In　this　article,　a　new　chemically-bonded　ceramic　composite,　called　magnesium　silicon　potassium　phosphate　cement　(MSPPC),　without　retarders,　is　prepared　from　dead-burnt　magnesium　oxide,　silica　fume　and　dipotassium　hydrogen　phosphate　(K2HPO4　or　DHP).　MSPPC　has　excellent　workability　and　mechanical　properties,　but　the　water　resistance　still　needs　to　be　improved　(its　strength　retention　rate　is　only　78.41%　after　90　days　of　water　immersion).　Notably,　in　contrast　to　the　acidic　or　near-neutral　pH　internal　environment　of　traditional　magnesium　potassium　phosphate　cement　(MKPC),　the　MSPPC　system　has　a　unique　high-alkaline　internal　environment,　which　appears　to　have　the　natural　alkali-activated　conditions.　In　order　to　improve　the　water　resistance,　the　effect　of　slag　on　MSPPC　is　studied.　The　results　show　that　with　the　increase　of　slag　content,　the　fluidity　decreases,　the　setting　time　increases　and　the　pH　value　of　the　hydration　process　rises.　After　90　days　of　air　curing,　the　specimens　containing　10　wt　%　of　slag　can　reach　a　maximum　flexural　and　compressive　strength　of　approximately　27.2　MPa　and　124.8　MPa,　respectively.　The　best　water　resistance　is　achieved　when　the　slag　content　is　20　wt　%,　with　a　strength　retention　rate　of　90.42%　after　90　days　of　water　immersion.　The　hydration　mechanism　of　slag　in　MSPPC　system　is　also　different　from　that　in　traditional　MKPC　system.　The　hydraulic　nature　of　slag　can　be　effectively　activated　by　KOH,　K2SiO3　and　MgSiO3　(the　hydration　products　of　MSPPC).　The　BSE-EDS,　29Si　MAS　NMR　and　SEM-EDS　tests　show　that　Ca,　Al　and　Si　elements　in　the　slag　are　released　into　solution　and　combined　with　Si　elements　in　the　MSPPC　system　to　produce　C-(A)-S-H　gels.　Such　gels　wrap　around　the　surface　of　the　soluble　substance　and,　together　with　the　physical　filling　effect　of　the　slag,　contribute　to　the　mechanical　strength　and　water　resistance　of　MSPPC.　©　2022　Elsevier　Ltd　and　Techna　Group　S.r.l.