Effect of wetting-drying cycles on compressive strength and microstructure of recycled asphalt pavement - Fly ash geopolymer

The usage of recycled asphalt pavement (RAP) and fly ash (FA) in pavement applications contributes to the sustainable usage of such waste by-products. Although RAP-FA geopolymer and RAP-FA blend without liquid alkaline activator have been proven as a pavement material based on strength and leachate requirement, the durability of these by-products when exposed to an aggressive environment has not been investigated to date. This research investigates the effect of wetting-drying (w-d) cycles on the strength and microstructural changes of RAP-FA blend and RAP-FA geopolymer. The strength characteristics of these materials were determined by unconfined compression strength (UCS) test. The micro-structure of the compound pavement material was also analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). Test results show that the UCS of RAP-FA blend increases with increasing the number of wetting-drying (w-d) cycles (C), reaching its peak at 6 w-d cycles. The XRD and SEM analyses indicate that the increased UCS of RAP-FA blend is due to stimulation of the chemical reaction between the high amount of Calcium in RAP and the high amount of Silica and Alumina in FA during w-d cycles leading to production of more Calcium (Aluminate) Silicate Hydrate [C-(A)-S-H]. For C > 6, the significant macro- and micro-cracks developed during w-d cycles cause strength reduction. For RAP-FA geopolymer, geopolymerization products [Sodium Alumino-Silicate Hydrate, N-A-S-H] co-existed with C-(A)-S-H results in increased UCS within the first 6 w-d cycles. The macro- and micro-cracks when C > 6 cause strength reduction of RAP-FA geopolymers. A better durability performance is observed when RAP-FA geopolymers are prepared with higher NaOH content that can be attributed to formation of a stable cross-linked alumino-silicate polymer structure. The outcome from this research confirms the viability of using RAP-FA blends and RAP-FA geopolymer as alternative sustainable pavement materials.