Sustainable Prospect for Entire Replacement of River Sand with Recycled Glass Aggregate in 3D Printing Concrete: Rheological Properties, Printability, and Alkali–Silica Reaction

Replacing river sand entirely with recycled glass (RG) aggregate in concrete is an intriguing approach to multiplying the sustainable impact of three-dimensional concrete printing (3DCP). This research aims to evaluate the prospects for utilizing solely RG as aggregate in concrete for 3DCP considering various factors, including rheological properties, shape stability, phase separation, and alkali–silica reaction (ASR). RG was prepared to obtain an approximate grading of river sand, which served as reference material. Varying quantities of fly ash were incorporated into the binder containing 8% silica fume to determine the appropriate concrete proportion that not only provided suitable properties for 3DCP but also hindered the ASR of RG. Actual 3D printing was conducted to examine the printability of concrete. The result indicated that when fly ash was not added, replacing river sand entirely with RG aggregate resulted in significant reductions in the rheological properties and thixotropy index of concrete while displaying negligible impact on shape stability, bleeding, and printability. Nevertheless, the addition of fly ash caused a decrease in rheological properties and thixotropy index, as well as an increase in the deformation and bleeding on both types of concrete, affecting the printability of the mixtures. The variation in the thixotropy index indicated that RG concrete appeared to be inappropriate for 3D printing since the incorporation of fly ash exceeded 30%. This observation agreed well with the findings related to shape stability and bleeding of concrete, where concrete using RG aggregate with more than 30% fly ash exhibited significant deformation and bleeding. Although the ASR of RG aggregate seemed substantially severe in mortar bars without fly ash, it was well hindered in RG concrete incorporating fly ash, making concrete using RG innocuous according to a recent standard. These findings present a promising prospect for the entire replacement of river sand by RG in concrete for 3DCP.