4.1.1. Yield Stress

Yield stress values were determined by Bingham model, Modified Bingham model, Herschel–Bulkley (HB) model and Casson model. The results are presented in Table 4. Due to slippage effect and smooth surface of parallel plates, low yield stress values were found [9]. Generally, the values of yield stress obtained by modified Bingham model were highest followed by Bingham, HB and Casson model. For all rheological models, the values of yield stress increased with the increase in the percentage of graphene in the cement matrix. This increase in yield stress values is in line with the available literature on the derivatives of graphene. The increase in yield stress values for graphene–cement paste might be due to higher surface area of graphene due to which it requires more amount of water for lubrication of graphene [33]. Shang et al. [6] carried out experimental investigation to study the effect of graphene oxide on rheological properties of cement paste and found that in comparison to plain cement, the value of yield stress increased by approximately four times for 0.08% of graphene oxide cement paste. It was noticed that negatively charged graphene nanoplatelets in water interacted with cement particles by electrostatic interaction and formed flocs. Wang et al. [7] also detected the increase in flocculation structures having higher dosage of graphene oxide (GO) in cement paste. According to literature [6], these large agglomerates entrap some water molecules, which reduce the content of free available water. Therefore, it can be concluded that by keeping the water cement (w/c) ratio constant, the values of rheological parameters will increase.


**Table 4.** Yield stress values determined by various rheological models.

For the case of shear rate cycle, the values of yield stress decreased when the shear rate cycle increases from 100–0.6 s<sup>−</sup><sup>1</sup> to 300–0.6 s<sup>−</sup>1. This is possibly due to destruction of potential agglomerated structures in cement paste. For high shear rate cycle, the resistance to flow was reduced and hence shear thinning behavior was dominant. Finally, for both control and GNP–cement paste, the yield stress values increased with the increase in resting time as shown in Table 4. The increase in rheological parameters may be attributed to: (a) hydration reaction of cement paste; and (b) presence of suspended particles (graphene).
