Study About Expansibility Of F-CaO In Steel Slag

Steel slag is the waste of steel mill.The primary mineral phase is C₂S and C₃S,which is similar to Portland cement.The existence of f-CaO in steel slag is the reason that causes bad stability of steel slag.According to the composition analysis of several steel slag in China,this paper syn-thesis some tri-component f-CaO,the chemical formula is CaO·aFeO_1.5·bMnO₂.The influence of its content and particle size on the volumetric change of cement was studied by adding it into cement.Fur-thermore,its influence on the microstructure and pore structure was also studied.By studying composition and hydration action of CaO aFeO_1.5 bMnO₂,two parts are detected in it,the single phase CaO and some sosoloid phase.The single phase can hydrate completely at room temperature after 24h while the sosoloid phase can’t hydrate at room temperature and only hydrate partly by boiling.And the hydration activity of single phase CaO is higher than that of sosoloid phase.F-CaO was mixed into the reference cement to study the expansibility by auto-claving.The relation between autoclave expansion rate y and mass frac-tion of f-CaO x is y=K1eK2X,and the correlation coefficient R2 was great than 0.9 after linearization.K2 becomes smaller with the increase of（a+b）in f-CaO,which means that the growth rate of expansion for f-CaO-cement specimen is slower.Using the method YB/T 140-2009<Methods of chemical analysis for steel slag>to measure the content of f-CaO in steel slag can only test the single phase in f-CaO,while the sosoloid phase in f-CaO can’t be measured.By this way the content of f-CaO in Xinyu thermal steel slag is 5.44%.And almost all of that is mono-component f-CaO and single phase CaO.So compared with tri-component f-CaO,its expandability is poor.When the content of tri-component and f-CaO in steel slag is close to 0.5%,the volumetric change of cement with tri-component is 0.4%and that with f-CaO in steel slag is just 0.15%.When the content of f-CaO in steel slag is 3.324%,the volumetric change of cement is just 0.3148%.When the content of tri-component f-CaO in cement is 0.375%,the volumetric change of specimen is bigger if the grain size of tri-component is bigger.When the grain size of tri-component is 1～2mm,the specimen broke up by boiling,while when the grain size of tri-component is less than 0.18mm,the volumetric change of specimen is close to that of pure cement.SEM shows that when the grain size of tri-component is big enough,it’s isolated with gel system and its expan-sion is intensive,which will squeeze and destroy the structure of speci-men.When the grain size of tri-component is less than 0.18mm,the tri-component will be conforming by gel system and its expansion can be absorbed partly.When the grain size of tri-component is big,the content of harmful hole in specimen after autoclaving will increase,while if the grain size of tri-component is small enough,the porosity of specimen de-creased,even less than pure cement.And the content of harmful hole de-creased obviously.In another word,it can improve the pore structure of specimen if the grain size of tri-component is small enough.