Chemical aspects related to using recycled geopolymers as aggregates

Abstract

Despite extensive research into sustainability of geopolymers, end-of-life aspects have been largely overlooked. A recycling scenario is examined in this study. This requires an investigation of alkali leaching potential from a geopolymeric matrix. To study the feasibility of geopolymer cement (GPC) recycling, the migration of alkalis was evaluated for the first time on a microstructural level through energy dispersive X-ray (EDX) scanning electron microscopy (SEM) elemental mapping and leaching tests. Macroscale impacts were assessed through an investigation of Portland cement (PC) mortar properties affected by alkali concentration. Leaching tests indicated that alkalis immediately become available in aqueous environments, but the majority remain chemically or physically bound in the matrix. This type of leaching accelerates the initial setting of PC paste. Elemental mapping and EDX/SEM analysis showed a complex paste-aggregate interfacial transition zone. Exchange of calcium and sodium, revealed by the maps, resulted in the migration of sodium into the PC paste and the formation of additional calcium-silicon-based phases in the geopolymeric matrix. Strength values of mortars with 25% and 50% recycled aggregates (RA) showed negligible differences compared with the reference sample. Screening tests indicated a low potential for GPC RA inducing alkali-silica reaction. Transport of GPC RA alkalis and the underlying mechanisms were observed. This transport phenomenon was found to have minor effects on the properties of the PC mortar, indicating that recycling of geopolymers is a viable reuse practice.

Recent progress in low-carbon binders

“…It has also been identified that there is a strong need for more refined and updated life-cycle inventory data for activator constituents including sodium silicate [303,305,313], as the currently available international databases do not reflect current production practice or processes. The assessment of recyclability [314,315] and release of potentially problematic elements (toxic or naturally occurring radioactive materials) [316][317][318] from AAMs in service has also received some attention as an essential constituent of a full cradle-to-grave or cradle-to-cradle environmental assessment. This will doubtless gain further importance as noncarbon emissions become more of a focus in material and product assessments worldwide, while the sources and characteristics of waste materials used in AAM production become ever more diverse.…”