How Concrete Is Made

What concrete is made of

The National Ready Mixed Concrete Association's definition is the clearest: "In its simplest form, concrete is a mixture of paste and aggregates (sand & rock)." The paste is cement and water, and it coats every grain of sand and stone and binds them as it hardens.

The Portland Cement Association gives a slightly different split on its own page, 7 to 15 percent cement and 14 to 21 percent water, which is a reminder that these are typical ranges, not a recipe. By weight the picture looks different again, because stone is heavy. The EPA's reference mix for a cubic yard is 1,865 pounds of coarse aggregate, 1,428 pounds of sand, 491 pounds of cement, 73 pounds of cement supplement and about 20 gallons of water.

The one ratio that matters most is water to cement. NRMCA defines it as "the weight of the mixing water divided by the weight of the cement," and says high-quality concrete comes from lowering it as far as the work allows. Every extra bucket of water in a batch makes the concrete easier to place and weaker once it sets, which is the subject of our guide to how to mix concrete.

How cement is made

Cement is the only manufactured ingredient, and the process has not changed in principle since the 1800s. These steps follow the EPA's technical description of portland cement manufacturing (AP-42 section 11.6, 2025) and the PCA's how cement is made.

1. Quarry the raw materials. Calcium is the main element, from limestone or another calcareous rock. The EPA notes that "Other elements included in the raw mix are silicon, aluminum, and iron," which come from "sand, shale, clay, and iron ore." Plants increasingly add fly ash and slag as alternative raw materials.
2. Crush and blend the raw mix. The rock is crushed, proportioned by chemistry, and ground into a fine kiln feed.
3. Preheat and calcine. In a modern plant the feed falls through a tall preheater tower, and a precalciner vessel at its base drives the carbon dioxide out of the limestone before the material reaches the kiln. That is calcination, and it happens between about 1,650 and 1,800 degrees F.
4. Burn it in the kiln. The rotary kiln is the heart of the plant. PCA describes "a roaring flame" that heats the material "to about 2,700 degrees Fahrenheit, making it partially molten," and the EPA gives approximately 1,510 degrees C (2,750 F). The minerals fuse into new compounds, mostly calcium silicates.
5. Cool the clinker. What leaves the kiln is clinker, which the EPA describes as gray, glass-hard nodules from about a third of a centimeter to 5 centimeters across, roughly 1/8 inch to 2 inches. A cooler recovers some of the heat for the kiln.
6. Grind with gypsum. The clinker is ground to powder with a little gypsum. In the EPA's words, "Up to 5 percent gypsum or natural anhydrite is added to the clinker during grinding to control the cement setting time." Without it, cement would stiffen almost the moment it met water. PCA puts the fineness at "about 150 billion grains per pound."

Two numbers explain why cement is in the news. The kiln step releases carbon dioxide twice, once from the fuel and once from the limestone itself, and the EPA puts the limestone part alone at about 500 kilograms of CO2 per metric ton of cement. And the scale is large: the USGS estimates US plants made about 82 million metric tons of portland and blended cement in 2025, at 97 plants in 34 states and Puerto Rico.

Types of cement, and why most bags now say Type IL

ASTM C150 defines the classic portland cement types. Type I is "For general use," Type II for "moderate sulfate resistance," Type III "for use when high early strength is desired," and Type V for high sulfate resistance. Type IV, a low-heat cement for massive pours, is, per ACI, "Not currently made in the United States."

The bigger change is portland-limestone cement, Type IL under ASTM C595. It replaces 5 to 15 percent of the clinker with ground limestone that never went through the kiln, which PCA says gives similar performance with "a CO2 footprint up to 10% lower." It has taken over fast: the USGS estimates blended cement was 63 percent of US shipments in the first nine months of 2025, and about 95 percent of that was Type IL. If you are wondering whether that changes how new concrete behaves, our post on Type IL cement and driveways looks at the evidence.

How ready-mix concrete is made

Most concrete never sees a shovel until it reaches the forms. The EPA estimates that about 75 percent of US concrete is made at plants that store, measure and discharge the ingredients into trucks. Its description of a batch plant is a good map of one:

• "The cement is transferred to elevated storage silos pneumatically or by bucket elevator."
• Sand and stone go up to elevated bins by loader, conveyor or bucket elevator.
• From there "the constituents are fed by gravity or screw conveyor to weigh hoppers, which combine the proper amounts of each material."

