Study On The Regulation And Mechanisms Of Cement Hydration At Negative Temperature

With the increasing demand for extreme environmental engineering in China and the fact that many important projects in the national strategy such as "One Belt And One Road" will span several winter construction periods,it is increasingly important to ensure the safety and reliability of concrete works in the cold environment and improve the key technologies and theoretical basis for winter construction.The early hydration and hardening of cement is the key factor to ensure the performance and quality of concrete in winter construction.How to promote the rapid hydration of cement under negative temperature,ensure the sustainable development of strength,and avoid the occurrence of freezing damage is the difficulty to be overcome.Although thermal insulation is one of the effective and reliable methods,such methods not only consume a lot of manpower and resources,but also consume a lot of energy,which goes against the concept of green environmental protection.Adding inorganic salt early strength agent is an efficient and economical method,which has been widely used in the infrastructure construction.However,with the extension of service time,the disadvantages brought by inorganic salts are gradually exposed,such as steel corrosion,alkali aggregate reaction,inorganic salt precipitation,and structural deterioration.When the durability problems are serious,they will lead to structural damage and security problems.Therefore,it is of great theoretical significance and application value to study a safe and reliable modification method to promote hydration and hardening of Portland cement under negative temperature.Firstly,the freezing and defrosting characteristics of early-age concrete were revealed by temperature,deformation and compressive strength tests.On this basis,it is proposed to describe the defrosting process in terms of defrosting time,minimum waiting time and minimum maturity.In addition,Taguchi-Gray relational analysis method was used to the degree of influence of temperature,specimen size and ice content on the above responses.Finally,a simplified formula of defrosting time based on the parameter analysis was estabilsed according to the parameter analysis,which provided the basis for the test parameters.Secondly,based on the early-age structure theory and thermal dynamics simulation,the calculation method of capillary water state and frozen water content was presented.Based on the calculated results of corresponded particle size range of completely frozen water,partially frozen water and non-frozen water,and frozen water content,the selection basis of pre-curing time and the modification idea of cement-based materials at negative temperature were put forward.Thirdly,using C-S-H seeds and nano SiO2 as modified components,the modification effects of C-S-H seeds and nano SiO2 on Portland cement at-5? was obtained by studying the performance at negative temperature,hydration characteristics,freezing characteristics and microstructure.The addition of accelerator of nucleation sites and the pre-curing of several hours can make the hydration products grow faster at the nucleation sites during the pre-curing period.This structure reduces the freezing point of capillary water at more locations and rapidly consumes the frozen water.On this basis,the hydration rate of the sample under negative temperature is accelerated,and there is enough unfrozen water to provide cement hydration and prevent frost-heaving damage,so as to obtain higher hydration degree and compressive strength under negative temperature.However,compared with nano-SiO2,C-S-H seeds combined with a period of pre-curing can significantly promote the early hydration of cement,reduce the freezing point,reduce the frozen water,and promote the development of strength.Forthly,using CSA as modified components,the modification effects of CSA on Portland cement at-5? was obtained by studying the performance at negative temperature,hydration characteristics,freezing characteristics and microstructure.The combination of CSA content and pre-curing has a great influence on the compressive strength,hydration mechanism,frozen characteristics and microstructure.When without a pre-curing,the initial hydration of large dosage of CSA samples at negative temperature is mainly dominated by the hydration of C4A3S.Needle stick ettringite can set up to structure at early stage,and frozen water was rapidly consumed.When with a short pre-curing,the risk of freezing was greatly reduced due to the combined effect of concentration effect and early structural effect by adding small dosage of CSA.The early hydration of the binder was dominated by the hydration of C4A3S,and the late hydration was dominated by the hydration of C3S.Then fromaiton of large amount of C-S-H make the samples for compact structure and high strength.Finally,using alkali activated GGBS as modified components,the modification effects of alkali activated GGBS on Portland cement at-5? and-20? was obtained by studying the performance at negative temperature,hydration characteristics,freezing characteristics and microstructure.The addition of alkali can significantly reduce the freezing point of cement-based materials under negative temperature,thus providing a basic guarantee for its continuous hydration.Alkali activated GGBS/PC cement-based materials have a more intense hydration reaction in the early stage,and can form hydration products with a higher degree of condensation and a denser matrix under negative temperature,thus improving the strength of cement-based materials under negative temperature.In a word,considering the temperature and the curing conditions of winter construction,scientifically choose C-S-H seeds,nano SiO2,CSA and alkali activated GGBS for modification method can significantly improve the negative temperature performance of Portland cement,which avoid the early frost damage and long-term durability problems,to ensure that the concrete winter construction of green environmental protection,safe and reliable and efficient to implement.