Effect Of Graphene Oxide On The Dissolution Of Cement Mineral Phases And The Formation Of Hydration Products

Graphene oxide（GO）has increasingly broad applications on cementitious materials due to the fact it enhances mechanical properties and durability of the materials by improving their microstructure.However,the hydration mechanisms of GO modified cement composites and the essential reasons for the performance improvement by adding GO are still unclear.In this thesis,the influences of GO on the hydration of C₃S,C₃A/gypsum,C₃S/C₃A/gypsum and cement systems were systematically investigated.More specifically,hydration process,hydration products and hydration mechanisms were revealed:saturation index of mineral phases and hydration products was calculated with the pore solution composition for the thermodynamic stability;dissolution rate of C₃S and precipitation rate of C-S-H were quantified with a set of kinetics equations;hydration degree and reaction rate of mineral phases were estimated with quantitative analysis of the solid residual phases after dissolution,aiming to clarify the impact of GO on the consumption of mineral phases and the precipitation of hydration products;the effect of GO on the microstructure of different systems was studied by scanning electron microscope（SEM）,transmission electron microscope（TEM）and nitrogen adsorption-desorption isotherm.The influence of GO on early hydration of C₃S and C₃A was studied.GO reacts with Ca²⁺in the pore solution,which reduces the concentration of Ca²⁺.The dissolution of mineral phases is facilitated with the addition of GO by increasing the undersaturation levels and enhancing the driving force for dissolution of minerals.The oversaturation levels and the driving force for C-S-H and AFt precipitation are reduced by the incorporation of GO.However,the hydrophilic groups on the GO with high surface energy act as nucleation sites for crystal growth,which counteracts the reducation of driing force.And the precipitation rate is promoted throughout the hydration process.Moreover,GO is found to increase the hydration degree and hydration rate of minerals,as confirmed by quantitative analysis.The early hydration of C₃S/C₃A/GO/gypsum composite system was studied.The results show that the concentration of Ca²⁺in the pore solution is reduced while the concentration of Si and Al is increased by adding GO.The dissolution of the starting materials（C₃S,C₃A and gypsum）is promoted by GO due to the higher undersaturation levels.The dissolution of C₃A might be more sensitive to the addition of GO than that of C₃S.Similarly,for the precipitation,although GO decreases the supersaturation level of ettringite and C-S-H,quantitative results confirm that GO can improve the hydration degree and accelerate the hydration rate because of its seeding effect.The early hydration behaviour of GO/cement system and the mechanical properties of hardened cement paste were studied.The results of inductively coupled plasma optical emission spectrometer（ICP-OES）show that GO reduces the concentrations of Ca²⁺and Al,and increases the element concentrations of Si and Fe.The thermodynamic calculation show that GO increases the unsaturation levels of C₃S,C₂S,C₃A and C₄AF.The influence of GO on the dissolution of C₃A and C₄AF is earlier than that of C₃S and C₂S phases.The quantitative results of XRD,TGA and thermodynamic simulation show that GO stimulates the consumption of mineral phases and the precipitation of CH,C-S-H and AFt.