10th American Ceramic Society Cements Meeting, Illinois, United States Of America, 16 - 18 June 2019, pp.1-10
The high environmental impact of ordinary portland cement (OPC) is due to its main mineral,
alite (C3S), which needs elevated kiln temperatures and high lime content to form. In calcium
sulfoaluminate (CS̄A) clinkers, ye’elimite (C4A3S̄ ) forms as the main clinker phase and alite does
not. Ye’elimite can form at 200-250 °C lower temperatures and has ~40 % lower lime content
than alite which makes CS̄A cements a promising alternative to OPC. Bauxite is typically used as
a raw material to satisfy the great alumina demand of ye’elimite formation. However, bauxite is
expensive as it is not abundant and is in demand by aluminum manufacturers. Hence,
incorporation of industrial wastes, particularly aluminous ones, into CS̄A cements can help
maintain their appeal.
This study used limestone, bauxite and gypsum as natural raw materials and some high-volume
industrial wastes in Turkey, a fly ash, red mud, and desulfogypsum, to produce CS̄A cements. Fly
ash and red mud were incorporated in the clinker raw meal to minimize the use of bauxite.
Desulfogypsum was added to the ground clinker as a source of calcium sulfate. Different CS̄A
clinkers were produced by varing calcination conditions (kiln temperature and residence time)
and using various raw mixtures. CS̄A cements and mortars produced with or without various
admixtures were investigated using strength tests, isothermal calorimetry, X-ray diffraction
(XRD), thermogravimetric analysis (TGA) and scanning electron microscopy (SEM). 1-d
compressive strengths of 15 MPa and 28-d strengths exceeding 35-40 MPa could be reached
using cements containing > 50 % industrial waste and as low as 10 % bauxite. Early strength gain
is due to ye’elimite hydration, which produces ettringite. Belite hydration which ordinarily
increases late strength can be accelerated with admixtures. XRD and TGA also indicate some
ettringite decomposition due to carbonation.