Hydration Activity And Expansibility Of Ro Phase In Steel Slag

The application of steel slag(SS)in cement and concrete is limited due to its poor volume stability.At present,the reasons for the poor stability of SS are insufficiently understood.Especially,the hydration activity and expansibility of the divalent metal oxide solid solution(RO phase)in SS are not clear.This study first analysed the hydration activity and stability of SS.Then,according to the composition analysis of RO phase in SS,the RO phases of different compositions were synthesized,and the hydration activity of RO phases of different compositions was studied.Finally,the RO phases of different compositions were blended with cement to prepare the cementitious materials,and the influence of the composition and content of RO phase on the volume stability of the cementitious material was studied.Compared with Portland cement,the hydration activity of SS was low.After hydrated at room temperature for 3 d,SS produced almost no hydration products.After autoclaved for 3 h,the hydration degree of SS was about 1/3 of that of cement.The hydration products of SS mainly included calcium silicate hydrate gel(C-S-H),rod-like ettringite(AFt)and layered structure of calcium hydroxide(CH).The SS was mixed into cement with different contents to prepare the steel slag-cement cmentitious material,and the boiling expansion rate and autoclave expansion rate were tested.The results showed that the expansion rates of specimen increased linearly with the increasing content of SS.When the content of SS was 60%,the specimen warped after boiled,and cracked after autoclaved,and the stability was unqualified.SS contains about 30%of RO phase.The RO phase in SS was about a phase of MgO·xFeO system,and the composition was not fixed and its distribution was wide.The hydration degree and activity of RO phase under autoclave condition decreased exponentially with the increasing molar ratio(x)of FeO to MgO in SS.When x≤0.5,the RO phase could be slowly hydrated at room temperature,and the hydration degree significantly increased after autoclaved.When x=1,the RO phase did not have hydration activity under room temperature and boiling condition,and only autoclave could promote its slow hydration.When x≥1.5,the RO phase had no hydration activity.When the value of x in RO phase was constant,the autoclave expansion rate(LA(x,y))of the cement paste specimen gradually increased with the increasing content(y)of RO phase.Meanwhile,when y was constant,the autoclave expansion rate of the specimen gradually decreased with the increasing value of x in RO phase.When x=0.2 or 0.4,y5%,the specimen was crushed after autoclaved.When x=0.5,y≥6%,the specimen was cracked after autoclaved.The relationship among LA(x,y),x and y was:when x≤0.5,the hydration activity of RO phase was relatively high,and the RO phase had a great effect on the poor stability of SS,and 5%content of RO phase could cause cracking of the specimen;when 0.5<x<1.5,the hydration activity of RO phase was low,LA(x,y)=7.10×x-5.38×y1.38+8.65×0.0356x+LA0;when x≥1.5,the RO phase had almost no hydration activity,LA(x,y)=0.76y+La0.LA0 was the autoclave expansion rate of the pure cement paste.