Abstract
Working Group 1 of RILEM TC 238-SCM ‘Hydration and microstructure of concrete with supplementary cementitious materials (SCMs)’ is defining best practices for the physical and chemical characterization of SCMs, and this paper focusses on their thermal analysis. Thermogravimetric analysis (TGA) can provide valuable data on the chemical and mineralogical composition of SCMs. Loss-on-ignition (LOI) testing is a commonly used, standardized, but less sophisticated version of TGA that measures mass at endpoints only, with heating generally in air. In this paper we describe the use of TGA and LOI to characterize Portland cement with limestone additions, coal combustion fly ashes, ground-granulated blast furnace slag, calcined clays, and natural pozzolans. This paper outlines the value and limitations of TGA and LOI (in the formats defined in different standards regimes) for material characterization, and describes testing methods and analysis. TGA testing parameters affect the mass loss recorded at temperatures relevant for LOI measurements (700–1000 °C) of slags and fly ashes, mainly associated with oxidation reactions taking place upon heating. TGA of clays and natural pozzolans is utilized to identify optimal calcination conditions leading to dehydroxylation and consequent structural amorphization, particularly for kaolinite. However, dehydroxylation and amorphization do not occur at similar temperatures for all clays, limiting the applicability of TGA for this purpose. Although TGA is widely utilized for characterization of SCMs, the testing parameters significantly affect the results obtained, and TGA results require careful interpretation. Therefore, standardization of TGA testing conditions, particularly for LOI determination of slags and fly ashes, is required.
Highlights
The class of supplementary cementitious materials (SCMs) encompasses a wide range of materials with different chemical compositions, mineralogies, and physical characteristics
In this paper we explore the use of LOI and Thermogravimetric analysis (TGA) to characterize several commonly used supplementary cementitious materials
The loss on ignition of fly ash is often used as a measure of the content of unburnt carbon in the material [7, 8], as the carbon can oxidize during heating in the presence of oxygen to CO and CO2, which is released during TGA measurement [9, 10]
Summary
The class of supplementary cementitious materials (SCMs) encompasses a wide range of materials with different chemical compositions, mineralogies, and physical characteristics Some of these properties are specified in standards that define limits with respect to oxide composition, mesh size, moisture content, and loss-on-ignition, for example. The mass loss is determined following heating in air to a specified maximum temperature for a specific time, this maximum temperature and time varies according to the standard followed. These discrepancies can lead to difficulty interpreting results. Attention is paid to the role of parameters selected during testing, as well as to the interpretation of results
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