DRY MIXTURES

[:ru]Дубина Э.[:en]Dubina, E.[:] [:ru]кафедра строительной химии, Мюнхенский технический университет, Гархинг, Германия[:en]Chair for Construction Chemicals, Technische Universität München, Garching, Germany[:]

[:ru]Планк Й.[:en]Plank, J.[:] [:ru]проф., доктор техн. наук, зав. кафедрой строительной химии, Мюнхенский технический университет, Гархинг, Германия[:en]prof, Dr. Eng. Sci., Chair for Construction Chemistry, Technische Universität München, Garching, Germany[:]

[:ru]Вадсё Л.[:en]Wadsö, L.[:] [:ru]Лундский ун-т, Лунд, Швеция[:en]Department of Building Materials, Lund University, Lund, Sweden[:]

[:ru]Блэк Л.[:en]Black, L.[:] [:ru]Ун-т Лидса, Лидс, Великобритания[:en]School of Civil Engineering, University of Leeds, Leeds, United Kingdom[:]

[:ru]Кёниг Х.[:en]König, H.[:] [:ru]Гейдельбергский центр технологии производства цемента, Лаймен, Германия[:en]HeidelbergCement Technology Center, Leimen, Germany[:]

Alitinform №3 (20) 2011 г. 38-45 стр.

Abstract

It is known that the performance of drymix mortars can change after a long period of storage in humid atmosphere. The principal consequences of this phenomenon, otherwise known as prehydration, include increased setting time, decreased compressive strength and heat of hydration, altered rheological properties, reduction in initial flowability and poor response to superplasticizer addition.

In the first part of this work, the physicochemical effects of water sorption on the surfaces of pure clinker phases such as C₃S, C₂S, C₃A and C₄AF, different sulfate components including CaSO₄ · 2H₂O, β-CaSO₄ ∙ 0,5H₂O and CaSO₄ as well as free lime (CaO) are investigated using a water vapour balance. From thence, these findings are extrapolated to commercial cement commonly used in drymix mortars.

For this investigation, the relative humidity was increased from 0 to 95 % within 11 h, and the difference in the mass of the samples due to water sorption was measured. Based on these data, it is possible to determine a threshold value for the relative humidity at which significant water sorption, and thus prehydration begins. As will be shown further, the onset of prehydration with respect to relative humidity depends strongly on the cement constituent and lies between 14–80 % RH for all cement constituents tested. Besides, it was found that the sorption of water onto the surfaces of the individual components can occur via physical (reversible) or chemical (irreversible) interactions. The degree of each interaction is also considerable different between the individual components present in cement.

The results show that CaO contained in cement sorbs a large quantity of water even when the air humidity level is low, thus it behaves as a drying agent (desiccant). Relatively large amount of water is sorbed by the orthorhombic modification of C₃A, while the cubic modification sorbs lower amount of water vapour. On the contrary, silicates (C₃S, C₂S) have low sensitivities for moisture.

Key words:

prehydration; clinker phases; relative humidity; sorbtion balance