Grounded　blast　furnace　slag　is　one　of　common　supplementary　cementitious　materials　in　Portland　cement　concrete,　and　is　capable　of　improving　the　performance　of　concrete　and　reducing　the　costs.　Once　hardened,　the　dosage　of　slag　in　concrete　matrix　is　hard　to　evaluate.　Three　types　of　ground　blast　furnace　slag　(GBFS)　are　used　to　prepare　cement-slag　pastes　with　different　water　binder　ratios,　and　the　finenesses　and　original　dosages　of　GBFS　are　various.　Backscattered　electron　(BSE)　images　combining　with　image　analysis　method　are　used　to　analyze　the　areal　fraction　of　GBFS　in　the　pastes.　The　feasibility　of　this　method　is　analyzed,　and　it　is　proved　that　the　method　is　effective　in　quantitative　analysis　on　the　content　of　GBFS　in　hardened　cement　pastes.　The　multivariate　linear　correlations　between　areal　fraction　of　GBFS,　curing　age,　GBFS　fineness,　original　dosage　of　GBFS,　are　created.　It　is　capable　of　obtaining　original　dosage　of　GBFS　based　on　existing　parameters.　(C)　2016　Elsevier　Ltd.　All　rights　reserved.