The　recovery　of　phosphorus　(P)　from　sewage　sludge　through　pyrolysis　is　one　of　the　most　promising　alternatives　to　alleviate　the　shortage　of　P　supply　in　the　future.　Adding　calcium　additives　is　beneficial　to　the　conversion　of　available　P　and　helps　to　improve　the　recovery　and　utilization　efficiency　of　P　in　biochar.　This　study　thoroughly　examined　the　influence　of　CaCO3　on　the　speciation　and　slow-release　behavior　of　P　in　sludge　during　pyrolysis　at　500　and　700　°C.　The　addition　of　CaCO3　could　enhance　the　conversion　of　non-apatite　inorganic　phosphorus　(NAIP)　into　more　accessible　apatite　phosphorus　(AP).　We　observed　that　adding　CaCO3　(2.5–20　%)　to　raw　sludge　at　500　°C　increased　the　proportion　of　AP　in　inorganic　phosphorus　(IP)　from　65.2　to　97.3　%　but　did　not　increase　the　concentration　of　AP,　which　remained　between　31.03　and　31.64　mg/g.　At　700　°C,　introducing　CaCO3　(2.5–20　%)　boosted　the　AP　content　from　35.08　to　41.38　mg/g　and　increased　the　proportion　of　AP　in　IP　from　67.7　to　98.6　%.　Moreover,　irrespective　of　the　pyrolysis　temperature　being　500　°C　or　700　°C,　the　final　main　form　of　P　in　biochar　with　CaCO3　was　Ca5(PO4)3OH,　according　to　experimental　characterization　results　(X-ray　photoelectron　spectroscopy　and　X-ray　diffraction)　and　thermochemical　calculations.　Additionally,　the　efficacy　of　citric　acid　(CA)　in　releasing　biochar-bearing　P　was　tested　in　a　laboratory　extraction　experiment　for　21　days.　CaCO3-amended　biochar　showed　a　significantly　slower　total　phosphorus　(TP)　release　than　the　biochar　without　CaCO3　daily,　and　the　amount　of　TP　decreased　with　increasing　amounts　of　added　CaCO3.　The　finding　of　this　study　indicates　that　biochar　generated　by　adding　10　%　CaCO3　has　the　highest　AP　concentration　(41.38　mg/g)　at　700　°C,　and　its　TP-release　efficiency　in　CA　is　0.29–1.98　mg/g　in　21　days,　which　shows　remarkable　potential　as　a　soil　fertilizer　with　slow　P　release.　©　2023　Elsevier　B.V.