Low-carbon materials: concrete, steel and mass timber

Lesson 3 showed that the structure dominates the upfront carbon of a typical mid-rise building, and that concrete and reinforcing steel together made up about 85 % of Cedar Row's product-stage emissions. This lesson looks at the materials that offer the biggest savings: concrete, steel and mass timber, with shorter notes on aluminium, insulation and finishes. CON-111 introduced these materials and their basic properties, including engineered wood and mass timber; here the focus is on how their carbon footprint arises, what a builder can do to reduce it, and the construction issues that come with each low-carbon option.

Why concrete is carbon intensive

Concrete itself is not especially carbon intensive per kilogram; it is used in such enormous quantities that it adds up. Most of its footprint comes from Portland cement, which typically makes up 10 to 15 % of a mix by mass but accounts for most of its embodied carbon. Cement is made by heating limestone and clay in a kiln to about 1,450 °C to form clinker. About 60 % of the resulting emissions come from the chemistry itself, as calcium carbonate breaks down into lime and carbon dioxide, and about 40 % from burning fuel to heat the kiln. Cement production is commonly estimated at about 7 to 8 % of global carbon dioxide emissions. Because the process emissions come from chemistry, burning cleaner fuel can only solve part of the problem; the main levers are using less clinker and, in the longer term, carbon capture at cement plants.

The practical rule for builders is that reducing clinker content is the biggest lever on concrete carbon. There are four main ways to do it: use a blended cement such as Portland-limestone cement (CSA type GUL), replace part of the cement with supplementary cementing materials (SCMs), optimize the mix so that less total binder is needed, and specify the strength at a later age so that slower-gaining SCM mixes can be used.

Low-carbon concrete strategies

Portland-limestone cement replaces part of the clinker with finely ground limestone, typically up to 15 %. The Cement Association of Canada describes GUL as having up to about 10 % lower carbon footprint than general use (GU) cement while giving similar performance, and it has become the default cement in much of Canada. Supplementary cementing materials include fly ash from coal power plants, ground granulated blast-furnace slag from iron production, silica fume, and newer materials such as calcined clays and ground glass pozzolan. Fly ash supply is shrinking as coal plants close, which makes slag and calcined clays increasingly important.

# Embodied carbon (A1-A3) of five 32 MPa slab mixes, per cubic metre (illustrative factors)
f = {'GU': 0.92, 'GUL': 0.83, 'fly ash': 0.02, 'slag': 0.12}   # kg CO2e per kg of binder
other = 0.005 * 1900 + 8        # aggregates (about 1,900 kg) plus batching, kg CO2e per m3
mixes = {
    'A: 100 % GU, 360 kg binder':            {'GU': 360},
    'B: GU + 15 % fly ash (baseline)':       {'GU': 306, 'fly ash': 54},
    'C: GUL + 15 % fly ash':                 {'GUL': 306, 'fly ash': 54},
    'D: GUL + 35 % slag':                    {'GUL': 234, 'slag': 126},
    'E: GUL + 50 % slag, 56-day strength':   {'GUL': 180, 'slag': 180},
}
for name, m in mixes.items():
    co2 = sum(kg * f[k] for k, kg in m.items()) + other
    print(f"{name:38s} {co2:5.0f} kg CO2e/m3")

A: 100 % GU, 360 kg binder               349 kg CO2e/m3
B: GU + 15 % fly ash (baseline)          300 kg CO2e/m3
C: GUL + 15 % fly ash                    273 kg CO2e/m3
D: GUL + 35 % slag                       227 kg CO2e/m3
E: GUL + 50 % slag, 56-day strength      188 kg CO2e/m3

Moving from a conventional mix with a little fly ash to Portland-limestone cement with 50 % slag cuts the slab concrete's carbon by more than a third, without changing the structural design. Even the simple switch to a limestone cement saves about 9 % per cubic metre. These factors are illustrative; real mixes vary by plant, and the supplier's product-specific EPD is the right source for a project.

Other strategies include specifying performance rather than prescription, so that suppliers can choose the lowest-carbon mix that meets strength, durability and workability requirements; setting a maximum global warming potential per cubic metre for each mix class, often set as a percentage below a benchmark such as the National Ready Mixed Concrete Association's regional benchmarks in the United States or the Canadian Ready-Mixed Concrete Association's industry-average EPD; using carbon dioxide mineralization, in which captured CO2 is injected into fresh concrete where it becomes a mineral and allows a small cement reduction; and right-sizing elements so that slabs, transfer beams and foundations are not thicker or stronger than needed.

These mixes bring construction issues that builders must plan for. High-SCM mixes gain strength more slowly, especially in cold weather, which affects formwork stripping, reshoring and post-tensioning schedules; they may need longer curing and more careful cold-weather protection; finishing may differ; and some specifications or engineers are cautious about using them in exposed or freeze-thaw conditions. A good approach is to agree on mix classes early with the structural engineer and supplier, set strength ages by element (for example, 56-day strength for foundations and walls that are not loaded early), and run trial batches.

Steel: recycled content and production route

Steel's carbon footprint depends mainly on how it is made. Blast furnace and basic oxygen furnace (BF-BOF) production makes new steel from iron ore and coke and emits roughly 2 tonnes of CO2e per tonne of steel. Electric arc furnace (EAF) production melts mainly scrap steel using electricity and typically emits a third to a half as much, with the exact figure depending on the scrap share and the grid. Most North American reinforcing bar and many hot-rolled structural sections are made in electric arc furnaces with high recycled content, while plate, sheet and some hollow sections are more often made by the BF-BOF route or imported.

