Preparation Of Calcium Silicate (C-S-H) And The Effect On The Cement Hydration Process

With the development of urban infrastructure construction,the construction of prefabricated houses,urban rail transit tunnels and other engineering projects are booming,and such projects with fast construction progress have higher requirements for the strength of precast concrete components.Therefore,how to improve the early strength of prefabricated concrete components is the key to technology.In general,there are mainly three ways to improve the early strength of concrete:First,the use of special cement or increase the amount of cement,but the high cost and high energy consumption makes it limited;Second,using steam curing to improve the early strength of concrete,but the concrete's volume stability and durability are difficult to guarantee,and its high energy consumption is not conducive to environmental protection;Third,the use of early strength agents,but the traditional early strength agents only pay attention to the early strength,ignoring the long-term adverse performance of the concrete.Inorganic salts,such as chlorine salts,sulfates,carbonates,and other early strength agents would severely reduce the durability of concrete.The presence of alkali(potassium,sodium ions)and other early strength agents lead to salt precipitation,excitation of alkali-aggregate reaction and poor suitability of cement;Organic compounds,such as triethanolamine,calcium formate and other early strength agent cause cement adaptability problems.In view of the defects of the conventional early strength agent,it is very necessary to develop a new type of concrete early strength agent.Hydrated calcium silicate(C-S-H)is the most important hydration product of cement-based materials and the most important source of cement stone strength.The growth structure,composition and final morphology of cement hydration product C-S-H in cement have a great influence in early strength of cement.Through the detailed analysis of the hydration mechanism of cement,it is known that the nucleation and crystallization of cement hydration products is one of the main factors that restrict the hydration reaction of cement.Therefore,this paper creatively synthesizes nano-C-S-H sol particles which is a new type of concrete early strength agent.Incorporates C-S-H seed crystals into the cement hydration system to investigate the effect of C-S-H on the cement hydration process.Firstly,the basic theory of crystallization chemistry was used to design the technical route for the synthesis of C-S-H seed crystals.C-S-H was prepared by solution precipitation method and the factors influencing the microstructure of C-S-H were explored by changing dispersant,Ca/Si ratio,reaction time and some positive ions.The particle size and structure of the prepared C-S-H were measured by various methods such as nanometer particle size analyzer,X-ray diffractometer(XRD),Fourier transform infrared spectrometer(FT-IR)and high resolution transmission electron microscope(TEM).From the microscopic analysis of morphology and other aspects,it is concluded that the differences in the microstructure and morphology of C-S-H prepared under different conditions and provide theoretical support for subsequent applications.In this project,C-S-H prepared under different conditions was also incorporated into the cement hydration slurry,and the effect of C-S-H seeds on the cement hydration process was explored.The experimental results show that,compared with the blank cement slurry,cement slurry with C-S-H has stronger early strength,shorter setting time,and accelerated hydration heat release rate.This is due to the fact that C-S-H is incorporated into cement paste as a nucleus to reduce the nucleation barrier of C-S-H,promote the acceleration of the hydration rate and generate more hydration products.Finally,the hardening of concrete hardens and the early strength increases.The DSC-TG analysis of the hydration products of the cement paste shows that the strength and mass loss of the dehydration endothermic valleys(TG curves)of the hydration products C-S-H and AFt in the cement pastes increased,indicating that C-S-H was incorporated into the cement paste making C-S-H gel and other hydration products increase significantly.By observing the micro-morphology of cement hydration products,it can be found that most of the blank cement pastes are flocculation-like substances with a low degree of crystallinity,and cements doped with C-S-H,most of the slurry is dense mesh or needle-like substance,which proves that the incorporation of C-S-H into the cement slurry can promote the formation of C-S-H in the cement,improve the crystallinity of the product,and ultimately improve the early strength of the cement.In addition,the chemical hydration degree of the cement hydration slurry mixed with different types of C-S-H was also calculated in the experiment,and the hydration reaction rate curve was simulated to determine the cement hydration process.Different types of C-S-H samples have different control mechanisms for cement hydration.The hydration reaction process of cement is more in line with the NG-I-D mechanism.The crystallization and nucleation stage(NG)at the initial stage of hydration is the controlling factor.With the increase of the degree of hydration,the reaction is gradually controlled by I or D.However,in the cement hydration reaction with C-S-H,the added C-S-H provides more nuclei,and the nucleation barrier decreases more significantly,so the hydration reaction rate is faster and the reaction proceeds from the crystallization and nucleation stages(NG)and rapidly enter the diffusion(D)phase.