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EPDs can be nationally averaged or plant specific. Plant specific EPDs provide more points in the LEED certification system as they are regarded as more accurate than nationally averaged EPDs. EPDs for concrete block masonry products are also now available. Figure 1 provides an excerpt from the Canadian Concrete Masonry Producers Association’s (CCMPA’s) nationally averaged EPD2 for Eastern Canada producers with an averaged global warming potential (GWP) at 205.38 kg (452 lb) of CO2eq when using GU Portland cement to manufacture the normal weight CMUs; while Figure 2 provides the Eastern Canada producers averaged GWP at 190.58 kg (420 lb) of CO2eq when using GUL to manufacture the normal weight CMUs. From these values, it can be seen the substitution of GUL for the GU cement in the manufacture of concrete block products results in a 7.7 per cent reduction in the GWP of concrete masonry products.
Carbonation of CMU
There are two methods as to how CMUs absorb CO2: weathering carbonation and pre-carbonation (carbon curing).
Weathering carbonation occurs as concrete, used to manufacture the concrete block and also in the mortar joints used to assemble a block wall, matures due to the reaction of CO2 in the presence of moisture with the hydrants (i.e. alkalis) in concrete block, mortar joints, and grout. Specifically, CO2 from the atmosphere diffuses into the capillary pores and combines with the water to form carbonic acid.2 The rate of CO2 diffusion depends on the relative humidity (RH); it is most rapid between 65 and 75 per cent ambient RH.
Carbonic acid then reacts with the hydrants—such as solid calcium hydroxide, calcium silicate hydrate (C-S-H) gel, and alkali/calcium ions in pore solutions—to form carbonate. Generally, calcium hydroxide has the highest concentration in Portland cement concretes and mortars.
Both mortar and CMUs, being more porous than cast-in-place concrete, experience higher rates of carbonation. In dense concrete, for example, penetration may be less than 20 mm (0.78 in.) after 50 years, whereas with the mortar in the joints of masonry structures, indicator tests have demonstrated carbonation of a 10 mm (0.3 in.) thick by 90 mm (3.5 in.) deep mortar joint is substantially complete after less than a decade.
A recent study by the National Concrete Masonry Association (NCMA) for ASTM discovered that a 198 mm (7.8 in.) concrete block will be 25 per cent fully carbonated within 28 days of manufacture and 49 per cent within 26 weeks of manufacture (Figure 3a and 3b).3 Although current life cycle analysis (LCA) does not account for carbon sequestration of concrete masonry products due to weathering carbonation in the GWP value, it will likely be included in the near future once more accurate methods of estimating weathering carbon sequestration by concrete masonry products are established.
Pre-carbonation of concrete masonry products occurs by either using CO2 to cure either the units at the time of manufacture or by using cement powder that has been pre-carbonated to manufacture the units. When CO2 is used to cure the units, CO2 sequestered from the atmosphere is introduced into the curing process of autoclaved CMUs. Autoclaved concrete blocks are steam cured in a high-pressure vessel—it is a process that can easily integrate CO2 into the steam curing process. In Figure 4, an existing autoclaving chamber that is in the process of being modified to include CO2 curing into the manufacture of CMUs (Figure 5).
Pre-carbonation typically increases the early strength of the CMUs and reduces shrinkage and defects in the finished units. These higher early strength gains can allow for producers to reduce their cement loading, further reducing their CO2 footprint.
It is estimated that pre-carbonation technologies have the potential to reduce CO2 emissions associated with the manufacturing of concrete blocks, pavers, and segmental retaining walls by up to 32 million tonnes (35 million tons) per year in North America.
