Micro-mechanical interaction of activated fly ash mortar and reclaimed asphalt pavement materials

• Low alkali activated fly ash increased the reactivity, thus, the strength of mortar.
• Three-fold increase in strength of RAP:VA mixes with fly ash activated with 1 M NaOH.
• Strength of activated fly ash is proportional to the peak area of Si-O-Si stretching.
• Typical random RAP aggregates have 15–70% exposed surface area.
• Strength is directly proportional to the amount of exposed RAP aggregate surface.

Reclaimed asphalt pavement (RAP) is a milled material obtained from the distressed pavement sections, containing a thin coating of aged bitumen on the aggregate surface. In spite of their inferior properties, RAP has been promoted to use in new pavement layers after blending with virgin aggregates (VA) and/or chemical stabilizers such as fly ash. Due to thin amorphous asphalt coating present on RAP aggregates, expected level of improvement is not witnessed. Hence, the non-reactive type fly ashes can be activated in an alkali environment, such as lime, sodium hydroxide, potassium hydroxide or a combination of sodium hydroxide and sodium silicate to further improve the reactivity to participate in the pozzolanic reactions. The present study investigates the degree of chemical and micro-mechanical interactions between the alkali (sodium hydroxide) activated fly ash mortar and RAP:VA base course mixes on strength development. To accomplish this task, specimens were prepared at various proportions of RAP and VA materials; stabilized with fly ash at 20% and 30% by dry weight with and without alkali activation at 2% and 4% sodium hydroxide (NaOH) solution. These mixes were cured for 1, 7 and 28 days and then tested for their unconfined compressive strength (UCS). Mineralogical studies such as X-ray fluorescence (XRF), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FT-IR) were then conducted on the powdered samples to identify and quantify the formation of possible hydration products. Since the RAP aggregates are coated with an amorphous asphalt coating, the amount of exposed aggregate surface of RAP particles plays a major role in the formation of pozzolanic compounds with fly ash. To verify the amount of exposed surface of RAP particles, surface area analysis was performed through high definition image processing. The interaction mechanism of RAP with other constituents of the mix is also studied. It is observed that a random RAP sample has exposed aggregate surface area between 15 and 70%, which is responsible for high pozzolanic reactions between the alkali activated fly ash mortar and RAP aggregates. The data are presented in terms of the exposed surface area and the strength characteristics of the mixes.

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