Ferronickel　slag　and　ground　granulated　blast-furnace　slag　(GGBFS)　are　solid　waste　by-products　from　the　metallurgical　industry.　When　incorporated　into　concrete,　they　help　promote　resource　utilization,　reduce　hydration　heat,　and　lower　both　solid　waste　emissions　and　the　carbon　footprint.　To　facilitate　the　application　of　ferronickel　slag–GGBFS　concrete　in　3D　printing,　this　study　examines　how　aggregate　size　and　nozzle　diameter　affect　its　performance.　The　investigation　involves　in　situ　printing,　rheological　characterization,　mechanical　testing,　and　scanning　electron　microscopy　(SEM)　analysis.　Results　indicate　that　excessively　large　average　aggregate　size　negatively　impacts　the　smooth　extrusion　of　concrete　strips,　resulting　in　a　cross-sectional　width　that　exceeds　the　preset　dimension.　Excessively　small　average　aggregate　size　results　in　insufficient　yield　stress,　leading　to　a　narrow　cross-section　of　the　extruded　strip　that　fails　to　meet　printing　specifications.　The　extrusion　performance　is　closely　related　to　both　the　average　aggregate　size　and　nozzle　diameter,　which　can　significantly　influence　the　normal　extrusion　stability　and　print　quality　of　3D　printed　concrete　strips.　The　thixotropic　performance　improves　with　an　increase　in　the　aggregate　size.　Both　compressive　and　flexural　strengths　improve　with　increasing　aggregate　size　but　decrease　with　an　increase　in　the　printing　nozzle　size.　Anisotropy　in　mechanical　behavior　decreases　progressively　as　both　parameters　mentioned　increase.　By　examining　the　cracks　and　pores　at　the　interlayer　interface,　this　study　elucidates　the　influence　mechanism　of　aggregate　size　as　well　as　printing　nozzle　parameters　on　the　mechanical　properties　of　3D　printed　ferronickel　slag–GGBFS　concrete.　This　study　also　recommends　the　following　ranges.　When　the　maximum　aggregate　size　exceeds　50%　of　the　nozzle　diameter,　smooth　extrusion　is　not　achievable.　If　it　falls　between　30%　and　50%,　extrusion　is　possible　but　shaping　remains　unstable.　When　it　is　below　30%,　both　stable　extrusion　and　good　shaping　can　be　achieved.　©　2025　by　the　authors.