The Microstructure of Cement Aggregate Interfaces
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THE MICROSTRUCTURE X.
ZHANG,
G.W.
GROVES AND
S.A.
OF CEMENT AGGREGATE
INTERFACES
RODGER.
Department of Metallurgy and Science of Materials, Road, Oxford, OXI 3PH, UK.
University of Oxford,
Parks
ABSTRACT
Mortar bars of tricalcium silicate or ordinary portland cement with small aggregate particles have been sectioned and examined in the transmission electron microscope (TEM). The presence of calcium silicate hydrate (CSH) gel at the interface is seen by TEM in many cases in intimate contact with the aggregate. In some cases calcium hydroxide (CH) is observed near the interface but is seldom in contact with the aggregate. Microanalysis in the TEM shows that in the case of reactive silica aggregate a gel of relatively low Ca:Si ratio is found at the interface. Aggregates of different degrees of reactivity and mortars of different ages are compared. It is suggested that interface fractures may occur beyond the coating of CSH rather than at the true interface in the case of the type of particles we have studied. INTRODUCTION There is considerable interest in the characterisation of the cement paste-aggregate interface because of its contribution to the strength of concretes. There have been suggestions that the strength of the cement aggregate bond may be the limiting factor in the determination of the ultimate strength of concrete (1) and may play a significant role in the durability of concrete (2). Similarly an examination of the interface when reactive siliceous aggregates are used may facilitate early diagnosis of deterioration which would lead to structural distress in concretes. Most of the microstructural studies reported in the literature have used X-ray diffraction (XRD) methods and scanning electron microscopy (SEM) sometimes supplemented by energy dispersive X-ray analysis (EDXA). Early workers reported the existence of an 'aureole de transition' which is described as an area extending radially from the aggregate surface which contains preferentially oriented CH crystals (3). Barnes el al (4) described the formation of the 'duplex film'. The nature and formation of this film has recently been reviewed by Diamond (5), who proposes that on mixing a mortar 0 5 with water there is rapid precipitation of a layer of CH about . '"m thick onto the aggregate surface. This is later covered with a layer of short fibres of CSH again about 0.5um in thickness. These two layers constitute the duplex film. At later stages a further layer of CH about 3,amthick is thought to form which separates the film from the area of less densely filled paste which itself is formed due to the high local water content often developed next to aggregate particles. There are many reports in the literature describing the detection of the duplex film, e.g.(6,7), but there are occasional studies where workers have failed to find evidence of the film. Javelas et al (8) used TEM studies of ion beam thinned mortars and found a layer of CSH some tenths of a micron thick separating both calcite and quartz particles from areas of paste whi
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