Urbanization has led to environmental challenges such as the Urban Heat Island effect and the reduction of green spaces, resulting in lower carbon dioxide absorption, increased air pollution, and higher noise levels. Incorporating vegetation into buildings through vertical or horizontal greening systems can address these issues. However, these systems face challenges like high irrigation costs, artificial substrates, low integration, and short lifespans compared to building infrastructure. Bioreceptive materials offer a promising alternative by allowing greenery to grow directly on the material, bypassing these limitations. This research focuses on enhancing the bioreceptivity of concrete while evaluating its sustainability performance and structural integrity in terms of compressive strength (CS), and freeze-thaw resistance (FT).

The type of concrete being evaluated is porous concrete, designed to include a growing substrate for hosting plants. A multi-criteria analysis is used to determine the optimal concrete mix, considering three design criteria: bioreceptivity, sustainability, and structural integrity (including compressive strength and freeze-thaw resistance). Three mix designs are developed: a reference mix with Ordinary Portland Cement (OPC) and limestone aggregates, and two mixes using a low-carbon Calcined Clay (C$^3$) binder—one with lava stone (Mix 1) and the other with recycled concrete aggregates (Mix 2). An iterative process is employed to adjust the binder and water content, evaluating workability, compressive strength after 5 days, and interconnected porosity. Once suitable proportions are achieved, the concrete samples are tested for their mechanical, bioreceptive, and sustainable properties. The incorporation of the substrate has also been tested, addressing a gap in the existing literature.

The compressive strength after 28 days was comparatively low for all tested mix designs due to high porosity, with achieved values as follows: Mix 1 (2.5 MPa), Mix 2 (3 MPa), and the reference mix (10 MPa). A decrease in compressive strength was observed when incorporating the growing substrate and after drying. Despite low compressive strength, all samples demonstrated resilience in freeze-thaw resistance, withstanding 28 FT cycles with low mass loss. However, Mix 1 and Mix 2 showed loosening of aggregates when pressed by hand after the standard testing process, indicating a need for further testing of CS after different FT cycles.

Various methods of substrate incorporation were evaluated, showing that the substrate did not penetrate the concrete pores but remained on the surface of the cubes. This did not negatively affect the bioreceptivity for plant growth. The volume of substrate incorporated per sample and mix design varied due to manual application and the surface connectivity of the aggregates for each concrete product. This variability in substrate volume affected the water absorption and retention results, suggesting that these outcomes do not consistently characterize the mix design but rather are variable. However, Mix 1 and Mix 2 exhibited high water absorption rates when considering the concrete product itself (Mix 1 = 164.4 g/L, Mix 2 = 134.1 g/L, Ref mix = 82.4 g/L). Lastly, leaching of elements from the concrete layer occurred, leading to an increase in the pH of the incorporated growing substrate. All samples showed germination after one month of testing, with Mix 1 displaying the highest average germination rate and dry biomass. However, Mix 2 and the reference mix exhibited high variance in plant growth results, likely due to varying outdoor conditions such as moisture and temperature. Further testing is recommended to better isolate the impact of these outdoor conditions, which could have contributed to the high variance in plant growth outcomes.

Finally, in terms of sustainability performance, Mix 1 and Mix 2 positively impact the environmental burden of the concrete product by reducing the ECI and CO$_2$ emissions by an average of 50\% per 1m$^3$ of concrete, considering LCA stages A1-A3. At the same time, the use of C$^3$ binder reduces the ECI and CO$_2$ emissions from the binder material extraction stage by 60\% compared to OPC binder, further demonstrating that this binder is a low-carbon alternative.

This research demonstrates that the tested mix designs, evaluated for bioreceptivity, sustainability, and structural integrity (in terms of compressive strength and freeze-thaw resistance), have great potential for use in vertical greening systems with Mix 1 identified as the most optimal concrete mix design. This finding is significant for urban environments, especially as urbanization is expected to increase in the coming years. Bioreceptive vertical greening systems made with sustainable materials can address environmental challenges in cities and align with the Sustainable Development Goals.