Lightweight concrete for 3D-printing with internal curing agent for Portland cement hydration
The development of 3D printing technology requires solving material science problems to ensure the rheology of concrete mixtures with controlled mobility and high retention of volume after extrusion and also the creation of favorable conditions for the hardening of the binder. This paper investigates lightweight concretes on hollow microspheres. The rheological properties of concrete mixtures and the peculiarities of the structure formation of cement stone in the presence of a superabsorbent polymer (SAP) have been studied. The paper used standardized test methods following EN 1015-3-2007, EN 12390-1-2009, and employing modern equipment and tools (K100 KRUSS processor tensiometer, comparator, calorimeter). It was found that concrete mixtures on hollow microspheres with an average density of 1400 kg/m3 have a high ability to retain volume (buildability) of more than 90 % after extrusion with mobility of not more than 135 mm of the spread diameter. It has been shown that the use of superabsorbent polymer solutions with controlled polymerization is an effective solution to provide internal curing for the hydration of Portland cement. Calorimetric analysis of cement stone showed a positive effect of SAP on the processes of structure formation of cement stone. This is expressed by an increase in the amount of hydration products, in particular portlandite. The number of SAP solution of 0.50–1.0 % of the mass of Portland cement in lightweight concrete provides the least reduction in strength. It was found that the flexural strength varies in the range of 5.5–5.8 MPa and the compressive strength – 45.3–47.8 MPa. An increase in the SAP content of more than 1.5 % of the mass of Portland cement is characterized by a decrease in compressive strength by 8.5 %. The permissible amount of SAP in concrete is limited to 1.0–1.5 % of the mass of Portland cement. The possibility of providing internal curing for the hydration of Portland cement in lightweight concrete through the use of SAP solution has been substantiated. The obtained results of the study of lightweight concrete show high printability of concrete mixtures on hollow microspheres for 3D printing. Moreover, the implemented soluble SAP composition instead of the granular SAP is capable of providing the function of internal curing for the hydration of cement with no loss in strength.