| Some parts of southwest China have environmental characteristics such as high altitude,low atmospheric pressure,large temperature difference between day and night,and the insufficient gas content of concrete leads to poor working performance,which can easily lead to the problem of frost durability of concrete.At the same time,under the background of our country’s "peak carbon dioxide emission and carbon neutrality" strategy,the adjustment of energy structure makes the resources of fly ash admixture scarce,and it is imperative to replace fly ash with stone powder industrial waste.At present,in view of the two major problems of high altitude construction conditions and the uncertainty of mineral admixture composition,it is urgent to develop a concrete admixture that can adapt to low atmospheric pressure environment and stone powder admixture system.Polycarboxylate superplasticizer(PCE),as the third-generation high-performance superplasticizer,has the characteristics of strong molecular structure design,high water-reducing rate and good adaptability to cement,and has been widely used in concrete systems.However,the traditional polycarboxylate superplasticizers are highly sensitive to stone powder,and their air-entraining performance decreases in high-altitude areas,which cannot guarantee the good working performance of concrete.Therefore,how to optimize the molecular structure of the modified polycarboxylate superplasticizer and improve its application performance in the mortar containing stone powder under complex environment has become an urgent problem to be solved.In this paper,vinyl glycol ether polyoxyethylene ether(EPEG)monomer and acrylic acid monomer are used as the basic units of side chain and backbone chain,and the solution polymerization process of the two monomers is studied in detail.On this basis,a series of vinyl ether-type polycarboxylate superplasticizers modified with different side chain lengths,carboxyl density and AMPS functional groups were synthesized.Through the experimental studies on the fluidity,rheological properties,adsorption behavior of PCE molecules,cement hydration process and mechanical properties of cement mortar,the structure-activity relationship between the molecular structure characteristics of EPEG-PCE and its performance was systematically expounded.In order to apply the synthesized EPEG-PCE to high-altitude engineering,the adaptability of PCE with different structures to stone powder and fly ash in cement mortar,and the effect of ambient atmospheric pressure on the foam performance of polycarboxylate water-reducing agent and air-entraining agent solutions were discussed.The foaming performance of PCEs and the change rule of the air content and pore structure in the mortar under different atmospheric pressures were summarized.Firstly,the process of binary copolymerization of vinyl ether macromonomer EPEG and acrylic acid(AA)monomer was studied.The structure and molecular weight of the copolymer were characterized by infrared spectroscopy(FT-IR)and gel permeation chromatography(GPC).The effects of monomer molar ratio,feeding method and temperature of reaction system on the conversion rate,molecular weight and distribution of monomers during EPEG-AA binary copolymerization were studied.The results show that,compared with TPEG monomer,EPEG monomer has higher reactivity,and EPEG-AA binary copolymerization can rapidly polymerize at room temperature.During the chain growth stage,the instantaneous conversion rate of EPEG-AA increases rapidly.The low molecular weight EPEG and AA were polymerized under the condition of high acid-ether ratio,showing higher reactivity.Secondly,a series of EPEG-PCE with different structures were added to cement mortar to investigate the effects of side chain length,carboxyl density of main chain and concentration of functional group AMPS on fluidity and time-loss of cement mortar,rheological behavior of mortar and adsorption behavior of PCE on the surface of cement particles.The change of surface tension of PCE solution and the hydrodynamic radius in cement pore solution were analyzed in order to reveal the structure-activity relationship and mechanism of EPEG-PCE in mortar.The experimental results show that the initial dispersion of short side chain PCE in cement mortar is better,while the dispersion retention of long side chain PCE is better,which can effectively reduce the plastic viscosity of mortar.With the increase of carboxyl density,the adsorption capacity on the surface of cement particles increases,the slurry fluidity first increases and then decreases,and the slurry shear stress first decreases and then increases.The results showed that the density of carboxylic acid group was the main structural factor affecting the adsorption capacity and viscosity of mortar.The introduction of 2-acrylamide-2methylpropanesulfonic