Rheology control and shrinkage mitigation of 3D printed geopolymer concrete using nanocellulose and magnesium oxide

3D printing concrete (3DPC) is an emerging technology that produces concrete using digital method and has revolutionized the traditional labor-intensive construction mode. However, the free formwork printing and layer-by-layer production of 3DPC induce severe shrinkage and plastic cracking during the early ages, especially for the geopolymer based materials. This research utilizes the nano-fibrillated cellulose (NFC) with the combination of magnesium oxides expansive agent (MEA) to mitigate the plastic and drying shrinkage of 3D printing geopolymer concrete (3DPGC), while optimizing its rheological behavior. The results show that after modification with proper dosages of NFC and MEA, 3DPGC showed reduced plastic and drying shrinkage at early ages, with improved printability, buildability, and mechanical strength. The underlying role of NFC and MEA on the performance of 3DPGC was thoroughly analyzed with rheometry, calorimetry, scanning electron microscopy, and internal humidity test. The water retention ability of nanocellulose can provide more moisture at early ages, thus mitigating cracking, while MEA can compromise the drying shrinkage at later ages. The contribution of the study shed light on the application of nanocellulose and MgO to increase the volume stability and mechanical performance of 3D printing geopolymer concrete.