Alumina-rich pozzolan modification on Portland-limestone cement concrete: Hydration kinetics, formation of hydrates and long-term performance evolution

Abstract

In order to further understand the modification mechanisms of Portland-limestone cement (PLC) concrete by alumina-rich pozzolan (ARP), and therefore achieve the proper use of ARP in PLC, the present work investigates the hydration kinetics, hydration products and long-term performance of concrete based on ARP and PLC. X-ray diffraction and thermogravimetry were employed to identify the hydration products, and the heat evolution was detected by a calorimeter. The pore structure, chloride resistance, accelerated/natural carbonation and free shrinkage were monitored. Besides, the chemical interactions between ARP and calcite induced modification on solid volume of hydrates were also evaluated. The results show that ARP can accelerate the hydration of PLC blends, favors monocarboaluminate (Mc) formation at low replacement level and hemicarboaluminate (Hc) at high incorporation levels. Carboaluminate formation significantly increases the solid volume of hydrates due to the lower densities compared to calcium silicate hydrate (CSH) gels, portlandite and calcite. This effect is amplified when Hc is favored due to its even lower density. These positive effects are beneficial to the improvement of pore structure, volume stability and chloride resistance. During long-term curing, PLC concrete with ARP tends to experience a pore structure degradation induced by the transformation of Hc to Mc, which results in nearly 8% reduction of solid volume. These findings help to achieve a better understanding on the modification mechanisms of ARP on PLC concrete, and benefit the design of concrete based on PLC and ARP in practical.