Magnesium　Phosphate　Cement　(MPC)　exhibits　high　strength,　exceptional　high-temperature　resistance,　and　outstanding　adhesion　properties　at　the　steel　interface,　rendering　it　an　ideal　material　for　fire　resistive　coatings　on　steel　structures.　In　this　study,　natural　brucite　powder　was　employed　as　the　magnesium　source,　while　hollow　glass　microspheres　(HGM)　were　utilized　as　functional　modifiers　to　formulate　brucite-based　MPC　fire　resistive　coatings　for　steel　structures.　This　research　explored　the　influence　of　the　mass　ratio　of　brucite　powder　to　ammonium　dihydrogen　phosphate　(M/P),　the　brucite　particle　size,　and　the　addition　of　HGM　on　the　physical,　mechanical,　and　fire-retardant　properties　of　the　coatings.　The　microstructure　of　the　brucite-based　MPC　before　and　after　fire　testing　was　examined　using　SEM,　XRD,　TG-DTG,　and　X-CT.　The　results　indicated　that　with　an　M/P　ratio　of　2.5:1　and　a　brucite　particle　size　of　75–150　μm,　the　material　exhibited　optimal　overall　performance　and　was　suitable　as　a　matrix　material　for　MPC.　The　incorporation　of　an　appropriate　amount　of　HGM　increased　the　compressive　strength　of　brucite-based　MPC　by　33.4　%,　reduced　the　dry　density　by　20.5　%,　extended　the　fire　resistance　of　the　structure,　and　decreased　the　thermal　conductivity　by　48.2　%.　This　can　be　attributed　to:　(i)　HGM　filling　the　pores　in　brucite-based　MPC,　creating　a　denser　microstructure;　(ii)　HGM　forming　a　closed　porous　structure　with　the　MPC　matrix,　reducing　thermal　conductivity　and　increasing　thermal　resistance;　(iii)　The　low　thermal　diffusivity　of　HGM/MPC　impeded　heat　transfer,　thereby　decelerating　thermal　decomposition　and　flame　propagation.　©　2024　Elsevier　Ltd