The Influence Of Stone Dust On Immobilization Of Chloride Ion Of Portland Cement And Its Mechanism

The use of manufactured fine aggregate (MFA) in Portland cement concrete isbecoming more common as sources of natural sands are being depleted. There is astrong need to better utilize MFA particularly the minus75μm-size stone dust. The sixkinds of stone dust with different lithologic, which are limestone, diabases, diorite,dolomite, granite and quartzite, were taken as the research objects in the paper, and theireffects on immobilization of chlorine ion of Portland cement were comparativelystudied. To analyze their mechanisms, the denseness of cement paste and pore structureof hydrated cement paste with different stone dust contents were tested, and the effect oflimestone powder on hydration product of Portland cement were studied with XRD,SEM and Raman spectrum, etc. The experimental results are as follow:(1) The six kinds of stone dust with different lithologic characters except forquartzite powder can improve chlorine ion immobilization of Portland cement. Theappropriate contents are respectively5%~20%limestone powder,3%~15%diabasesand diorite powder,3%~5%dolomite powder, and10~15%granite powder. Quartzitepowder is advantageous to the chlorine ion immobilization of Portland cement only atearly stage (earlier than7d), but the chlorine ion immobilization of Portland cement isimproved with an increasement in the specific surface area of quartzite powder.Furthermore, the chlorine ion immobilization of Portland cement gradually increases asconcentration of chlorine ion increases.(2) The five kinds of stone dust with different lithologic characters, which arelimestone, diabases, diorite, granite and quartzite, can improve the denseness of cementpaste. Their replacing percentage by weight are as follow:10%limestone powder,15%diabases powder,15%diorite powder,15%granite powder, and10%quartzite powderwhen the denseness of mixture (including cement and stone dust) reaches a maximum.The denseness of the mixtures increases as the specific surface areas of the limestoneand the granite powder increase.(3) The pore structure of hydrated cement paste can be improved by stone dust, justas limestone powder does, and the greater its improvement capability is, the smaller thelimestone powder size is. (4) The amount of calcium hydroxide in the hydration productions, which isobtained wtih X-ray quantitative analysis, shows that a smaller amount of limestonepowder delays the hydration of silicate minerals at earlier than or equal to28days, but alarger amount of limestone powder can accelerate the hydration of cement; after28days,the hydration degree of cement increases with an increase in percentage of limestonepowder, but the greater the percentage of limestone powder is, the smaller the increaseratio from28days to180days is. The limestone powder can react with tricalciumaluminate into C3A·3CaCO3·32H2O, and it can prevent the AFt phase from changinginto AFm phase.These results indicate that the particle size of stone dust is very close to Portlandcement, and it can participate the formation of hydration products and affectmicrostructure of hydrated cement paste. Therefore, it can affect the immobilization ofchlorine ion of Portland cement when stone dust is mixed into Portland cement andform a new system of cement and stone dust. It has a better guidance meaning for theuse of MFA with a percentage of stone dust in Portland cement concrete.