Effects And Mechanism Of GO/CNTs Hybrid On The Enhancement Of Cement Composites

Cement-based composites retains its popularities as the main building materials in the world with extensive applications due to abundant raw materials,low cost,high strength and long-lasting durability.However,the brittle and porous nature of cement-based composites limits its further application.At present,a variety of reinforcing materials incorporated in cement-based composites are proven effective for addressing the aforementioned drawbacks.Nowadays,the advancements of nano-technology provide the nanoscopic perspective.As graphene derivatives.Carbon nanotubes（CNTs）are a type of hollow tubular fibers crimped from graphene,which possesses the characteristics of high aspect ratio,high tensile strength and Young’s modulus;while graphene oxide（GO）consists of mono-layer of carbon rings bearing many oxygen-containing groups such as-OH and-COOH,which also is a competitive candidate due to the high strength,high specific surface area and excellent dispersive behavior.Both CNTs and GO have been proven capable to enhance the strength,improve pore structure,and impart piezoelectric properties to cement-based composites.However,CNTs suffer from the extremely low dispersibility and re-agglomeration in cement environment.Although GO is soluble in water,the abundance of Ca²⁺in the cement environment would complex with-COO^-on the surface of GO,leading to agglomeration and the discount of strengthening effect.Uniform dispersion of nanomaterials is the prerequisite for strengthening cement-based composites.Combining GO and CNTs together with a suitable hybrid ratio has exhibited the advantages to enhance the dispersion performance and stability.Moreover,GO and CNTs in the state of GO/CNTs hybrid can also connect to each other and form overlapped 3D microstructures.In order to systematically study this synergistic enhancement system,this study conducted in-depth research on the dispersion characteristics of GO/CNTs hybrid by comparing and selecting different dispersants and hybrid ratios,and prepared corresponding cement-based composite to investigate the enhancements and reveal the mechanism.This synergistic reinforcement system of GO/CNTs hybrid were also utilized in magnesium phosphate cement and PVA fiber-reinforced strain hardening cement to extend the applications in cement-based composites.The main results obtained are as follows:1.The aqueous dispersion of GO/CNTs hybridThe GO/CNTs hybrid modified by different surfactants was evaluated by contact angle experiment,optical microscope,sedimentation experiment,UV-vis and nano particle size analyzer.It was found that the polycarboxylic acid dispersants showed the best dispersive performance,because the polycarboxylic acid surfactants can disperse the GO/CNTs hybrid through the two mechanisms:electrostatic repulsion and steric hindrance.The experiment also found that the GO:CNTs hybrid ratio of 3:2 showed the greatest UV-vis absorbance,strongest zeta potentials and the smallest particle sizes,indicating that an appropriate hybrid ratio is crucial to the dispersion and ionization of GO/CNTs.2.The influence of GO/CNTs hybrid on the performance of ordinary Portland cement-based composites and its mechanismThe GO/CNTs hybrids reinforced cement composites with different dispersants modification and various GO:CNTs hybrid ratios were prepared,and the fluidity and mechanical properties were investigated.It was found that the polycarboxylic acid dispersants modified GO/CNTs hybrids reinforced cement composites has the best fluidity and mechanical performance.As to the hybrid ratio,the best mechanical performance occurred when the hybrid ratios is 3:2.The cement compressive strength is increased by 45.20%,which is significantly higher than the same content of GO（28.00%）and CNTs（32.58%）.The properties of GO/CNTs hybrid in aqueous solution and cement-based composites are consistent.The mechanism of GO/CNTs hybrid reinforced cement-based composites can be summarized as follows:（1）GO and CNTs promotes dispersion mutually and enlarge the space between nanoparticles through electrostatic repulsion and steric hindrance;（2）GO and CNTs could work as overlapped 3D structure through covalent connection,which possesses higher enhancement efficiency;（3）GO/CNTs hybrid provides a large number of nucleation sites and carries many water molecules.The nucleation effect and the internal curing effect could work together promoting the hydration process and refining the microstructure;（4）GO/CNTs hybrids could bond with hydration products through covalent connection,improving the efficiency of load transfer and modernizing the hydrates.3.The effects of GO/CNTs hybrid on magnesium phosphate cementThe GO/CNTs hybrid reinforcement system was migrated to magnesium phosphate cement,a totally new cement type with completely different components.The results proved that GO/CNTs hybrid still performed better than GO and CNTs individuals in the magnesium phosphate cement.The characterizations of the reinforced magnesium phosphate cement by TGA,MIP,SEM,XRD and FTIR confirmed that the mechanism of GO/CNTs hybrid enhancing magnesium phosphate cement mainly contains that GO/CNTs hybrid promoted the hydration of magnesium phosphate cement and improved the microstructure and pore properties.4.The co-effect of GO/CNTs hybrid and PVA fibers on cement compositesThe nano-scale GO/CNTs hybrid was combined with the macro-scale PVA fibers,and it showed that the GO/CNTs hybrid helped the PVA fiber-reinforced strain hardening cement bearing stronger flexural load and deformation.It was because the surface of the PVA fiber was modified due to the adhesion of nanomaterials and the bonding between PVA fibers and cement matrix would be strengthened accordingly.Moreover,the abundance of free GO/CNTs hybrid can promote the properties of cement together with the modified PVA fibers.