The effects of MgO, Na2O and SO3 on industrial clinkering process : phase composition, polymorphism, microstructure and hydration, using a multidisciplinary approach

The present investigation deals with how minor elements (their oxides: MgO, Na2O and SO3) in industrial kiln feeds affect (i) chemical reactions upon clinkering, (ii) resulting phase composition and microstructure of clinker, (iii) hydration process during cement production. Our results show that all these points are remarkably sensitive to the combination and interference effects between the minor chemical species mentioned above. Upon clinkering, all the industrial raw meals here used exhibit the same formation temperature and amount of liquid phase. Minor elements are preferentially hosted by secondary phases, such as periclase. Conversely, the growth rate of the main clinker phases (alite and belite) is significantly affected by the nature and combination of minor oxides. MgO and Na2O give a very fast C3S formation rate at T > 1450 K, whereas Na2O and SO3 boost C2S After heating, if SO3 occurs in combination with MgO and/or Na2O, it does not inihibit the C3S crystallisation as expected. Rather, it promotes the stabilisation of M1-C3S, thus indirectly influencing the aluminate content, too. MgO increseases the C3S amount and promotes the stabilisation of M3-C3S, when it is in combination with Na2O. Na2O seems to be mainly hosted by calcium aluminate structure, but it does not induce the stabilisation of the orhtorhombic polymorph, as supposed to occur. Such features play a key role in predicting the physical-mechanical performance of a final cement (i.e. rate of hydration and hardening) when used as a bulding material.