO2 absorption, the fluidity of slurry begins to decrease. Adding water reducer to fresh cement paste can improve its fluidity. In order to reveal the influences of ultrasonic vibration and water-reducer addition on the rheological properties of cement pastes after absorbing various amounts of CO2, changes in the rheological properties of yield stress and plastic viscosity (PV) were analysed. The results show that ultrasonic vibration can effectively increase the shear stress and PV of cement paste. Moreover, shear stress and PV are positively related to the CO2 absorption amount. Meanwhile, a new rheological model of cement paste carbonated under ultrasonic vibration was established based on the basic principles of rheology. Microstructural changes in cement paste before and after water-reducer addition were observed by scanning electron microscopy (SEM). A microrheological model of cement paste carbonated under ultrasonic vibration and with water reducer added was constructed. It describes the influencing mechanisms of ultrasonic vibration and water-reducer addition on the rheological properties of carbonated cement paste. Next, a molecular model was constructed in which CO2 was added into a C-S-H gel. Changes in intermolecular repulsion in the CO2+ C-S-H gel structure and in the CO2− water-reducer molecular structure were analysed. Finally, the rheological mechanism was further analysed in terms of the dispersion effect of the C-S-H gel. The results will play a major role in improving the fluidity of cement paste.

Rheological Properties and Flow Behaviour of Cement-Based Materials Modified by Carbon Nanotubes and Plasticising Admixtures

In this study, the rheological properties of cement paste modified by a suspension containing both multi-walled carbon nanotubes (MWCNT) and carboxymethyl cellulose (CMC) (MWCNT/CMC suspension) with different types of plasticising admixtures (Pl), such as lignosulphonate (LS), sulfonated naphthalene formaldehyde condensate (NF), and polycarboxylate ether (PCE) were evaluated. The increase in yield stress and plastic viscosity up to 20% was established in the case of the modification of cement-based mixtures by MWCNT in the dosage up to 0.24% by weight of cement (bwoc) without Pl and with LS and NF. The complex modification of cement paste by MWCNT and PCE increases the yield stress and plastic viscosity from the MWCNT dosage of 0.06% and 0.015% bwoc, respectively. The yield stress and plastic viscosity of cement paste with PCE enhanced by 265% and 107%, respectively, in a MWCNT dosage of 0.12% bwoc. MWCNT do not have a significant influence on the flow behaviour index of cement paste; however, in the case of usage of PCE, the shear thickening effect decreased from a MWCNT dosage of 0.03% bwoc. The significant reduction in the volume coefficient of water bleeding by 99, 100, and 83% was obtained with LS, NF, and PCE, respectively, with an increase in MWCNT dosage up to 0.24% bwoc.

Effect of aliphatic-based superplasticizer on rheological performance of cement paste plasticized by polycarboxylate superplasticizer

Polycarboxylate superplasticizer (PCE) and aliphatic superplasticizer (AS) are widely used water-reducing agent in concrete. In actual practice, the addition of AS has been developed as a common way to alleviate the bleeding and segregation of concrete plasticized by PCE, while the main reason for this phenomenon is not clear. In this research, the effect of AS on dispersion of PCE was investigated. Adsorption behavior of AS and PCE and conformation of these polymers in liquid phase were analyzed with total organic carbon (TOC), conductivity, and dynamic light scattering (DLS). Finally, the experimental results were verified by molecular dynamics simulation, and the dispersion model was proposed to better understand the mechanism behind. The results showed that the effects of AS on the rheological performance of cement paste containing PCE was closely related to the added dosage, and it was observed that three stages were divided according to the dosage of AS. The main reason was due to competitive adsorption between AS and PCE. In addition, PCE molecules, which existed in the liquid phase due to competitive adsorption, would be crosslinked through calcium ions in pore solution, leading to the increase in plastic viscosity of cement paste despite with the decreased yield stress. These results explained why the addition of AS in PCE system could enhance the resistance to bleeding and segregation. These results would offer usefully experience for the application of multiple superplasticizer systems in practical engineering.

THE INFLUENCE OF PLASTICISING ADMIXTURES ON DRYING SHRINKAGE OF CEMENTITIOUS COMPOSITES

This article presents a research into the impact of plasticising admixtures on drying shrinkage of cementitious composites by evaluating the effectiveness of plasticising. Materials used in the study: Portland cement CEM I 42.5 R, plasticiser LS (modified lignosulphonates based), superplasticiser PCE (synthetic polycarboxylate esters based), superplasticiser MAP (modified acrylic polymers based), sand and water. The percentage change of plastic viscosity of cement paste indicates the effectiveness of plasticising of cementitious composites. The change of plastic viscosity has been tested by adding a specific amount of plasticising admixtures. The effectiveness of plasticising of pastes were tested using rotational viscometer. Chemical admixtures content in pastes varied from 0 to 1.2%. Drying shrinkage was tested based on length change of the specimens with fine aggregates. The method to minimise drying shrinkage of composites was established through use of plasticising admixtures. It has been found that by increasing plasticising admixture dosage from 0 to 1.2% the following changes occur: LS plasticising effectiveness increases by 35% while drying shrinkage decreases by 8%; MAP plasticising effectiveness increases by 70% while drying shrinkage decreases by 16%; PCE plasticising effectiveness increases by 80% while drying shrinkage decreases by 20%.

