3.1. Structural Characterization of GONs
The FTIR spectra of GONs and graphite are shown in Figure 1
. The results indicate that the GONs contain hydroxyl groups (–OH, 3350 cm−1
), carboxyl groups (COOH, 1740 cm−1
), carbonyl groups (C=O, 1660 cm−1
), and ether bonds (–C–O–C–, 1450, 1360, 1320, 1260, 1100, 1050 cm−1
), which are not present on the FTIR spectra of graphite. The XPS spectra of GONs are shown in Figure 2
, indicating that the carbon bonds in GONs were C=C, C–OH/C–O–C, C=O, and COOH in a proportion of 6.57%, 38.76%, 43.32%, and 11.35%, respectively. The results suggest that GONs contain hydroxyl, epoxy, carbonyl, and carboxyl groups compared with graphite.
The AFM images of GONs are shown in Figure 3
. The results reveal that the thickness of the GONs is less than 7.67 nm, and their length/width is in the range of 50–600 nm. The results also indicate that the surfaces of GONs are usually not flat and have a very rough surface. This may be attributed to the random overlap of many GONs. A laser particle analyzer was used to confirm the size distribution of the GONs, and the result is shown in Figure 4
. The size distribution of the GONs was in the range of 10–800 nm, and 90% of the GONs were in the range of 100–600 nm. The results of the AFM images and the size distribution indicated that GONs are multi-layer nanosheets.
shows a possible formation mechanism for GONs. Original graphite is comprised of compact aggregates of a flat sheet of carbon one atom thick, and it is hard to disperse into nanosheets (Figure 5
a). When graphite is oxidized, the oxidizer can soak slowly into the lamellar structure of graphite and produce many hydrophilic chemical groups on its interfaces especially at its edges, which resulted in edge dilation (Figure 5
b). The enlarged edges provide a pathway for oxidants to penetrate into the deeper level and make it easy dispersion, exfoliated in later ultrasonic processing (Figure 5
3.2. Microstructure of GON/Cement Composites
The effect of GONs on cement composites were investigated by comparing the microstructure and performances of the composites in both the absence and the presence of GONs. The microstructure of cement composites without GONs was first investigated by observing the SEM images of its fracture interfaces. The SEM images are shown in Figure 6
. Figure 6
a,b is the SEM images of the fracture interfaces magnified 500 times, showing that the whole cement matrix has an irregular microstructure. Figure 6
c–f shows SEM images magnified 5000 times, showing that the structure characteristic of the cement matrix is irregular and disordered, and contains many holes and cracks, as well as some needle-like, bar-like, and sheet-like crystals with an irregular aggregate state.
The cement composites mixed with GONs for the preparation of GON/cement composites. The SEM images of GON/cement composites at 28 days are shown in Figure 7
. The results indicate that the entire cement matrix in GON/cement composites have an ordered microstructure that consists of polyhedron-like crystals with flower-like patterns. Figure 7
a is a SEM image in a low magnification of 500 times, showing that the entire sample formed the ordered microstructure. Figure 7
b is a SEM image in a magnification of 1000 times; from the image, it can be seen that that the ordered microstructure consists of polyhedron-like cement crystals via interweaving. Figure 7
c–f shows four typical ordered microstructures with flower-like patterns, which assemble via the polyhedron hydration crystals. All cement hydration products became polyhedron-like crystals, and these polyhedron-like crystals assembled into ordered microstructures with flower-like patterns in the presence of GONs. These results suggest that there is a very capable organizer in the formation process of cement composites.
Generally, in the cement hydration process, producing some cracks and holes are inevitable. However, the above results show that GON/cement composites had dense and ordered microstructures with flower-like patterns at 28 days. In the research process, the microstructure of GON/cement composites in the initial stage of its formation was also investigated to reveal the regulation mechanism of GONs on the structure of cement composites. Figure 8
shows interesting SEM images of the composite at 7 days. The common feature of these SEM images is that the flower-like and polyhedron-like cement hydration products generate in the holes or the cracks of the composites. These results can help us to understand the formation process of cement hydration products under the control of GONs. Figure 8
a shows that flower-like hydration products are easy to produce in the holes of cement composites [31
]. Figure 8
b shows that flower-like hydration products tend to form dense microstructures via growing aggregation. Figure 8
c shows that flower-like products can form ordered microstructures with flower-like patterns and exhibit repairing effects for the holes and cracks. Therefore, Figure 8
a–c exactly indicates the generating, growing, and forming process of the ordered microstructures of cement composites. Identically, Figure 8
d–f shows that the polyhedron crystals are easy to produce in holes and cracks, and their growth can repair those holes and cracks. Figure 8
shows that cement hydration products have a repairing function for cracks and holes in a cement matrix.
