Concrete 101
What It Is, How It Works, and Where It's Used
Executive Summary
Concrete is a man-made stone: a composite of cement, water, aggregates, and admixtures that hardens through chemical reaction.
Cement is the binder โ produced by heating limestone in a kiln, releasing COโ both from energy use and the chemistry itself.
Concrete doesn't 'dry' โ it cures through hydration, a chemical reaction between cement and water that produces C-S-H, the glue that binds everything together.
Strength (PSI) is just one spec โ workability, durability, air content, and curing conditions are equally critical in real-world performance.
Concrete is less like a single product and more like a family of recipes, each engineered for a specific job, exposure, and placement method.
Concrete is a man-made stone. It starts as a fluid you can place into forms and finish with hand tools, and it hardens into a durable solid that can last decades (or longer) when designed and installed correctly. It's everywhere โ foundations, floors, sidewalks, bridges, ports, tunnels, warehouses, parking garages, water tanks โ because it's strong in compression, fire resistant, widely available, and cost-effective at scale.
This page is a simple primer: what concrete is, what it's made of, how it hardens, the common types of concrete, and how PSI strength maps to real-world uses.
Five Facts That Should Change How You Think About Concrete
The scale of concrete is hard to overstate. Here's why it demands climate-sector attention.
Concrete is the world's #2 most used material โ after water.
Nothing else humans make comes close in volume. That's roughly 1.7 tonnes per person per year, every year, globally.
Source: Chatham House; GCCA Global Cement and Concrete Industry Statistics
We produce ~14 billion cubic meters of concrete per year.
That's enough to fill 5.6 million Olympic swimming pools โ every single year. And demand is still growing, especially in the Global South.
Source: GCCA, Global Cement and Concrete Industry Statistics (2020 baseline)
That's roughly ~30 billion tonnes of concrete used annually.
Nearly 4 tonnes for every person on Earth, every year. No other manufactured material operates at anything close to this scale.
Source: Derived from GCCA volume data; consistent with industry mass-equivalent framing
The world makes >4 billion tonnes of cement per year โ the "glue" that drives the footprint.
Cement is 10โ15% of concrete by weight but responsible for ~90% of its emissions. Over half a tonne of cement per person per year, globally.
Source: GCCA, ~4.2 billion tonnes in 2020
Human-made mass now outweighs all living biomass on Earth โ dominated by construction materials.
Around 2020, total anthropogenic mass surpassed global living biomass for the first time. That mass is overwhelmingly concrete, aggregates, bricks, and asphalt. We have literally built more than nature grew.
Source: Elhacham et al., Nature (2020) โ "Global human-made mass exceeds all living biomass"
This is not a niche material. This is infrastructure at planetary scale. And its emissions profile โ ~7โ8% of global COโ โ makes it one of the most consequential climate sectors to solve.
What Concrete Is Made Of
At its core, concrete is a composite material made from four ingredient groups:
1) Cement (the binder)
Cement is the "glue" in the system. When cement reacts with water, it forms new compounds that bind the mixture into a solid mass. The most common binder is Portland cement, though many mixes also include additional cementitious materials.
2) Water (activates the binder)
Water starts the chemical reaction that turns cement into glue. Water also controls workability โ how fluid the concrete is while placing and finishing. Too little water can make it hard to place; too much can reduce strength and durability.
3) Aggregates (sand + gravel / crushed stone)
Aggregates are the bulk of concrete. Think of them as the "rock skeleton" that gives concrete volume and stability. In most mixes, aggregates make up the majority of the material, and their size distribution and quality strongly influence performance.
4) Admixtures (small additives that tune behavior)
Admixtures are chemical tools used in small quantities to modify how concrete behaves. Common functions include:
- Improving workability without adding water
- Speeding up or slowing down set time
- Improving freeze-thaw durability
- Reducing shrinkage or cracking risk
- Helping concrete pump or finish better
Concrete is less like a single product and more like a family of recipes. Two mixes can both be "4,000 psi concrete" and still behave very differently depending on aggregate, water content, cement content, and admixture package.
