Infrastructure Technology Institute
Work on the application of nanoparticles to enhance the properties of cement-based materials is a growing area in cement and concrete research. In this work, two types of nanoparticles are explored: nanoclays and nanoCaCO3. Small additions of nanoclays can significantly increase the fresh-state stiffness of self-consolidating concrete (SCC) with little compromise to initial flowability. Such properties make applications such as reduced SCC formwork pressure and slipform paving possible to increase efficiency in construction. Through filler and/or seeding effects, nanoCaCO3 particles can accelerate rate of hydration and improve early-age mechanical properties of cementitious materials, including those with fly ash (a coal combustion byproduct) to lead to a more environmentally-friendly material.
In this study, the influence of the method of dispersion of nanoCaCO3 powder on the early-age properties of high-volume fly ash systems (50% replacement of cement by mass) are evaluated. Fly ash slows rate of hydration, delays setting and lowers early-age compressive strength gain, all of which slow construction. NanoCaCO3 powders that were dispersed by an ultrasonication technique were found to accelerate rate of hydration of cement paste and completely offset the delay in setting caused by a 50% fly ash replacement at additions of 5% by mass of cement. It also improved 7d compressive strength of 30% fly ash-cement paste by 20%. To further improve compressive strength, ongoing work on achieving stable nanoCaCO3 suspensions is being done, where stability is measured through absorbance spectroscopy using a spectrophotometer.
To effectively manipulate the fresh-state properties of SCC using clays, it is necessary to obtain a good understanding of their effect on thixotropy: decrease in viscosity under shear followed by a recovery upon the removal of shear. Shear rheological results show that nanoclays significantly increase the thixotropy of cement pastes at additions as low as 0.5% by mass of cement. A novel method of evaluating the rheological properties of granular materials, the tack test, is being explored to obtain a quantitative measure of the clay effect: high fresh-state stiffness at rest and high flowability during casting. Results show the potential of the test to measure the influence of clays, fly ash and superplasticizers (all constituents of SCC) on these two properties as cohesion and flow resistance. This will ultimately help to develop mix design methodologies for SCC to make it suitable for efficient construction applications.