## 4 Results and Discussion
### Effect of GONF on the Properties of Fresh Cement Paste  
The effect of GONF on the properties of freshly mixed cement paste was investigated by performing the mini-slump test and measuring the setting time. The mini-slump flow test was employed to determine the influence of GONF on the workability of cement paste. For the mini-slump test, a 600 g of OPC was used with 0.5 w/c ratio. OPC and water was mixed in a mixer for 1 min. Then the mixture was poured into a mini slump cone, compacted, struck off and lifted at 2 min. The area of each pat was determined and reported. Table 5 shows the results of mini-slump tests of OPC paste with different percentages of GONF. The diameter of the mini-slump flow test is approximately 110 mm. The addition of GONF in OPC paste results in the decrement of mini-slump flow. With the increment of the dosages of GONF, the diameter of mini-slump flow was decreased proportionally. This result indicates that the addition of GONF reduces the fluidity of OPC paste mix. This phenomenon might be explained with the physical and chemical properties of GONF. GONF has a large surface area compared to OPC (see Table 4). Therefore, GONF could absorb and hold more water molecules around it during mixing. The other reason of the reduction of fluidity can be due to oxygen groups at the edge of GONF. These oxygen groups may initiate the cement hydration process faster by reacting with ions from the dissolved cement grains [9; 13].  
Figure 6: SEM images of various carbon-based materials ((a) carbon nanofiber (CNF); (b) grapheme; and (d) carbon nanotubes (CNT) and (e) GONF which used in this study. (conversion: 1 \(\upmu\)m = 0.0394 mil) (reproduced with permission from [28]. Copyright IOP Publishing, 2014).  
One of the indirect methods to determine the process of cement hydration is the time of setting. The strength of cementious materials is gained with the growth of cement hydration products. The time of initial setting can be measured when OPC paste begins to stiffen and the time of final set can be found when the cement has hardened to sustain some load. For the setting time test, a 400 g of OPC was used with 0.5 w/c ratio. OPC and water was mixed in a mixer for 1 min. Then the mixture was placed in the Vicat mould under the rod bearing the needle. The needle was lowered until it comes in contact with the surface of cement paste and the needle was released quickly, allowing it to penetrate into the paste. Repeat this procedure every 2 min until the needle fails to penetrate the paste about 5 mm (initial setting) measured from the bottom of the mould and no penetration (final setting). Table 5 shows the results of the initial and final setting of OPC paste with the different dosages of GONF and different mix designs. Both PD and PW shows similar trends in the results. The initial and final settings were decreased with an increasing increment of GONF dosage until 0.05%. This result indicates that the addition of GONF up to 0.05% in OPC paste mix helps accelerate strength gain within the mix. In contrast, more than 0.05% of GONF in OPC paste leads to an increase in time of setting. One of the reasons for this behavior may be that a high surface area of GONF results in the reduction of free water which is necessary for cement hydration. The water which was absorbed and held by GONF may affect the speed of cement hydration. The other reason for this increment of setting time can be H-bonds connection between water and GONF. It could affect the speed of cement hydration process.