Multiscale Properties of Fly Ash Modified Recycled Fine Aggregate Mortar for Stone Masonry

In recent years, construction and demolition wastes (CDW) have been used to produce recycled fine and coarse aggregates. However, the non-sorted presence of large quantities of crushed old concrete results in discontinuous particle gradation along with the presence of brick, mud, and wood in a portion of the CDW, renders the recycled aggregates unsuitable for recycled mortar. An effective optimization method involves the use of fly ash to replace a portion of the cement in recycled mortar to reduce the water–cement ratio and the mortar and brick interface transfer zone thickness. In this paper, we report the results of a comprehensive study on the feasibility of CDW as a fine aggregate for stone masonry. The effects of the fly ash content and natural sand substitution rate on the mechanical properties of the recycled mortar were quantified. Based on the experimental results, with an increase in the fly ash substitution rate, the 28-day compressive strength of the recycled mortar first increased and then decreased. When the substitution rate of fly ash was 20%, the strength reached a maximum value of 20.8 MPa. The results suggest that the fly ash substitution rate used in recycled mortar should be maintained at 20%, and the natural sand substitution rate should be within 20–30%.