Abstract
Cementitious materials are the most widely used materials in our built environment even though the production of cement requires considerable energy and causes significant global carbon dioxide emissions, leading to various environmental and social impacts. As the performance of cement-based composites is strongly affected by their nano-scale properties, one potentially transformative approach to introduce superior performance in these composites is nanoengineering. Graphene, which offers outstanding mechanical properties at a low-cost, can serve as an exceptional nano-reinforcement in cement composites. However, certain challenges currently hinder the use of graphene in cement composites. In particular, graphene sheets often aggregate into flakes of weakly interacting monolayered sheets due to their strong hydrophobicity and van der Waals attraction. To leverage the excellent mechanical properties of graphene in cement-based composites, its dispersion problem needs to be addressed. In addition, though there have been various studies where the graphene-based nanomaterials have been used in either cement paste or mortar composites to improve their mechanical properties, only a few studies have explored the effects of graphene in cementitious composites with coarse aggregates that possess different physico-mechanical behavior.
This study investigates an effective dispersion method and an optimal morphology and concentration of graphene-based nanomaterials for the incorporation into cement composites. First, the use of commercially available graphene nanoplatelets (GNPs) in concrete mixtures is investigated. For the dispersion of GNPs, high shear mixing at different durations with and without additional ultrasonication is used. A polycarboxylate-based superplasticizer (PCE) is utilized to assist the dispersion. The compression and flexural strength are evaluated on concrete mixtures, while direct tensile tests are conducted on mortar specimens to evaluate tensile properties of GNP-reinforced cement composites. As the performance enhancement obtained from PCE-assisted dispersion of GNP into cement composites have remained modest, the use of several other surfactants, including two ionic surfactants, one non-ionic surfactant, and ultrasonication are explored for the fabrication of graphene-reinforced cement composites. Taguchi method of experimental design is employed to minimize the number of experiments needed to assess the effects of selected factors on the dispersion process. Two multi-criteria decision making methods, namely Technique for Order Preference by Similarity to an Ideal Solution (TOPSIS) and Principle Component Analysis (PCA) methods, are employed to determine the optimal values of experimental factors. The findings of the optimal dispersion study are used to fabricate GNP-reinforced mortar composites. The mechanical and durability properties of the graphene-reinforced cement composites is thoroughly investigated. Finally, a life-cycle assessment framework is used to evaluate environmental performance of the developed cement composites over stages from “cradle-to-gate”. Results show that the incorporation of GNPs at 0.1 wt.% of cement can increase the compression strength by 36%, tensile strength by 48%, and significantly reduce permeability and sorptivity of mortar composites.
