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• 28-day flexural strength of geopolymer increases by 25% with 0.30 wt% GO. • GO/rGO accelerates the dissolution of fly ash microspheres and promotes the growth of geopolymer chain. • Electron binding energy of O1s, Si2p and Al2p confirms the change in geopolymerization degree. • The interfacial bonding mode of GO/rGO-geopolymer and reaction mechanism are proposed. This work aims to examine the impact of graphene oxide (GO) and reduced graphene oxide (rGO) on mechanical properties and microstructure of fly ash (FA) based geopolymer. Different schemes were proposed to prepare GO/rGO-geopolymer composites. The chemical composition, microstructure and hydration degree of GO/rGO modified geopolymer were characterized by using FTIR, XRD, DTG, SEM, XPS and BET. The experimental results show that GO/rGO changes the microstructural composition and pore diameter distribution of geopolymer. The increased average electron binding energy of the tetrahedron of [AlSiO 4 − or [AlSi 3 O 8 − leads to the acceleration of geopolymerization process. The decreased average pore volume and porosity confirms that GO/rGO accelerates the dissolution and hydration process of FA microspheres. GO and rGO exhibit different efficiency in promoting the hydration degree of geopolymer, which may be due to different functional groups and dispersion efficiency. Meanwhile, the flexural strength changes with addition of GO/rGO, which increases by 25% for the use of 0.30 wt% GO and 10% for the use of 0.10 wt% rGO. In general, GO and rGO are promising to promote the hydration of FA and increase the mechanical strength of geopolymer. [ABSTRACT FROM AUTHOR]

The effects of 10% metakaolin addition on compressive strength, water absorption, shrinkage and microstructure evolution of cement paste after elevated temperatures exposure from room temperature to 800 °C were evaluated. The experimental results show that compressive strength increases at 200 °C and 400 °C compared with that obtained at ambient temperature. Up to 800 °C, compressive strength decreases rapidly. The addition of 10% metakaolin leads to the enhancement of compressive strength regardless of exposure temperatures. After thermal exposure at 400 °C, compressive strength reaches the maximum value. Thermal exposure degrades pore structure. A polynomial equation was used to indicate the shrinkage of cement paste or metakaolin-blended cement paste with testing days. Mechanical properties, permeability resistance, and shrinkage in cement pastes are closely related to the microstructure development. 10% metakaolin addition presents better thermal resistance, lower shrinkage and denser microstructure compared with pure cement paste before and after thermal exposure. [ABSTRACT FROM AUTHOR]