| Concrete is a composite material composed of cement,auxiliary cementitious materials,aggregate and water.In order to improve the hydration speed and physical properties of concrete,certain humidity and temperature must be provided when curing.With the continuous improvement of concrete performance requirements in the application field,different curing methods have emerged to improve temperature and humidity,and even pressure.The change of curing system leads to the significant change of reaction products and reaction rate of concrete,which significantly affects the microstructure and properties of concrete.With the continuous development of concrete technology and the emergence of new types of concrete,the effects of different curing systems on its microstructure and properties also need to be further studied,especially the most promising ultra-high performance concrete（UHPC）.Based on the National Natural Science Foundation of China General Project“Preparation of lightweight and low-shrinkage ultra-high performance concrete and its microstructure formation mechanism”（51878003）,on the basis of the design and preparation of lightweight UHPC,The effects of standard curing,steam curing and autoclaved curing on the composition and morphology,interfacial transition zone and pore structure of lightweight UHPC hydration products were studied by using29Si NMR,27Al NMR,TG-DSC,SEM-EDS,DSI,CT and low-field nuclear magnetic field testing techniques.The main research achievements of this paper are as follows:The effects of different curing systems on the hydration degree of lightweight UHPC cementing slurry,the composition and morphology of hydration products,the microstructure of C-S-H gel,the content of CH and the composition of hydration products of aluminum phase were investigated.The hydration products of light UHPC under standard curing and steam curing conditions mainly include C-S-H gel,CH,AFt,AFm and TAH,while the hydration products of light UHPC under autoclaved curing conditions mainly include C-S-H gel,CH,AFm TAH and tobermorite.Under different curing systems,with the prolonging of curing age,the hydration degree of the cementified material in lightweight UHPC increased,AFm content increased,CH and TAH content decreased,and the MCL and Al[4]/Si of C-S-H gel increased and decreased with the prolonging of curing age.The relation of CH content in lightweight UHPC is as follows:autoclaved curing<steam curing<standard curing,while the degree of hydration of cementified materials is opposite to that of MCL.Compared with standard curing,the high temperature condition of steam curing promotes the hydration of cementified slurry,which increases the degree of polymerization of C-S-H gel.The high temperature and high pressure conditions of autoclastic curing not only promoted the hydration of cementified slurry,but also promoted the transformation of C-S-H gel to the denser tobermorite.The effects of different curing regimes on the morphology,width and mechanical properties of the lightweight UHPC interface transition zone（ITZ）were revealed.In the three curing systems,the lightweight aggregates in the lightweight UHPC are all closely bound to the matrix without obvious boundary.The width of ITZ increased with the increase of water absorption of light aggregate,and the microhardness increased with the prolonging of curing age.The ITZ microhardness is the highest when the water absorption of light aggregate is 3%,and the microhardness is more stable with the distance from the light aggregate.Compared with standard curing,steam curing and autoclaved curing can increase the ITZ microhardness.Under standard curing conditions,the hydration degree,micro-elastic modulus and hardness of ITZ in lightweight UHPC of prewetting lightweight aggregate are higher than those of non-prewetting lightweight aggregate.With the increase of curing temperature,ITZ hydration was promoted,and more HD C-S-H and UHD C-S-H gels were generated,which made the micro-mechanical properties of ITZ in light UHPC under steam curing and autoclavic curing better than standard curing.The pore structure characteristics of lightweight UHPC under standard curing,steam curing and autoclaved curing conditions were characterized by different pore structure measurement methods,and the influence rule and action mechanism of three curing systems on the pore structure of lightweight UHPC were investigated.Under the three curing systems,the initial adsorption coefficient of light UHPC water adsorption firstly increased and then decreased with the increase of the water absorption rate of light aggregate.The law of the secondary adsorption coefficient was consistent with the initial adsorption coefficient in standard curing and autoclaves curing,while the steam curing resulted in the coarsing of the pores inside UHPC,making the secondary adsorption coefficient increase gradually.The porosity of light UHPC increases with the increase of water absorption of light aggregate under the three curing regimes.The porosity and gel hole diameter of light UHPC were the largest under steam curing,and the standard curing and autoclaved curing were basically the same.Compared with standard curing,C-S-H gel generated by steam curing has fewer interlayer pores and more compact,which reduces the volume of hydration products,and then leads to the coarsening of lightweight UHPC pores,the increase of harmful pores,the increase of porosity,and the decrease of compressive strength.Compared with steam curing,autoclaves curing can transform part of C-S-H gel into tobermorite and fill the pores,thus reducing the porosity,refining the gel pores,reducing the number of harmful pores and improving the overall mechanical properties of lightweight UHPC. |
