The a
ddition of CaCO
3 an
d CaSO
4 to Portlan
d cement clinker influences the hy
dration an
d the strength
development. An increase of the CaSO
4 content accelerates alite reaction
during the first
days an
d results in the formation of more ettringite, thus in a higher early compressive strength. The late compressive strength is
decrease
d in Portlan
d cements containing higher quantities of CaSO
4. The re
duce
d late compressive strength seems to be relate
d to an increase of the S/Si an
d Ca/Si content in the C-S-H.
The presence of calcite leads to the formation of hemicarbonate and monocarbonate thus indirectly to more ettringite. Only a relatively small quantity of calcite reacts to form monocarbonate or hemicarbonate in Portland cement. Although hemicarbonate is thermodynamically less stable than monocarbonate, hemicarbonate formation is kinetically favored. Monocarbonate is present only after 1 week and longer independent of the quantity of calcite available and the content of sulphate in the cement.