# Recycling 3D Printed Concrete Waste for Normal Strength Concrete Production
## 2 Materials and Methods  
### Materials  
The material employed in the fabrication of 3D-printed concrete is the premixed white powder known as Sikacrete(r)-752 3D, combined solely with water. Sikacrete(r)-752 3D incorporates a fine aggregate with a maximum size of 2 mm, along with Portland cement and additives [18]. The specific type and proportions of these constituents are proprietary to the Sika company (Biatorbagy, Hungary). When mixing this powder with water, the company prescribes a water-to-powder ratio (w/p) ranging from 14% to 16%, based on the weight of the powder [18].  
The 3D-printed concrete waste, obtained after [PERSON] [19] conducted a comprehensive investigation into the material properties of Sikacrete(r)-752 using both conventional and 3D printing casting methods for her MSc thesis work, is outlined in Table 1. The table includes relevant information on casting types, the material's water-to-powder ratio (w/p), and printing directions, with the symbol N/A indicating 'Not Applicable'. Following this study, the 3D-printed concrete waste was processed by crushing it into fine and coarse aggregates using a jaw crusher in one step.  
In the recycling approach, all crushed particle sizes, including both fine and coarse particles, were used as recycled aggregates. This approach towards recycled aggregate usage emphasizes a comprehensive and sustainable recycling strategy. By incorporating the full spectrum of particle sizes, the aim is to optimize the potential for reusing 3D-printed concrete waste in a manner that enhances the overall material sustainability and minimizes environmental impact.  
In this experimental study, a two-stage investigation was conducted to evaluate the performance of concrete specimens in terms of compressive strength using a 150 mm cube mold [20]. The initial stage concentrated on producing reference specimens, representing conventional concrete with no replacement. The materials used in this phase included CEM II/AS 42.5 N [21], river sand, and quartz gravel, aiming to establish a baseline for standard or reference mixes. These reference specimens, following conventional practices, serve as a reference point for subsequent comparisons. Having completed the first stage, the transition to the second phase emphasized the incorporation of recycled aggregate (RA) sourced from 3D-printed concrete. Visual representations of both quartz gravel and recycled aggregates from the 3D printed concrete are presented in Figure 1.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Type of** **Casting**} & \multirow{2}{*}{**w/p (\%)**} & \multirow{2}{*}{**Direction** **of Printing**} & \multirow{2}{*}{**Density** **(kg/m3)**} & \multicolumn{2}{c}{**Tensile Strength**} & \multirow{2}{*}{**Compressive Strength** **(MPa)**} \\  & & & & & **(MPa)** & \\ \hline \multirow{4}{*}{Conventional} & 14.0 & N/A & 2084 & 9.4 & 3.9 & 63.8 \\  & 14.5 & N/A & 2084 & 8.8 & 3.2 & 63.1 \\  & 15.0 & N/A & 2066 & 8.7 & 4.2 & 61.1 \\  & 14.0 & horizontal & 2116 & 6.9 & 3.3 & 53.8 \\  & & horizontal & 2140 & 5.4 & 2.8 & 53.4 \\
3D & 14.5 & vertical & 2140 & 2.5 & 3.2 & 44.2 \\ Printing & & horizontal & 2119 & 3.6 & 3.5 & 53.0 \\  & & vertical & 2119 & 2.4 & 3.1 & 48.1 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Physical and mechanical properties of waste concrete.  
The specimens in this stage, still utilizing a 150 mm cube mold, showcase the potential of recycled materials in concrete production.