Polyethylene terephthalate PET is a material with which each of us is confronted daily and which forms the largest part of the plastic waste which is designated for recycling (RPET). Society strives to achieve that the majority of RPET produced disposables are returned back into the production line after their use. Since our planet has only a limited supply of natural materials, we need to replace some of these with artificial materials, ideally recycled. Recycling can be done many times (not just once), however it depends on the quality of purity or impurity of RPET. The research report presents the current state of research to apply RPET using existing and proven technology for blow-molding bottles for use in architecture and construction. The basic element is a brick of rPET material. In the first phase we present the geometric properties of the future brick with respect to use of the blow-molding technology and stackability of the bricks without using connecting materials.