3D-printed concrete (3DPC) enables automated and material-efficient construction, but its layer-wise manufacturing process introduces pronounced interfacial heterogeneity that significantly reduces structural integrity. In particular, weak bonding and particle segregation in the interlayer region, often accompanied by elongated air voids, induce stress concentrations and can promote premature fracture under flexural loading and other deformation modes [1]. A quantitative understanding of how this microstructure governs interlayer failure is therefore essential for improving the mechanical performance of 3DPC and remains an open issue to date. In this contribution, a microstructure-based discrete lattice fracture model for 3DPC is presented. The material is represented by a regular three-dimensional truss lattice in which aggregates, cement matrix, and interfacial transition zones are explicitly resolved. Heterogeneity is introduced through spatially varying particle distributions and statistical variations in the strength of the lattice members. This allows localized fracture patterns and crack paths to emerge naturally from the numerical model, without the need for predefined interfaces or macroscopic cohesive laws. The model is applied to numerical 3DPC specimens subjected to tensile or flexural loading, with particular focus on fracture initiation and propagation along layer interfaces. The influence of particle volume fraction, aggregate size, and their spatial arrangement across the layer interface on bond strength and failure mode is systematically investigated. The simulations demonstrate how microstructural features control stress redistribution, crack localization and propagation, and the resulting macroscopic load–displacement response. The potential of embedding the lattice model into a multiscale Quasicontinuum framework [2, 3] is discussed as a route toward efficient simulation of larger structural domains while retaining full microstructural resolution in critical regions. The proposed modelling strategy provides a practical and efficient approach for simulating fracture in layered cementitious materials and supports the design of improved interlayer bonding in 3DPC.