Microstructure of high-lime fly ash cementitious mixes

CEMENT and CONCRETERESEARCH. Vol. I I , pp. 291-294, 1981. Printed in the USA. 0008-8846/81/020291-04502.00/0 Copyright (c) 19E7 Pergamon Press, Ltd.

MICROSTRUCTURE OF HIGH-LIME FLY ASH CEMENTITIOUS MIXES M.W. Grutzeck, D.M. Roy and B.E. Scheetz Materials Research Laboratory The Pennsylvania State University University Park, PA 16802

(Received Dec. 17, 1980)

Reactive high-lime f l y ashes (of bituminous, sub-bituminous and lignite varieties) incorporated in blended cements have hydration behaviors differing considerably from those of "normal" high aluminosilicate fly ashes. Such fly ashes havebeen reported to increase in reactivity in proportion to their lime contents.(1) Current research in our laboratory(2) has confirmed this higher re. activity inwhich the f l y ash i t s e l f has cementin9 properties in contrast to the behavior of low-lime f l y ashes.(3) Diamond (4) recently described the hydration of such f l y ash which contained C3A. Current studies have been made of various cementitious mixtures incorporating a high-lime fly ash with composition given in Table I. In addition to the dominant glassy phase, the ash contains C3A , anhydrite and some free CaO. A low water/solids ratio (~X).30) mixture containing 68g of a low-C3A cement, 23g f ly ash with additions of calcium sulfate, sodium chloride and minor amounts of other additives was cured at 38°C [in sat. Ca(OH)2] and the sequences of phase formation and microstructure development were followed. Substantial e t t r i n gite formation was observed at 3 days and also at 7 days, which was related to their expansion behavior. X-ray diffraction measurements detected much ettringite and Friedel's salt in addition t o t h e usual calcium hydroxide, and f l y ash and cement residual components. The ettringite normally grows as radiating clumps of hexagonal prisms or needles in void spaces (Figs. l and 2). The f l y ash surfaces are usually smooth and relatively free of crystal growth, but crystals suggesting epitaxial growth are observed on some surfaces (Fig. 3), and some thin shells show extensive reaction, apparently to form e t t r i n g i t e inside the sphere i t s e l f (Fig. 4). Though the samples were very dense at 28 days, ettringite forms were s t i l l observed at that age. Honeycombed C-S-H was present, but observed only rarely, in both 3-day (Figs. 5 and 6) and 7-day samples. The more massive C-S-H which dominates at 7 days (Fig. 7) incorporates f l y ash particles of differing stages of reaction, 291

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Table I.

Chemical Analysis of Fly Ash # B26

Component Si02 A1203 Fe203 CaO MgO S03 Na20 K20 Ti02 P20s Mn203 BaO SrO LOI Moisture Content TOTALS Other C02 Total Alkalies as Na20

33.2 17.6 6.03 29.91 5.07 2.77 1.03 0.41 1.55 0.95 0.08 0.84 0.37 0.44 0.06 100.31 0.22 1.30

while at 28 days (Fig. 8) massive dense structures are even more predominant.

The other properties of the cementitious mix, rapid (and high in the particular mixture) strength development are consistent with the relatively high reactivity of the fly ash. Thus, the results of these and continuing studies here and elsewhere (5) illustrate the usefulness of such reactive fly ashes as cementitious components, and for preparing expansive cements. REFERENCES (1)

(2)

(3)

Sersale, R., Proceedings, 7th Intl. Congr. Chem. Cement, Paris 1980, Vol. I, IV-I/3-1Vl/18. Roy, D.M., Geochemical Factors in Borehole and Shaft Plugging Materials Stability, Proceedings, OECD/ONWI International Workshop on Borehole and Shaft Plugging, Columbus, OH (7-9 May, 1980), 14 pp. Roy, D.M., and N. Setter, Cement and Concrete Research 8, 621-632 (1978).

(4)

Diamond, S., Proc. 7th I n t l . Congr. Chem. Cement, Paris 1980, Vol. I I I , IV-19-1V-23.

(5)

U.S. Army Engineer Waterways Experiment Station, Vicksburg, Mississippi.