3.1 Compressive strength
The compressive strength of the foam concrete after adding different amounts of carbon fiber is displayed in Figure 2.
From this figure, whether at the age of 3d or 28d, the compressive strengths of the four groups of specimens change with the same trend: group C1 > group C2 > group O > group C3. Compared with that of group O, the compressive strength of group C1 at 3d and 28d increases by 0.34 MPa and 0.22 MPa, respectively. However, the strength of group C2 increases slightly at 3d and 28d compared with that of group O, only 0.07 MPa and 0.02 MPa, respectively. As more carbon fiber is added, the compressive strength drops dramatically. The compressive strength of group C3 at 3d is only 1.77 MPa, decreased by 41.58% compared to that of group O at the same age. Thus, the addition of carbon fiber in the range of 0.6wt% slightly improves the compressive strength of specimens, and the enhancement effect is relatively good when the content is 0.3wt.%. Lu et al.[33] added carbon fibers to the concrete matrix and found that as the amount of carbon fiber admixture increased, the strength of the concrete showed a tendency to increase and then decrease. This is because the carbon fiber with low content can be evenly distributed in the slurry and fully bonded to the hydration products. And the elastic modulus of carbon fiber is much higher than that of foam concrete, effectively inhibiting the growth of cracks in foam concrete and improving compressive strength. However, when the carbon fiber content exceeds a certain value, it is easy to agglomerate during stirring due to its Van der Waals force and further reduces the processability of the foam concrete slurry. It also damages the pore structure inside the foam concrete, increases the connecting holes, and causes stress concentration after being exposed to external forces.
Figure 3 exhibited the compressive strength of graphite-carbon fiber foam concrete.
It follows that the compressive strengths of the three groups with the addition of graphite and carbon fiber decrease at different ages. The compressive strength of group C2S2 decreases the most compared to that of group O at 3d, accounting for 84.82% of group O. At 28d, the compressive strength of group C2S3 decreases the most compared to that of group O, accounting for 81.08% of group O. The three groups have similar compressive strength values. It indicates that when the content of graphite is less than 10wt.% during adding graphite and carbon fiber, little effect will be generated on the compressive strength. As can be seen from the previous article, the compressive strength of group C2 with 0.6wt.% carbon fiber is slightly higher than that of group O, proving that graphite is unfavorable to the compressive strength. This is because graphite is an inert material with poor cementing ability[34]. In the preparation process of the composite group specimens, we also find that the consistency of the cement slurry increases with increasing graphite content. With the large specific surface area of graphite, a large amount of free water is absorbed, with the result that the carbon fiber cannot be dispersed in the cement slurry uniformly and causing agglomeration. It adversely affects the compressive strength of the specimens.