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The application of asphalt hot mix recycling is one challenge in sustainable road pavement research. In addition to the vast amount of research on the performance of recycled asphalt–concrete, the research on the frictional resistance of recycled hot mix asphalt is still limited. The effects of aged asphalt and aged aggregates on the skid resistance of recycled hot mix asphalt were investigated in this research. The aged asphalt and aged aggregates were carefully extracted from the field-reclaimed asphalt pavement, and the engineering and mechanical properties of aged and virgin aggregates were measured. The degradation of recycled hot mix asphalt was simulated using an accelerated polishing machine to mimic road surface abrasion. Accordingly, the initial and final skid resistances of the recycled hot mix asphalt were determined and correlated with the properties of the aged asphalt and aggregates. The initial skid resistance of recycled hot mix asphalt decreased with reductions in penetration and ductility of the blended asphalt. However, the changes in the blended asphalt properties contributed only small variations to the final skid resistances of the recycled hot mix asphalt. The gradations of recycled hot mix asphalt correlated only with the final skid resistances. The aggregate gradations controlled the characteristics of the final skid resistance since the covered binder was partially polished off from the road surface at this stage.

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With a lack of standard lateritic soil for use in road construction, suitable economical and sustainable soil-stabilization techniques are in demand. This study aimed to examine flue gas desulfurization (FGD) gypsum, a by-product of coal power plants, for use in soil–cement stabilization, specifically for ability to strengthen poor high-clay, lateritic soil but with a lower cement content. A series of compaction tests and unconfined compressive strength (UCS) tests were performed in conjunction with scanning electron microscope (SEM) analyses. Therefore, the strength development and the role of FGD gypsum in the soil–cement–FGD gypsum mixtures with varying cement and FGD gypsum contents were characterized in this study. The study results showed that adding FGD gypsum can enhance the strength of the stabilized substandard lateritic soil. Extra FGD gypsum added to the cement hydration system provided more sulfate ions, leading to the formation of ettringite and monosulfate, which are the hardening cementitious products from the cement hydration reaction. Both products contributed to the strength gain of the soil–cement–FGD gypsum material. However, the strength can be reduced when too much FGD gypsum is added because the undissolved gypsum has a weak structure. Examinations of FGD gypsum in the soil–cement–FGD gypsum mixtures by SEM confirmed that adding FGD gypsum can reduce the cement content in a soil–cement mix to achieve a given UCS value.

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