Because scrap supply is limited, buying more recycled steel for one project does not necessarily reduce global emissions; it may only shift scrap from another buyer. For that reason, many carbon policies focus on the product's actual GWP rather than recycled content alone. Newer production routes, such as direct reduced iron made with natural gas or hydrogen, could reduce primary steel emissions in the future. Meanwhile, builders can reduce steel carbon by avoiding waste, optimizing rebar detailing and coordinating so that bars are not left over or replaced.

Mass timber

Mass timber uses large engineered wood panels and members, such as cross-laminated timber (CLT), glued-laminated timber (glulam), nail-laminated timber (NLT) and dowel-laminated timber (DLT), to form floors, walls, roofs and frames. Its fossil emissions per cubic metre are typically much lower than those of concrete, and it stores biogenic carbon, although, as Lesson 3 explained, that storage must be reported separately and is not permanent. The 2020 National Building Code of Canada introduced encapsulated mass timber construction for buildings up to 12 storeys, with mass timber protected by gypsum board or other encapsulation, and the 2025 edition and several provinces have expanded the options; check the edition your province has adopted.

For Cedar Row, Option B uses a mass timber superstructure (CLT floors on glulam columns and beams) over a two-storey concrete podium, with concrete stair and elevator cores, a concrete topping on the floors for acoustics, and gypsum encapsulation for fire. The structural engineer has produced preliminary quantities, which the code below compares with Option A on a fossil-only basis.

# Structure only, A1-A3 fossil emissions: Cedar Row Option A (concrete) vs Option B (mass timber)
A = [('Foundations', 1400, 330), ('Suspended slabs', 3200, 300), ('Columns and walls', 900, 380),
     ('Rebar', 550000, 0.85), ('Misc. steel', 60000, 1.22)]
B = [('Foundations', 1150, 330), ('Podium slabs', 800, 300), ('Podium columns and walls', 300, 380),
     ('Concrete cores', 450, 380), ('Rebar', 270000, 0.85), ('CLT floor and roof panels', 1286, 130),
     ('Glulam columns and beams', 260, 150), ('Concrete topping', 315, 250),
     ('Steel connections', 45000, 1.8), ('Misc. steel', 30000, 1.22), ('Fire encapsulation gypsum', 14700, 2.5)]
def total(items):
    return sum(q * fct for _, q, fct in items) / 1000
ta, tb = total(A), total(B)
wood_m3 = 1286 + 260
stored = wood_m3 * 0.75          # about 0.75 t CO2 stored per m3 of softwood product
print(f"Option A structure: {ta:,.0f} t CO2e")
print(f"Option B structure: {tb:,.0f} t CO2e  (saving {ta - tb:,.0f} t, {1 - tb / ta:.0%})")
print(f"Biogenic carbon stored in Option B wood, reported separately: {stored:,.0f} t CO2")

Option A structure: 2,305 t CO2e
Option B structure: 1,573 t CO2e  (saving 731 t, 32%)
Biogenic carbon stored in Option B wood, reported separately: 1,160 t CO2

Option B saves about 730 tonnes, or roughly a third of the structural embodied carbon, even after including the concrete podium, cores, topping, steel connections and fire encapsulation. The lighter building also needs smaller foundations. The wood stores about 1,160 tonnes of biogenic CO2, but that figure is reported separately and does not reduce the upfront total. Published comparisons of mass timber and concrete buildings typically find savings in a similar range, although results depend heavily on the scope, the concrete mix assumed for the baseline and how biogenic carbon is treated.

Mass timber changes construction in ways a builder must manage. Panels are prefabricated, so erection is fast and quiet with small crews, but procurement lead times are long and design must be frozen early; CON-410 covers the prefabrication logistics. Wood must be protected from moisture during construction with temporary roofing, membranes and sequencing, and moisture content should be monitored before encapsulation. Fire safety during construction needs particular attention, since unprotected timber is combustible until encapsulation and sprinklers are complete. Insurance, acoustic separation, vibration and connections to the concrete cores all need early coordination. Finally, the carbon benefit depends on responsible forestry, so specifications usually require certified wood from sustainably managed forests.

Other materials

Once the structure is optimized, other materials become relatively more important.

Insulation shows how operational and embodied carbon interact. Adding insulation always reduces heat loss, but each additional increment saves less energy while adding the same embodied carbon. For mineral wool or cellulose, the embodied carbon is usually repaid by operational savings within a few years; for foams blown with high-GWP agents, the payback can be decades or never. Specifying low-GWP products for the same thermal performance avoids the trade-off entirely.

Procurement and specification

Low-carbon materials only end up in the building if specifications, procurement and submittal review protect them. Common tools include maximum GWP limits per product category written into the specifications; requirements for product-specific EPDs with submittals; a carbon budget for the structure that the contractor must report against; and review of substitutions for carbon as well as cost and performance. Public owners in Canada and some US states now set such limits through green procurement policies, which Lesson 12 discusses. For a builder, the key is to involve suppliers early, check that the specified products are available locally in the needed quantities, and price the alternatives honestly.