Due to changes in climate, cities and urban areas are more and more tormented by issues such as the Urban Heat Island Effect and extreme precipitation. One way to tackle these issues is the implementation of green spaces, found to control local climate and improve water management. However, densification within cities threatens the space for green envelope. The application of moss, a type of plant that can adapt to many habitats, directly on the concrete surface of buildings could provide a solution. This requires the concrete to possess certain characteristics that allow and promote moss growth. The general term for a material’s ability to be colonized by biological growth is called its bioreceptivity. Surface roughness, surface pH, chemical composition, porosity and water retention have been documented to be strong predictors for bioreceptivity of concrete.

In this thesis, the bioreceptivity of 3D printed concrete (3DPC), an upcoming and promising concrete manufacturing technique with near prospects in urban context, is aimed to be improved by incorporating lightweight aggregates (LWA). A starting 3DPC mixture was adopted. Samples were 3D-printed with sand partially replaced by 30% Crushed Expanded Clay (CEC), Expanded Glass (EG), Expanded Vermiculite (EV), and Pumice (PU). Additionally, a separate set of samples incorporated a 45% replacement using only pumice.

Moss was planted on the specimens and allowed to grow in a climate controlled glass house. After 7 weeks, the green coverage on all specimens had severely decreased. The maximum quantum yield measured by Pulse Amplitude Modulation Fluorometry (PAM-F) suggested decreasing plant health as well. Improper environmental conditions and watering regime were held accountable for this. The results yielded insufficient information to discuss the impact of added LWA on bioreceptivity.

The samples were evaluated on their bioreceptivity factors, compressive and flexural strength. Surface roughness was similar across all the variants. All samples had a surface pH that exceeded 12. Samples withg with 30% replacement of expanded vermiculite (EV30) showed highest porosity, but lowest water retention. Mechanically, EV30 showed weakest compressive and flexural strength, but this can be partially allocated to its curing conditions. Contrarily, CEC30 and PU30 samples showed similar or higher values for compressive and flexural strength than the reference samples without any treatment.

The findings in this research implied that 3DPC open porosity increases with increasing aggregate porosity, and implied a reduced ability to retain water with increasing open porosity, although both these characteristics are reliant on the pore microstructure as well. Of all the LWAs, implementation of EV showed to be most promising in terms of habitat provision for moss growth. However, due to the marginal sample size the statistical significance is questionable.

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.