# On the Stability of Graphene-Based Aqueous Dispersions and Their Performance in Cement Mortar
###### Abstract
Wet ball milling provides shear forces in the liquid medium and promotes the vertical expansion of the layers and exfoliation [31]. In micro-fluidization, high pressure is exerted on the fluid to cause exfoliation of the material layers [32]. However, re-aggregation of the carbon nanomaterials continuously occurs and competes with exfoliation due to significant interlayer binding energies. This reversible process can also be regulated by modifying the nature of surface-solvent interactions. It can be obtained by using selected surfactants that bind with the carbon nanomaterials through van der Waals interaction. A promising surfactant to be used is sodium cholate (SC) (tri-hydroxy salt) [24]. It is characterized by a four-ring steroid nucleus with two hydroxyl groups and a five-carbon side chain terminating in a carboxylic acid. SC belongs to the facial amphiphiles class, as the arrangement of hydrophobic and hydrophilic groups provides it with a planar polarity. The above-mentioned peculiarities allow SC to present distinct properties that differ significantly from ordinary aliphatic surfactants, as they are relatively bulky and rigid with a distinct behaviour with respect to self-association and molecular solubilization. SC molecules not only intercalate between graphene sheets, but their hydrophobic moieties interact with hydrophobic graphene sheets, stabilizing them. Furthermore, SC can hinder re-aggregation through van der Waals interactions and the repulsion of the like-charged hydrophilic head-groups of the surfactant bound to each of the nanosheets [33]. An interesting method to provide external energy for exfoliation is represented by hydrothermal treatment. It generates shear force by the Brownian motion of various molecules at high temperature and high pressure [34]. In recent scientific literature, two methods have been adopted to better evaluate the dispersibility and stability of graphene, the former being the visual observation method and the latter measuring the ultraviolet-visible absorbance at the intrinsic absorption peak of carbon nanomaterials [5; 35; 36; 37]. Specifically, higher absorbency values correspond to better dispersion in water, with the absorbance directly proportional to the concentration, according to the Beer-Lambert law. However, the use of the ultraviolet-visible spectrophotometry (UV-vis) requires collecting the absorption spectra of graphene-based dispersions not only in terms of different concentrations but also distinguished for each amount of the specific dispersants used to obtain calibration curves. Moreover, as reported in different papers [37], the dispersants, by recombinating with carbon nanomaterials or interfering with their absorption curves, can affect the accuracy and reliability of analyses. Furthermore UV-vis spectrophotometers work by passing a beam of light through the sample and measuring the amount of light that is absorbed at each wavelength at a certain time, requiring the dilution of highly concentrated dispersion, which alters their state and structure. Static multiple light scattering (SMLS) offers the ability to investigate the dispersion state and its evolution over time in their native state, without any dilution, even on highly concentrated samples. The TURBISCAN technology, employing SMLS, measures transmission or backscatter intensities versus the sample's height and ageing time. During the analysis, an infrared light source scans the sample vertically, while two sensors collect the transmitted (T) and backscattered (BS) light at different heights. The stability of the suspensions over time is tracked by the accurate and reliable Turbiscan Stability Index (TSI). In this study, various graphene-based additives, namely graphene technical grade (Gr-TG), graphene super grade (Gr-SG), and GO, have been characterised and added at different percentage contents (\(0\div 1\%\) by weight of cement, a range that allows enhanced thermal and self-sensing performances [38]) to a reference mortar.