Bound Water in Cement Pastes and its Significance for Pore Solution Compositions
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BOUND WATER IN CEMENT PASTES AND ITS SIGNIFICANCE FOR PORE SOLUTION COMPOSITIONS H.F.W. TAYLOR Department of Chemistry, AB9 2UE, Scotland, UK
University of Aberdeen, Meston Walk, Old Aberdeen
ABSTRACT The problem of defining bound water in a cement paste is discussed; a reasonable definition is one that includes interlayer water in C-S-H and AFm phases, structural water in ettringite, and adsorbed water, but not water in micropores or in larger pores. On this basis, structural considerations indicate a value of around 32% on the ignited weight for a fully hydrated paste. 'Non-evaporable' water, typically around 22% on the ignited weight at full hydration, cannot be identified with bound water, because dehydration to the state in which only non-evaporable water remains causes major loss of interlayer water and destruction of ettringite. In the interpretation of pore solution data, the definition of bound water, and the value assumed for this quantity, are important, because the ionic concentrations in the pore solution are greatly affected by the volume of free water available to dissolve them. If cement is partially replaced by low calcium fly ash, the quantity of bound water at any given age is substantially reduced. This effect contributes to the relatively low concentrations of alkali metal and hydroxyl ions that are observed in the pore solutions of many portland-fly ash cement pastes.
INTRODUCTION The need for a correct measure of bound water in cement pastes has been accentuated in recent years by interest in the compositions of the pore solutions, since in any attempt to explain these theoretically it is necessary to subtract from the total water content the amount that is bound in the hydration products. Diamond [1] and subsequently others used nonevaporable water for this purpose. This quantity, which for a fully hydrated portland cement is typically about 22% on the ignited weight, is strictly defined as the amount retained after D-drying, i.e., to constant weight at 23°C and a water vapor pressure, p(H 2 0), of 5 x 10-4 torr, achieved by equilibration with ice at -79°C [2]. In practice, oven drying at I05°C, which removes approximately the same amount of water, has often been substituted. However, Powers and Brownyard [3], who originated the concept of non-evaporable water, recognized that, even at the higher p(H 2 0) of 8 x 10-3 torr which they used, much of the water present in ettringite was lost, together with some of that from monosulfate. Feldman [4,5] concluded from sorption studies-that major loss of interlayer water from C-S-H took place at p(H 2 0) below about 2 torr (11% rh) at ambient temperatures. He found that D-drying of a fully reacted, bottle hydrated cement previously equilibrated at 11% rh produced a weight loss of 7.54%, referred to the D-dried weight [4]. This suggests that the total amount of water retained at 11% rh is about 30%, referred to the ignited weight. In the present paper, it is shown that this water content is approximately consistent with the amounts and probable deh
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