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Performance of alternative supplementary cementitious materials

Year: 2026
Type: dissertation
Abstract: One approach to reduce the environmental impact of cement production is to substitute a proportion of the clinker by more sustainable, so-called supplementary cementitious materials (SCM). The present cumulative dissertation consists of five peer-reviewed publications investigating the suitability of different SCMs. The overall aim of the thesis was to gain a better understanding on the factors determining the performance of SCMs. It includes calcined clays of different compositions and reactivity, also in combination with ground calcium carbonate, pozzolans from self-ignited coal tailings dumps (referred to as autogenously calcined clays), conventional siliceous fly ashes from hard coal combustion, plant-based biomass ashes (wood ashes and a spelt husk ash) and a leaching residue from lab-scale phosphorus recovery of sewage sludge ash. All materials where physically, chemically and mineralogically characterized. The main methods used to evaluate performance were the R3 test and mortar compressive strength testing. The base cement used was a CEM I 42.5 R. The hydration in the cement system of most of the materials where studied by using isothermal calorimetry and X-ray diffraction. The hydration of selected systems was modelled thermodynamically. The reactivity determining factors for different material types where analyzed with statistical methods. Literature analysis on calcined clays revealed that the strength contribution mainly depends on the raw clay composition in combination with the calcination parameters. In addition to the most reactive meta-clay phase, meta-kaolinite, other frequently occurring clay phases such as smectite and illite can also exhibit relevant reactivity when calcined appropriately. However, their optimal calcination temperatures differ. It was clearly shown that the reactivity is not well correlated to the total X-ray amorphous content, as different X-ray amorphous meta-clay phases may occur together. As indicated by the literature analysis, experimental investigations on 11 industrially calcined clays using multiple regression analysis revealed that the content of X-ray amorphous Al2O3 is the most relevant indicator for reactivity. It is assumed that with increasing X-ray amorphous Al2O3 content the probability for higher contents of meta-kaolinite increases. The study on calcined clays in combination with ground calcium carbonate revealed that a highly reactive calcined clay according to the R3 test does not necessarily perform better in cementitious systems than lower reactive ones. After a rapid initial reaction, the further course of the reaction was significantly decelerated, as confirmed with a thermodynamic hydration model. Accordingly, the full potential of the calcined clay was not realized. Possible explanations are discussed. For a comparatively low reactive sample of autogenously calcined clays from a coal tailings dump in a further study, no reactivity determining factors could be identified, except for contained unreactive quartz-rich fractions, which lower the overall reactivity. However, the suitability of this material as an SCM was proven, as it showed a strength contribution and had no negative impact on clinker hydration. Multiple regression analysis was also applied on physical, chemical and mineralogical data of a multitude of siliceous fly ashes (sFA) concerning their mortar compressive strength and the reactivity according to an old version of the R3 test. Whereas no good model could be calculated for the relative mortar compressive strength, for the R3 reactivity, the TiO2 and chloride content were identified as relevant factors. The investigations on biomass ashes showed that their properties vary considerably more than the properties of sFA. The wood bottom ash showed no significant reactivity and the wood fly ashes even showed a negative impact on hydration and strength development. A spelt husk ash and the mineral residue of a sewage sludge ash after phosphorus recovery showed significant pozzolanic reactivity, but other parameters such as the high content of unburned organic matter and the low bulk density reduce their potential. Limitations of the thesis and further research needs are discussed at the end of the dissertation.
DOI: 10.18154/rwth-2026-06815
Language: English
Open Access status: green
OpenAlex ID: W7204526890
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