# Carbstone Paves: A Sustainable Solution for the Urban Environment
## 4 Conclusions  
In the range of 10-40 MPa, a clear trend is observed between compressive and tensile split strength for the carbonated pavers. The trend is similar to that observed for concrete, with a significant decline in \(\rm f_{\rm bsp}/f_{c}\) ratio with increasing compressive strength. The trend shows that a minimum compressive strength in the range of 30-35 MPa must be reached to meet the requirement of 3.6 MPa split tensile strength. Further investigation is needed to verify the trend observed for higher compressive strengths, and other materials with carbonation potential. The environmental properties of the Carbstone pavers comply with the current Flemish regulations, and the Dutch soil quality decree, when applied in contact with seawater or brackish water. After their service life, the Carbstone pavers can be recycled as aggregate in concrete without any environmental concerns. Concrete in which all natural aggregates have been replaced with recycled Carbstone pavers complied with all leaching limit values of VLAREMA and the Dutch soil decree. Conceptualization, H.K.K., P.N. and L.H. ; Data curation, H.K.K. and P.N. ; Investigation, H.K.K., P.N. and L.H. ; Methodology, H.K.K. and P.N. ; Writing--original draft, H.K.K., P.N. and L.H. ; Writing--review and editing, H.K.K., P.N., L.H. and P.V.M. All authors have read and agreed to the published version of the manuscript. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \begin{tabular}{c} **Column** \\ **7 Fractions -A** \\ \end{tabular} & \begin{tabular}{c} **Column** \\ **7 Fractions -B** \\ \end{tabular} & \begin{tabular}{c} **VLAREMA** \\ **Annex 2.3.2B** \\ \end{tabular} & \
\begin{tabular}{c} **VLAREMA** \\ **Future** \\ \end{tabular} &
\begin{tabular}{c} **Dutch Soil** \\ **Decree** \\ \end{tabular} \\ \hline
**Metals** & & & & **limit values** & **proposed limit value** & **limit values** \\ \hline As (mg/kg) & 0.007–0.020 & na & 0.8 & 0.8 & 0.9 \\ \hline Cd (mg/kg) & \textless{}0.005 & na & 0.03 & 0.03 & 0.04 \\ \hline Cr (mg/kg) & 0.31 & 0.43 & 0.5 & 2.6 & 0.63 \\ \hline Cu (mg/kg) & \textless{}0.03 & 0.01 & 0.5 & 0.8 & 0.9 \\ \hline Hg (mg/kg) & na & na & 0.02 & 0.02 & 0.02 \\ \hline Pb (mg/kg) & \textless{}0.02 & na & 1.3 & 1.3 & 2.3 \\ \hline Ni (mg/kg) & \textless{}0.03 & na & 0.75 & 0.75 & 0.44 \\ \hline Zn (mg/kg) & \textless{}0.15 & na & 2.8 & 2.8 & 4.5 \\ \hline Ba (mg/kg) & 7.4 & 8.0 & & 20 & 22 \\ \hline Co (mg/kg) & 0.035 & 0.02 & & 0.5 & 0.54 \\ \hline Mo (mg/kg) & 0.33 & 0.37 & & 55 & 1 \\ \hline Sb (mg/kg) & \textless{}0.020 & na & 1 & 0.32 \\ \hline Se (mg/kg) & 0.010 & na & & 2 & 0.15 \\ \hline Sn (mg/kg) & 0.042 & \textless{}0.010 & & 1 & 0.4 \\ \hline V (mg/kg) & \textless{}0.010 & \textless{}0.005 & & 2.5 & 1.8 \\ \hline
**Anions** & & & & **target value (1)** & \\ \hline Bromide (mg/kg) & 0.56 & 1.2 & 20 & 20 (2) \\ \hline Chloride (mg/kg) & 250 & 219 & 1000 & 616 (2) \\ \hline Fluoride (mg/kg) & 24 & 24 & 55 & 55 (2) \\ \hline Sulphate (mg/kg) & 97 & 90 & 2200 & 2430 (2) \\ \hline \hline \end{tabular}  
* For anions, the future VLAREMA legislation only imposes target values. These must be monitored, but are not grounds for exclusion. (2) When using building materials in places where direct contact with sea water or brackish water with a natural chloride content of more than 5000 mg/L is possible, no maximum emission values for chloride and bromide are imposed. Maximum emission values for fluoride and sulphate are multiplied by a factor of 4.  
\end{table} 
Table 5: Results leaching concrete mix (350 kg/m\({}^{3}\) CEM I), with replacement of fine (0–4 mm) and coarse aggregates (4–20 mm) with recycled aggregate from crushed Carbstone pavers. na = not analyzed. **Funding:** This research was funded by the Flemish Government under the funding scheme of Vlaanderen Circulair (project number 153).