Zu einigen Problemen der Chemie des Portlandzements

After describing the main chemical reactions going on during the hydration of portland-cement, the influence of small increases in the alkali- and sulphate content, coming from the introduction of new energy saving cementproduction processes, are discussed. By investigating pore- solutions squeezed from, hardened cement pastes it was shown that they contain alkalihydroxides up to 1 mol/l. These alkalihydroxide solutions react with some types of aggregates in the concrete giving alkalisilicate gels, which swell in the presence of humidity, resulting in crack formation and demolishing of the concrete constructions. In addition to this reaction the amount of ettringite (Ca6[Al(OH)6]2 · [SO4]3 · 26H2O) formed from the cement clinker mineral 3CaO · Al2O3 is increased, because of the higher level of sulphate resulting from the fact, that most of the alkalies in the portland cement clinker are present as alkalisulphates. This ettringite is decomposed if the concrete is treated in the temperature range from 50–100°C to accelerate the hardening process. At room temperature ettringite crystallizes again which is accompanied by volume increase resulting in a deterioration of the concrete too. Possibilities to avoid the alkali-aggregate reaction as well as the secondary ettringite formation are shown.