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

Abstract

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 6w–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 6w–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.