Microstructure of Cement Blends Including Fly Ash, Silica Fume, Slag and Fillers
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MICROSTRUCTURE OF CEMENT BLENDS INCLUDING FLY ASH, SILICA FUME, SLAG AND FILLERS MICHELINE REGOURD Microstructures Department, CERILH,
23 rue de Cronstadt,
Received 13 January,
75015 PARIS,
FRANCE
1987; refereed
ABSTRACT The hydration of a blended cement through hydraulic or pozzolanic reactions results in heterogeneous polyphase materials. Because portland cement clinker is the major component in most cement blends, the microstructural development of portland cement hydrates, including C-S-H and pore structures, is first discussed. Slag, fly ash, silica fume and limestone filler cements are then compared to portland cement with regards to C-S-H morphology and composition, aluminate crystallization, cement paste interfaces and pore size distribution. INTRODUCTION The microstructure of hydrated cements has been recently reviewed for the 8th International Congress on the Chemistry of Cement (Rio de Janeiro, 1986) in principal reports by Taylor [1], Diamond [2] and Uchikawa [3]. The microstructural description of a cement paste includes the characterization of the structural elements (particles, crystals, films, voids) and of their juxtaposition and mutual arrangement [2]. Progress has been made thanks to new procedures of sample examination under electron microscope (wet cell, cryostage, ion-beam thinned specimens) and to the combination of such electron techniques as TEM, HVEM, EPMA, SEM, STEM, AEM and image analysis [1,2]. This paper will present the microstructure of blended cements containing mineral additives, i.e. blast furnace slag, fly ash, silica fume, and limestone filler. Because in most of these cements the portland clinker is the major component, the microstructure of ordinary portland cement (OPC) pastes will be described first. MICROSTRUCTURE OF PORTLAND CEMENT PASTES Portland cement hydrates are C-S-H (calcium silicate hydrate) and CH (calcium hydroxide) generated from silicates (C3 S, tricalcium silicate (alite), and C2 S, dicalcium silicate (belite)), along with AFt (ettringite) and AFm (monosulfates) phases from aluminates (C3 A and C4 AF). Quantitative phase determinations using XRD, TG, DTA, DSC and chemical extraction methods allow the calculation of an approximate distribution of the structural elements. Table I shows that in volume proportion, C-S-H and voids are the most important components of a largely hydrated cement [4,5]. Microstructural Development New results on the microstructural development during cement hydration have appeared since the Paris Congress (1980). Clearer pictures of the cement paste microstructure have been made possible through the following combination of techniques and careful preparation of specimens: high voltage transmission electron microscopy (HVEM) with a wet cell [6]; scanning transmission electron microscopy (STEM) of ion-beam thinned sections [7]; scanning electron microscopy (SEM) with either backscattered electron images (BEI) of polished sections [8] or secondary electron images of matched surfaces [9] and using a cryostage to avoid dehydration [10
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