Lignin fiber reinforced gypsum-cement composite materials: Investigation of fracture properties and freeze-thaw behaviors

The weak crack resistance and water stability of gypsum-cement composite materials limit their further development in the construction sector. In order to enhance the performance and extend the application of gypsumcement composite materials, gypsum-cement composite materials were prepared using gypsum, granulated blast furnace slag, cement, and lignin fiber. Cement content ranged from 10 % to 20 %, while lignin fiber content varied between 0 and 0.4 % by weight. To assess their fracture performance and moisture durability, three-point bending (TPB) fracture tests and compressive tests were conducted on these mixtures. Furthermore, Scanning Electron Microscopy (SEM) was utilized to explore the role of fibers in improving the mixture's properties. The results demonstrated a clear correlation between increased cement content and improvements in both fracture toughness (KIC) and fracture energy (GF). Optimal performance regarding the stress intensity factor was observed at a 0.2 % lignin fiber content after curing for both 7 and 28 days. Despite this, the inclusion of 0.4 % fiber content in specimens with 20 % cement resulted in the highest fracture energies, suggesting an enhanced deformation capacity even though peak loads decreased. After 5 F-T cycles, specimens incorporating 0.2 % fiber exhibited the lowest loss rate of KIC and GF. With a cement content of 10 %, the compressive strengths rose with higher fiber content across all specimens undergoing identical freeze-thaw cycles. Conversely, at a 20 % cement content, maximum strength was achieved with a 0.2 % fiber content, establishing it as the optimal fiber concentration due to its resistance to compressive strength loss during freeze-thaw testing. SEM analysis revealed that lignin fiber ends were well-integrated within the matrix, with Calcium Silicate Hydrates (C-S-H) clearly visible on fiber surfaces. Additional entrapped air voids in the gypsum-cement composite material were generated due to the porous structure of lignin fiber, which could arrest the crack growth energy provided optimum fiber was incorporated. In addition, the bridging effect fiber also contributes to the overall strength and durability of the mixtures.