Study On Cement-bonded Wheat Straw Composites

With forest conservation and utilization of agricultural residues becoming more and more significant recently,manufacturing cement-bonded composites with wheat straw instead of wood particles and conventional fiber has been a research trend in related fields.In this research wheat straw and portland cement were used as raw materials to press composite in the process of cold pressing.With the purpose of exploring the composite mechanism of cement and wheat straw,the effects of five types of pretreatments(alkali solution,hot water,cellulase degradation,xylanase degradation,non-starch polysaccharide enzyme degradation) and three types of additives (CaCl₂,Al₂(SO₄)₃,Na₂SiO-3) on chemical constituent and organic texture of wheat straw, compatibility of wheat straw-cement-water mixture,and bonding performances of cement and wheat straw were studied by SEM,FTIR and the evaluation system of wettability(θ-t curve and wetting coefficient-K) and hydration characteristics(compatibility coefficient-C_A, hydration eigenvalue-T_max and t).Optimum experiments of wheat straw size,water/cement ratio,cement/particle ratio,pretreatments methods and types of additives were conducted to make the composite meet the requirement of wood/cement composite standard(JC411/T-2007). The following conclusions were got by the corresponding test according to the above thoughts.(1) Five types of pretreatments methods changed the chemical constituent and organic texture of wheat straw,improved the wettability of external surface of wheat straw and the compatibility of wheat straw-cement-water mixture,and finally improved physical properties of composites.Among the five types of pretreatments methods,non-starch polysaccharide enzyme degradation and alkali solution were the optimum.(2) The wheat straw pretreated with 1%NaOH solution changed as follows:â‘ SEM analysis showed:smooth external surface was damaged and a large number of holes and grooves appeared on it.Inner surface was destroyed severely,leading to the abscission of original organization structure.â‘¡FTIR analysis showed:Within 3500～3300 cm^-1,peak value of—OH was increased;Within 1710～1300 cm^-1,a series of peak values were increased significantly;Within 1300～1000 cm^-1,characteristic peak of O—Si—O was increased.â‘¢Penetration and spreading coefficient K determined by 6-t curve became 11 times more than before.â‘£By means of time—temperature curve of wheat straw-cement-water mixture,T_max of 525^#and 425^# cement increased by 20.8℃and 8.2℃respectively,t of 525^# and 425^# cement decreased by 14.5 h and 18.5 h respectively,and C_A of 525^# and 425^# cement increased by 45.1%and 48.4%respectively.⑤Composite's MOR,24h MOR,MOE and IB increased to 9.41 MPa,8.07 MPa,3879.8 MPa and 0.41 MPa respectively.All mentioned above shows: â‘ Wax layer of external surface of wheat straw can be hydrolyzed by NaOH solution, producing higher fatty acid and higher fatty alcohol.â‘¡Some cellulose,hemicellulose and lignose can be hydrolyzed by NaOH solution.â‘¢NaOH solution can reduce SiO₂ in wheat straw.(3) The wheat straw pretreated with non-starch polysaccharide enzyme changed as follows:â‘¡SEM analysis showed:stomata of external surface were damaged and appeared shape of "Z";A large number of tearing holes and grooves appeared on inner surface,size and number of which were increased remarkably compared with cellulase or xylanase degradation.â‘¡FTIR analysis showed:Within 3500～3300 cm^-1,peak value of—OH was increased;Within 1710～1300 cm^-1,a series of peak values were increased significantly.According to peak intensity,degradation effect of non-starch polysaccharide enzyme was more significant than cellulase or xylanase.â‘¢Penetration and spreading coefficient K determined byθ-t curve became 3 times more than before.â‘£By means of time—temperature curve of wheat straw-cement -water mixture,T_max of 525^# and 425^# cement increased by 22.1℃and 13.6℃respectively,t of 525^# and 425^# cement decreased by 16.0 h and 19.0 h respectively,and C_A of 525^# and 425^# cement increased by 53.7%and 58.1%respectively.⑤Composite's MOR,24h MOR,MOE and IB increased to 10.13 MPa,7.79 MPa,4012.7 MPa and 0.45 MPa respectively. All mentioned above shows:non-starch polysaccharide enzyme has more significant degradation effect on inner surface and external surface,which is consist of cellulase and xylanase.Cellulase is mainly used to degrade cellulose in fibers to create simple sugars,and xylanase is able to breakdown xylan at theβ-1,3-D xylanosidic bond.The two types of enzymes promote each other,resulting in a better effect than single enzyme.(4) By the compatible inhibitory coefficients(T_max,t and C_A) based on hydration temperature curve of wheat straw-cement-water mixture,three types of additives decreased t and increased T_max and C_A in different degrees,improving the compatibility of wheat straw-cement -water mixture.The effects of the three additives in turn were as follows:CaCl₂>Al₂ (SO₄)₃>Na₂SiO₃.CaCl₂ can promote hydration reaction and so improve the compatibility of wheat straw-cement-water mixture.By means of CaCl₂,T_max of 525^# and 425^# cement increased by 26.9℃and 19.4℃respectively,t of 525^# and 425^# cement decreased by 17.0 h and 20.5 h respectively,and C_A of 525^# and 425^# cement increased by 52.7%and 55.3% respectively.(5) The levels at which the wheat straw size,water/cement ratio,cement/particle ratio were used in the experiment had significant effects on physical properties of composites.Based on the available data,optimum process conditions were obtained as follows:wheat straw size of 0.60～2.00 mm,water/cement ratio of 0.45:1,cement/particle ratio of 4:1,non-starch polysaccharide enzyme degradation of wheat straw,additive concentration from CaCl₂ and Na₂SiO₃ applied at 3%of the cement mass respectively,cold-pressing at 2.5 MPa for 4 h,and curing for 28 days to the final moisture content within 12%.The composites manufactured under the optimum process conditions presented acceptable properties,meeting the requirement of the China Building Material Standard(JC411/T-2007).