Research And Application Of Kenaf Straw Lightweight And Crack-resistant Cement-based Material

Cement-based materials are widely used in the field of construction engineering,however,common cement-based materials have such disadvantages as high brittleness,easy cracking,high density and poor insulation effect.In order to solve the above problems,the current research is mainly carried out from the perspective of crack resistance and lightweight.Adding high performance fiber can effectively solve the problem of brittleness and easy cracking,but these fibers are relatively expensive and are not ecologically and environmentally friendly;Adding gypsum and other industrial products can make cement-based materials lightweight and insulation,but they also have poor economy,pollution and other problems.Kenaf is a kind of phloem fiber crop which is planted widely in China and can repair soil polluted by heavy metals such as cadmium,lead and arsenic.The straw contains core with lightweight and heat-insulating and bast fibers with high tensile strength.In this study,the core of kenaf straw was used as a cement-based admixture to make it lighter and reduce the thermal conductivity.Meanwhile,kenaf fiber was substituted for high-performance fiber to achieve crack resistance and toughening.This research can not only improve the shortcomings of common cement-based materials,but also realize the resource,scale and industrial utilization of kenaf straw,especially the heavy metal enriched kenaf straw.So as to provide a sustainable way of "industry and agriculture complement each other" for the use of kenaf to remediation heavy metal contaminated soil.In this paper,the sensitivity of the effects of kenaf straw core(KSC)content,kenaf fiber(KF)content,microsilicon powder content and fly ash floating bead content on the performance of cement mortar was studied by orthogonal test,and two key factors,KSC content and KF content,were selected as key factors affecting the performance of kenaf straw cement mortar.Based on orthogonal experiments,experiments was conducted to study the KSC content(2%,4%,6%,8%),KF content(0.5%,1.5%,2.5%)and KSC particle size(0~2.36 mm,2.36~4.75 mm,4.75~9.50 mm)on the mechanical and thermal properties of cement mortar under the same working performance;Based on the compound admixture test,the influence of only mixed KF with different length and dosage on the mechanical properties,working properties,toughening and crack resistance of cement mortar and its dispersion in cement mortar were deeply studied;In addition,the mechanical properties of slight Cadmium-enriched kenaf straw cement mortar were compared with ordinary kenaf straw cement mortar,the safety and possibility of slight Cadmium-enriched kenaf straw in cement mortar were studied by heavy metal leaching test;Xray diffraction(XRD)analysis,thermogravimetry and scanning electron microscope(SEM)were used to study the microstructure of cement mortar;Finally,the research results were applied to the yiheyuan rural complex homestay reconstruction project in Yiyuan County,Zibo City.The main findings are as follows:(1)The research results of compounding KSC and KF showed that the influence of KSC content on compressive strength,flexural strength,dry density and thermal conductivity of cement mortar is the largest among the four factors;The influence of KF content on the flexural strength and dry density of cement mortar was lower than that of KSC content,but higher than that of microsilica powder and fly ash floating bead content,and its impact on compressive strength and thermal conductivity was least.The compressive and flexural strength of cement mortar decreased with the increase of KSC content,but with 8% KSC content,the 28-day compressive strength of mortar still reached 5.55 MPa,while the thermal conductivity and dry density decreased by 37% and 27%,respectively;KSC particle size has no significant effect on compressive strength,flexural strength,dry density and thermal conductivity of cement mortar;KF can inhibit the development of cracks,and improve the bond strength of mortar.The bond strength is the best when the content is around 1.5%.(2)The research results of only mixed KF showed that KF cement mortar has significant crack resistance.With the same mass fraction and the same length,KF has better anti-crack effect than polypropylene fiber in cement mortar;The fiber content and length increase,the cracking index and dispersion coefficient decrease,but the dispersion of KF in cement mortar becomes worse;In engineering application,it is recommended that the dosage and length of KF should be controlled at 1.5%(mass fraction)and about 15 mm respectively,and the dispersion coefficient should not be lower than 0.88.(3)The microscopic test and heavy metal leaching test showed that the Cadmium-enriched kenaf straw cement mortar has the same mechanical properties as ordinary kenaf straw cement mortar.The maximum leaching concentration of cadmium is 0.0094 mg/L,which is far lower than the national standard limit of 1.0 mg/L.It is safe and reliable in engineering applications;Through XRD,thermogravimetric test and SEM analysis,it can be seen that no new hydration products appear when kenaf straw is mixed with cement mortar,but the hydration of cement mortar is inhibited to some extent,and the internal structure of kenaf straw cement mortar is more loose.