The Effect of Reclaimed Asphalt Pavement (RAP) Aggregates on the Reaction, Mechanical Properties and Microstructure of Alkali-Activated Slag

Juliana O Costa,Augusto Cesar S Bezerra,Johan Blom,Wim Van Den Bergh,Paulo H R Borges,Flávio A Dos Santos

doi:10.3390/civileng2030043

Abstract

Reclaimed asphalt pavement (RAP) is a recyclable aggregate produced during the demolition of old flexible pavements and consists of natural aggregates (NA) coated with aged bitumen. The detrimental effect caused by the bitumen coating on strength and porosity has limited the use of RAP on traditional cementitious systems. This study investigates the potential use of fine RAP to substitute NA in the production of alkali-activated slag mortars (AAM). The effect of different activator dosages was assessed, i.e., either 4% or 6% Na2O (wt. slag) combined with a modulus of silica equal to 0, 0.5 and 1.0. The characterisation of 100% RAP-AAM consisted of hydration kinetics (Isothermal Calorimetry), pore size distribution (Mercury Intrusion Porosimetry), mechanical performance (Compressive and Flexural strength), and microstructure analysis (Scanning Electron Microscopy and Confocal Laser Scanning Microscopy). The results show that RAP aggregates do not compromise the reaction of the matrices; however, it causes a significant strength loss (compressive strength of RAP-mortars 54% lower than reference NA-mortar at 28 days). The higher porosity at the interface transition zone of RAP-AAM is the main responsible for the lower strength performance. Increasing silicate dosages improves alkaline activation, but it has little impact on the adhesion between aggregate and bitumen. Despite the poorer mechanical performance, 100% RAP-AAM still yields enough strength to promote this recycled material in engineering applications.

Highlights

Reclaimed asphalt pavement (RAP) is generated during the removal of existing flexible pavements
Most recycling efforts focus on using RAP in new hot-mix asphalt or as granular materials for unbound pavement layers [1,2,3], but for both alternatives, there is a limit to the replacement level and a relatively high amount of RAP is left unused [4,5]
High RAP replacement levels lead to a loss of mechanical properties, which can be compensated by using stabilisers, i.e., agents that increase the compressive and impact strength and the overall integrity of the layers [5,6]

Summary

Introduction

Reclaimed asphalt pavement (RAP) is generated during the removal of existing flexible pavements. Alkali-activated materials (AAM) emerged in the last three decades as sustainable building materials to replace Portland cement for some applications [13] These binders rely on the alkaline activation of mainly natural or calcined clay materials and industrial residues or wastes, such as pulverised fly ash, ground granulated blast furnace slag (GGBFS), mining residues, among others [13,14,15,16]. Hoy et al and Horpibulsuk et al [19,20] presented results of RAP stabilised with alkaliactivated high calcium (CaO > 10%) pulverised fly ash (PFA) for base courses. Avirneni et al [5] Saride et al [22] Mohammadinia et al [23] Horpibulsuk et al [20] Jallu et al [24]

Main Findings

Testing Procedures

Isothermal Calorimetry