## 4 Results and Discussion
### Effect of GONF on the Strength of Hardened Cement Paste and Concrete
First, GONF particles were well dispersed in PW specimens due to the sonication. The sonication can disperse GONF into water well thus there is high possibility that GONF particles can be dispersed well in the cement paste mix. Second, GONF can form H-bond connection with water thus it could affect the fluidity of the cement paste mix. From the results of mini-slump tests, the Wet-mix design method produces bigger areas of slump than the Dry-mix design method. This indicates that the Wet-mix design method may reduce the workability of the cement paste mix below than of the Dry-mix design method. It is known that reduction of workability can affect the strength of the cement composite. Figure 8: Compressive and flexural strength results of GONF-combined paste with Wet-mix design method. (**a**) Mechanical properties of PW (7 days); (**b**) Mechanical properties of PW (28 days). Figure 7: Compressive and flexural strength results of GONF-combined paste with Dry-mix design method. (**a**) Mechanical properties of GONF-combined paste with Dry-mix design (PD) (7 days); (**b**) Mechanical properties of PD (28 days). However, even though the ratio of strength increment with PW specimen (Wet-mix design) is higher than PD specimen (Dry-mix design), note that Dry-mix design can still be considered a very practical mix design for GONF-combined cement composites based the results of strength tests. To investigate the feasibility of GONF use in the concrete industry with no regard for the sonication, dry GONF powder was incorporated into concrete mix. The effect of GONF on the strength of hardened concrete was investigated. The specimen IDs with CD indicate that GONF-combined concrete specimens were made with Dry-mix design. Figure 9 shows the results of compressive and flexural strength tests of CD specimens. As expected, the results indicate that the compressive and flexural strengths of CD specimens are higher than the control Portland cement concrete (PCC) specimens at 7 and 28 days. This result confirms that the addition of GONF in cement composite can lead to the improvement of strength. Another important point to note about the increment of strength is that 0.01% GONF addition results in 14.2% strength increment compared to the control PCC specimen. From the strength test results of GONF-combined paste, 0.05% GONF addition was found as the optimum GONF content. And, this optimum GONF content was the reason to choose the range of GONF content (0.01-0.1%) for PCC specimen. For GONF-combined concrete, however, 0.01 wt. % of GONF led to the highest strength increment of CD specimen. The phenomenon that the different optimum contents of GONFs for cement paste (0.05%) and concrete (0.01%) might be explained with the interaction between cement paste, GONF and aggregates in concrete. As previously mentioned in Section 4.3, the optimum content of GONFs for cement paste (0.05%) might be due to more cement hydration products and two-dimensional interlocking between hydration products even though there was the reduction of workability (Table 5). In other words, the optimum content of GONFs for cement paste (0.05%) was the outcome of offsetting between positive and negative effects of GONFs in cement paste. However, for GONF-mixed concrete, the present of dry-aggregates might amplify the reduction of workability even though water correction factor was applied to the concrete mix. Thus, the lowest GONF (0.01%) resulted in the highest strength even though there were positive effects of GONF in concrete mix.