One　of　the　key　parameters　for　the　performance　of　concrete　repairs　is　the　quality　of　the　interface　between　the　repair　material　and　concrete　substrate,　which　is　determined　by　cement　hydration　and　microstructure　development.　The　moisture　exchange　between　the　repair　material　and　concrete　substrate　plays　an　important　role　in　the　cement　hydration　and　porosity　of　cementitious　repair　materials.　To　better　understand　the　influence　of　moisture　exchange　on　the　hydration　of　cementitious　repair　materials,　this　paper　presents　a　numerical　simulation　of　cement　hydration　and　microstructure　development　of　repair　materials,　considering　moisture　exchange.　The　simulation　results　reveal　that　the　moisture　exchange　between　the　repair　material　and　concrete　substrate　results　in　a　water　content　change　in　two　parts.　Before　the　repair　material　setting,　the　water　absorption　of　an　unsaturated　concrete　substrate　causes　a　reduction　in　the　w/c　ratio　in　the　repair　material,　decreasing　the　hydration　rate　of　the　repair　material.　After　the　repair　material　setting,　the　water　migrates　from　the　concrete　substrate　to　the　repair　material　to　provide　additional　water　to　accelerate　the　hydration　of　unhydrated　cement　in　the　repair　material.　Copyright　©　2022　Lan,　Zhou,　Li　and　Wang.