Early age shrinkage phenomena of 3D printed cementitious materials with superabsorbent polymers

3D printing of cementitious materials is a novel construction method, capable of producing complex geometries without the use of expensive formwork. However, due to the lack of molding, additional shrinkage will be induced and the risk of crack formation will increase. As cracks introduce ingress paths for chemical substances, it will harm the durability of the printed element. One potential way to tackle this disadvantage is by including superabsorbent polymers (SAPs) in the cementitious material. As these polymers are able to absorb part of the mixing water and to release it during hardening, they induce internal curing and can mitigate self-desiccation, plastic and autogenous shrinkage. Additionally, as drying shrinkage and the related early-age crack formation are major issues in printed structures, the mitigating effect of the SAPs on the latter is also investigated. Three different SAPs (bulk-polymerized monovalent salt polyacrylates with an irregular shape and different size) were used in this research to fabricate printed elements and their influence on the durability and the mechanical properties was correlated with the microstructural changes. First results showed that in general, the addition of superabsorbent polymers mitigates shrinkage in printed materials up to 200%. Inclusion of SAPs also reduced the nanoporosity in the pore size range of 100 nm-500 nm and increase the amount of voids with a diameter above 700 nm, resulting in less microcracks and a decreased amount of preferential ingress paths for chemical substances. On the other hand, the total air content increases with the addition of SAPs, proportional to the amount of SAPs and additional water added, due to the formation of macropores. The addition of SAPs did not seem to have a pronounced influence on the mechanical properties of the printed specimens with a reference water-to-cement ratio of 0.37.