Effect of fly ash on rheological properties of graphene oxide cement paste

While graphene oxide (GO) is found to have a potential to enhance the strength and toughness of cement based composites, it may reduce the cement paste fluidity and thus the workability of concrete. In this paper, a simple and economical additive, fly ash, is investigated to improve the rheological properties of cement paste when GO is present. Based on the quantitative analysis of the rheological parameters, it’s shown that fly ash can offset the reduction of fluidity by GO. The effect of fly ash was studied with two dosages of GO, 0.01wt% and 0.03wt%. The yield stress and plastic viscosity of cement paste decreased with the increase of fly ash. At 0.01wt% of GO and 20wt% of fly ash, the yield stress of the paste decreased 85.81% and the plastic viscosity decreased 29.53% in comparison to the control sample (no fly ash or GO). At 0.03wt% of GO and 20wt% of fly ash, the yield stress of the paste is 50.33% lower and the plastic viscosity decreased slightly by 5.58%.The hysteresis area of the composite paste also decreased with the increase of fly ash. Meanwhile, the results indicated a good correlation between the fluidity and the plastic viscosity. The “ball” effect, grain size gradation and less water demand of fly ash can play an important role in improving the fluidity of the GO-cement systems. Moreover, GO can offset the delay in early-stage strength gain of fly ash-cement systems. When the dosage of fly ash is less than 15wt%, the compressive and flexural strength of fly ash-GO-cement composites are all higher than the control sample at 3, 7, and 28d. This indicates that the addition of fly ash is an economical and effective method to obtain desirable properties of GO-cement paste. The benefits of mixing fly ash and GO can help counteract the fluidity problems of the GO-cement paste on the workability of concrete.

고성능 감수제 종류에 따른 실리카흄이 혼입된 시멘트 수화 특성

In this study, the effect of superplasticizers on the dispersibility and hydration properties of cement with silica fume were investigated. Superplasticizers are used Naphthalene type, Ligno-sulfate type and polycarboxylate type. Compared with plastic viscosity of cement paste without superplasticizers(Plain), plastic viscosity of cement paste with superplasticizers are reduced and yield stress of cement pastes with each superplasticizer is increased rather than Plain. And then, plastic viscosity of cement paste with Naphthalene type superplasticizer was the least of all types. Compared with other two types, setting and hydration of cement paste with Polycarboxylate type(PC) superplasticizer is very fast. At compressive strength after 3-day, PC is higher than other two types, But, compressive strength of after 7-day and 28-day, PC is smaller than other two types.

Genetic algorithm rheological equations for cement paste

The rheological behaviour of cement pastes incorporating superplasticisers is influenced by the ambient temperature and mixing time; which is critical for hot weather concreting. Rheological parameters including yield stress and plastic viscosity of cement pastes with a water/cement ratio of 0·38 and incorporating polycarboxylate-, melamine sulfonate-, and naphthalene sulfonate-based superplasticisers were measured as a function of the superplasticiser dosage and temperature (22–45°C) over 20 to 110 min from the time of mixing, with 30 min between successive measurements. The rheological tests were conducted using an advanced shear-stress/shear-strain controlled rheometer. Test results indicate that the yield stress and plastic viscosity of cement paste vary in a linear fashion with the elapsed time, whereas their variations with the temperature and superplasticiser dosage follow power and inverse power functions, respectively. In this study, curves showing shear stress plotted against shear rate for cement pastes incorporating the various superplasticisers at different ambient temperatures and mixing times were predicted using genetic algorithms. Rheological equations were then derived from these curves using the Bingham model and employed to predict the yield stress and plastic viscosity of cement pastes. A sensitivity study was performed to evaluate the effects of mixing time, ambient temperature, and superplasticiser dosage on the calculated yield stress and plastic viscosity. It was shown that the computed yield stress and plastic viscosity values compared well with corresponding experimental data

Investigations about the influence of fine additives on the viscosity of cement paste for self-compacting concrete

In this work, the effect of fine additives (limestone, silica fume, fly ash, pozzolan, nano-silica fume) on the plastic viscosity of cement paste is being investigated. Towards this direction, twenty-four samples were designed and produced. Those pastes consisted mainly of cement (type CEM I 42.5) and specific proportions of one or two fine additives. Plastic viscosity and yield stress were measured, as well as micro-structure of selected 28-days hardened samples was studied through means of mercury porosimetry. Results showed that limestone (40%) can improve the rheological behavior of cement pastes, and the synergy of limestone (20%) and fly ash (20%) can lead to higher packing density. Cement pastes that combine those two characteristics, could serve as the base for self-compacting concrete (SCC) production.

초미분말 고로슬래그를 혼합한 시멘트 페이스트의 유동특성

Rheological properties of cement paste containing ultrafine blastfurnace slag (UBS, 9600 ㎠ /g) were investigated by mini-slump test, pH meter, conduction calorimeter and coaxial cylinder viscometer. In order to improve rheological properties of the cement paste, granulated blastfurnace slag (GBS, 3500㎠ /g) and polycarboxylate type superplasticizer (PC) were also used in this experiment. The fluidity of cement paste containing UBS was decreased. The yield stress and plastic viscosity of cement paste was increased with increasing UBS. But the rheological properties were improved when GBS and PC were added to UBS blended cement paste. In the relationship between the yield stress and the plastic viscosity or the mini-slump value, the yield stress of the cement paste was proportional to the plastic viscosity of it. However the cement paste mini-slump value was in inverse proportional to the yield stress.