# Recycling 3D Printed Concrete Waste for Normal Strength Concrete Production
## 1 Introduction  
In recent years, the construction industry has witnessed a transformative shift, with the advent of 3D printing technology revolutionizing traditional building methods. One of the most groundbreaking developments in this domain is 3D-printed concrete, a cutting-edge construction technique that has the potential to reshape the way we build structures. 3D-printed concrete, also known as the 3D printing of concrete or concrete 3D printing, is an innovative construction technology that uses computer-controlled robotic systems or large-scale 3D printers to deposit layers of concrete material in a precise and predetermined manner [1, 2]. 3D-printed concrete (3 DPC) finds applications across residential, commercial, and infrastructure projects, ranging from walls and buildings to bridges and smaller structures, all while continuing to evolve and shape the future of construction. However, as the adoption of 3 DPC grows, the need for sustainable and environmentally-responsible practices becomes increasingly critical. This environmental consideration is expected to gain further prominence in the future, particularly as 3D-printed concrete is applied in the construction of temporary buildings. The swift production pace inherent to this technology necessitates the prompt encounter and management of demolition waste in such projects. This dual emphasis on sustainability and efficiency positions 3D-printed concrete as a promising solution not only for current construction needs but also for the evolving demands of future structures. The construction industry's increasing inclination towards environmentally-conscious and resource-efficient solutions underscores the transformative potential of 3D-printed concrete in shaping the future of construction methodologies [3, 4]. The incorporation of recycled coarse aggregate (RCA) into concrete mixes presents a promising avenue for sustainable construction practices. Various studies have delved into the impact of RCA replacement ratios on both the mechanical properties and durability of concrete structures. Research indicates that up to 30% RCA replacement has no significant impact on mechanical properties [5]. At 50% RCA replacement, there is typically a 10-15% reduction, and at 100% replacement, it can reduce mechanical properties by 30-40% [6; 7; 8; 9]. Over a 2-year period, [PERSON] et al. [10] investigated the compressive strength of concrete, incorporating 0%, 25%, and 50% of RCA alongside supplementary cementitious materials, revealing increased strength in recycled aggregate (RA) mixes over natural aggregate (NA) mixes and suggesting enhanced long-term performance attributed to the continuous hydration of attached mortar on the RCA, even during freeze-thaw cycles. [PERSON] and [PERSON] [11] recommend that in regions prone to severe seismic activity, it is sufficient to replace up to 30% of conventional NA with RA to maintain acceptable structural performance. [PERSON] and [PERSON] [12] assessed the mechanical properties of RA produced from parent concrete (PC) with 28-day compressive strengths ranging from 30 to 100 MPa. After 28 days, concrete derived from PC with 80 and 100 MPa compressive strengths exhibited strengths exceeding 65 MPa, slightly surpassing those of natural aggregate concrete. Over a 90-day period, both mixtures achieved compressive strengths of 75 MPa. The highest compressive strength reported by [PERSON] et al. [13] was 42 MPa after 91 days at a water-cement (w/c) ratio of 0.32. In addressing the potential adverse effects on strength and durability in structural applications due to RCA, various guidelines have been established to provide specific recommendations. One such guideline, HB155-2002 [14], proposes measures to control the use of RCA in different concrete grades. For instance, it suggests limiting the replacement of conventional coarse aggregate with Class 1A RCA to 30% in Grade 1 structural concrete. In contrast, Grade 2 non-structural concrete is permitted to have a 100% replacement with RCA.