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Title: Portland Clinker-Fly Ash Cements – Relation between Compressive Strength and Microstructure

Author(s): Harald Justnes, Klaartje De Weerdt and Tone A. Østnor

Publication: Symposium Paper

Volume: 349


Appears on pages(s): 188-209

Keywords: clinker, compressive strength, fly ash, microstructure, sulphate

DOI: 10.14359/51732747

Date: 4/22/2021


Portland cements were made by mixing 4 different clinkers with 2 gypsum levels and 0-5% limestone powder. The compressive strength after 28 days of curing varied from 40 (5802) to 70 MPa (9718 psi) for mortar with equal w/c. Some of the clinkers were replaced with 4 different fly ashes and the response on strength differed. To explain the relatively large differences in strength evolution, the clinker and fly ash composition was investigated by SEM-BSE/EDS, the oxide compositions were determined by XRF and differences in clinker mineralogy determined by XRD Rietveld analysis. The microstructure of hydrated cement pastes of clinker/fly ash was investigated by SEM/EDS.

The highest strength was achieved with the white clinker containing no C4AF, produced using CaSO4/CaF2 flux, and therefore contained a higher total calcium sulphate content. The white cement also seemed to contain two calcium aluminate phases with potentially some fluoride in one of them, one probably glassy as Rietveld analysis underestimated C3A. The C4AF content of the other clinkers have low reactivity within the 28 days explaining some of the difference. One “fly ash” was actually a fluidized bed ash with higher calcium and sulphate content and different morphology explaining the different behavior from the other fly ashes.