In order to confirm the organizer of ordered microstructures, the chemical element compositions of the polyhedron aggregates with flower-like patterns were determined by EDS, and the results are shown in Figure 9
. The EDS results indicate that the carbon content in the center of the flower-like patterns is relatively high compared with other places. This can act as evidence that GONs influence the shape and aggregate state of cement hydration products, especially in the center of the ordered microstructures with flower-like patterns. The results confirm GONs’ ability to regulate cement hydration products and microstructures. The ordered microstructures with a crosslinked network are beneficial for reducing cracks and improves the strength and toughness of cement composites.
The crystal structure of the cement hydration products of GON/cement composites was investigated via XRD spectra. Four randomly selected test samples were investigated, and the results are shown in Table 2
. The results indicate that GONs have an important effect on the pore structure of GON/cement composites. GON/cement composites have a small total pore area, a median pore diameter, and an average pore diameter and porosity. The median pore diameter’s closeness to the average diameter indicated that the pore diameters were uniform. The results indicate that GONs can promote the formation of ordered microstructures with smaller, fewer cracks and holes. This explanation appears consistent with the SEM images of the GON/cement composites.
3.3. Strength and Durability of GON/Cement Composites
The compressive and flexural strength of GON/cement composites are shown in Table 3
, from which it can be seen that GON/cement composites have made great improvements in compressive and especially flexural strength. The compressive and flexural strengths of GON/cement composites at 28 days increased by 51.1% and 85.1% compared with the control sample, respectively. The results are consistent with SEM morphology and the pore structure of GON/cement composites.
The microstructure of cement composites is very closely related to its durability. Properties closely related to durability, such as penetration resistance, freeze–thaw resistance, and carbonation resistance, were determined, and the results are shown in Table 4
. The results indicate that durability parameters such as seepage height, freeze–thaw mass loss (mloss
), the retention rate of relatively dynamic elasticity modulus (p
), and carbonation depth have markedly improved compared with the control samples. The results suggest, therefore, that GON/cement composites will have an improved service life.
3.4. Formation Mechanism of Regular Cement Hydration Products
Cement mainly consists of tricalcium silicate C3
), dicalcium silicate C2
), tricalcium aluminate C3
), and a small amount of gypsum (CaSO4
O). In the hydration process, C3
S, and C2
S will carry out a complex hydration reaction to form ettringite [Ca6
O, AFt], monosulfate [Ca4
O, AFm], calcium hydroxide [Ca(OH)2
, CH)], and calcium silicate hydrate [3CaO·2SiO2
O, C–S–H] gel, the corresponding chemical reactions of which are represented by Figure 10
. Generally, CH, Aft, and AFm exhibit rod-like and needle-like shapes with disorder.
According to the above results, a possible formation mechanism of GONs on the microstructures ofthe cement matrix is proposed, as shown in Figure 11
. The surfaces of GONs have many chemical groups, such as –OH, –O–, and –COOH. These chemical groups react preferentially with C3
S, and C3
A when cement meets with water (Figure 11
a). The initial products form growth points for the hydration products (Figure 11
b,d), after which the hydration reaction continues as the formation of hydration products aggregate with flower-like patterns (Figure 11
c,e). These flower-like crystals consist of AFt, AFm, CH, and C–S–H, and its shape is controlled by GONs. On aGON surface, a multitude of hydration crystals can interweave into a column of crystals growing from the GON surface. Once the column-shaped crystals grows into a pore, crack, or loose structure, they grow apart and form flower-like crystals, which disperse into pores and cracks, acting as fillers and crack arrestors (Figure 11
a–c). When the cement hydration reaction has been in a dense environment, it produces a dense structure with a flower-like pattern and expands further (Figure 11
d–f). Ordered microstructures with flower-like patterns can greatly contribute to improving strength [31