Carbon Smart Attributes
Kiln type matters for cement
The different kiln types used for cement production, listed in descending order of energy intensity, are: wet, long dry, dry with preheater, and dry with preheater and precalciner. Dry with preheater and precalciner kilns use on average 85% less energy than wet kilns1. Understand what type of kiln your concrete supplier uses for cement production, and request cement that comes from ‘dry with preheater and precalciner’ kilns whenever possible.
Less cement = less carbon
After reducing the carbon impact of cement production, additional carbon reductions can be made by reducing the amount of cement used per unit volume of concrete. Substitute cement with supplementary cementitious materials (SCMs) (including, but not limited to, fly ash, slag, calcined clays, agricultural byproducts, silica fume, etc.), and/or use larger sized aggregate (e.g. 1” vs ¾” coarse aggregate) where appropriate.
Consider new mixing methods
New methods for mixing concrete are being developed that can create high-strength concrete with a lower volume of cement. For example, the scattering-filling aggregate process adds an additional “10-30% (by the volume of the finished concrete) of coarse aggregate while the concrete is being poured, paved or placed, then vibrating the mixture to form a consolidated concrete”2. This method results in 10-30% less cement that conventional concretes, reducing the carbon emissions while increasing the compressive strength of the mix.
Utilize carbon sequestration (CO2 injection)
New technology captures the carbon naturally emitted during the cement manufacturing process and injects it back into the concrete mix during mixing, thereby sequestering that carbon for the lifespan of the building. Encourage concrete suppliers to use carbon sequestration/CO2 injection methods.
Specify hard, clean, and strong aggregates
Weak aggregates require the addition of more cement to achieve the necessary mix strength. Soft, porous aggregates can also result in weak concrete with low wear resistance, reducing the life-span of the material. Whenever possible use hard aggregates to reduce the required cement quantity and create concrete with a high resistance to abrasion and a longer life-span3.
Consider using recycled aggregate, where appropriate
Aggregates and recycled materials from construction and demolition waste are relatively low-cost and have lower embodied carbon than new materials. However, recycled aggregates typically have a high variability in shape and texture and have been found to have higher absorption values, therefore requiring more water and ultimately more cement. Use of recycled aggregates may be appropriate for certain applications that are not sensitive to aggregate absorption, such as pavements.