# Development of Cementitious Mortars for Aerial Additive Manufacturing
## 1 Introduction
However, the applications have typically dealt with topographical surveillance, building inspection  
Figure 1: A flying, untethered Unmanned Aerial Vehicle (also known as a ‘drone’) 3D printing multiple layers in a Peano curve design using a formulated cementitious material with suitable rheological properties ([12]). and data gathering to assist cost estimations [22], rather than the AM construction or repair of a building in situ. An aerial approach is intended to free AM from ground-based design and logistical constraints, and is designed to be particularly effective in reducing safety risks and costs when confronted by elevated, hostile or inaccessible environments [12; 19] which present human hazards or topography challenging for ground-based methods. Examples include post-natural, or human-made, disaster reconstruction and working at height on projects such as bridges and stadia, an inherently dangerous task with falls from height causing over a third of construction industry fatalities [23] and scaffolding related operations being a leading cause of falls [24]. An alternative miniature syringe-based deposition device, light enough to be carried by an aerial robot and capable of drawing up and extruding high viscosity liquids, has previously been developed for horizontal attachment to the base of a UAV [19]. This study modifies the miniature deposition device in order to investigate cementitious mortars and pastes suitable for AAM with additional additives and admixtures to assess the impact upon rheological properties. In addition to layer adhesion, the following parameters have been identified as important for fresh, wet cementitious materials in AM processing: 'pumpability' (the ease and consistency at which a material may flow through a deposition device), 'extrudability' (the ability of the material to be deposited through a nozzle), 'open time' (the duration of time in which material properties remain within the range required for printing, prior to full curing) and 'buildability' (a measure of how freshly extruded material can be deposited in layers and resist deformation under loading) [7]. In the context of AM, buildability may be further defined as a term describing the ability possessed by a freshly deposited filament to both support its own self-weight and support the weight of subsequently deposited layers. Pumpability and extrudability will be dealt with in this study by encompassing the parameters into the general term 'workability'. Crucial to the success of a material developed for AM extrusion is the recognition of the trade-off between buildability and workability [15]. A desirable balance encompassing both qualities in the chosen material is sought. A plasticiser is an important admixture in this respect, allowing a reduction in the water:binder ratio (thus improving strength and buildability), while aiding workability [13]. Mixes with high workability have lower stiffness and lower initial strength (thus reducing buildability), but encourage greater cohesion between layers by maintaining a chemically active surface longer, promoting inter-facial bonding where properties are closer to bulk material [9], thus enhancing the durability of the extruded element. Other previous AM studies using cementitious materials have used mix proportions from which AAM had to differ notably. Ground-based concrete printing studies have used mortar mixes with a sand:binder ratio of up to 3:1 [13]. It became clear during preliminary mix formulation that such ratios were impractical with a miniature single component deposition method as employed in this study, and 1:1 provided a maximum sand:binder ratio which could facilitate the workability required for successful material transport. Previous studies using high strength cementitious materials have used water:binder ratios below 0.30 [13; 25]. The XtreeE 'Democrite' project used 10% by weight of Silica fume and a low water/(cement + sand) mass ratio of 0.1 [26].