Research On Microstructure And Property Of Recyclable Expired Cementitious Material

Construction concrete waste recycling is a significant approach to fulfill the four promise of China in The Paris Agreement.Expired cementitious material,as kind of construction waste suffered vapor hydration and air carbonation,is in the lack of research consideration.To save this high energy cost produced material,this paper will study its original characteristics deeply and aim to utilize this expired cementitious material combining with another two supplementary wastes by referring the designing method of ultra-high performance concrete.And this combined cementitious material will be tested in detail,including the viscosity and setting time,fresh and hardened paste microstructure development,mult iple shrinkages during hydration process,simulation of porous structure and the relationship between its pore volumetric reductions with shrinkages.Finally this material will be trialed in laboratory as a concrete surface reinforcement to resist the chloride ions invasion.In first,the expired cement was tested with laser particle analyzer,X ray diffraction meter,X ray fluorescence spectrometer,thermal gravimetric analysis and scanning electron microscopy.The following results were obtained including expired cement had two forms of agglomerated blocks and granular particles,after simple grinding and filtering both forms could be combined and used together,and this combination had a larger amount of small particles with diameter less than12.476?m compared with ordinary cement;four kinds of main clinker minerals still existed but with weaken crystals in this cement,and the calcite crystal peaks were obvious which corresponds to the carbonating reaction products;according to the elements oxides mass percentage,Bogue empirical equation and thermal mass loss,the main clinkers percentages were obtained with 76.3%,and notably that expired cement got 12.68% of calcium carbonate;micrometer and nanometer scale calcium carbonate particles were tested respectively,and the difference between two mass loss vs.temperature lines proved that the carbonate in expired cement was nanometer which was covering on the particles surface and bonding with hydration produced gels.After this expired cement material was mixed with water and super-plasticizer as paste,the surface tension meter and viscometer were used to measure the relevant properties in paste and different super-plasticizer solutions.The results showed that when the water to cement ratio was lower than 0.2,the expired cement in paste would adsorb about 3wt% super-plasticizer first before gained the similar viscosity level of ordinary cement paste.After the surface adsorption was fulfilled,when water to cement ratio lower than 0.2,the viscosity of expired cement paste and ordinary cement paste would controlled by viscosity concentrations which was different with paste viscosity controlled by water percentage when water to cement ratio bigger than 0.3.And the contact angle test was used to measure the super-plasticizer coating angle on cementitious material,and 19° coating layer would become a gel like coverage which would significantly retard the cement setting time.Referring on the close packing and filling effect designing method,modified Funk and Dinger model was used to combine the recycled expired cement with another two supplementary waste(silica fume and slag)in the closest fi lling situation.Several groups were prepared and cured for microstructure analysis(including mercury porosimetry,nitrogen adsorbing and nuclear magnetic resonance)to verify the feasibility of pre-designed filling effect and super-plasticizer modification on the combined recycle of cementitious waste.Then the following results were observed that: expired cement had the same hydrating reaction as ordinary cement and the products could also fill partially the porous space,while combined with another two supplementary wastes could highly enhance the expired cement filling effect through volcanic reaction;close packing design together with super-plasticizer could decrease the total porosity of hardened paste further which had the minimum as 2% after 28 days curing;the hysteresis loop of nitrogen adsorbing line could distinguish the pores as lamellar structure and cylinder structure,both of which helped create the lamellar & cylindrical porous model,and the pore volume differences within 6nm-11 nm and 11nm-50 nm helped explain the small particles filling effect which also confirmed through scanning electron microscopy pictures.When the expired cement paste was practically used,the nuclear magnetic resonance test presented the internal liquid migration phenomenon in the paste without super-plasticizer,while this phenomenon was not happened to the groups mixing with super-plasticizer.And the liquid migration obviously proved the hydration process and porous structure liquid storing situation.With super-plasticizer viscosity modification,the paste would catch part of air inside during mixing and these air pores would keep original diameter in high viscosity ordinary cement paste but agglomerate partially in expired cement paste with relatively low viscosity.To further verify the close packing design and super-plasticizer modifing effect on dense and stable structure,chemical shrinkage,autogenous shrinkage and dry shrinkage were measured for over 28 days,the results showed that: expired cement paste chemical shrinkage strain was in accordance with hydration degree,and supplementary waste would highly increase dry shrinkage but not autogenous based on the retarded hydration speed and degree,when the super-plasticizer was added the expired cement paste autogenous shrinkage would reduce by 1/3;based on the previous obtained surface tension and contact angle results of water and super-plasticizer on cementitious material,Laplace equation and lamellar &cylindrical porous model were used to estimate the influence of lamellar pores and cylindrical pores on autogenous shrinkage and dry shrinkage.After the dense microstructure and shrinkage reducing property were verified through the above tests,this recycled combining material from expired cement and supplementary wastes was practically applied in laboratory to trial the feasibility in resisting chloride ion invasion on cement concrete surface.The results from splitting tensile strength and chloride ion impermeability proved this recycled material could prolong the lifespan of pavement concrete.As a whole,all consequence proved the recycle feasibility of this valuable high-energy cost cementitious material and it was worth to give this kind of waste more attention in terms of sustainable construction development in 2020 of China.