While the effective alkali content of Portland cements rises in line with an increase in the total alkali content, the effective alkali content of cements containing several main constituents may deviate from their total alkali content to varying degrees. This can be traced back to discrepancies in the solubility of the alkalis contained in the cement constituents on the one hand, and to the different alkali absorption by the reaction products on the other hand. As a consequence, the regulation according to which NA cements are classified on the basis of their total alkali content may in some cases lead to an exclusion of certain cements from use as NA cements due to their total alkali content, even though their effective alkali content is low. In order to be able to classify further cements as NA cements if applicable, it is necessary to determine their effective alkali content. In this context, particular importance is conferred on the dynamic combination and dissolution processes of the alkalis. However, knowledge of these processes was fairly inadequate so far, especially as far as cements with several main constituents are concerned.

The Research Institute therefore conducted investigations to examine the influence of blastfurnace slag, hard coal fly ash and silica fume on the effective alkali content of the cement. Pore solution investigations were conducted to gain an insight into the very phenomenon of how the alkalinity of the pore solution of hardened cement paste is changed by the utilisation of latent hydraulic or pozzolanic main cement constituents, respectively. Investigations of solid matter were to reveal which chemico-mineralogical mechanisms are crucial for alkali binding and whether or how long, respectively, the bond is stable in the hardened cement paste.

The investigations show that the CSH phases constitute the most important combination partner for the alkalis in all hardened cement pastes. The binding capacity of the CSH phases changes in conjunction with their stoichiometric composition, however. As the calcium content in the phases decreases, their tendency to sorb alkalis increases. The CSH phases with the highest calcium content are formed during the reaction of the Portland cement clinker. When the clinker is replaced by the latent hydraulic or pozzolanic main constituents, respectively, which have a lower calcium content, the average Ca/Si ratio of the CSH phases in the hardened cement paste declines. The degree to which the Ca/Si ratio is reduced on average depends on the calcium content of the other main constituents apart from the clinker and their proportion in the cement. When hard coal fly ash was utilised, additional alkalis were found to be combined in zeolite-type compounds at least temporarily.