Study On Properties Of Silica Sol Modified Concrete Under Standard Curing And Steam Curing

The resource consumption and carbon emissions brought by the construction industry have always been at the forefront.Therefore,the construction industry needs to be reformed to adapt to the sustainable development strategy.With its high environmental benefits,prefabricated buildings have brought an effective way for the reform of the construction industry.Precast concrete members are the basic components of prefabricated buildings,and steam curing has become the most economical and effective method for the production of prefabricated components.Although steam curing can bring higher early strength,it has certain damage to the later performance.How to alleviate or even eliminate this negative impact has always been the concern of relevant personnel.The development of nanotechnology and nanomaterials has brought great changes to traditional cement-based materials.Nanomaterials can give concrete more excellent performance due to their unique properties.Therefore,nanomaterials can provide a new way to solve the thermal damage caused by steam curing.This paper aims to study the effect of silica sol（nano-silica,NS）on the performance of concrete under standard curing and steam curing,and provide more and more comprehensive theoretical supplements for the study of nano-concrete,steam curing concrete and nano-steam curing concrete.In the process of optimizing the benchmark mix proportion,it is found that 8%stone powder content,45%sand ratio and 0.2wt%superplasticizer can maintain the strength and slump of concrete at an optimal level.The adjustment of stone powder and sand ratio can ensure sufficient paste or mortar to enwrap aggregate and increase the compactness of concrete system.The adjustment of superplasticizer can reduce the agglomeration of cement particles and release free water,ensuring the normal hydration of cement.Mechanical properties are the basic indicators to ensure the normal use of concrete structures.The mechanical properties test results show that under standard curing,NS has a large increase in the early strength of concrete（1-7 d）,and a slight increase in the later strength.Steam curing increased the 1d,3d strength of concrete,but 7d,28d,56d strength is lower than the standard curing.The synergistic effect of NS and steam curing gives concrete higher 1d and3d strength,and the loss of concrete strength in the later period is also alleviated.Durability is related to the service life of concrete structures.Durability test showed that NS increased the water absorption,water permeability and chloride ion permeability of concrete under standard curing.The water absorption decreased by 27.58%-40.5%and 13.88%-35.47%at 28d and 56d,respectively.The water penetration height decreased by 14.34%-39.15%and10.82%-33.77%at 28d and 56d,respectively.The chloride ion migration coefficient decreased by 30.91%-65.45%and 23.40%-68.09%at 28d and 56d,respectively.Steam curing will lead to the increase of water absorption and the decrease of water permeability and chloride ion permeability,but this damage can be alleviated by adding NS.In addition,NS significantly improves the sulfate resistance of concrete.NS can reduce the apparent deterioration degree after corrosion.At the same time,NS endows concrete with higher corrosion resistance coefficient.In particular,the synergistic effect of NS and steam curing endows concrete with higher corrosion resistance than that of concrete only using NS.The development of cement hydration can be understood through phase analysis.XRD results show that NS accelerates the conversion of Ca（OH）₂,C₃S and C₂S into C-S-H,which is particularly obvious in the early stage.Steam curing promoted the consumption of early C₃S and C₂S,thereby increasing the formation of Ca（OH）₂ and C-S-H.The synergistic effect of NS and steam curing consumed the most Ca（OH）₂,C₃S and C₂S in the early stage.Microscopic characterization can further reveal the reasons for the change of macroscopic properties of concrete.SEM images show that NS increases the density,uniformity and continuity of hydration products,and the interface transition zone also shows good bonding.Steam curing destroys the uniformity and continuity of hydration products,and there are many holes and cracks.The synergistic effect of NS and steam curing can restore the dense and uniform state of early hydration products,which is the basis of late hydration products.MIP results showed that NS reduced the total porosity and transformed the pores into’harmless and less harmful pores’.However,steam curing significantly increased the porosity,and most pores were’harmful and more harmful pores’.Adding NS could restore the pores to’harmless and less harmful pores’after steam curing.