Preparation And Properties Of Geopolymer Based Plant Fiber Composites

In recent years,geopolymers have attracted much attention as a new type of green inorganic cementitious material.Based on the brittleness of the geopolymer itself,it can be toughened by adding plant fibers.Since the organic matter such as sugars in plant fibers will delay the solidification of geopolymers,this paper uses three different methods to pretreat rice straw.Orthogonal experiments and single factor experiments were used to explore the preparation parameters of the geopolymer-based rice straw composite materials.The effects of rice straw before and after treatment on the bending strength,internal bonding strength,water absorption and frost resistance of the composite material were also comparatively studied;at the same time,the performance of mixed rice straw(rice straw before treatment and rice straw after treatment)to enhance geopolymer composites was also been studied.Because the current research cycle of plant fiber reinforced geopolymer is relatively short,in order to further study the durability of geopolymer-based plant fiber composite materials,this paper used water glass,slag,metakaolin,modified water glass,and fir fiber as raw materials.The slag base geopolymer plant fiber composite material and the slag-metakaolin base geopolymer plant fiber composite material were prepared,and the durability of the two composite materials was studied.In this paper,X-ray diffractometer(XRD),Fourier infrared spectroscopy(FTIR),scanning electron microscope(SEM)and energy spectrometer(EDS)were used to analyze and characterize composite materials.The results of the study are as follows:(1)Explored the effect of rice straw on geopolymer composite before and after treatment.The rice straw treated with sodium hydroxide has the best effect on enhancing geopolymers,and the bending strength reaches 9.24 MPa.Therefore,the rice straw reinforced geopolymer treated with sodium hydroxide was selected,and the optimal preparation parameters were obtained through orthogonal experiment and single factor experiment:water glass modulus is 1.7,water-binder ratio is 0.4 and fiber content is 12 wt%.(2)The properties of rice straw reinforced geopolymer composites before and after alkali treatment were studied.Compared with the pure geopolymer,the rice straw before and after the treatment can significantly improve the bending strength of the geopolymer composite,but the enhancement effect of the treated rice straw is better.The water absorption thickness swelling rate of the sample before treatment is better than that of the treated sample,but the water absorption was inferior to that of the treated sample.From the analysis results of FTIR and XRD,it is consistent with the results of other fiber-reinforced geopolymer.The combination of rice straw and geopolymer had no effect on the bonding of geopolymer and no new phases are formed.The results of SEM indicated that the combination of treated rice straw and geopolymer matrix was better than that of the untreated rice straw.In addition,the dissolution of rice straw not only reduced the compactness of the geopolymer,but also reduced the binding effect of the geopolymer matrix and fibers.(3)The freeze-thaw cycle experiment shows that the water absorption rate of the sample before the treatment during the freeze-thaw cycle is greater than that of the treated sample.After the freeze-thaw cycle,the bending strength of the pre-treatment sample and the treated sample are reduced,but it still meets the requirements of the national standard;and the mass loss rate of the sample before treatment is greater than that of the treated sample.FTIR analysis showed that the polymerization of Si-O-Si increased after freezing and thawing,and XRD after freezing and thawing detected more calcite crystal peaks,indicating that the geopolymer was more carbonated after freezing and thawing.The SEM analysis after freezing and thawing showed that the combination of fiber and geopolymer of the treated sample is worse than that of the sample before treatment.(4)Researched mixed fiber(rice straw before treatment and rice straw after treatment)reinforced geopolymer composite material.The bending strength of the composite material reaches more than 12MPa,which can meet the requirements of the national standard for cement particleboard(static flexural strength>10MPa),and it can also improve the water resistance of composite materials.The use of mixed fibers for preparing geopolymer-based plant fiber composite boards is of great significance for the rational use of resources and the improvement of environmental protection.(5)The durability of the slag base geopolymer plant fiber composite material was studied.The results show that the bending strength retention of the sample placed indoors is the best,followed by bagged,and the worst outdoor.From the phenolphthalein detection analysis,it can be seen that the free alkali inside the indoor and outdoor samples has signs of migration,but the bag does not.According to the results of XRD and FTIR analysis,indoor,outdoor,and bagged samples have been carbonated,and the degree of carbonation deepens with time.This indicates that alkali-activated slag geopolymers have poor carbonation resistance.The SEM analysis results show that the outdoor environment causes the most serious damage to the geopolymer and the fiber,and the combination between the geopolymer and the fiber;there are many harmless microporous structures in the matrix of indoor samples,which is one of the reasons for the obvious alkali migration phenomenon;while the internal matrix of the bagged sample is intact,but the fiber toughness is reduced.In addition,the apparent analysis of the sample showed that the surface of the outdoor sample was obviously whitened and chalked,while the bagged sample did not change significantly.Efflorescence appears on the surface of indoor samples,and the main ion involved in efflorescence is Na⁺.According to EDS and FTIR analysis,both Na⁺and Ca²⁺involved in efflorescence are carbonated to form carbonates.(6)Through the durability study of the slag-metakaolin base polymer plant fiber composite material,it can be seen that the durability of the samples placed in bags is better,followed by indoor samples,and outdoor samples are the worst.The alkalinity change of the sample is greatly affected by the environment.Among them,the carbonation rate and the alkalinity decrease rate of the outdoor sample are the fastest,followed by the indoor sample,and finally the bagged sample.XRD and FTIR analysis showed that the geopolymer samples of the slag-metakaolin system were significantly affected by carbonation,and the geopolymers were mainly carbonated to produce calcium carbonate.SEM analysis shows that the reason for the rapid decrease in the strength of the geopolymer of the slag-metakaolin system is that the geopolymer cannot be maintained in a high alkaline environment for a long time,which leads to insufficient reaction of the geopolymers in the early stage and carbonation in the later stage.As well as the deterioration of the bond between the fiber and the matrix in the later stage,those are the reasons for the decrease of the strength of the sample.EDS analysis shows that carbonation leads to a decrease in the Ca/Si ratio in the geopolymer gel and an increase in the Si/Al ratio.