What Is Concrete Made Of: Ingredients Explained

Most people use “cement” and “concrete” interchangeably, and it drives contractors crazy. Cement is just one ingredient — usually only 10 to 15 percent of the mix by volume. So what is concrete made of, exactly? Four things: portland cement, water, aggregates (sand and gravel), and in most modern mixes, chemical admixtures that fine-tune how the material behaves. Understanding what each ingredient does explains why some slabs last 50 years while others crack in five.

The Four Core Ingredients

A typical cubic yard of concrete breaks down roughly like this: 60 to 75 percent aggregate, 10 to 15 percent cement, 15 to 20 percent water, and 5 to 8 percent entrained air. The proportions shift depending on the job — a 4,000 PSI driveway mix carries more cement than a 2,500 PSI footing mix — but the cast of characters stays the same.

• Portland cement — the binder. A fine gray powder made by firing limestone and clay at about 2,700°F in a rotary kiln, then grinding the resulting clinker with a little gypsum.
• Coarse aggregate — crushed stone or gravel, typically 3/8 inch to 1 inch. It provides bulk, strength, and dimensional stability.
• Fine aggregate — sand that fills the voids between the larger stones so the paste doesn’t have to.
• Water — triggers the chemical reaction that hardens the cement and makes the mix workable enough to place.

If you’re comparing bagged products or batching your own, our concrete mix guide covers how these ingredients show up in common pre-blended options.

Portland Cement: The Glue That Holds It Together

Portland cement isn’t a brand name — it’s a material standard dating to 1824, when English bricklayer Joseph Aspdin patented a binder that resembled the pale limestone quarried on the Isle of Portland. Modern cement plants blend limestone, clay, shale, and iron ore, fire the mixture into marble-sized clinker, and grind it to a powder fine enough that a single pound contains roughly 150 billion particles.

In the US, cement is classified under ASTM C150. Type I is the general-purpose workhorse in most residential work. Type II resists sulfates in soil, Type III is ground finer for high early strength (handy in cold weather), and Type V handles severe sulfate exposure common in parts of the Southwest. Many ready-mix plants now substitute 15 to 30 percent of the cement with fly ash or slag cement — industrial byproducts that cut cost, reduce heat buildup, and often improve long-term strength.

Aggregates: The Skeleton of the Slab

Aggregate does the heavy lifting, literally. Stone is stronger and far cheaper than cement paste, so a good mix packs in as much rock as workability allows. Gradation matters more than most homeowners realize: a well-graded blend of stone sizes and sand nests together tightly, leaving fewer voids for expensive paste to fill. Poorly graded aggregate produces a harsh mix that demands extra water — and extra water is the enemy of strength.

Aggregate also needs to be clean. Clay coatings, organic silt, or salt contamination prevent cement paste from bonding to the stone. That’s why batch plants wash their aggregate, and why “cheap fill sand” has no business in structural concrete. In freeze-thaw climates, aggregate must be durable too — soft, porous stone near the surface pops out as the water inside it freezes, leaving the pockmarks contractors call popouts.

Water and the Water-Cement Ratio

Water does two jobs: it makes the mix placeable, and it fuels the hardening reaction. The critical number is the water-cement ratio (w/c) — pounds of water divided by pounds of cement. A w/c of about 0.40 to 0.50 produces strong, durable concrete. Push it to 0.60 or beyond and strength drops sharply, because every drop of water beyond what the chemistry needs eventually evaporates and leaves a microscopic pore behind.

This is the mistake that ruins more DIY slabs than any other. A soupy mix flows beautifully off the shovel, but adding one extra gallon of water per bag of cement can cut compressive strength by 1,000 PSI or more and dramatically increase shrinkage cracking. Professionals adjust workability with admixtures, not the hose.

Hydration: How Concrete Actually Hardens

Concrete doesn’t dry — it cures. The moment water hits cement, a chemical reaction called hydration begins. The two key compounds in cement, tricalcium silicate and dicalcium silicate, react with water to form calcium silicate hydrate (C-S-H), a rigid gel that grows crystal-like tendrils that interlock and bind the aggregate into a solid mass. The reaction also releases calcium hydroxide and a surprising amount of heat, which is why a curing footing feels warm to the touch.

Hydration is why keeping fresh concrete damp matters so much. If the surface dries out during the first week, the reaction stalls and the top layer never reaches its potential strength — a leading cause of dusting and scaling. It’s also why concrete keeps getting stronger for months: standard mixes hit about 70 percent of design strength in 7 days, the full rated strength around 28 days, and continue gaining slowly for years as unhydrated cement grains find moisture.

Admixtures: The Modern Chemistry Set

Nearly every truckload of ready-mix carries at least one chemical admixture, dosed in ounces per hundred pounds of cement. They’re what let crews pour in July heat, January cold, and everything between.

• Air-entraining agents create billions of microscopic bubbles that give freezing water room to expand — essential for exterior slabs in northern states.
• Water reducers and superplasticizers make a stiff mix flow without extra water, preserving strength.
• Accelerators (calcium chloride or non-chloride versions) speed set time in cold weather.
• Retarders slow the set on hot days so the crew can finish before the surface locks up.
• Fibers — polypropylene or steel — reduce plastic shrinkage cracking and add toughness.

How Mix Design Changes by Application

Concrete is a recipe, and the recipe changes with the job. A residential footing might call for 2,500 PSI with 1-inch stone. A driveway in Minnesota wants 4,000 PSI, 5 to 7 percent entrained air, and a 0.45 w/c ratio to shrug off salt and freeze-thaw cycles. Countertop mixes run small aggregate, high cement content, and superplasticizer for a dense, finishable surface. Pump mixes use smaller stone and more sand so they move through a hose without jamming.

When you order from a batch plant, you’re really ordering a specification: strength (PSI), slump (stiffness), air content, and aggregate size. Get those four numbers right for your application and the ingredients take care of the rest. For a broader look at slabs, finishes, and flatwork applications, see our main concrete guide.

Cement vs. Mortar vs. Concrete: Clearing Up the Confusion

Since the terms get tangled constantly, here’s the family tree. Cement is the powder. Mortar is cement plus sand plus water — no coarse stone — used to bond brick and block, and it’s deliberately weaker than the masonry it joins. Grout is a soupier cement-sand blend made to flow into gaps. Concrete is the only one of the four built around coarse aggregate, and that stone is why it carries structural loads the others can’t. Buy accordingly: a bag labeled “mortar mix” will never make a strong slab, and concrete mix is too rocky to lay brick with. The label on the bag matters more than the gray color, which is identical across all of them.

Why Ingredient Quality Shows Up Years Later

Bad concrete rarely announces itself on pour day. Excess water shows up as hairline shrinkage cracks at 60 days. Missing air entrainment shows up as surface scaling after the second winter. Dirty aggregate shows up as delamination when the bond fails. The material’s strength comes from getting boring details right: accurate proportions, clean ingredients, a low water-cement ratio, and a proper 7-day cure. Concrete is simple chemistry — cement, stone, sand, and water — but it punishes shortcuts on a delay, long after the finisher’s trowel marks have faded.