What Is Concrete Made Of? (Cement, Aggregate, Water & Admixtures)

Concrete is not cement. That's the most common confusion in residential building, and it matters because the two are priced differently, behave differently, and are used differently. Cement is one ingredient — a fine grey powder that makes up about 11% of a typical concrete mix by volume. Concrete is the hardened composite that comes out the other side when cement, aggregate, water, and a few small additives have all been combined.

The four main ingredients

Every batch of normal-weight concrete is built from four basic ingredients. The proportions vary slightly by mix design, but the makeup is consistent across nearly all residential and light-commercial pours:

1. Coarse aggregate (gravel or crushed stone)

Coarse aggregate is the bulk of the mix — about 41% by volume. It's usually 3/4-inch crushed limestone, granite, or river gravel. The aggregate carries the load; the cement just glues it together. Choosing a clean, well-graded aggregate is the difference between a strong slab and a crumbly one. Avoid aggregate with clay coatings or organic material — it weakens the bond.

2. Fine aggregate (sand)

Fine aggregate fills the gaps between the coarse aggregate, packing the mix tight and giving the surface its workability. Most residential mixes use about 26% sand by volume — washed concrete sand, not play sand or fill sand. The cleaner the sand, the stronger the final concrete; silt and clay in the sand are major causes of weak, dusty surfaces.

3. Portland cement

Portland cement is the binder. About 11% of the mix by volume, it's made by heating limestone, clay, and other minerals to about 2,700°F (1,480°C) in a rotary kiln. The result, called clinker, is ground into a fine grey powder. When you add water, a chemical reaction called hydration begins; the cement crystallizes into rigid networks of calcium silicate hydrate and bonds the aggregate into a solid mass. Hydration is why curing matters — concrete needs water to harden, so keeping a fresh slab moist is what produces full design strength.

"Portland" is a trademark name from 1824, chosen because the cured material resembled limestone quarried near Portland, England. It's not a brand — most US suppliers carry Type I (general use), Type II (moderate sulfate resistance), or Type III (high early strength).

4. Water

Water makes up about 16% of the mix by volume and is the trigger for hydration. The single most important number in mix design is the water-to-cement ratio (w/c), the weight of water divided by the weight of cement. Lower w/c means stronger, more durable concrete:

• 0.40 w/c: high-strength structural concrete (5,000+ PSI) — needs plasticizers to stay workable.
• 0.50 w/c: typical residential 3,000 PSI mix — good balance of strength and workability.
• 0.60 w/c: low-strength 2,500 PSI mix — easier to place, but weaker and less durable.

Adding water on site to make a stiff truck "pour better" — called tempering — is the most common reason residential pours end up weaker than spec. Every extra gallon raises the w/c ratio and reduces strength. Most suppliers won't honor warranty claims if the truck has been tempered after discharging more than 25% of the load.

The fifth ingredient: admixtures

Modern concrete almost always includes one or more admixtures — chemicals added at the plant in small percentages (usually 1–5% of the cement weight) to tune the mix's behavior. The five most common:

• Air-entrainer: Microscopic bubbles intentionally introduced to give freezing water somewhere to expand. Specify for any exterior pour in a freezing climate — without it, surface scaling is nearly guaranteed within five winters.
• Water reducer / plasticizer: Lets you place a stiff, low-w/c mix that would otherwise be unworkable. Strength improves without losing slump.
• Superplasticizer: Extreme water reduction for high-strength and self-consolidating concrete. Used in structural and architectural pours.
• Retarder: Slows hydration to extend working time. Useful in hot weather or for long pours.
• Accelerator: Speeds early strength gain for cold-weather pours or fast-track schedules.

Fiber reinforcement — short polypropylene or steel fibers — is also commonly added at the plant for residential slabs to control shrinkage cracking. Fibers don't replace rebar in structural pours, but they can replace wire mesh in a 4-inch residential slab.

Mix ratios in practice

For most residential pours you don't need to specify proportions yourself — you call the supplier with a PSI number and they batch a published recipe. Useful approximations:

• 2,500 PSI: 1 : 3 : 4 (cement : sand : gravel) — sidewalks, light-traffic patios.
• 3,000 PSI: 1 : 2.5 : 3.5 — driveways, residential foundations, garage floors. The most common residential mix.
• 3,500 PSI: 1 : 2 : 3 — light commercial, foundation walls.
• 4,000 PSI: 1 : 2 : 3 with a lower water content — heavy driveways, freeze-thaw exposure.
• 5,000+ PSI: Engineered mix — needs lab-tested design and admixtures.

What concrete is not

Three things often confused with concrete:

• Mortar contains only sand, cement, and water (no coarse aggregate). It's used to bond bricks, blocks, and stones. Weaker than concrete, more workable.
• Grout is a high-flow, high-strength mix used to fill gaps — think tile joints or post-tension cable ducts. Sometimes contains aggregate, sometimes not.
• Cement paste is just cement + water with no aggregate. Used as a bonding agent or skim coat, never structurally.

Why this matters for ordering

When you call the ready-mix dispatcher, the breakdown above is what you're agreeing to on the delivery ticket. The ticket will list the cement type, total cubic yards, water added at the plant, and any admixtures. If the ticket says "3,000 PSI air-entrained, max w/c 0.50, slump 4–5 in," that's a complete spec. Anything missing is an opportunity for a misread later — always confirm the air content if you're pouring exterior, and never let the truck add water past the slump on the ticket.

For the math side of estimating concrete, see how to calculate concrete or run your numbers on the concrete calculator.