Mechanical Properties And Fracture Behavior Of C_sf(Al₂O_3p) Reinforced Geopolymer Matrix Composites

Two dimension short carbon fiber reinforced geopolymer matrix composites (Csf/geopolymer composites) with different fiber lengths and contents were prepared by the method that mixing geopolymer resin and a kind of carbon fiber preform pre-developed with the help of the ultrasonic scattering treatment. The composites with different interface properties between fiber and geopolymer matrix, different properties of geopolymer matrix were also obtained by electroless deposition Ni-P coating on the surface of short carbon fibers, addingα-Al2O3p into geopolymer matrix or high temperature treatment on the composites. The effects of fiber length, content, interface properties between fiber and matrix, coating thickness, content ofα-Al2O3p and treatment temperatures on the mechanical properties and fracture behavior of the obtained Csf/geopolymer composites were systematically analyzed by X-ray diffraction (XRD), optical microscope (OM), scanning electron microscopy (SEM), environment scanning electron microscopy (ESEM) et al and measured by three-point flexure test. The fracture and toughness mechanisms were studied, the crack propagation and extension models of the composites were built and the mechanical properties and fracture behavior were estimated.The results show that the obtained Csf/geopolymer composites own a two-dimension homogenous distribution of short fibers, and short fibers are undamaged during the process. Comparing with the geopolymer matrix, the Csf/geopolymer composites exhibit drastically improved mechanical properties due to the incorporation of short carbon fiber, especially to the composites reinforced by 3.5vol.% and 7mm short carbon fibers, the flexural strength, fracture toughness and work of fracture increase 4.4 times, 10.6 times and 118 times, separately. With the increase of the fiber content, the flexural strength of the composites exhibits a maximum value when fiber content is 4.5vol.%, then gradually decreases.Cracks propagation and extension on the side of the Csf/geopolymer composites samples were in situ observed and correlated with the load-displacement curve during a three-point flexural test. The results show that the Csf/geopolymer composites exhibit an typical elastic response in the initial stage; The Young's modulus of the Csf/geopolymer composites decrease but the load-displacement curve of the composites still exhibits a nearly linear elastic change, when a lot of microcracks generate on the tensile side of the Csf/geopolymer composites, especially for the composites with 3.5vol.% in content, and 7mm or 12mm in length, 4.5vol.% in content and 7mm in length. The main crack gradually extend with the maximum load is gradually reached, which results in the stress on the minor cracks are released somewhat and lots of minor cracks tend to close to some degree. The Csf/geopolymer composites show a typical pseudoplasticity behavior with the extension of the main crack.The flexural strength and Young's modulus of 3.0vol.% Ni-plated short carbon fiber reinforced geopolymer composites exhibit maximums, 55.2MPa and 5.45GPa separately, as the average fiber coating thickness reaches to 0.15μm. The Young's modulus, flexural strength and fracture toughness of the composites can effectively improved by incorporation ofα-Al2O3p. However, the mechanical properties of the Csf-Al2O3p reinforced geopolymer matrix composites is less than that of the composites reinforced by Csf only, and they become more poor as the increase ofα-Al2O3p content.The shrinkage of the composites at the high temperature gradually decreases as the increase of the content of Csf andα-Al2O3p. Csf andα-Al2O3p have no influence on polycondensation and phase change of the composites after high temperature treatment. The amorphous geopolymer matrix crystallized and formed leucite at high temperature. Incorporating Csf orα-Al2O3p into geopolymer matrix increases the onset crystalline temperature of the geopolymers, especially for Csf andα-Al2O3p are added together.The flexural strength and Young's modulus of the Csf/geopolymer composites were estimated on basis of traditional strength theory and concerning the effects of fiber-orientation, fiber length and porosity. The results show that forecast strength is near to experiment strength with the relative error in the range of -12.9~4.2%, and the forecast Young's modulus have a good match with the experimental value with the relative error in the range of -2.9~-1.5%.In general, the mechanical properties and fracture behavior of the Csf/geopolymer composites were influenced by not only fiber length, fiber content but also interface bonding strength, the modulus of fiber and matrix and distribution of microcracks etc. The flexural strength can be improved to an extent by increasing interface strength between fiber and matrix, however, exorbitant interface strength will result in decrease of work of fracture and catastrophic fracture. Low Young's modulus and appropriate interface strength favor to generate a lot of microcracks during the test and improve application safety.