Hybrid Modified Waste Rubber Powder And Its Influence On The Structure And Properties Of Cement-Based Materials

With the development of automobile industry large amounts of waste rubber tires are produced all around the world. More and more researchers have concerned on the application of crumb rubber powder in cement-based materials. This is of great interest because it can reduce the environment pollution caused by waste rubber. Besides, cement-based materials containing rubber is superior to conventional ones with following advantages, such as lower density, higher toughness and impact resistance, enhanced ductility and better sound insulation and so on. But it is also found a fatal weakness that is a sharp decline in the strength of cement-based materials incorporating rubber powder, because the surface of rubber is hydrophobic and is not compatible with cement-based materials. This greatly limits the application of waste rubber in cement-based materials. Therefore, the modification of rubber powder is one of the key techniques associated with its application in cement-based materials. With improved performance, modified rubber powder may serve as a functional additive of cement-based materials. Thus, it is of prime importance for comprehensive utilization of rubber powder and preparation of functional cement-based materials.In this study, rubber powder (RP) as a cross-linked elastomer was hybrid modified by sol-gel method under swelling condition with reactive precursor tetraethyl orthosilicate (TEOS) or sodium silicate, respectively, and also inducting organic siloxane with hydrophilic groups. Then the hydrolysis and condensation reactions took place in situ and the hybrid modified rubber powder TRP (modified with TEOS) and SRP (modified with sodium silicate) were produced. The structures of the obtained products were investigated with Fourier Transform Infrared Spectroscopy (FTIR), Thermalgravimetric Anylsis (TGA), Nuclear Magnetic Resonance (NMR) of 29Si, Electron Probe Microanalysis (EPMA), Scaning Electron Microscope (SEM) and so on. The results show that Si-O-Si hybrid network has interpenetrated through the network of RP and a lot of hydrophilic groups–OH have also inducted onto the surface of RP at the same time. Contact angle of water on the surface of modified rubber powder is decreased by 26o (from initial 94o to 68o). The surface of RP has been turned from hydrophobic to hydrophilic. Cement mortar specimens with hybrid modified rubber powder addition were prepared and cured to definite ages, thereafter their strength, elastic modulus, shrinkage, impact toughness, impermeability, sound absorption and thermal insulation properties were tested. Compared to the mortars with RP, NaOH modified rubber powder (HRP) and coupling agent modified rubber powder (CRP), the mortar with hybrid modified rubber powder has the highest compressive and flexural strength. It is found that the type and content of precursor of hybrid modified rubber powder have greatly effect on the strength of cement mortar. When TEOS addition is 5phr, the 28day compressive and flexural strength of TRP-cement mortar is increased by 51.5% and 29.0% respectively, compared with that of RP-mortar. On the same way, the 28day compressive and flexural strength of SRP-cement mortar is enhanced by 48.8% and 17.2% respectively with 6phr sodium silicate. In comparison to TEOS, sodium silicate is cheaper, more safety and environment friendly. So in this study researches were mainly performed on cement mortars with SRP addition in the subsequent experiments.In addition, the results show the anti-impact performance of SRP-cement mortar with 9% SRP addition is increased by 7times, while the average sound absorption coefficient of SRP-cement mortar with 7% SRP addition is improved by 17%, at the same time, its thermal conductivity is decreased by 67%. Furthermore, compared with the reference specimens the anti-permeability pressure of the mortar with 0.8% SRP addition is increased by 40% and that of mortar with 2% SRP addition is improved by 180%.It is expected that the hybrid modified rubber powder can be used as a new additive in the production of functional cement-based materials. Using SRP one kind of thickening powder applied for making waterproof mortar was prepared, with it and only 12.8% cement addition, dry-mixed waterproof mortar with strength grade of M10 and impermeability level of P10 was prepared. Compared to using common waterproof additive, production cost of the dry-mixed waterproof mortar is reduced by about 50%. This shows SRP can also find its application in the production of dry-mixed waterproof mortar by increasing impermeability, water retention and adhesive strength and reducing production cost as well.The result shows that the interfacial adhesive strength between SRP and cement mortar is increased by 3 times in comparison to that of reference specimens. Morpological observations confirm that the hydration product between SPR and cement mortar is C-S-H gel, thus, interpenetrating networks structure of C-S-H gels is formed, leading to improvement of interfacial adhesive strength. The results of XPS indicate that the O-Si-OH group on SRP's surface can react with Ca and Si in hydrating cement paste and generate Si-O-Si and Si-O-Ca group, which makes the rubber powder and the cement paste combine closely together by chemical bonds. To sum up, SRP itself and the bond between SRP and cement paste block the capillary pores of hardened cement paste so as to improve impermeability of cement mortar. As a result, it has been originally put forwarded that the reaction model of hybrid modified rubber powder with hydrating cement during different hydration stages, and the effect and mechanism of SRP on cement mortar performance.