Preparation Of Cement Based Composites Reinforced With Plant Fiber And Their Mechanical Properties

Fiber is one of necessary compositions of cement based materials. Plant fibers has many advantages over most synthetic fibers, including polypropylene fibers and glass fibers, such as low density, low cost, widely available, excellent specific strength, high specific modulus and their renewability. Therefore, the use of plant fibers in the production of cement based composites has drawn an increasing interest in many countries. However, the long-term durability of plant fibers reinforced cement based composites undergoes reduction, resulting in limiting the application of plant fibers in building materials. In addition, the properties of plant fibers are affected by many factors, such as growth climate condition, harvest period, pretreatment process and technology, which could lead to unpredictable properties of fibers/cement based composites. Aiming at these problems, the pristine rice straw and three fibrous materials during the components separating process from rice straw, were used as the reinforcement of cement based composites. The relationship among different kinds of straw fibers as well as the hydration of the straw fiber filled cement based composites was studied. Moreover, the detailed study of plant cellulosic fibers filled cement based composites, including the mechanical properties, the strengthening mechanism and the improvement of durability of the composites, were systematically studied in this thesis.1. The morphological and structural characteristics of four kinds fibers, named pristine rice straw (RF1) and three fibrous materials (RF2, RF3 and RF4) during the components isolating process from rice straw, were investigated. The results indicated the pretreatment of rice straw could eliminate amorphous hemicellulose and lignin, and thus increased the crystallinity and improved the thermal stability of the rice straw fibers. Additionally, the effect of four kinds of straw fibers on hydration of the straw fiber filled cement based composites were studied using isothermal calorimetry, X-ray diffraction (XRD) and thermogravimetry (TG/DTG) techniques. For this purpose, the composites containing 10 wt.% of the pristine and the pretreated rice straw fiber were prepared. As a result, the presence of both the pristine and the pretreated rice straw fibers led to delay of the setting time and retard of the hydration of cement. Compared with the pristine straw fibers filled cement based composites, the pretreated rice straw fibers filled cement based composites displayed better mechanical properties, because the principal compounds of rice straw, especially hemicellulose and lignin, exerted a negative impact on the setting and hydration of cement. The influence order of main component of rice straw on the hydration of cement was hemicellulose, lignin, cellulose. The corresponding results further proved the necessity of the pretreatment of rice straw as reinforcement of cement based materials.2. The cement based composites reinforced with cellulosic fibers isolated from rice straw were fabricated by a slurry vacuum de-watering technique. The physical structures and mechanical properties of the composites with fiber contents ranging from 2 wt.% to 16 wt.% were investigated. Moreover, the composites reinforced with bamboo cellulosic (BF) fibers and the control cement paste, samples without cellulosic fibers, were also fabricated as reference materials. As a result, the loading rather than the type of the fibers displayed more obvious influence on the physical properties of the composites. The bulk density of the composites was decreased by 12.4%-37.3% as a result of the introduction of cellulosic fibers. Furthermore, the cement based composites reinforced by cellulosic fibers showed a remarkable improvement in the mechanical properties. The optimal flexural strength of rice straw cellulosic fibers (RF) reinforced cement based composites (RFRCC) samples is reached when the fiber content is 8 wt.%, which is 124.3% of the control specimen at 28 days. As a comparison, the optimal flexural strength of bamboo cellulosic fibers reinforced cement based composites (BFRCC) is 102.7% of the control specimens when using 10 wt.% BF. At the fiber content of 16 wt.%, the fracture toughness of rice straw and bamboo cellulosic fibers reinforced cement based composites at 28 days is 37 and 45 times respectively higher than those of the control samples.3. Low velocity impact tests of bamboo cellulosic fiber filled cement base composites were conducted using CEAST 9350 impact test machine with a drop weight system. The damage models and mechanisms of the composite samples with different fiber content, which were subjected to impact at the different velocity, were investigated. The results showed that the damage models and the impact resistance were affected by both the cellulosic fiber content and impact velocity. When the composite samples reinforced with 0-12 wt.% fiber were subjected to impact at 25 J (impact velocity of 3.0 m/s), the impact resistance of bamboo cellulosic fibers/cement based composites increased with increasing the fiber content. Beyond fiber content of 12 wt.%, the constant increase of the fiber content could not increase the impact resistance of the composites. At different velocity, the bamboo cellulosic fibers/cement based composite samples had the different damage models and energy absorption patterns. Under the lower impact velocity, the composite samples stratified in the direction perpendicular to the impact, the energy absorption pattern was interfacial debonding between the matrix and fibers. With the increase of the impact velocity, the delamination of the composite samples were accompanied with matrix cracking, interfacial debonding and fiber fracture, the energy absorption pattern was fiber fracture and interfacial debonding.4. The bamboo cellulosic fibers/cement based composites, samples with 40 wt.% partial cement replacement by silica fume, were fabricated. Moreover, the bamboo cellulosic fibers/cement based composites without silica fume were also fabricated as reference materials. The composite specimens were subjected to cycles of wetting and drying to accelerate aging evaluation. The microstructure, flexural strength and fracture toughness of the composite samples as well as X-ray diffraction and thermal analyses of the materials were characterized. As a result, the introduction of 40 wt.% silica fume showed a remarkable effect of improving the durability of the composites. Due to addition of 40 wt.% silica fume, the fracture toughness of the composites remained steady in 25 cycles of wetting/drying. By comparison, for the composite specimens without silica fume, whose fracture toughness was 18.2% of the initial fracture toughness after 10 cycles of wetting/drying. The SEM observation, TG/DTG results and XRD patterens indicated that addition of 40 wt.% silica fume decreased and eliminated the content of calcium hydroxide in the matrix, due to the secondary pozzolanic reaction between calcium hydroxide and silica fume, which probably resulted in the remarkably improving in durability of the composites. Moreover, compared with the influence of calcium hydroxide, the pH value of alkaline solution of the matrix was a minor factor.