Study on the Nano-modification Mechanism of Cement-based Materials

Author:Du Tao

Supervisor:li hui

Database:Doctor

Degree Year:2019

Download:141

Pages:122

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Cement-based materials are currently the most consumed artificial materials which are widely used in the construction of large-scale infrastructure.However,cement-based materials have the disadvantages of large self-weight,low tensile strength and high brittleness,which the cement industry is often accompanied by high energy consumption and high emissions.It is of great scientific and engineering significance to study the performance modification of cement-based materials.The development of nanomaterials and technologies provides new ways for researching and modifying traditional cement-based materials.In fact,the particle size of C–S–H—the main binder phase of cement-based materials,is on the nanometer scale.Due to its unique physical and chemical properties,nanomaterials can effectively improve the macroscopic properties of cement-based materials.However,the internal mechanism of nanomaterial modified cement is still not well understood.This thesis systematically studies the inherent mechanism of nanomaterial modified cement.The main contents are as follows:Effect of nanomaterials on the C–S–H morphology and nanostructure.In order to study the influence of the properties of nanomaterials on the microstructure of C–S–H,four different properties of nanomaterials(zero-dimensional nano-TiO2and nano-SiO2,one-dimensional CNT(Carbon nanotube)and two-dimensional GO(Graphene oxide))were selected to prepare the pure phase C–S–H and nanomaterial modified C–S–H by the metathesis reaction of Na2SiO3·9H2O and Ca(NO3)2·4H2O.Through the microstructural characterization,it is found that the pure phase C–S–H is a sheet-like structure,and with nano-TiO2it looks like a short rod-like structure,and becomes a larger particle cluster after the addition of nano-SiO2.Both CNT and GO will adsorb C–S–H on the surface of the nanomaterial.From the structural point of view,nano-TiO2and GO can induce C-S-H to form nanocrystalline regions on the surface and improve crystallinity of C–S–H.Effect of non-pozzolanic nanomaterials on C3S hydration.Nano-TiO2with different polymorphs were selected to study its influence on the hydration process and product of C3S.The results show that anatase nano-TiO2with higher reactivity can adsorb Ca2+in solution more effectively,thus promoting the dissolution of C3S.Combined with the results of calorimetry,the driving effect of nano-TiO2on promoting C3S hydration was verified.Through phase analysis and electron microscopy observations,it was found that TiO2uniformly distributed in the hydration space can promote the formation of C–H,induce multi-point growth of hydration products with smaller sizes,thus improving the microstructure of hydration products matrix.Molecular dynamics study of silica sol-gel reaction.In order to reveal the mechanism ofnanomaterials on the C–S–H gelation process at the atomic scale,it is necessary to study the growth process and structural evolution of early C–S–H in the nanoenvironment by means of molecular dynamics simulation.Based on the method of reactive molecular dynamics simulation,a simplified version of hydration reaction—the sol-gel reaction process of silica was studied.The sol-gel process simulation method of silica system was determined by comparing different ReaxFF force field parameterizations,reaction temperature,initial system size and precursor concentration.Compared with experi-mental results,it is verified that the method can effectively simulate the polymerization kinetics and structural properties of silica glass.Based on the simulation method,the polymerization mechanism of the sol-gel reaction of silica was revealed.Molecular dynamics studies of the early C–S–H generation process.Based on the silica-based simulation method,a reactive molecular dynamics simulation method for the early-age C–S–H precipitaion process was proposed.The formation of C–S–H in different initial compositions was investigated by constructing CaO–SiO2–H2O systems with different Ca/Si ratios(0.0,0.5,1.0,1.5,1.8,2.0 and 2.2).The simulation results show that the degree of polymerization of the early C–S–H decreases with the increase of the Ca/Si ratio.When the Ca/Si ratio is≥1.0,the Qnstructure of early-age C–S–H structure is consistent with experimental characterizations.In addition,the kinetics of different C–S–H formation processes were studied.Combined with the formation enthalpy of C–S–H after precipitation,it was revealed that the average stoichiometry of the C–S–H gel forming upon the hydration of ordinary portland cement(i.e.,Ca/Si=1.75)is determined by the composition of the C-S-H phase presenting the fastest precipitation kinetics,rather than the highest thermodynamic stability.Molecular dynamics study of early C–S–H generation in nano-environment.Based on the previous research results,the precipitation process and structural changes of early-age C–S–H in TiO2nano slits were studied.The TiO2nano slits with the spacing of19?,38?and 59?were constructed.The CaO–SiO2–H2O solution with a Ca/Si ratio of 1.2 and a H2O/Si ratio of 3.0 was added into the slit to simulate the precipitation of early-age C–S–H under nanoconfined environment.Results show that with the TiO2nano slit,the content of Q3and Q4in the C–S–H structure increases significantly as the slit spacing decreases.In addition,it was found that the composition of C–S–H along the vertical direction of the interface changed gradually,Ca2+enrichment occurred near the surface with oxygen dangling bonds,while Q3and Q4units appeared in the silicon-rich regions.TiO2surface with oxygen dangling bonds has a strong adsorption effect on C–S–H,resulting in a more dense C–S–H structure.Based on the results of this chapter,the nucleation effect of nano-TiO2was confirmed and the inherent mechanism for C-H promoting effect was revealed.