Influence Of Temperature And Humidity Change On Microstructure Formation Of Cement Paste And Its Improvement Mechanism

As an important building material,concrete has been widely used in the infrastructure all over the world.The temperature and humidity change during curing process has a significant impact on the structure formation and mechanical development of cement paste.It's not appropriate to determine the service performance of cement paste and concrete actual environment by standard curing.But current studies are mostly carried out at constant high or low temperature curing.It's quite different from the actual environment.Based on actual environment,this paper selects more typical temperature and humidity change: the summer of South China Sea area and internal mass concrete.Research its effect on the structure and properties of cement.For the adverse effect of temperature and humidity change on structure and properties of cement paste,researching the effect of improving by adding fly ash or nano-carbon material.Main work and conclusions were as follows:Compared with the standard curing,the higher temperature in the summer of South China Sea area and internal mass concrete would accelerate the early cement hydration,improving the early compressive strength,portlandite content and the degree of C-S-H gel polymerization of cement paste.While the above two kind of temperature and humidity change had a negative impact on the development of late compressive strength of cement paste.Besides,it decreased its 28 days compressive strength.The faster of early hydration rate,the stronger of late adverse effects.The main reason was that the uneven distribution of the compact external hydration products formed at early ages would hinder the late transport of moisture and ion.Besides,the internal hydration products formed at early ages would be wrapped around the cement particles,impeding the contact of cement particles and moisture.These were not conducive to the cement hydration at late stage.It made the loose regional structure developing slowly and even more deteriorated.Leading to the increase of porosity and slow develop of compressive strength at late stage.Fly ash could improve thermal damage of microstructure and properties of cement paste caused by the temperature and humidity change of the summer of South China Sea area and internal mass concrete at some degree.The main reason was that the low degree of hydration of fly ash could slow down the formation rate of the overall structure of cement paste.And it provided sufficient time for hydration products to separate from the cement grains and precipitate evenly into the interstitial space,forming a more compact overall structure with the development of age.Compared with porosity of cement paste,incorporating fly ash could decrease the porosity at 28 days.The pozzolanic reaction of fly ash was significantly affected by temperature.The pozzolanic reaction of fly ash was mainly carried out after 7days in the summer of South China Sea area,contributing greater to the later compressive strength growth.While internal mass concrete,the pozzolanic reaction of fly ash was mainly carried out within 7days,contributing greater to the early compressive strength growth.The pozzolanic reaction rate of fly ash slowed after 7day,contributing smaller to the later compressive strength growth.Incorporating graphene oxide(GO)and carbon nanotubes(CNT)could also improve thermal damage of microstructure and properties of cement paste caused by the mentioned two kinds of temperature and humidity change at some degree.And the improvement effect of CNT was better.The main reason was that GO can regulate the morphology of cement hydration products.This made forming rod and the columnar hydration products at dense region and flowerlike hydration products in the loose region.The flowerlike hydration products filled in the pores of cement paste,playing a role of compacting cement matrix structure.Finally it leaded to the decrease of porosity and increase of compressive strength at 28 days.When the CNT dispersed evenly in cement paste,it interspersed in the cement hydration products and acted as a "suture" role.So CNT could make the cement hydration products closely integrated.And CNT filled in the cement matrix,densing cement matrix structure at nanoscale.Finally CNT could decrease the porosity and increase the compressive strength of cement paste at 28 days.