Structure Changes Of Calcium Silicate Hydrate After Heating And Its Rehydration Characteristic

Calcium silicate hydrate?C-S-H?gel,as the main hydration product of Ordinary Portland cement,plays a crucial role in the performance of cement-based materials.It has the rehydrate ability after dehydration.C-S-H in cement paste is generally amorphous and in gel form,with a complex structure.This paper aims to study the structural evolution of C-S-H gels with different structural properties during the dehydration process,and the rehydration performance of the dehydration phase.The research work is Based on the National Key Research and Development Plan?No.2017YFB0310000?and the National Natural Science Foundation of China?No.51772226?.In this paper,we synthesized a series CSH samples with initial Ca-Si ratio of 1.0-3.0.After calcination and heat preservation,the dehydrated phase was prepared and then subjected to hydration experiments.Comprehensive use of XRD,FT-IR,²⁹Si NMR,TG-DSC,microcalorimetry and X-ray computed tomography,exploring the dehydration characteristics of C-S-H,the phase changes and structural changes in the dehydration process,and the dehydration characteristics of rehydration phase.The free water and bound water in C-S-H are gradually removed during calcining.Most of the water is lost after 400°C heated,and the layered structure of C-S-H also collapsed.However,the structure of C-S-H in high calcium-silicon ratio samples is more unstable than that of lower,showing in the XRD pattern that the intensity of low angle peak and other peaks had further reduced after 200°C calcination,so the C-S-H is easier to disintegrate.As the calcination temperature increases,new phases are generated.C-S-H with Ca/Si=1.0 was converted into wollastonite,C-S-H with 1.0<Ca/Si<2.0 was converted into wollastonite and?-C₂S,and the higher ratio of Ca/Si had the higher content of?-C₂S.After heated,the C-S-H with Ca/Si?2.0 changed to?-C₂S.Difference in calcium-silicon ratio of C-S-H causes the difference in the polymerization of silicon tetrahedral and the structure of Calcium oxide layer.The polymerization degree of tetrahedron in C-S-H with Ca/Si=1.0 is higher.During the calcination process,tetrahedrons occurs polymerization reaction.The degree of reaction is higher as temperature increases,and tetrahedron single-chain structure is formed finally.The increase of the ratio of calcium to silicon makes the polymerization degree of silica tetrahedron in C-S-H lower,and both polymerization and depolymerization occur during the dehydration process.C-S-H with a high calcium-silicon ratio contains portlandite.After the portlandite is decomposed,free calcium ions could promote the depolymerization of silicon tetrahedrons,then more silicon tetrahedral monomers are generated.The calcium-silicon ratio and calcination temperature both affect the rehydration performance of C-S-H dehydration phase.Dehydration phase of C-S-H with Ca/Si=1.0show low rehydrate activity and low heat of hydration.Dehydrations of C-S-H with high calcium-silicon ratio have better hydration activity,and the 650°C dehydration shows best hydration activity and releases maximum hydration heat and generates a new C-S-H structure.Q⁰ is the component with the highest hydration activity in the dehydration phase,which could be polymerized again to form a new Q¹ and Q²structure after rehydration.