Powder accelerated high volume slag cement for lower carbon sprayed concrete linings

The early age strength development required for typical wet mix sprayed concrete tunnel lining (SCL) necessitates higher binder contents than strength class cast equivalents, and is dependent on liquid accelerating admixtures specifically formulated to work with CEM I. Consequently, SCL concretes have relatively high embodied carbon contents that are difficult to mitigate using supplementary cementitious materials such as ground granulated blast furnace slag (GGBS). Contemporary low carbon alternatives based on conventional spraying technology achieve a 23 % carbon reduction, although calcium aluminate (CA) based powdered accelerators have the potential to facilitate significantly greater reductions through the replacement of a high proportion of CEM I. The paper reports the trial spraying of a concrete employing a CA based powdered accelerator to achieve 70 % CEM I replacement with GGBS, representing a 57 % reduction in embodied carbon. Partially intermittent spraying led to some statistically significant in-situ material variability in nominally identical test panels. Nonetheless, the material was of consistent good quality evidenced by there being no statistically significant difference in the mean strengths of parallel and perpendicular cores. Early age (up to 24 hours), 28 and 90-day compressive strengths were conformant, although 1 to 7-day strength development was lower than specification requirements. This was attributed to factors unique to the prototype trial set up, which would necessarily be addressed in SCL construction practice. Residual flexural tensile strength ductility performance was also conformant, although requirements relating to immediate post cracking brittleness and absolute residual strength were below that required. Given the concurrent compressive strength conformance, this was attributed to inadequate fibre performance that is easily addressed through fibre type and dosage selection. The aim of demonstrating potential conformance with a contemporary SCL specification mechanical performance requirements was therefore clearly met. Durability related testing, including petrographic and SEM analyses, provided assurance that hydration products were as expected which, along with reduced heat of hydration, demonstrate enhanced potential for overall carbon reduction by meeting the quality requirements associated with single pass permanent SCL.