INFLUENCE OF SUPERPLASTICIZERS ON LONG TERM VARIATION IN C3A HYDRATION AND FORMATION OF AFM PHASES – EFFECTS ON DURABILITY

摘要

Superplasticizers are applied in concrete production to improve the performance and to produce high strength concretes. Most previous investigations discuss the mechanisms of liquefaction, hydration retardation and cement-superplasticizer incompatibilities. Little information is given about the influence of superplasticizers on durability of concretes, especially their effect on long term C3A hydration and AFt / AFm formation. The present study discusses the risk of increased C3A hydration and AFm formation of Portland cements in the presence of superplasticizers. It is generally known that under normal conditions in concrete AFm phases are metastable and their recrystallisation to ettringite is probable (Delayed Ettringite Formation, DEF). DEF is one severe risk for durability of concrete. It is shown by quantitative X-ray diffraction (Rietveld method), isothermal heat calorimetry measurements and high resolution scanning electron microscopy in combination with durability testing, that superplasticizers may increase the AFm to AFt ratio and the degree of C3A hydration. Findings are supported by dissolution experiments on pure C3A in dependence of superplasticizer addition. Also it is shown that for Portland cements with increased content of soluble alkali sulphates the risk of increased AFm formation is limited.