Chemical ddmixtures used in 3D printing of building materials

Abstract

Chemical admixtures present the key to successful 3D printing technologies. Moreover, many similarities between admixtures for dry-mix mortar and 3D printed mortar exist. This paper highlights current technologies and provides an outlook into which technologies might be required in the future. Generally, the kind of admixtures differ, depending on the type of 3D printing applied: extrusion, shot-crete or printing in particle bed. As of today, about 90% of actual work uses extrusion printing. In extrusion, immediate stiffening of the mortar is required once it leaves the nozzle. To achieve such sudden structuration, liquid alumina cement or crosslinking polymer systems are applied. Moreover, in order to avoid the sag of printed mortar layers, thixotropic agents based on nanoclays including attapulgite or meta muscovite are admixed. Additionally, adhesive polymers based on synthetic latex copolymers facilitate sufficient cohesion between individual printed layers and provide the necessary form stability of the structure. Early strength development can be achieved by nucleation enhancing admixtures ("seeding materials") such as those based on C-S-H-PCE nanocomposites. The working mechanisms of those admixtures and the benefits achieved from them will be disclosed in the presentation. A major hitherto unresolved problem is excessive shrinkage of 3D printed objects. It derives from chemical shrinkage (most mortars are very high in cement content) as well as physical/dry shrinkage (the absence of formwork to protect the surface from desiccation being the main reason here). So far, no technically satisfying and economical solution in actual application (i.e. not under controlled laboratory conditions) has been found, which requires future development work. In the future, more work will be devoted to print actual concrete which should be the ultimate goal, as it by far represents the most attractive application. However, to print concrete with steel embedded currently is considered to be unfeasible. To solve this problem, completely new approaches need to be taken to devise a realistic, field- suitable method.