An investigation into the colloidal stability of graphene oxide nano-layers in alite paste
Introduction
Graphene oxide (GO) is composed of a distorted graphene mono-layer where a fraction of carbon atoms have been functionalised by various oxygen-containing chemical groups such as carbonyl and carboxyl [1]. In recent years, the use of GO as a potential strength-enhancing additive in Portland cement (PC) paste has been the focus of much research [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. Previous studies have found that GO improves the compressive strength of PC paste, however the reported results are somewhat erratic and difficult to interpret in detail. For instance, Lv et al. [11] found that adding GO to PC paste with an GO:PC mass ratio of 0.02% yields a maximum increase of 60% in the compressive strength, whereas the same authors in a recent article [12] required a higher optimal GO:PC ratio of 0.06% to achieve the same level of strength improvement. Many factors might explain these discrepancies (e.g. the size and oxidation level of the GO layers [10], [14]), but the mechanism by which the presence of GO leads to the strength improvements in PC pastes has remained controversial.
Two mechanisms have so far been put forward to describe the role of GO in enhancing the mechanical properties of PC paste: (i) as a derivative of graphene, the GO layers possess a high in-plane tensile strength and therefore could act as a nano-size reinforcing agent in the PC matrix, thereby delaying the formation of cracks [2], [3], [4], [5], [6], [7], [8], [9], [15], [16]; (ii) the GO layers could serve as nucleation-growth sites during the hydration of PC, stimulating a higher degree of hydration [10], [11], [12], [13], [14], [17].
With respect to the reinforcing mechanism, studies postulated that the functional groups of GO play a pivotal role in providing strong interfacial bonding between the GO nano-layers and C-S-H [2], [5], [16]. In a detailed molecular dynamics simulation study, Sanchez and Zhang [16] showed that a sufficient number of oxygen-containing functional groups should exist on the GO surface to achieve a strong interfacial bonding between GO and C-S-H (modelled as 9 Å tobermorite structure). They suggest that the nature of the interaction between GO and C-S-H is electrostatic, and that Ca2 + ions in the pore solution of paste could act as a bridge between the polarised oxygen atoms of GO and C-S-H [16]. Regarding the role of GO as extra growth sites, some studies suggest that GO accelerates the hydration of PC, resulting in an increased early-age compressive strength [13], [17]. Others report that GO induces the formation of a new micro-structure with a highly regular flower-like pattern [10], [11]. According to the latter, the GO layers may not act directly as a reinforcing agent, but rather stimulate a micro-structural pattern that gives the PC-GO paste enhanced mechanical properties.
Whether GO directly reinforces the PC matrix or increases nucleation, the mechanisms proposed to date are underpinned by a number of assumptions. First, the individual GO layers must remain well-dispersed in PC paste so that a homogeneous reinforcement and/or nucleation-growth is achieved. While it is established that due to the presence of oxygen-based functional groups, GO forms a stable aqueous colloid [18], [19], the stability of its dispersion in a PC paste environment is so far unknown. Second, the source of interfacial bonding, i.e. the GO functional groups, must remain chemically stable during the hydration of PC, otherwise the reinforcing role would not be effective. Using Fourier Transform Infrared Spectroscopy (FT-IR) on solid PC-GO paste, Lin et al. [17] reported that the hydration of PC has no detrimental effect on the functional groups of GO. However, the FT-IR of PC-GO could have been easily misinterpreted due to the overlapping of various stretching vibrations associated with the hydration products and GO.
To avoid the complexities involved in the hydration of PC, we herein focus on the hydration of alite, the main constituent of Portland cement. First, the overall effect of GO on the alite hydration is investigated by conducting a series of isothermal calorimetry measurements. The calorimetry patterns show that GO accelerates the alite hydration, but the extent of observed acceleration is quite low. A theoretical boundary nucleation-growth (BNG) model was used to analyse the calorimetric data. According to the BNG analysis, the acceleration observed in the hydration of alite-GO system may stem from a combination of extra surface for the nucleation of hydration precipitates provided by GO; higher nucleation density on the GO surface compared to that of alite; higher rate of precipitation in alite-GO paste compared to alite paste. However, the BNG results indicate that both the amount of extra surface as well as the higher nucleation density added by GO is only a small fraction of what GO could potentially provide. This, together with direct microscopic observation pointed to a clear aggregation of GO in the pore solution of alite paste. This led us to investigate the underlying mechanisms controlling the interaction of GO with various calcium-containing aqueous electrolytes using Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES), FT-IR, thermogravimetric analyses (TG), and X-ray diffraction (XRD).
Section snippets
Synthesis and characterisation of materials
Alite was synthesised by sintering pelleted powders of 3:1 stoichiometric mixture of high-purity CaCO3 and SiO2 (≥ 99 wt%, Sigma Aldrich) which were doped with 1.1 wt% MgO and 0.7 wt% Al2O3 (≥ 99 wt%, Sigma Aldrich) according to the procedure explained by Wesselsky and Jensen [20]. The pre-mixed powders were wet-homogenised in a mixer, and then dried and calcined for 5 h at 1000 °C. The de-carbonated mix was pressed into pellets and subsequently heated at 1500 °C for a period of 8 h in a muffle
Effect of GO layers on the hydration of alite
Fig. 4 shows the rate of heat evolution for the reaction of alite with water in the absence and presence of GO, measured for various water to alite mass ratios. In general, all the heat evolution curves follow the typical pattern of alite hydration, including a period of accelerating heat flow which thereafter starts to decelerate. It can be observed that for all water to alite mass ratios (Fig. 4a–d), the alite pastes containing GO have a slightly altered hydration pattern compared to the
Conclusions
In this paper, the hydration of alite was first investigated in the presence of GO nano-layers using isothermal calorimetry. Results indicated that the presence of GO accelerates alite hydration. However, the observed acceleration was found to be quite low. A boundary nucleation-growth (BNG) model was used to better understand the role of GO as a nucleation-growth site. The BNG analysis showed that the amount of extra surface and nucleation sites added by GO was several orders of magnitude
Acknowledgments
The financial support provided by the University College London (UCL) to the first author is gratefully acknowledged. The authors would like to thank Dr Christoph Salzmann and Mr Martin Rosillo-Lopez for their support throughout this study. The first author greatly thanks Mr Tobias P. Neville for access to his furnace. Dr Enrico Masoero is also greatly acknowledged for his discussions on the modelling of hydration. The authors would also like to thank Mr Warren Gaynor from UCL Laboratory of
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