Assessment of the Potential for Delayed Ettringite Formation in Heat Cured Mortars and Concrete Using Australian Materials

Abstract

Delayed ettringite formation (DEF) is a recognised durability issue in concretes where the temperature during curing has been elevated. To address the potential risk of DEF, Australian specifications for heat and steam cured concretes, such as TfNSW B80, MRTS 70, and MRS 820, restrict the maximum concrete temperature during heat or steam curing to 70 or 80 °C (depending on the jurisdiction). The wide range of road authority specifications in Australia has led to uncertainty among precast concrete manufacturers, designers, and contractors, as there is a lack of clarity on how less durable the concretes become when they breach these temperature limits. Moreover, the role of supplementary cementitious materials (SCMs) in mitigating DEF in the specifications is unclear. This paper addresses these concerns by reporting some of the outcomes from research carried out over the last 8 years at the University of Technology Sydney investigating the factors that raise the risk of deleterious DEF. The work indicates that the risk of DEF is low if the cements conform to Australian specifications (AS 3972 and ATIC-SP43). The risk is further reduced if fly ash (FA) is used as part of the binder composition. As the risk of DEF is low if a limit is placed on the alkali and sulphate contents in the cement and is further mitigated if FA is used to partially replace the cement, a more practical and standardised approach to heat cured concrete specifications across the Australian jurisdictions could be adopted.

Keywords

• delayed ettringite formation (DEF)
• precast concrete
• SO₃
• Na₂O_e
• heat curing
• mortar