Stability of recycled plastics in an aqueous alkaline cementitious matrix

The rising problem of plastic waste, coupled with a shortage of construction materials, has prompted research into the replacement of traditional aggregates with recycled plastic aggregates (rPA) in cementitious structures. However, the impact of the highly alkaline pore solution present in cement on the long-term stability and performance of rPA is still not fully understood. This research examined the alkaline stability of two types of commonly recycled plastics that increasingly serve as aggregate substitutes in concrete: polyethylene terephthalate (rPET) and high-density polyethylene (rHDPE) in two size ranges, when exposed to extremely alkaline conditions. The effects of exposure to simulated alkaline cement pore solutions on rPA stability were analysed by assessing alterations to polymer mass, surface features, functional groups, and crystallinity. Prolonged exposure (up to 75 days) to simulated cement pore solution significantly decreased the stability of rPA, while fine particle sizes underwent faster degradation, losing up to 40 % of weight. Recycled HDPE demonstrated greater alkali resistance than rPET, suggesting better suitability as an alternative aggregate in concrete, although factors like surface hydrophobicity should be considered. The amorphous regions of rPET surfaces proved more susceptible to hydroxyl reactions compared to crystalline regions, resulting in inferior stability of rPET compared to rHDPE, and therefore raises questions about the use of rPET as an alternative aggregate. Overall, this study elucidated the physical and chemical stability of recycled plastics in alkaline cementitious matrices, revealing how plastic type, intrinsic properties, particle size, and exposure duration govern their suitability as aggregate replacements.