Everything is weighed, not measured by volume. ASTM C94, the ready-mix specification, says "cement, aggregate, and admixtures shall be measured by mass." State specifications put tight tolerances on that: Michigan DOT, for example, allows cement within 1 percent of the target weight and aggregates within 2 percent. That is the real difference between a plant's 4,000 psi concrete and a hand-measured batch.

Then it is mixed one of two ways. NRMCA describes central-mix plants, which "include a stationary, plant-mounted mixer," and transit or truck mixing, in which all the raw ingredients are loaded into the truck drum and mixed on the way. Some plants partly mix at the plant and finish in the truck, called shrink mixing. ACI's guidance is that "truck mixers should adequately mix the concrete in 70 to 100 revolutions," after which the drum turns slowly to keep it agitated.

The 90-minute rule is no longer in the standard. You will read everywhere that ready-mix must be placed within 90 minutes or 300 drum turns. Older editions of ASTM C94 did say 1-1/2 hours. California's hospital building agency, in a 2025 policy notice, records that the current edition "no longer provides specific guidance on the duration from the start of mixing materials to the placement of the concrete," and leaves the limit to the purchaser. Many agencies and contracts still set one, and Michigan's ranges from 45 minutes in hot weather to 90 in cold. If time matters on your pour, the limit is whatever your order says.

For the practical side of ordering, see ready-mix delivery and how much concrete is in a truck.

The other ingredients: admixtures, fly ash and slag

Modern concrete is rarely just four ingredients. Plants tune it with small doses of chemicals and with other cementing materials:

• Air entraining admixture (ASTM C260) creates billions of microscopic bubbles. They give freezing water room to expand, which is why exterior concrete in cold climates is air entrained.
• Water reducers, retarders and accelerators (ASTM C494 Types A to G) let the plant cut water without losing workability, slow the set in hot weather or speed it in cold.
• Fly ash, the fine residue from burning coal, replaces part of the cement. The FHWA notes that "Typically, 15 percent to 30 percent of the portland cement is replaced with fly ash," and that it improves workability.
• Slag cement (ASTM C989), ground blast-furnace slag, does a similar job and is common in sulfate-resistant and mass concrete.

None of these are available to someone mixing bags in a wheelbarrow, which is the main reason bagged and site-mixed concrete can't match a plant mix for exterior work in a freezing climate.

How to make concrete yourself

You can make concrete from the raw ingredients: bags of portland cement, sand and gravel. Quikrete's portland cement data sheet gives the proportions by volume as 1 part cement, 2 parts sand and 3 parts gravel, and says five to six 94 lb bags of cement typically make a cubic yard.

1. Measure by volume with the same bucket for each ingredient: 1 cement, 2 sand, 3 gravel.
2. Mix the dry ingredients until the color is even.
3. Add water a little at a time until the batch holds its shape when squeezed. Stop there; extra water costs strength.
4. Place it within the working time, then keep it damp while it cures.

The concrete mix calculator works out how much cement, sand and gravel a given volume takes, including why dry ingredients measure more than the concrete they make. The step-by-step method, and whether to use a bucket, a wheelbarrow or a mixer, is in how to mix concrete. For most small jobs, a bagged premix is easier and more consistent: the proportions are already right, and the bag calculator tells you how many.

Wet cement is caustic. OSHA warns that "Skin exposure to wet cement can result in cement burns," and construction safety material puts wet cement at a pH of 12 to 13. Wear alkali-resistant gloves, long sleeves and eye protection, and wash skin with clean water promptly. A cement burn can keep getting worse after the concrete is washed off.

Concrete hardens by reacting, not by drying

Concrete gets hard because cement and water react to form new compounds, not because the water evaporates. NRMCA puts it this way: "Through a chemical reaction of cement and water called hydration, the paste hardens and gains strength." That is why fresh concrete is kept wet, not left to dry. PCA notes that "The longer it is kept moist, the stronger and more durable concrete will become," and that most of the strength gain happens in the first month.

How long that takes, and when you can walk, drive or build on it, is in the concrete cure time guide. What strength you need for each job is in concrete psi.