Study On Synthetical Calcium Silicate Hydrate And Its Improvement Of Micromechanical Properties Of Cement-based Materials

The product of ordinary Portland cement has a calcium silicate（referred to as C-S-H）,while C-S-H is a main contributing product of cement strength.Owing to complicacy of cement hydration and difficulty of separation of hydration products,the study on the strength contribution of C-S-H to cement materials has some limitations.The calcium hydrated silicate with high synthetic purity is helpful in studying its structure,composition,and micro-morphology,which has further awareness and understanding of C-S-H.This study mainly focuses on the synthesis of C-S-H,study on the factors affecting the composition and micro-morphology of C-S-H,analysis of nano-scale mechanical properties of synthetic C-S-H on cement materials.This paper controls the reaction temperature,calcium silicon ratio,reaction time,etc.Different types of hydrated calcium silicic were prepared by hydrothermal method using calcium oxide as calcareous material and gaseous silica as siliceous material.The prepared product was tested by XRD,analyzed composition structure of the product and whether the material was completely reacted;the microstructure of synthetic hydrated silicic was observed by SEM and investigate the morphology of different types C-S-H under different reaction conditions.It is fully understood that the composition,structure,and morphology of C-S-H can further recognize the effect of hydrated silicic acid in cement-based materials.The hydrated calcium silicate,which was prepared by reaction at120℃with a Ca/Si ratio of 1:1.5 for 6h,was mixed into cement-based materials.The influence of the C-S-H on the early micromechanical properties（M and H）of cement was studied by nano indentation test.（1）When the reaction time is extended from 3 h to 6 h,it is mainly C-S-H（Ⅰ）gel phase,and when extended to 9 h and 12 h,the gel phase of the product is transformed into tobermorite and xonotlite;with the extension of time,the C-S-H gel phase gradually transformed into tobermorite and xonotlite with good crystallization,and the raw materials also reacted completely.（2）As the temperature rises,different types of hydrated calcium silicate transform each other.When the temperature is 100℃and 120℃,the morphology of the product is flocculent and the crystallinity is relatively dispersed.When the temperature is increased to 160℃and180℃,the morphology of the product is obviously changed,showing the shape of coiling-plate,and the products are tobermorite and xonotlite.Elevated with initial Ca/Si,the surface of product becomes loose from dense by degrees,and the micro-morphology transitions from flocculent to irregular particles gradually,and the surface pores increase;When the initial calcium-silicon ratio is low,the calcium material Si O₂remains.（3）Synthetic C-S-H can enhance early nano-scale mechanical strength of cement materials,and the mechanical properties of 1day and 3days are enhanced obviously;C-S-H can also promote cement hydration and fill the pores in cement paste;with the increase of age,the volume fraction of LD C-S-H gel increases greatly,and the volume fraction of HD C-S-H gel also increases.（4）Hydration products mainly spread slowly around unhydrated cement,calcium hydroxide is distributed around unhydrated cement particles.the unhydrated cement particles,high density C-S-H is distributed around Ca（OH）₂,and low density C-S-H gel is distributed at the outermost of hydration products.LD C-S-H and HD C-S-H gels increase greatly,and unhydrated cement particles decrease greatly.