This paper presents a new form of biomimetic cementitious material, which employs 3D-printed tetrahedral mini-vascular networks (MVNs) to store and deliver healing agents to damage sites within cementitious matrices. The MVNs are required to not only protect the healing agent for a sufficient period of time but also survive the mixing process, release the healing agent when the cementitious matrix is damaged, and have minimal impact on the physical and mechanical properties of the host cementitious matrix. A systematic study is described which fulfilled these design requirements and determined the most appropriate form and material for the MVNs. A subsequent series of experiments showed that MVNs filled with sodium silicate, embedded in concrete specimens, are able to respond effectively to damage, behave as a perfusable vascular system and thus act as healing agent reservoirs that are available for multiple damage-healing events. It was also proved that healing agents encapsulated within these MVNs can be transported to cracked zones in concrete elements under capillary driving action, and produce a recovery of strength, stiffness and fracture energy.

The selection of an appropriate healing agent is critical to the success of vascular and mini-vascular networks. In self-healing concrete technology, commercially available cyanoacrylate (CA) adhesives have been shown to produce good strength recoveries; however, their rapid curing rate and short shelf-life make them unsuitable for site application. The aim of this study was to develop a modified cyanoacrylate (n-CA) with an extended shelf-life suitable for incorporation in a self-healing system. A series of n-CAs were formed from a commercial Ethyl Cyanoacrylate adhesive mixed with acrylic acid (AA) and nitro-anthraquinone (nAq) in varying ratios. When encapsulated within 3D printed mini-vascular networks (MVNs), the n-CAs remained dormant in liquid form for up to 5 days. The contact angle between the n-CAs and the cement mortar substrate, as measured via the sessile drop technique, decreased significantly with increasing AA content. The mechanical properties (bond strength) and the polymerization hardening of the n-CAs were evaluated over a curing period of 7–21 days, via a series of pull-off tests using cement mortar cubes. The 4:1:02 (CA:AA:nAq) n-CA formulation showed a significant increase in bond strength from 14 to 21 days, with a ceiling value of 2.6 MPa, while the 2:1 (CA:AA) n-CA formulation exhibited a good bond strength after 21 days (1 MPa). Nuclear Magnetic Resonance (NMR) conducted on the n-CAs suggested the formation of several new polymeric species, whilst differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) on the pre- and post-printed MVN material confirmed no significant changes in chemistry with no evidence of polymer degradation. Considered together, the experimental results show the potential for different n-CA formulations to act efficiently as a healing agent.

Transitioning to more sustainable materials than cement is vital for reducing the carbon emissions and ecological footprint associated with cement production. Ternary cement blends, particularly Limestone Calcined Clay Cements (LC3), are gaining attention as sustainable alternatives to Ordinary Portland Cement to reduce clinker content. Three clay samples from the western area of Chania, Crete, Greece, were calcined at 700 °C and reacted with saturated lime to assess their pozzolanic activity as potential cement substitutes. Additionally, a local marl was calcined at 800 °C and then hydrated to evaluate its binding capacity for designing mortars with hydraulic binders. The silica and alumina minerals in the calcined clays and marl, after reacting with lime and undergoing hydration, produced respectively calcium silicate hydrate (C-S-H) and calcium aluminate hydrate (C-A-H), which contribute to the hardening of plasters and mortars. The study includes Attenuated Total Reflection (ATR-IR) analysis, compression tests on mortar samples aged 1 to 4 months, and workability tests on fresh mortars. The results found that calcined clays, particularly metakaolin-rich samples, reacted more significantly with lime compared to those with high calcite content and low kaolinite. Mortars containing clays and lime required a similar water-to-binder ratio to achieve workability comparable to cement-based mortars. Compressive strength showed that the portlandite produced from the hydration of the marl was insufficient to fully react with calcined clay and cement to form C-S-H. This paves the way for ongoing studies to identify the minimum and optimal lime content required to effectively promote the hydrolysis reaction forming C-S-H, in accordance with LC3 principles.