How Concrete Hardens: The Key Chemistry
Concrete doesn't "dry" into stone โ it cures through a chemical reaction.
Cement from limestone (where it starts)
Most Portland cement starts as limestone (calcium carbonate, CaCOโ) plus smaller amounts of clay or shale (sources of silica and alumina). In a kiln, limestone is heated and breaks down into lime (CaO) and carbon dioxide:
CaCOโ โ CaO + COโ
The fundamental chemistry behind cement's carbon footprint
That lime then reacts in the kiln to form clinker, which is ground into a fine powder (with a small addition of gypsum to control set time). That powder is Portland cement.
Cement + water (where concrete becomes concrete)
When water hits cement at the batch plant or in the mixer truck, cement begins hydration. Hydration creates C-S-H (calcium-silicate-hydrate), the primary "glue" that binds the aggregates into a solid mass. Strength develops as hydration continues.
This is why curing matters. Keeping concrete from drying out too quickly โ especially early โ helps hydration proceed, improving strength, durability, and surface quality.
The concrete production chain: from raw materials quarry through the clinker kiln to final mixing and delivery.
The Concrete Lifecycle on a Jobsite
Concrete is a practical material, which means field conditions matter. The basic sequence looks like this:
Mix design is selected for the project (strength, durability, workability, placement method).
Batch plant measures cement, aggregates, water, and admixtures precisely.
Mixer truck keeps the material workable while transporting.
Placement happens into forms or onto a prepared subgrade.
Finishing creates the surface texture and flatness needed.
Curing protects the concrete while it gains strength and durability.
Because concrete begins to set once water meets cement, it's typically produced and placed within a local region. Timing, temperature, and workmanship can be as important as the mix itself.
Strength (PSI): What Those Numbers Mean
Concrete strength is usually described as compressive strength in PSI (pounds per square inch). This is measured by crushing standardized test cylinders at specific ages โ most commonly 28 days.
PSI mainly describes compressive strength. Concrete is weak in tension, which is why structural concrete often uses rebar or fibers.
Strength gain is time-dependent. Many mixes reach much of their design strength at 28 days, but some continue gaining strength beyond that.
A "higher PSI" isn't automatically "better." Projects often optimize for durability, constructability, and cracking control โ not just peak strength.
Common PSI Ranges & Typical Uses
| PSI Range | Typical Uses |
|---|---|
| 2,500โ3,000 | Light-duty flatwork (walkways, patios) |
| 3,000โ4,000 | Residential slabs, footings, foundations, driveways |
| 4,000โ5,000 | Commercial slabs, heavier flatwork, some structural elements |
| 5,000โ6,000+ | Structural beams/columns, elevated slabs, parking structures |
| 8,000โ12,000+ | High-rise cores/columns, long spans, specialized structural work |
In practice, engineers specify strength alongside exposure and durability requirements. A coastal structure may require durability measures that matter more than simply increasing PSI.
Concrete Properties Beyond PSI
Concrete specs often include additional parameters because "strength" doesn't tell you everything.
Higher slump means easier placement and finishing, but must be designed โ not achieved by "just adding water."
Tiny intentionally-created air bubbles relieve pressure from freezing water. Carefully controlled because it affects durability and strength.
One of the biggest drivers of strength and durability. Lower w/c improves both, but can reduce workability unless admixtures are used.
Affects pumpability, finishability, and strength. Pumping mixes may use smaller aggregate than mass pours.
Some pours need fast set (open to traffic, strip forms quickly). Others need slower set (hot weather, long hauls, large placements). Admixtures tune this.
Common Types of Concrete
Categorized by how it's produced, reinforced, or performs.
A Simple Bottom Line
Concrete is a composite of cement, water, aggregates, and admixtures. Cement is produced by transforming limestone into a reactive binder. When water is added, cement hydrates and forms C-S-H โ the glue that binds rock into a hard, stone-like mass. Concrete is used everywhere because it is practical, local, durable, and adaptable, with performance commonly described by PSI strength alongside key real-world requirements like workability, durability, and curing.
Now that you understand what concrete is and why it matters, explore where the emissions actually come from.