Mechanical and durability characteristics of gypsum-free blended cements incorporating sulphate-rich reject fly ash

To accommodate the wide variations among the different types of fly ash and the growing need for its greater utilization in the construction sector, relevant standards have been established that differentiate the appropriate qualities from the unacceptable ones. Unfortunately, potentially reject fly ashes (rFA), which do not comply with standard requirements, comprise a significant part of the total amount produced in coal or lignite burning stations. High-sulphate fly ash (HSFA) is a typical example of reject material, since all relevant standards (both European and ASTM) clearly define that sulphur trioxide should be kept under a certain low limit (approximately 3–5% depending on the standard) or else concrete’s durability may be threatened. The aim of this work was to design, produce and monitor the properties of a series of blended cements prepared by mixing clinker with a fly ash of high-sulphate content. No gypsum was added in the mixtures, since it is believed that sulphate ions necessary for the prolongation of the setting process (commonly provided by gypsum) could be provided by fly ash enriched in sulphates. All samples tested exhibited satisfactory initial and final setting times as well as decent compressive strength values when compared to the reference specimen containing only gypsum and no fly ash. Additionally, this paper reports on the performance aspects of blended pastes exposed to chloride binding tests and aggressive (a) 2% H2SO4 and (b) simulated marine environment solutions. Results revealed that waste materials not up to relevant standards could still contribute to the production of quality products of energy and economical efficiency.