Mechanism and performance study of cement-organic integrated materials

Abstract

Cement-based materials has high compressive strength but is relatively weak in tension, and its porosity can lead to physical and chemical deterioration. Polymers, on the other hand, are weak in compression but have relatively high tensile capacities and provide good resistance to physical and chemical attack. Integration of these two materials can yield composites with excellent strength and durability properties. In this thesis, the preparation and characterization of cement-organic composites were conducted. Also, the mechanism for performance enhancement of such composites was further studied. Firstly, by controlling the temperature in the hydration process of Ca₃SiO₃, non-aggregated Ca(OH)₂ nanoparticles were synthesized, with diameters < 5 nm. These nanoparticles were applied to improve the mechanical properties of polyacrylamide hydrogels. The tensile stress and stretch ratio at rupture of the enhanced NC gel could achieve 430 KPa and 121 with only 40-p.p.m. nanoparticle content. The NC gel containing 200-p.p.m. nanoparticles could revert to 90% of its original size after enduring 100-MPa compressive stress. Moreover, the possibility of using cement particles to replace Ca₃SiO₃ for generating nano particles and enhancing PAM hydrogels was investigated. It turned out that the nano Ca(OH)₂ particles could be obtained from cement suspension, with Polycarboxylate ether (PCE) as the dispersant. And the enhancement effect to the polymer network was similar. It implied that nanoparticles for significant hydrogel enhancement could be mass-produced as Portland cement were cheap and available at every corner of the world, which made it possible to produce the cost-effective engineering hydrogels with excellent mechanical properties. Secondly, in addition to employing cement generated nano particles to enhance the organic materials, the effect of organic material on property enhancement of cement-based materials was investigated. Based on the strong chemical bonding between Ca²⁺ and PAM, the PAM/cement composite with the continuous interpenetrating network was produced by a simple in-situ polymerization process. The continuous polymer-cement network in the composite, as evidenced by SEM, EDS and Tof-sims results, could greatly improve the flexural strength of the composite. With only 1% polymer content, the flexural strength increased almost three times and at the same time the compressive strength could still be maintained almost the same value with OPC reference specimens. Inspired by the high viscosity of cement slurry after adding the PAM gels, a double framework lightweight cement composite was developed by applying high water/cement ratio in the fabrication process. This unique double network provided the possibility of achieving lightweight cement composite by simple adjusting the proportion of gel solution. Also, the density, mechanical and thermal properties of LW cement composite were controllable. Finally, besides Ca₃SiO₃ and cement, the feasibility of using other inorganic salts like magnesium oxide to produce tiny nanocrystals to enhance the hydrogels was investigated. It had been demonstrated by the experiments that MgO could also be used to generate nano particles to strengthen hydrogel. It could be summarized to a general method to produce nano particles, i.e. controlled ion release and nano crystal particle formation from an inorganic filler. Such method would have a great potential to be used in future.

Date of Award	2018
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology