Mechanical Properties And Durability Of Sodium Silicate Modified Bamboo Fibers Reinforced Cement Based Composites

Cement and cement-based composites have high compressive strength and low toughness which can easily produce cracks under external loads.Cracks are one of the major disease of cement and cement-based composites when applied in engineering.Bamboo fiber has many superior properties including high specific strength and modulus,renewability,degradability,light weight and low price.Incorporating bamboo fiber into cement base composite is expected to improve the tensile strength and durability of the material.In considering the fact that the bamboo resources are abundant in Fujian Province and the development and application of bamboo fibers is not sufficient,the bamboo fiber reinforced cement-based composites were investigated in this paper.However,the interfacial bonding between bamboo fibers and cement matrix is poor.Thus,the bamboo fibers were chemically modified in order to improve interfacial adhesion between fibers and cement matrices,thus leading to increased mechanical properties and durability of the resulting composites.In this study,the surface chemical composition of bamboo fibers was analyzed by FT-IR and X-ray photoelectron spectroscopy?XPS?and nuclear magnetic resonance?NMR?.Then,the bamboo fibers were chemically modified by sodium silicate in order to improve the interfacial bonding between bamboo fibers and cement hydration products.The hydration process of cement materials and the hydration products were analyzed as well.XPS and NMR were used to characterize the surface chemistry of modified bamboo fibers;SEM was used to evaluate the microstructure and the interfacial adhesion between the modified bamboo fibers and cement matrices.The influence of bamboo fiber modification on the mechanical properties of cement-based composites was investigated.The volume stability of cement mortar was evaluated by drying shrinkage test and the permeability of mortar was characterized by capillary absorption coefficient tests.The main conclusions are drawn as follows:?1?Bamboo fiber has a large number of polar hydroxyl groups and phenolic hydroxyl groups on the surface,leading to an incompatibility with cement hydration products.The cement hydration results in an alkaline environment around the bamboo fibers,which is harmful to the strengths of bamboo fibers and their reinforced composites.?2?When bamboo fibers are treated by sodium silicate,the substances containing Ca²⁺can be adsorbed on the surface of bamboo fibers by sodium silicate.On one hand,it improves the roughness of bamboo fibers so that enlarges the contact area between bamboo fibers and cement;on the other hand,it mitigates the damage of bamboo fibers caused by alkaline.The Si-OH groups of sodium silicate can react with the hydroxyl groups of bamboo fibers,and the Si-OH groups can also participate in the hydration of cement.Thus the treatment of bamboo fibers with sodium silicate effectively improves the interfacial bonding between bamboo fibers and cement matrix,thereby increasing the mechanical properties of bamboo fiber cement-mortar composites.The modified bamboo fiber cement mortar had 8.9%and 15.6%increment in the flexural strength and tensile strength respectively,compared with the fiber cement mortar without addition of bamboo fibers,while its compressive strength decreased by 8.2%.?3?Compared with cement matrix,bamboo fibers have low elastic modulus.Thus,the compressive strength of cement-based composites was reduced when adding bamboo fibers into cement matrix.The compressive strength of cement mortar with modified bamboo fibers decreased by 9.0%when compared with that without bamboo fibers addition,but it still meet the engineering application requirements.The compressive strength of unmodified bamboo fiber concrete is 23.6%lower than that of plain concrete,while modified bamboo fiber concrete had 17.9%lower than that of plain concrete in compressive strength.?4?The addition of bamboo fibers reduces the shrinkage of cement mortar and improves the volume stability of cement-based materials.The capillary water absorption tests showed that the water permeability of cement-based materials was improved by adding bamboo fibers,and the cement-based materials with unmodified bamboo fibers had the highest water permeability.The results of anti-freezing and carbonization experiments showed that durability of modified bamboo fibers cement composites is better than that of unmodified bamboo fibers.