Abstract
In laboratory experiments, we found the unexpected influence of zinc addition, which was the acceleration of the Portland cement setting. This untypical cement behaviour was caused by the addition of anhydrite II as the setting time regulator, by its producer. Anhydrite II dissolves very slowly and the sulphate ions concentration in the paste of the hydrating cement, was very low. However, zinc was soluted very quickly and its hydroxide precipitated on the surface of C 3 A crystals, limiting the formation of the continuous ettringite layer. Thus, ettringite is formed in the free areas in the solution, linking the cement grains, with the formation of the quick “ettringite” setting. Ettringite precipitated in the free areas within the solution, binds to the cement grains and causes the ‘quick’ setting of ettringite. However, the alite crystals remain continuously covered by the amorphous layer of Zn(OH) 2 , and under this condition the induction period with water reaction of this phase remains hang-up. The elongation of this period is increasing with the increase in the addition of zinc
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