This　study　focused　on　the　interfacial　transition　zone　(ITZ)　between　magnesium　phosphate　cement　(MPC)　paste　and　Portland　cement　paste　(PCP).　Multi-scale　analyses　were　conducted　to　explore　bonding　characteristics　and　interfacial　shear　strengths　of　the　ITZ　with　an　innovative　composite　specimen.　Meanwhile,　ITZ　was　investigated　by　the　Nano-indentation　test,　the　scanning　electron　microscope　(SEM),　and　the　energy-dispersive　spectrometer　(EDS).　Based　on　the　damage　mechanics　and　the　interface　kinematics　formula,　the　viscous　behavior　and　friction　behavior　were　coupled　together　for　the　first　time　to　develop　the　constitutive　relation　of　the　interface　damage.　A　finite　element　(FE)　model　of　MPC-PCP　interface　shear　strength　was　developed　and　conducted　to　validate　the　proposed　constitutive　relation　of　the　interface　damage.　Sensitivity　analysis　of　repair　size　was　predicted　by　the　effect　of　different　sizes　on　shear　strength.　The　7d　shear　strength　of　the　repair　interface　reached　5.2　MPa,　which　indicates　that　MPC　has　good　bonding　with　PCP.　Investigated　by　SEM/EDS　analysis,　the　cracks　appeared　at　10　μm　away　from　PCP　edge　and　the　width　at　6　h　and　3d　was　around　3–5　μm.　According　to　the　nano-indentation　test,　the　elastic　modulus　varied　greatly　in　the　ITZ　and　the　lowest　value　located　10　μm　away　from　the　PCP　side.　Numerical　results　from　FE　models　were　found　consistent　with　the　experimental　results,　which　validates　the　proposed　constitutive　relation　of　the　interface　damage.　©　2020　Elsevier　Ltd