Research On The Micro-nano Structure Of Calcium Silicate Hydrates And Its Modification By Organics

Calcium silicate hydrate（C-S-H）is the main component of the hydration product of Portland cement.And its micro-nano structure determines properties of Portland cement.Studying the micro-nano structure and modification methods of C-S-H benefits to understand the rule of developed performance,optimization and create new cementitious materials.This paper studied the early structure evolution of C-S-H,according to the non-classical crystallization theory;and then analyzed the chemical structure of C-S-H by quasi-in situ techniques.The organics were adopted to modify C-S-H structure,and its synthesis method,structure changes and mechanical properties were studied.The toughness and failure mechanism of organic-modified C-S-H were studied by molecular dynamics simulation.The C-S-H intermediates were proposed and then prepared,and the nanostructure of both intermediates and evolved C-S-H were revealed.（1）The early C-S-H formation process follows the non-classical crystallization theory.There are differences in chemical structure between C-S-H precursor and long-cured C-S-H.The silicate chains of the amorphous C-S-H precursor contains the segments which is similar with tobermorite,jennite and xonotlite.The chemical structure evolution from C-S-H precursor to long-cured C-S-H is involving the forming of Q² or Q³ silicate chains.The Ca/Si ratio and temperature control the evolution process.There are resting periods and active periods of silicate chains evolution existing in C-S-H precursors.During the resting periods,the mean chain length（MCL）and polymerization degree varies little;while during the active periods,the MCL and polymerization degree varies considerably.At the early hydration stage of C₃S,amorphous C-S-H precursors forms at regions away from the C₃S surface,while C-S-H precursors containing crystalline aeras forms at regions near the C₃S surface.Dense liquid phase clusters determine the C-S-H precursor types and affect the final C-S-H structure.（2）The micro-nano structure of C-S-H was characterized by quasi-in situ characterization.BSE-Raman can analyze the morphology,composition and chemical structure of C₃S hydration products.AFM-IR can obtain the morphology and chemical structure of C-S-H nano stacked particles.At the nanoscale,the outer C-S-H and inner C-S-H,which formed by C₃S hydration,mixes with calcium hydroxide（CH）and then form a composite phase.There is an inner transition region（ITZ）between the inner C-S-H and the unhydrated C₃S,which chemical structure is similar with C₃S.The semi-quantitative analysis,which involving the relative content of tobermorite-like,jennite-like chemical structure in the stacked C-S-H nanoparticles,can be obtained by the deconvolution of the IR spectra.Based on the CM-II model,there are Q² tetrahedral chains connecting the adjacent nano-C-S-H particles.（3）PDMS,APTES and pregelatinized starch changed the chemical structure and stacking of C-S-H.PDMS molecules coats on the surfaces of C-S-H,and are also covalently bonds to C-S-H at both ends,which acting as molecular bridges between C-S-H particles.Compared with pure C-S-H,the elasticity modulus of PDMS/C-S-H reduces,and their stacking units are dominated by ULD C-S-H.APTES hydrolyzed molecules are linked to the inorganic silicate chains of C-S-H by Si-O-Si bonds,which cause brick-like C-S-H nanoparticles aligning in the same direction.The brick-like C-S-H nanoparticles are separated by middle layer with a low elastic modulus.The APTES modified C-S-H structure contains UHD C-S-H,which decreases with the increasing of APTES content.The pregelatinized starch is adsorbed between the C-S-H nanoparticles and acts as a binder.The Ca/Si ratio has an important influence on the nano structure of pregelatinized starch modified C-S-H.When the Ca/Si ratio is 0.83and adding 0.1%pregelatinized starch,the C-S-H particles are rod-shaped at the nano scale with a lower elasticity modulus than pure C-S-H.（4）Both PDMS and APTES are involved in the middle of the C-S-H stacking structure,which causing a higher toughness.The formation and expansion of initial cracks cause failures of C-S-H stacking structures.The initial crack first generates in the middle layer between C-S-H stacking particles,and its expansion path is related to the tensile direction and organics which existing in the middle layer.Both PDMS and APTES molecules affects initial crack expansion path,which causing the failure process needs more energy.The PDMS organic molecular“bridge”existing between the C-S-H stacking particles hinders and branches the initial crack expansion.APTES molecules form hydrogen bonds with water molecules,which indirectly affects the failure process of C-S-H.When stretching,hydrogen bond fractures first and then reforms.（5）Intermediate is an effective phase to regulate C-S-H nanostructure.In the"sol-gel"method,the formation of C-S-H intermediates is related to the polymerization of silico-oxygen tetrahedral chains.The C-S-H intermediate can be divided into organic parts,inorganic parts and hybrid parts.The structure of C-S-H intermediates can be tailoring by changing the Ca/Si ratio.In alkaline solution,the C-S-H intermediate can evolve into C-S-H which is the stacking of tobermorite-like nanocrystals.The crystal structure,chemical bonding,silicon-oxygen tetrahedral chains and morphology of evolved C-S-H can be tailoring by altering the C-S-H intermediate structure.The evolved C-S-H is hollow particles,and the"shell"of the hollow particle is composed of the dense C-S-H which located in the interior and the sheet-like C-S-H which located on the surface.The C-S-H intermediate provides a new method for C-S-H structure modification.