Porosity in the Microstructure of Blended Cements Containing Fly Ash
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POROSITY IN THE MICROSTRUCTURE OF BLENDED CEMENTS CONTAINING FLY ASH H.H. PATEL,a P.L. PRATT' and L.J. PARROTTb Department of Materials, Imperial College, Prince Consort Road, London SW7 2BP, UK a. Now at Redland Technology Ltd, Graylands, Horsham, W. Sussex RH12 4QG, UK b. British Cement Association, Wexham Springs, Slough SL3 6PL, UK Received 30 November, 1989; refereed ABSTRACT The changes in porosity of OPC and an OPC-fly ash blended cement during hydration have been studied at water/solids ratios of 0.35, 0.47 and 0.59, cured for times of up to 1 year at 25 0 C. The porosity was measured indirectly by methanol exchange and methanol adsorption techniques and, directly, by quantitative image analysis using backscattered electron imaging in the scanning electron microscope. Measurements of porosity and of remaining anhydrous material by image analysis showed good correlation with indirect methods. Measurement of the diffusion of methanol and of the compressive strength were made in parallel with the determination of the porosity during hydration and attempts were made to relate the properties to the microstructure. For both binders the reduction of total porosity with increased reaction was small. The major change in pore structure was the subdivision of coarse pores by gel to form finer pores. Compressive strength and diffusion properties were dominated by the relative volume of coarse pores. INTRODUCTION The properties of hardened cement paste are commonly related to the principal phases present in the microstructure with varying degrees of success. With three major solid phases and a liquid phase filling part or all of the porosity, whose proportions and composition change with time of curing, the interpretation of properties in terms of composite theory is bound to be difficult. Furthermore the experimental determination of the proportion and composition of each phase can prove difficult while their relationship in space and the degree of bonding between them can play a major role in determining properties. This paper describes some of the results of a study [1] of the development of the microstructural features of an ordinary Portland cement (OPC) and an OPC-fly ash blended cement for periods of curing up to one year. In an earlier paper [2], a quantitative comparison was made of the volume fractions of anhydrous material, porosity and calcium hydroxide derived from backscattered electron image (BSE) analysis with those obtained by traditional techniques. A good correlation was found for the measurements of anhydrous material and of porosity. Since then a similar correlation has been found for calcium hydroxide also [3]. EXPERIMENTAL MATERIALS AND TECHNIQUES The cement used was an ordinary Portland cement (OPC) provided by the Cement and Concrete Association. The fly ash came from Fiddlers Ferry power station provided by CEGB Ash Marketing Division. The specific surface areas of OPC and fly ash were 405 m2 /kg and 370 m2/kg respectively. The oxide compositions are given in Table I and the phase composition o
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