The increasing urgency of global warming and resource depletion demands more sustainable construction methods capable of reducing material use. Additive manufacturing (AM), in particular selective cement activation (SCA), offers such potential; however, its adoption remains limited due to challenges in mechanical performance and shape accuracy. Current SCA systems, including the custom-built printer used in this study, often exhibit low strength and require high water dosages to compensate, resulting in excessive penetration depth and thereby a loss of detail in the product.
This research aims to develop a concrete mix suitable for printing reef-like structures using SCA by optimising both strength and shape accuracy. Eighteen mix designs were investigated through small-scale mechanical testing, including compression and bending. Additionally, four different curing regimes were tested. Shape accuracy was evaluated using a visual grading method applied to complex printed geometries. Key mix parameters—particle-size distribution, cement-to-aggregate (C/A) ratio, binder composition, and additives—were systematically studied to identify their influence on water penetration behaviour and resultant print quality.
The results show that increasing the C/A ratio and adjusting the PSD significantly improve mechanical performance, while cellulose ether considerably enhances shape accuracy when applied within a narrow dosage range. A ternary binder of 90% ordinary Portland cement (OPC), 5% calcium sulphate (CS), and 5% calcium aluminate cement (CAC), combined with a C/A ratio of 0.4 and 0.25% cellulose ether, produced the most favourable balance between strength and geometric accuracy. This final mix design increased bending strength from 0.37 MPa to 1.62 MPa and compressive strength from 0.38 MPa to 1.15 MPa without sacrificing shape accuracy.
Overall, the study advances the understanding of material design for SCA-based 3D concrete printing and highlights sustainable binders and improved layer compaction methods as key directions for continued development. At the same time, it underscores that the recommended mix design is not universal but must be tailored to the specific requirements of the intended application.

The geographical features in the Southern part of Limburg forces precipitation from upstream located areas to flow through a bottleneck, which is exactly located at the city centre of Valkenburg. This makes increasing the safety level more complicated than in other areas. The safety level of Valkenburg has a lower standard in comparison to the rest of the country, namely 1 in 25 years. The combination of those two characteristics is not desirable. Official documents
state that this lower standard is based on detailed (societal) Cost-Benefit Analyses. In reality however, the safety standard is based on simple back of the envelope calculations. The Limburg Waterboard has indeed developed a Cost-Benefit tool which they could use to find out whether the implementation of safety measures are cost effective, however they have not been able to
implement it until now. Additional safety measures to increase the safety level are assumed too costly based on the same brief calculations. It is doubtful whether individual risk laws are met, since the Limburg Waterboard assumes no casualties in the Geul area. The 2021 flood however showed that this might be false for future floods which get more severe over time due to climate change.

The citizens and entrepreneurs in Valkenburg were not completely aware of the risks they were exposed to and their sense of safety related to flooding decreased after the flood. Most of the people questioned in a survey demanded a higher safety level than the current standard. They would even be open for an increase in tax to realise this improvement. Raising the quay walls would be a cost-effective solution according to some of the citizens. However, the entrepreneurs who rely on tourist based income, do not prefer this option due to loss in aesthetic value.

Hydraulic, structural, and non-technical solutions which are investigated in this report, have the aim to increase the safety level or make the safety level more acceptable for citizens. The hydraulic, and structural solutions focus on four main aspects. The first aspect is related to the redesign of bridges in the city centre. This is mainly done by applying a flat bridges design, which is further elaborated with a case study for the collapsed Emmalaan bridge, and a liftable bridge design. The second aspect is related to closing the gaps in the quay walls, and increasing the height of the quay walls. The third aspect is related to the implementation of water tunnel concepts with six different design concepts. The fourth aspect is related to implementing parts of Meerssen’s 4-step approach. The first three aspects of the hydraulic and structural solutions are focused on increasing the discharge capacity of the Geul, while the latter aspect focuses on retaining, delaying, and storing the precipitation. Non- technical solution are also proposed
that focus on making people more aware of the risk they are exposed to. This could eventually lead to more acceptance and thus more pleased citizens.

The first order estimations for investment costs and safety level for the hydraulic, and structural solutions are graphically displayed in order to provide an overview of possible interventions to the municipality of Valkenburg and the Limburg waterboard. Although preliminary, and based on limited available data, these results should encourage both stakeholders, and other relevant parties, to reconsider safety standards and search for measures that could increase the safety level of Valkenburg when desired.