The Composition Of The Cement - Stem Wood Fiber Composite Sound-absorbing Material, Performance, And Sound Absorption Model

To develop cement-based wood fiber sound absorption material that have good compatibility between wood and cement, excellent performance of sound absorption and outstanding durability, the author has systematically studied the properties of wood and cement, optimized the composition of cement-wood fiber system, and the durability of the composite material. It is also discussed the factors that influence sound absorption of ideal porous sound absorption material. A model is established to describe sound wave absorption in ideal porous material.It is indicated that the main factors influencing the setting time of cement is the composition of wood fiber, that is, cellulose, hemicellulose and lignose. During the initial stage of hydration (less than 1 hour), cellulose and hemicellulose affect the hydration rate of cement severely, to a same extent. However, in the first ten minutes, lignose decreases the hydration rate of cement significantly. From 1 hour to 3 days, hemicellulose and lignose retarded the hydration of cement strongly. Compared with that, the effect of cellulose is much small. Based on the observations, the compatibility between wood and cement has been established. The compatibility can be improved by measures of adding silica fume into cement paste, pre-heating the wood fiber or pre-treating it using saturated lime. In addition, chemical admixtures can be used to improve the compatibility between cement and wood fiber.The composition of cement-based wood fiber sound absorption materials can influence the acoustic and mechanical performances. The result of experiment showed that, the size of wood fiber affects the performance composite greatly, and the composite made of large aspect ratio wood fiber has much better sound absorbing performance. Increasing the length of wood fiber, bending strength and toughness will be improved. The composite made of certain proportion of long and short wood fibers has excellent sound absorbing performance. In addition, thickness and density of cement-based wood fiber sound absorption materials influence the sound absorption performance. Creating cavum behind the composite or density gradient distribution will cause the first resonance frequency transfer towards lower frequency.The cement-based wood fiber sound absorption material has poor stability in dimension because wood fiber has larger distortion after several drying and wetting cycles. The result of experiment showed that, there was a groove at the interface of wood fiber and cement paste after several drying and wetting cycles. This leads to the bond reduction between wood fiber and cement paste, which results in the decrease in flexural strength and toughness. Increasing drying and wetting cycles, has the same effect with large decrease rate. Pre-treating wood fiber in 10% acrylic emulsion (by volume), or soaking in organic silicon repellent agent, the dimensional stability of composite will improve greatly. But its stability still gets worse and worse with drying and wetting cycles. Because of the rather high alkalinity of cement paste, the brown and white rot fungi cannot survive. As a result, the composite has an excellent fungi resistance. A model of micro-structure and anti cycle destruction is established through the study on destruction of cement-based wood fiber composite undergoing drying and wetting cycles. It shows that there are two interfaces in the composite, one is between cement paste and wood fiber, the other is between cellulose fiber and hemicellulose as well as lignose. The first bond is molecule force, and the second one is chemical force. Without undergoing drying and wetting cycles, the destruction occurs at the interface between cement paste and wood fiber; while after several cycles, hemicellulose and lignose will decompose under alkali environment, the bond between cellulose fiber and hemicellulose, lignose becomes very weak, therefore this interface is the weakest part in the composite.Through the study on the sound absorption property of ideal porous material, it can be concluded that pore structure is the main factor affecting sound absorption property of a composite. At the same porosity, absorption performance can be improved by reducing aperture. On the other hand, the absorbing performance can be improved with the porosity increasing at the same aperture. With the increase of pore length, the first resonance frequency transfers towards lower frequency. In addition, the distribution of higher porosity with much bigger pores at the surface and lower porosity with smaller pore inside the materials is the best pore structure for sound absorption. At the same time, a model is established to show the diffusion and absorption of sound wave. Sound absorption of material is determined by its porous structure. The higher the porosity and the bigger the aperture is, the better sound diffusion will be. On the other hand, inner porous structure of the composite is the main factor for sound absorption, the higher the porosity and the smaller the aperture is, the better sound absorption will be.