Mechanical properties, life-cycle assessment, and costs of alternative sustainable binders to stabilise recycled aggregates

Cement-stabilised subbases provide superior bearing capacity and durability to road pavements compared to unbound aggregate layers. However, stabilisation reduces the environmental benefits derived when recycled aggregates are used. This research compares alternative binders to Portland cement to highlight mechanical, environmental, and economic advantages and disadvantages in a cradle-to-production scenario. Three low-clinker cements with different proportions of pozzolana and three alkali-activated (AA) binders derived from (i) construction and demolition waste fines, (ii) municipal incinerator bottom ash and (iii) waste clay, were compared to Portland limestone cement. The compressive strength of binder pastes was measured after 7 and 28 curing days. Pozzolanic cements proved viable alternatives to Portland ones, while AA pastes exhibited lower strengths. The crystallinity of alkali-activated silica- and alumina-rich waste precursors was responsible for their limited strength. The life cycle assessment indicated that the replacement of clinker with pozzolana significantly reduces the environmental impact. AA binders with waste precursors can reduce the environmental impact only with a limited quantity of alkaline solution. If the lower strength achieved by AA binders is compensated by adding higher quantities to recycled aggregate, the increase in environmental impact and cost would make them less competitive. The option of using AA binders would be further strengthened with the production of environmentally friendly alkaline solutions and greater local availability of amorphous precursors. At present, cements are cheaper than AA binders due to the current massive production, widespread availability, and competition between producers.