Alisa Machner; Jürgen Neubauer; Friedlinde Götz-Neunhoeffer
Influence of portlandite with different reactivity on alite hydration
Portlandite, known as a major hydration product in the OPC hydration, also can be used as an admixture to dry cements for accelerating the hydration or improving the rheology and plasticity. Although portlandite is used in cementitious systems to accelerate the hydration, its effect is still controversially discussed in literature. The accelerating period of alite hydration starts simultaneously with CH precipitation. As alite hydration is controlled by the dissolution rate of Ca2+ and OH-, Kondo et al. came to the conclusion, that Ca(OH)2 shortens the induction period . And additionally the higher alkalinity should lead to an earlier portlandite precipitation and therefore to an earlier end of the induction period , providing that CH precipitation is the cause for ending the induction period not one consequence among others . The aim was to investigate which influence portlandite has on the alite hydration and how this impact can be controlled by changing the reactivity and the specific surface of portlandite respectively. As being the major clinker phase in OPC, the hydration studies were carried out on pure alite. The used synthesized CH had specific surfaces from 5 to 43 m2/g. To demonstrate the impact of CH addition on alite hydration, heat flow calorimetry and in-situ XRD measurements were carried out with pure alite and mixtures of 80 wt.% alite and 20 wt.% CH. Heat Flow calorimetry results show a shortening of the induction period. The results of the in-situ XRD measurements confirm this trend as all measured mixtures show earlier alite dissolution, respectively CH and C-S-H precipitation compared to the measurements of pure alite. In addition the amount of dissolved alite respectively precipitated CH was higher when CH was added to the dry phase what results in higher heat of hydration values after 24 h. As known the hydration kinetics depend strongly on the ionic concentrations of the pore solution. For this reason the Ca2+ concentrations were measured with AAS at different times of hydration. For checking the influence on the silica reaction the Si4+ concentrations will be measured on an ICP-MS. Among the used CH there were no big differences in their influence on alite hydration. There is a trend in higher initial heat flows and higher heat of hydration values for the CH with the finest particles respectively the highest specific surface. Mixtures with fine disperse CH show the highest Ca2+ concentrations ab initio but the curve progression doesn´t change. In summary it was shown that CH addition accelerates the alite hydration and its influence is not dependent on the reactivity of the used portlandite, if it´s specific surface is in the area of the used ones.
 Kondo et al., Journal of Applied Chemistry and Biotechnology 1977, 27, 191 – 197.  Kumar et al., Cement and Concrete Research 2012, 42, 1513 – 1523.  Markar, J. & Chan, G. W., Journal of the American Ceramic Society 2008, 91, 1292 – 1299.  Odler, I. & Doerr, H., Cement and Concrete Research 1979, 9, 277 – 284.
Dewey Decimal Classification:
Book / Congress title:
1st International Conference on the Chemistry of Construction Materials
Society of German Chemists e.V. (GDCh), Division of Chemistry of Construction Chemicals