Nucleation Mechanism,Mutiscale Configuration,and Cohesion Of The C-S-H Gel In Cement Hydration

Cement concrete is the most widely used artificial material in the world.The well mechanical and durability is the basis for large-scale construction and long-term safe application of modern infrastructure.The mechanical and durability properties of cement concrete are basically controlled by calcium silicate hydrate（C-S-H）gel,the main hydration product.However,C-S-H gel generally exists in form of amorphous phase in the multiphase cement-based system,which makes it difficult to capture and quantify its formation mechanism and multi-scale structure.In this case,despite the discussion for nearly a century,it is still not completely clear for topic of generation-growth and structure-performance.Our study utilizes molecular dynamics,crystal chemistry,micromechanics and a series of nano-microscopic characterization to study the formation path and growth mechanism of C-S-H grains,the multi-scale structure of C-S-H gel from molecular to microscopic,C-S-H cohesion,and the influence of C-S-H cohesion on the micro-macro durability behavior.The main results are as follows.（1）In cement hydration system,in addition to the classical nucleation-growth model,C-S-H should also follow the large-scale grain self-assembly and amorphous-crystal generation path.A model of nucleation and growth mechanism according to ionic concentration-temperature-growth space is established.Experiments confirm that the C-S-H foils generated by amorphous-crystal path is abundant in the cement hydration system,and also calculates the adsorption and agglomeration behavior of the C-S-H foils driven by the electrostatic interaction between（002）faces.（2）The evolution of atomic arrangement and growth dynamics in the amorphous-crystal path are clarified.It starts from ionic complexes,which transforms rapidly to calcium silicate layers.Then the disordered calcium silicate layers are gradually rearranged into an ordered layered structure,accompanied by polymerization and interlayer compactness.The defects reduce the surface energy and inhibit the growth along the a-axis and c-axis,but promote b-axis growth.The polymerization of silicate chain along the b-axis needs to cross a reaction energy barrier,that is,the formation of five-coordinate silicon.However,silicate chain defects transform b-axis polymerization mechanism into Ca²⁺electrostatic linking,without energy barrier.（3）It quantifies the structure,dynamics,chemical reactions,and mechanical properties of C-S-H molecules subject to decalcification.It is shown by the transformation of C-S-H molecular structure from lamellar crystal into cross-linked glassy structure,transformation from bound water into-OH groups,and transformation of dynamics characteristics from lattice vibration at fixed sites into diffusion,which would be exacerbated by the dissociation of intralayer calcium.The dissolution of each calcium ion is accompanied by the decomposition of 0.6 water molecules.（4）C-S-H cohesion force is quantified based on coarse-grained simulation for the first time.The C-S-H cohesion,the capacity of cracking resistance of bulk C-S-H,depends on inter-grain interaction and packing fraction that evolve continuously during hardening and service.The microstructure and mechanical properties of bulk C-S-H gel can be well quantified.It also interprets the huge gap of mechanical properties between molecular and micro/macro scale,like from single inter-grain interface to packing structure and from local dense packing to porous structure or submicron cracks.（5）The degree of dry shrinkage cracking and sulfate attacking of cement paste are controlled by C-S-H gel cohesion.The X-CT characterization method has been innovated to realize the dynamic in-situ tracking of the crack propagation of sulfate-etched cement-based materials.It quantifies the transition threshold between the pore expansion mode and crack propagation mode based on the cracking stress.It clarifies the regulation direction of inhibiting sulfate attacking.Introducing closed pores can slow down the sulfate attack by interrupting the crack propagation.