Reactive Incompatibility between Organic Asphalt-Coated Aggregates and Inorganic Cement Phases: Mechanistic Insights from Suspension-Based Analysis

Abstract

Incompatibility between organic asphalt-coated aggregates and the inorganic cement matrix led to a reduction in mechanical strength in reclaimed asphalt pavement (RAP)–based concrete mixtures; however, the fundamental chemical mechanisms governing this behavior remain insufficiently understood. In this study, a fundamental approach was used to understand the leachate compounds within the alkaline cement solution, and the effect of asphalt-coated aggregates on cement hydration was investigated using a novel suspension technique. Fine RAP aggregates with different levels of asphalt oxidation (high, moderate, and low) were used. A suspension system was prepared using fine RAP aggregates mixed with cement powder at a 1 ∶ 1 weight ratio and deionized water at a liquid-to-solid ratio of 10 ∶ 1 . The mixture was continuously stirred at 30 ± 2 revolutions/min (rpm) for 24, 48, 72, and 96 h, followed by vacuum filtration to obtain and isolate the solid and liquid phases for subsequent analysis. The liquid cement solution was analyzed to determine the leachate compounds using Fourier transform infrared spectroscopy (FTIR) and inductively coupled plasma optical emission spectroscopy (ICP-AES), and pH measurements. The solid (cement powder sample) was further analyzed to understand the variation in cement hydration products caused by asphalt-coated aggregates in the suspension using thermogravimetric analysis (TGA), FTIR, and X-ray diffraction (XRD) analysis. The results showed elevated calcium concentrations and reduced sodium and barium mobility in the cement pore solution, accompanied by a decrease in pH, attributed to the formation of calcium–carboxylate complexes and neutralization of alkalinity by oxidized asphalt functional groups. FTIR analysis of the liquid phase indicated the transformation of carbonyl groups into carboxylate species. Solid-phase analyses exhibited reduced portlandite and bound water content, delayed C─S─H formation, and weakened Si─O stretching vibrations, indicating inhibited early cement hydration in the presence of asphalt-coated aggregates. These findings indicate that chemical interaction between aged asphalt coatings and the cement matrix alters ionic equilibrium and inhibits early hydration reactions.