Through SEM analysis of PVAEGC containing PVA Fibers with different lengths and contents, the microstructure and bonding situation between fibers and the phosphogypsum-based matrix under different influencing factors were obtained, and a preliminary analysis of the influence mechanism was conducted based on the above experimental results. Due to the small range that can be observed in SEM scanning, it is not possible to distinguish the changes in fiber length.
When the fiber length and content are constant, as the diameter of PVA fibers increases, the number of fibers decreases, and the specific surface area of the fibers gradually decreases. As a result, the amount of free water adsorbed by PVA fibers in the slurry also decreases, leading to a slower decrease in fluidity and an increase in setting time. In addition, when the fiber length and content are constant, fibers with smaller diameters have more roots, which makes it easier to form a three-dimensional network structure in the slurry, resulting in a faster decrease in the fluidity of the slurry.
As the length and content of PVA fibers increase, a three-dimensional network structure is formed in the slurry, which increases the internal friction of the slurry and leads to a decrease in fluidity. In addition, as shown in , the molecular structure of PVA fibers contains hydroxyl groups, which are hydrophilic groups that can adsorb a part of free water, causing a change in the water distribution in the slurry and thus making the PVAEGC slurry lose its plasticity earlier. The black arrow in the figure indicates the moving direction of water.
As can be seen from a, the hardened phosphogypsum is a porous material and PVA fibers have good hydrophilicity which can be better combined with the phosphogypsum matrix, making the internal structure of PVAEGC more compact and thereby improving the strength of PVAEGC. In addition, the addition of PVA fibers can effectively transfer stress and play a good bridging role. The bridging effect of PVA fibers changes the internal stress distribution of PVAEGC, limits the extension of stress, and makes the specimen bear the external load together with the matrix, achieving a toughening effect and improving the flexural strength [24].
From and , it can be seen that when the content of PVA fibers is too low, the fibers cannot be completely and uniformly dispersed in the gypsum matrix, and the distance between the fibers is relatively large (as shown in b and a). Although the bridging effect occurs in the matrix under external force, the strength is improved but the crack restriction is not significant and the strength improvement is limited. When the content of PVA fibers is moderate, PVA fibers are evenly distributed in the matrix without entanglement or agglomeration (as shown in b), and the hydrophilic hydroxyl groups in PVA fibers are conducive to the precipitation and crystallization of calcium sulfate dihydrate on their surface, resulting in better adhesion between fibers and phosphogypsum matrix, making the internal structure of PVAEGC more compact, and thus improving the bridging effect of fibers [37]. When the content of PVA fibers is excessive, the dispersion ability of PVA fibers in the slurry is poor, and it is prone to phenomena such as crossing, entanglement, and agglomeration of uneven distribution (as shown in f and c), which will increase the internal pores and defects of the specimens, leading to an increase in porosity and a decrease in the compactness of the matrix. After the slurry hardens, the content of phosphogypsum in these unevenly distributed areas is relatively small, becoming stress concentration areas, which leads to a decrease in the strength of PVAEGC [37].
From , it can be seen that when short PVA fibers are added, although they can play a certain bridging role, the length is too short ( a), making it easy for the fibers to be pulled out when PVAEGC cracks, so the strength improvement effect is not very significant. As the length increases, the bonding force between PVA fibers and the gypsum matrix hinders the pull-out of the fibers, thereby preventing the development of cracks and improving the strength. When the length of PVA fibers continues to increase, their dispersion ability in the slurry becomes worse, and it is prone to phenomena such as crossing, entanglement, and agglomeration of uneven distribution (as shown in c). After the slurry hardens, the content of phosphogypsum in these unevenly distributed areas is relatively small, becoming stress concentration areas, which leads to a decrease in the strength of the specimens. In addition, when the length of PVA fiber is too short to reach the critical length of the fiber, the phosphorus building gypsum base may not be able to effectively transfer the load to the PVA fiber, resulting in low flexural strength of the specimen. When the length of PVA fiber is moderate and meets the critical length of fiber, PVA fiber can effectively share load and provide enough deformation capacity, which greatly improves the flexural strength of the specimen [39].
The mechanical strength of PVAEGC depends on the strength of the fiber bridging stress. From the perspective that the bridging stress of the fiber is a function of the fiber quantity, the finer the fiber, the more the number of fibers under the same content (as shown in ), which is more beneficial to the bridging stress, so the strength is higher. In addition, the strength of PVAEGC is also related to the wrapping force of the fibers [35]. Under the same content, the finer the fiber diameter, the greater the number of fibers, and the larger the specific surface area of the fibers, which increases the wrapping force of the fibers, thereby leading to an increase in the strength of PVAEGC [40,41].