Reinforcing Mechanism of Reduced Graphene Oxide on Flexural Strength of Geopolymers: A Synergetic Analysis of Hydration and Chemical Composition

Abstract

With the development of nanotechnology, reduced graphene oxide (rGO) has been used to improve the flexural strength of geopolymers. However, the reinforcing mechanism of rGO nanosheets on the flexural strength of geopolymers remains unclear. Here, this reinforcing mechanism was investigated from the perspectives of hydration and chemical composition. The effect of the reduction degree on rGO-reinforced geopolymers was also studied using isothermal calorimetry (IC), X-ray diffraction (XRD), and nuclear magnetic resonance (NMR) tests. Results show that the hydration degree and flexural strength of geopolymers effectively increase due to rGO addition. After alkali reduction at a temperature of 60 °C, rGO nanosheets have maximum reinforcement on the flexural strength of geopolymers with an increment of 51.2%. It is attributed to the promotion of slag hydration, as well as the simultaneous formation of calcium silicate hydrate with low Ca/Si ratio (C-S-H(I)) and calcium aluminosilicate hydrate (C-A-S-H) phases due to the inhibiting effect of rGO nanosheets on Al substitution on the end-of-chain silicates of C-S-H and C-A-S-H gels. In addition, different reduction degrees have almost no effect on the chemical composition of rGO-reinforced geopolymers, while excessive reduction impairs the improving effect of rGO nanosheets on the hydration process and flexural strength of geopolymers due to significant structural defects.

Geopolymers vs. Cement Matrix Materials: How Nanofiller Can Help a Sustainability Approach for Smart Construction Applications—A Review

“…Long et al [ 69 ] conducted a study on the reinforcing mechanism of reduced graphene oxide (rGO) nanosheets on GBFS-based geopolymer mortars. The addition of rGO nanosheets remarkably increased the flexural strength properties of the samples.…”

Structural Applications of Thermal Insulation Alkali Activated Materials with Reduced Graphene Oxide

“…This result was due to the increase of low-density waste EPS (28 kg/m 3 ). Note that the addition of rGO slightly accelerated the alkali reaction, which leads to more reaction products and increases the density of rGO-WEA [46,48]. However, the rGO content adopted in this study was relatively low (0.04 wt.%).…”

“…Particularly, reduced graphene oxide (rGO) is used in construction for its large specific surface area, favorable mechanical strength, and high toughness [44][45][46][47]. In addition to this superior performance, rGO can effectively disperse in the highly alkaline environment of AAMs matrix (pH~12.5) due to electrostatic stabilization and significantly improve its mechanical properties [48][49][50].…”

Mechanical Strength of Graphene Reinforced Geopolymer Nanocomposites: A Review

“…It is vital to retain a small amount of water in the cured geopolymer to avoid dehydration and excessive shrinkage due to long curing times at high temperatures (Van Jaarsveld et al, 2002). It was found that the flexural strength increases to about 29% for slag geopolymer composite filled with rGO steamed cured for 48 h at 60°C (Long et al, 2019a). There are three reasons for the geopolymer strength improvement.…”

“…At the same time, The residual oxygen functional group helps in dispersing rGO in alkaline solution. However, when stirring temperature is further increased to 80°C, the flexural strength is observed to decline due to severe structural defects of RGO (Long et al, 2019a). Extreme stirring heat in alkaline solution not only causes reduction of graphene to GO, but also exhibits wrinkles on GO structure (Deng and Berry, 2016).…”