Study On Properties And Strength Prediction Of Metakaolin And Modified Cotton Stalk Fiber Reinforced Cement-based Materials

Xinjiang,as the main base of cotton production in China,the annual output of cotton straw is more than1100 million tons.How to make efficient use of cotton straw resources is a questiont of concern in this region.Using cotton stalk fibers（CSFs）as reinforcement material mixed with cement to make lightweight,environment-friendly and low-cost cotton stalk fiber cement-based composite（CSFCC）is a feasible ecological solution.However,as a kind of plant fiber,CSFs is rich in cellulose,lignin,sugar and other substances.When it forms a composite with cement,it will affect the early setting and hardening of cement,so its application is limited.In order to improve the effect of CSFs on cement-based materials and improve the early mechanical properties of CSFCC,CSFs was modified by sodium hydroxide（Na OH）solution and sodium silicate（Na₂Si O₃）solution respectively,and metakaolin（MK）was used to replace part of cement into the composites.The effects of different modification conditions,MK content and water-binder ratio on the fresh workability,early mechanical properties,water absorption and microstructure of CSFCC were studied.The crystallinity,phase composition and microstructure of CSFs and CSFFCC 28-day samples were characterized by X-ray diffraction（XRD）,Fourier transform infrared spectroscopy（FT-IR）and scanning electron microscope（SEM）.The grey correlation method was used to analyze the relationship between the factors in the mix ratio of CSFCC and the compressive strength of CSFCC,and the prediction model of CSFCC compressive strength based on BP neural network was established,which provided some reference for the effective utilization of cotton stalk resources.The main contents and conclusions of this paper are as follows:（1）The addition of CSFs has retarding and water-reducing effect on cement.When CSFs is modified by Na OH solution,the mass loss rate increases with the increase of Na OH concentration and soaking time,and then tends to equilibrium.It is recommended that the best treatment method of CSFs is soaking in 4%Na OH solution for 6 hours.The surface voids of CSFs fiber treated with Na OH solution increased,and the crystallinity of cellulose increased by 14.5%.After treatment with Na₂Si O₃ solution,some pores on the surface of CSFs were closed,and the crystallinity index decreased by 2.3%.（2）For the CSFs modified by NaOH solution,the collapse degree of CSFCC decreases and the cohesion increases with the increase of CSFs content.CSFCC shows plastic failure in the test of mechanical properties,the 3-day compressive strength of the specimen decreased gradually with the increase of CSFs content,but the flexural strength increased at first and then decreased.When the content of CSFs was 3%,CSFCC had the best mechanical properties,the compressive strength and flexural strength of 3-day was23.5MPa and 5.1MPa respectively,which was 9%and 96%higher than that of unmodified CSFs cement-based materials.At the same time,the diffraction intensity of Ca（OH）₂ peak increased in the early stage of hydration reaction.Scanning electron microscope showed that a large number of hydration particles were attached to the pores and cement cementation surface of CSFs,which enhanced the mechanical bite force with cement-based materials and improved the compatibility with cement-based materials to some extent.Under the condition of Na₂Si O₃ solution modification,the 28 days compressive strength of CSFCC is 10.01%higher than that of Na OH solution modification,but the flexural strength is only 81%of that of Na OH solution modified CSFCC.The splitting tensile strength is consistent with the compressive strength.（3）With the increase of MK content,the collapse degree of CSFCC decreases.At the age of 28 days,compared with the specimens without MK,the specimens with 10%MK content had a denser surface,The reaction with MK produced more C-S-H and the compressive strength of 3-day,7-day and 28-day increased by 17.8%,11.7%and 26.6%respectively,but the early flexural strength of CSFCC decreased with the addition of MK.When the water-binder ratio is 0.7,compared with the CSFCC with water-binder ratio of0.6,the compressive strength decreases 6.26 MPa.It is suggested that the optimum content of MK is 10%,and under the condition that the workability of CSFCC meets the requirements,properly reducing the water-binder ratio is beneficial to the development of its early strength.（4）According to the grey correlation degree analysis,the correlation coefficient between each factor and compressive strength is as follows:CSFs treatment mode（0.50697）>water consumption（0.48816）>MK content（0.47832）>CSFs content（0.37739）>CSFCC water absorption（0.32741）.Finally,the regression values of the CSFCC compressive strength prediction model based on BP neural network in training,verification and testing are 0.99696,0.99895 and 0.99976 respectively,all close to 1,and the relative error range between the predicted value and the actual value is-3.0%-2.4%.The prediction effect of the model is good.