Nano-modified concrete at sub-zero temperatures: experimental and statistical modelling

In cold regions, concrete practitioners face challenges when trying to achieve quality results with concrete produced under low temperatures. The addition of nano-silica, which has vigorous reactivity, to concrete can produce mixtures with a dense microstructure and improved hardened properties under cold temperatures. Thus, this research focused on gaining a fundamental understanding of the performance of nano-modified concrete which was mixed, cast and cured at a temperature of −5°C, without any method of heating or insulation. This study adopted the response surface method as a statistical modelling approach to assess the effect of different parameters on the performance of 28 mixtures. Four factors were implemented in this model – water/binder ratio, fly ash content (0–25%), nano-silica dosage (0–4%) and type of antifreeze admixtures – followed by optimisation scenarios. The mixtures’ performance was assessed based on multiple responses: initial and final setting times, early- and late-age compressive strengths and resistance to freezing–thawing cycles. In addition, mercury intrusion porosimetry, thermogravimetry and backscattered scanning electron microscopy were conducted to capture the microstructural evolution of the mixtures. Nano-modified mixtures with and without fly ash, especially with a low water/binder ratio (0·32) and high calcium nitrite content, showed promising performance when cast under cold weather conditions without any protection method.