Influence of off-spec fly ash and surfactant-coated nano-iron-oxide on the fresh and hardened properties of cement pastes: An exploratory study

Millions of tons of Fly Ash (FA) is generated worldwide each year, and this generation is anticipated to grow alarmingly. Despite the application of FA in cement composites and other soil stabilizers, more than 30% of the total FA is considered off-spec and is landfilled, causing different environmental, economic, and health problems. Owing to the scantly available literature and knowledge gap related to incorporating the off-spec FA in cement composites, limited mass applications of off-spec FA have been reported so far. This study aims to investigate the influence of the off-spec FA and surfactant-coated nano-iron-oxide (SNI) solution on the fresh and hardened properties of cement pastes. For this reason, fresh-state flow percentage, setting time, capillary water absorption rate, compressive strength development, pozzolanic activity, and chemical durability (in acid solutions) were compared among the cement pastes containing 40% off-spec FA with and without an SNI addition (0%-3%). Results showed that the SNI addition enhanced workability, compensating for the negative effect caused by the off-spec FA substitution. SNI addition also retarded the initial rate of capillary water absorption and improved the acid resistance of hardened cement paste. Furthermore, slow development of compressive strength of cement pastes is observed by the off-spec FA substitution. Thermogravimetric analysis revealed that the SNI addition made the lesser formation of C-S-H gel and CH at early ages, resulting in a delayed pozzolanic activity. The development of advanced cement composites using off-spec FA and SNI may be a positive step towards an efficient and eco-friendly consumption of industrial wastes in construction materials.