Effects Of Low Temperature And Admixtures On Rheological Behavior Of Cement-based Materials

For the vast northern region in China,many engineering projects are facing the severe challenges from winter construction.When preparing cement-based materials in cold environment,chemical admixtures such as water-reducing,air-entraining and anti-freezing agents are often employed to improve early-age performance and longterm durability of materials.However,the variation of temperature usually disturbs the effects of chemical admixtures,and the rheological performance and workability response of cement-based materials is not still clear under the coupled effect of low temperature environment and admixtures.This study carried out some systematical experimental investigations on rheological behavior of cement-based materials with various admixtures at low temperature.The influential mechanism of temperature on the interaction of admixtures with different molecules to cement particles,and on the rheological parameters of materials was clarified,through characterization of paste microstructure and analysis of pore solution.The low-temperature fresh properties of flowable concrete was also analyzed.The conclusions will provide theoretical support for scientific and reasonable preparation of cement-based materials in cold areas and early-age performance control of modern concrete at various temperatures.Time-dependent rheological properties of cement-based materials plasticized by naphthalene sulfonate formaldehyde(NSF),sulfonated acetone formaldehyde(SAF)and slump-retaining type polycarboxylate(PCE)adxmitures were investigated at 0～30℃.It is found that the decrease of temperature leads to the increase of initial plastic viscosity,decrease of yield stress,the enhancement of thixotropy and the weakening of shear thickening characteristics.The rheology of mortar containing PCE is greatly affected by temperature variation,but the fresh mortar with NSF or SAF is much less sensitive to temperature.The lower temperature decelerates the yield stress evolution of mortar with PCE,while the influence of ambient temperature on the evolution of dynamic yield stress of mortar with NSF or SAF presents complicated laws.Time-dependent rheological properties of cement-based materials plasticized by PCE with different side chain densities was investigated at 0～30℃.It is found that with the decrease of temperature,the adsorption capacity of PCE on cement surface decreases and the adsorption rate slows down;while the binding ability between PCE and calcium ions increases,which can play a compensation effect on adsorption loss.At low temperature,more residual non-adsorbed polymers exists in the pore solution of paste with high side-chain density PCE,which leads to higher plastic viscosity and thixotropic features.The early-age yield stress of mortar with high side-chain density PCE is almost equal under 0～30℃,while the early-age yield stress of mortar with low side-chain density PCE decreases sharply with the lower temperature.The coupled effects of air-entraining agents(AEAs)with different ionic groups and ambient temperature on the rheological parameters and air-void system of mortar were explored.It is found that the influence of bubbles on rheology of cement mortar depends on the interaction of AEA with cement.If the interaction is strong,bubbles play a bridging role and increase yield stress;if the interaction is weak,bubbles exert the ball lubrication effect and reduce yield stress.At lower temperature,the average chord length and spacing factor of voids increase,the adsorption capacity of anionic AEA together with the air content retention decreases,while the adsorption capacity of cationic AEA along with air content increases.When AEA and PCE are compound,the size of bubbles significantly increases,air-entrained mortar shows obvious shearthinning behavior,and its plastic viscosity become substantially lower.The rheological response of cement-based materials that contains single organic or inorganic anti-freezing component or combined with other admixtures,along with relevant mechanisms,are explored under low temperature and sub-zero temperature environments.It is found that calcium nitrite(CN)changes the early-age rheological properties of fresh mortar significantly and the variation laws are complicated,while ethylene glycol(EG)has less influence on rheological parameters.CN possesses poor adaptability to NSF or SAF,which leads to a much worse plasticizing effect if they are mixed.PCE has better tolerance to low dosage CN,but its function will also be substantially weakened if the CN dosage is higher.PCE with low side chain density provides a better low-temperature flowability of mixture with CN.For air-entrained mortar,the air content is greatly reduced after adding CN,and the number of small bubbles increases after adding EG,and both of them increase the yield stress.To solve the problem of poor bubbles stability in air-entrained concrete material in practical application,a novel idea that utilization of nanoclay(NC)with a small addition as a thixotropy-modifying agent to enhance air-void structure is proposed.It is found that the improvement effect of NC on thixotropic features of slurry is more pronounced at low temperature than that at room temperature.Adding 0.5%NC can greatly increase the static yield stress and thixotropic loop area.NC can effectively promote the rapid structural build-up of flocculations in low-temperature slurry and thus significantly improve the stability of bubbles.After adding 1%～2% NC into airentrained low-temperature hardening mortar,the spacing factor and average chord are greatly reduced by 21%～30% and 34～43%,respectively.In view of the general applications of pumping concrete and self-compacting concrete(SCC)in modern civil engineering,a two-phase discrete element method(DEM)model is established to characterize the flow behavior of the mixture.Based on the model,the physical method of optimizing multi-scale particles distribution is adopted to improve the workability of flowable concrete,and the regulation criteria are proposed.The fresh performance response of concrete under low temperature was also analyzed.It is found that the decrease of temperature leads to a small increase in flow spread,a substantial reduction in flow speed,and an improvement in workability retainability.For vibration operation,the lower temperature expands the liquefaction area,while the segregation degree induced by single vibration in vertical direction is reduced with lower temperature.