Layer interface in 3D printed cement-based materials: Heterogeneous phase distribution and new insights into formation mechanism

An in-depth understanding of the formation mechanism of layer interfaces is crucial to improving interface quality and advancing the application of extrusion-based 3D printed cement-based materials (3DPCM). Here, a multi-scale analysis of phase distribution at layer interfaces of 3DPCM containing silica fume (SF) and ultrafine fly ash (UFA) have been conducted, and new insights into the formation mechanism of the layer interface were provided. The interface, as observed via scanning electron microscopy coupled with energy dispersive spectroscopy, is notably more porous than the matrix and shows both a deficiency in aggregate and an enrichment in calcium hydroxide (CH). The concentrated pores and moisture at the interface provide sufficient conditions for the growth of CH. Wall effect of particle accumulation causes aggregates to move away from the interface, while the material deformation and the aggregate settlement during printing lead to aggregate redistribution. As the printing height increases, the heterogeneous distribution of aggregates and pores becomes more pronounced. CH enrichment is enhanced with longer time intervals. Due to the improved deformation resistance and water retention, SF promotes a more homogeneous phase distribution at the interface, hence reducing variations in interlayer bond strength across different printing heights and time intervals. UFA has a limited impact on materials’ deformation resistance but contributes to the reversible structural build-up. This helps to mitigate the discrepancy in phase distribution between matrix and interface with increasing time intervals, hence reducing the dependence of interlayer bond strength on time intervals.