Time dependent changes in the internal chemical evolution of blended OPC cements over a 20 year period

Changes in the composition of ordinary Portland cement blended with either Class F fly ash or blast furnace slag at 30% replacement of cement have been investigated from 1 hour to 20 years. A nearly “steady state” aqueous phase composition is achieved during the initial 6 hours with respect to Ca, Na, K and SO4. Renewed activity, 6-24 hours, showed a rapid decrease in Ca and SO4 while Na and K increased gradually. Over the 20 year span the internal chemical composition continued to evolve finally reaching some degree of equilibrium. Changes in mineralogy were determined by XRD and Ca(OH)2 determination, while aqueous phase composition was determined from pore solutions extracted under pressure. Aqueous composition showed minor changes over the long term whereas mineralogy showed variable reactivity particularly between fly ashes. The significance of the results is discussed in relation to durability.

Author: K. Luke, E.E. Lachowski

Section: Application of cement

Keywords: blended cement; fly ash; blast furnace slag; hydration; phase composition