Proceedings pro035 : International RILEM Workshop on Internal Sulfate Attack and Delayed Ettringite Formation

Two forms of internal sulfate attack (ISA) have long been known:

* ISA due to contamination of the aggregates by sulfates
* ISA due to over sulfation of the cement

This workshop dealt with a third form of ISA, which has only come to attention relatively recently. This form of ISA has been observed in concretes (and mortars) which have experienced elevated temperatures during curing, either through the external application of heat as in steam curing, or from internal temperature rise due to the heat evolved during hydration. This form of ISA is widely known as Delayed Ettringite Formation or DEF. Despite its general use, this term can be misleading as it has long been known that in concretes exposed to moisture ettringite will recrystallised harmlessly into pore, void and previously existing cracks. This phenomenon is more usually referred to as secondary ettringite formation. Furthermore, it has also been observed that in concretes which have been heat cured ettringite may form at late ages (delayed formation) without any detrimental effect on the concrete.

In particular the workshop aimed to establish the extent to which the following questions could be answered:

* Does the DEF form of ISA occur only in concretes which have experienced temperatures in excess of some threshold temperature (around 70°C). Can it also occur in concretes which have been maintained at ambient temperatures?
* What parameters in the cement and concrete influence the likelihood of expansion after exposure to elevated temperatures during curing?
* Can high sulfate contents originating in the clinker (rather than added during grinding) induce this form of ISA even in the absence of elevated temperatures?
* How prevalent is this form of ISA in the field? How may it be identified in the field concrete given the confusion of other forms of degradation - frost damage or alkali silica reaction, coupled with the formation of secondary ettringite in the cracks formed by these deterioration processes?
* Can the formation of ettringite in pre-existing cracks lead to the lengthening and/or widening of these cracks? Can the deposition of ettringite in air-voids jeopardise the resistance of the concrete to frost damage?

The workshop was organised with sessions designed to address these key questions.
A combination of keynote papers and extensive discussion, also reported in these proceedings, is believed to give a complete picture of the current state of the art relating to this form of Internal Sulfate Attack.

Contents

Theme 1: Sulfates in Portland cement clinker and cement

Theme 2: Basics of ettringite stability

Theme 3: Case studies of DEF

Theme 4: Mechanisms of expansion

Theme 5: Parameters affecting expansion after exposure to elevated temperatures

Theme 6: Cracks and Airvoids

Theme 7: What have we learned from this workshop

Summary of discussions during workshop