Cement additives that control shrinkage of concrete

Active, swelling-based shrinkage-control additives can be divided into 3 groups, which either create hydrates or ettringite crystals. 2 types of these additives are based on burnt lime treated with a special process or on magnesium oxide that has not been burned to shrinkage. The third group is based on crystals that create swelling ettringite. In our product range, the Denka CSA #20 type additive represents this proven, reliable and most widespread technology in our country.

CSA #20 is a swelling calcium sulfoaluminate-based cement additive that exerts its effect chemically. During its hydration, it creates ettringite, which results in a rapidly forming, swelling and pore-reducing crystal structure. During the setting of the cement paste, the colloidal crystals continue to develop and fill the cavities of the already formed crystal skeleton. Therefore, this process is fundamentally different from the aluminum powder shrinkage reduction method that involves gas formation, the effect of which occurs in a very short time, typically before the pulp strength increases.

Namex XA is a highly effective magnesium oxide based admixture containing special minerals developed to reduce drying shrinkage of concrete. It is not “fired to shrinkage”, therefore it behaves differently compared to the magnesium oxide found in Portland cement and does not cause long-term post-swelling.
Namex XA reacts with water to form new hydrates. During the hardening of the cement paste, the volume of Namex XA increases as hydrates are formed, which reduces shrinkage. This volume increase pre-stresses the crystal structure of the concrete, which reduces drying shrinkage.