Ghazizadeh, Seyedamirhossein; (2018) Understanding the behaviour of graphene oxide in Portland cement pastes. Doctoral thesis (Ph.D), UCL (University College London).

Text PhD thesis - Sam Ghazizadeh.pdf - Accepted Version Access restricted to UCL open access staff until 1 July 2025. Download (81MB)

Abstract

Graphene oxide is a distorted graphene mono-layer in which a fraction of carbon atoms have been oxidised into various functional groups, such as carbonyl and carboxyl. Graphene oxide has the potential to enhance the low tensile strength of Portland cement matrices, but research is required to understand how it behaves in blends with cement and water. This thesis investigates the behaviour of graphene oxide in pastes made with either mineral alite (the main constituent of Portland cement), clinker (alite with aluminate and ferrite minerals) or Portland cement. This study shows that graphene oxide is susceptible to agglomeration in alite and Portland cement pastes. Various experimental techniques, namely isothermal calorimetry, inductively coupled plasma atomic emission spectroscopy, X-ray diffraction, Fourier-transform infrared spectroscopy, and thermogravimetric analysis were used to establish why graphene oxide agglomerates in the environment of cement pastes. Results show that the carboxylate sites in graphene oxide form complexes with multivalent cations (mainly calcium) in the pore solution of cement pastes. This reaction bridges the graphene oxide layers, causing their agglomeration. In a separate reaction, the hydroxide ions present in the paste solution deoxygenate graphene oxide, reducing it into an almost hydrophobic material. Calorimetry measurements showed that graphene oxide significantly retards clinker hydration, whereas it accelerates the hydration of alite and Portland cement only marginally. Retardation occurs as graphene oxide binds to the surface of hydrating clinker grains, while acceleration results from a seeding effect in which graphene oxide provides extra surface for the nucleation and growth of hydration products. In Portland cement pastes, sulphate ions released from the dissolution of gypsum prevent graphene oxide from retarding the hydration. Simulations using a boundary nucleation-growth model confirmed that graphene oxide is not particularly active as a nucleation surface in alite paste and behaves in a similar way to inert fillers, such as quartz. Mechanical tests indicated that graphene oxide slightly strengthens hardened cement pastes. This effect is shown to be directly related to the change of cement hydration degree, as opposed to the reinforcing effect previously proposed in the literature. Finally, a simple strategy was proposed to counteract the agglomeration of graphene oxide in cement pastes. The method uses organic acids and copolymers that mitigate the reaction of graphene oxide with divalent cations and hydroxide ions. These allowed the dispersion of graphene oxide in alite paste for the first time.

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