With the familiarity of slag and fly ash as SCMs,7 the obvious solution to many was to simply reduce cement (and therefore embodied carbon) and use more of these SCMs. This especially makes sense if for ready mix suppliers already selling slag, and fly ash products. However, this seemingly simple increase in SCMs represents a complex material change which sets the stage for bad concrete.
Similar to just about any dry-add pozzolan, slag is a moisture-demanding material because of its size and surface. It can also inhibit cement formation due to its alkalinity. This increase in alkalinity, creating counterproductive reactions with sodium hydroxide (NaOH) and calcium hydroxide (Ca[OH]2), will lower the internal relative humidity (IRH). Once IRH is 80 per cent or less, there is no longer cement formation and porosity of the concrete is increased. Slag’s success in concrete is dependent on cement formation. Higher alkalinity will also destroy bonds between adhesives and floor coverings. Higher alkaline concrete can be a benefit to reducing corrosion with embedded steel reinforcement, but with newer composite low-carbon reinforcement such as basalt rebar, this justification becomes less and less necessary.
Portland Limestone Cement
PLC is a fantastic method for reducing embodied carbon.8 It is similar to PC but with a crushed limestone replacing some of the cement. This higher limestone content can help reduce embodied carbon by nine to 10 per cent. However, PLC is not the same as PC. Though PLC (Type IL) can perform similarly to a non-thinned Portland cement-based concrete, it does not behave exactly the same. This is very important to understand when it comes to curing. Both workability and hydration are affected. According to Eric Traffie, owner of Premier Concrete, concrete researcher and educator, “finishers notice the behavioural changes of the PLC compared to ordinary portland cement mixes. Workability and finishing can be challenges due to higher water demands.” Like slag, crushed limestone is also finer than the regular cement and has a greater surface area9 meaning it will increase water demands during curing. However, because adding water would mean lowering concrete strength and introducing potentially serious structural problems, the concrete is instead less than optimally cured and suffers from higher permeability.
Cement kiln dust
The path for using CKD began decades ago,10 but similar to these other SCMs, it represents a material change in concrete. CKD is fine-grain, solid, and highly alkaline waste removed from cement kiln exhaust gas by air pollution control devices.11
With much of it still being reactive (still able to create calcium silicate hydrate [C-S-H]) with concrete, CKD can be returned to cement production versus heading to landfills. This is part of a great solution in dealing with a waste material, but again, the apparently simple solution of moving CKD into the production process has also created colonial cobra-level consequences. CKD’s alkalinity—similar to slag—leaves cement competing for moisture, prohibiting important reactions for proper concrete formation, and increasing porosity which directly adds to increased costs for both initial construction as well as facility maintenance.
Low carbon mix designs
These new generations of low carbon mixes are not only seeing high percentages of slag, PLC, and cement kiln dust, but encouraged for use in concert with recipes such as Type IT (AX)(BX) ternary blended cement, and other mixes falling into the ASTM C595, blended hydraulic cements. These low carbon mixes are starving for water and fail to react without proper hydration, not creating important calcium silicate hydrate (C-S-H). This cascading effect of less water and fewer reactions leaves the concrete stretching itself out of shape (i.e. volume change) searching desperately for water that is not there and thus not forming a sturdy structure. Volume change aids in creating highly permeable, uneven, cracked concrete. This not only has the potential to damage the ability of a structure to meet its structural requirements, but will damage its serviceability and esthetics. It will likely increase the chance of deflections and certainly dry-shrinkage, which in turn can lead to other complications. These factors must be considered when designing low-carbon concrete—removing cement is not enough on its own.