The Quest for Carbon-Neutral Concrete

From Concrete International, President’s Memo, Charles K. Nmai, ACI President

In Part 1 of this President’s Memo published last month, I noted that proven supplementary cementitious materials (SCMs), such as fly ash, slag cement, silica fume, and high-reactivity metakaolin, are available and are being used to meet carbon-neutral goals of owners and the design community. This month, I’ll explore other options that are available or emerging but are not readily accepted for various reasons.

Traditional SCMs have been used successfully to significantly reduce the carbon footprint of concrete; however, there are issues that must be acknowledged. Issues include availability, depending on geographic location; lack of storage silos at concrete plants for SCMs; and code limitations on SCMs for air-entrained concrete mixtures exposed to deicing chemicals in service. These issues can translate into higher costs. For example, portland cement and SCMs must be shipped to Hawaii, making SCMs more expensive for design professionals on the islands. The need to lower the carbon footprint of concrete has led to increased production of portland limestone cement (PLC) and increased focus on alternate cements and binder materials. In 2018, ACI Innovation Task Group (ITG) 10 published two documents on these materials—ITG-10.1R-18: Report on Alternative Cements, and ITG-10R-18: Practitioner’s Guide for Alternative Cements. Readers should note that an alternative cement is defined as “an inorganic cement that can be used as a complete replacement for portland or blended hydraulic cements, and that is not covered by applicable specifications for portland or blended hydraulic cements.” Binder materials covered by this definition include geopolymers, activated glassy cements, activated fly ash cements, activated slag cements, calcium aluminate cements, calcium sulfoaluminate cements, magnesia cements, and CO2-cured cement. Several of these alternative cements have been available and used in niche applications prior to the current focus on low-carbon concrete. I personally evaluated magnesium phosphate cements for use in rapid repair concrete applications over 30 years ago!

The publication of the two ITG reports preceded the inclusion of alternative cements in ACI 318-19 with the stipulation that the use of such cements is permitted if approved by the licensed design professional and the building official. ACI 318-19 also requires the approval to “be based upon test data documenting that the proposed concrete mixture made with the alternative cement meets the performance requirements for the application including structural, fire, and durability.” The need to have test data documenting performance for the application points to the need for proper assessment and validation of alternative cements and other innovative materials for the concrete industry that are not covered by existing standards and specifications. In February 2022, International Code Council Evaluation Service, Inc. (ICC-ES) published ICC-ES AC529, Low-Carbon Alternative Cements for Use in Concrete, for the use of such cements in concrete in lieu of conventional code-compliant cements. The document is applicable to low-carbon alternative cements that are activated by water or a proprietary activator supplied by the manufacturer; and it was developed to show compliance with the International Building Code (IBC) and the International Residential Code (IRC)—the predominant codes in the United States for building construction that currently do not have provisions for evaluation of alternative cements. Readers should note that the current IBC and IRC codes do reference ACI 318-19.

The pursuit of low-carbon concrete has inspired the introduction of a wide range of products promoted to minimize the portland cement content of a concrete mixture. These include, but are not limited to, CO2 injection systems, admixtures based on synthetically produced crystalline calcium silicate hydrate (CSH) nanoparticles, synthetic limestone aggregates manufactured from waste CO2, and a powdered admixture based on a blend of dry-expanded polymeric microspheres and mineral powder that eliminates the need for air entrainment in frost-resistant concretes. In their quests to reach informed decisions, owners and design professionals are often challenged with sorting through sets of marketing literature, fancy graphs, animations, and videos, looking for true performance data! Quite often, the provided data is limited to compressive strength and a few other concrete properties, perhaps with additional information regarding evaluations that are not readily available for review. One of the core functions of NEU: An ACI Center of Excellence for Carbon Neutral Concrete will be “assessment and validation” of manufacturer product claims. This will provide owners and design professionals with unbiased reports to facilitate decision-making and consideration of alternatives. Successful validation will also provide credibility to proven technologies and help expedite their acceptance and use in concrete construction.