acid monomer increases the adsorption capacity of PCE on the surface of cement particles,and the flow retention of mortar is enhanced.In cement pore solution,the hydromechanical radius of PCEs with short side chain is larger,and the carboxyl group of the long side chain PCE complex with Ca2+and curled up on the surface of cement particles,which is one of the reasons leading to its low initial dispersion performance.At the same time,the influence of molecular structure of EPEG-PCE on hydration heat release,microstructure and phase composition of hydration products,as well as the compressive strength of cement mortar material after the hydration process was terminated was systematically studied.By summarizing the experimental data,the internal relationship between the molecular structure of PCE and cement hydration reaction and mechanical properties of materials was found,and the law of the molecular structure of PCE and the microstructure evolution of cement mortar hydration process was summarized.The results show that the addition of EPEG-PCE can significantly delay the hydration reaction in the initial stage of cement hydration.Compared with short side chain PCE,the hydration rate increased after adding long side chain PCE,but the side chain length and carboxyl density had little effect on the maximum hydration temperature peak of cement mortar.After introducing sulfonic acid group,the temperature peak of cement hydration heat release is the lowest,which is beneficial to reduce the degree of hydration heat release.The compressive strength of mortar increases with the increase of side chain length and the decrease of carboxylic acid density.The addition of sulfonate group can improve the 7d compressive strength of mortar.Furthermore,stone powder composed of two different minerals in Sichuan Province was used as mineral admixture to replace part of cement.The fluidity,rheological properties,hydration process,mechanical properties of EPEG-PCE with different structure in cement mortar containing stone powder and the adsorption behavior of PCE on the surface of cement and stone powder particles were studied.The interaction principle of EPEG-PCE,stone powder and its influence on mortar performance were analyzed.The results show that when the amount of stone powder are 2%and 10%,the fluidity loss can be improved.When the amount of two kinds of stone powder reaches 40%,the mortar basically loses its fluidity.Compared with perovskite powder(GS),mortar containing dolomite powder(LS)has better fluidity and lower plastic viscosity.The mineral phase structure,particle size distribution and hydration activity of stone powder have great influence on the application performance of stone powder.The stone powder-containing mortar mixed with EPEG-PCE has good initial fluidity,while TPEG-PCE can improve the fluidity stability of stone-containing mortar over time and reduce the fluidity loss of 2h.Finally,the effects of high altitude and low air pressure on the entraining performance of different molecular structure superplasticizer and different types of entraining agent solutions were studied,and the changing rules of foaming height and foam half-life of superplasticizer molecular structure solutions were analyzed.At the same time,20%dolomite powder and fly ash were used to replace cement,respectively,to explore the influence of stone powder and fly ash on the fluidity of cement mortar,slurry gas content,pore structure and mechanical properties of hardened slurry.The results show that the number and stability of bubbles at low atmospheric pressure are lower than those at atmospheric pressure.The type of entraining agent has little effect on foaming ability,and the main difference is reflected in foaming stability.EPEG-PCE introduced more small bubbles,the foam half-life is longer than TPEG-PCE,TPEG-PCE solution produced foam diameter is large,the defoamer speed is fast.The foaming ability of the mixture of entrainment agent and superplasticizer is slightly higher than that of pure superplasticizer.The gas content of the mortar containing stone powder is greater than that of the mortar containing fly ash and the fluidity is less than that of the mortar containing fly ash.After the addition of stone powder,the transition pores in the test block decreased,the capillary pores and harmful pores increased,and the early compressive strength decreased.The addition of stone powder and fly ash has little effect on the hydration process of cement slurry.Tpeg-pce and short side chain EPEG-PCE have good adaptability to stone powder under atmospheric pressure.Under low pressure,TPEG-PCE can better adjust the pore diameter and pore distribution of stone-bearing mortar.Long side chain EPEG-PCE can improve the flow performance of cement mortar containing stone powder. |
PDF Full Text Request