Study On The Effect Of Fiber On The Properties Of Cement-based Materials At Micro And Nano Scale

Cement is the most widely used building cementing material at present.Cement-based materials have high compressive strength,but relatively low flexural strength.The flexural resistance of cement-based materials can be improved by adding fiber materials.At present,the influence of fiber on the properties of cement-based materials has been studied extensively,but most of them focused on macro properties.In order to further clarify the mechanism,the influence of different fibers on the properties of cement-based materials was studied at micro and nano scale in this paper.In this study,five kinds of fiber and its powder were selected,including steel powder(IP),wood powder(WP),basalt powder(BP),carbon fiber powder(CFP),glass fiber powder(GFP),steel fiber(IF),wood fiber(WP),basalt fiber(BF),carbon fiber(CFP)and glass fiber(GFP).Firstly,X-ray diffraction(XRD),thermogravimetric analysis(TGA)and other methods were used to determine the physical and chemical properties of each fiber powder.Then,the simulated solutions of pH,Ca2+,Na+,K+and SO42-were prepared,and the adsorption properties of the fiber powders for Ca2+,Na+,K+and SO42-were studied by zeta potential.According to the particle size and morphology of different fiber particles,the influence of different fiber particles on the rheology of cement paste was studied.Meanwhile,the nucleation and growth of hydration products on the surface of different fiber powders were observed.Then,the effect of fiber powder on the porosity of cement paste at different hydration ages was studied with mercury intrusion porosimetry.Afterward,scanning electron microscopy(SEM)was used to observe the fracture path and fracture surface morphology of hardened cement paste mixed with different fiber particles.Finally,the influence of different fibers on the properties of cementbased materials was further determined by testing the flexural and compressive strength.The results showed that:(1)Five kinds of fiber powders have little effect on hydration products,and no new hydration products were found in the cement paste mixed with the fiber powders.IP particles have a strong adsorption capacity for Ca2+.The adsorption capacity of BP,CFP and GFP for Ca2+was similar but weaker than IP.GFP particles have a certain adsorption capacity for K+.WP particles have the lowest adsorption capacity for Ca2+.(2)The addition of IP and GFP can improve the fluidity of cement paste,while the addition of BP,CFP and WP has a negative impact on the fluidity of cement paste.(3)IP particles have a strong adsorption capacity for Ca2+,which can promote nucleation of C-S-H on its surface.The adsorption capacity of BP,CFP and GFP particles for Ca2+is weaker than that of IP.However,since the surface of CFP and GFP particles is smooth and not conducive to the growth of hydration products,BP particles’ surfaces have more early hydration products than CFP and GFP particles’surfaces.Because WP is complexed with Ca2+in the pore solution of cement paste,no C-S-H is formed on its surface.(4)The porosity of cement paste is increased by the addition of fiber powder.Adding IP increases the porosity of cement paste.The addition of IP is beneficial to the pore structure of cement paste.The porosity of CFP and GFP paste is similar but slightly larger than that of BP paste.The pore structure of cement paste mixed with WP is the worst.(5)In cement paste blended with IP,cracks propagate along the matrix or directly through IP particles.It indicates that the adhesion between IP and hydrates is strong.In cement paste mixed with BP,CFP and GFP,the cracks are mostly developed around the particle surface.For WP,most of the cracks develop along the surface of particles,and a few of them pass through WP particles because of the low strength of WP particles themselves.It suggests that their interfaces are very weak.(6)The cement paste mixed with IF has the most obvious increase in flexural strength without significantly reducing the compressive strength.The addition of BF,GF and WF similarly increases the flexural strength and decreases the compressive strength of cement paste.Due to its small size,adding CF improves the flexural strength of cement paste.