Concrete and mortar ingredients, and the role of aggregate

People often use "cement" and "concrete" as if they meant the same thing, and mortar gets lumped in too. They are related but different, and the difference starts with what goes into each. This page lays out the basic ingredients of concrete and mortar (cement, water, fine and coarse aggregate, and admixtures), explains the job aggregate does, and says why clean, well-graded material matters. It stays general and does not recommend mixes or products. Sources are American and the reader's own local standards take priority.

01The basic ingredients

The Federal Highway Administration (FHWA) lists the basic components of portland cement concrete as coarse aggregate (crushed stone or gravel), fine aggregate (usually natural sand), portland cement and water. The aggregate acts as a filler, and it is bound together by hardened cement paste formed by a chemical reaction, called hydration, between the cement and the water (FHWA-RD-97-148, an archived 1997 report). A University of Illinois teaching guide makes the same point in plainer words: the density and strength of concrete are determined in part by the proportions of three key ingredients, water, cement and aggregate (University of Illinois, The Basic Mix).

Cement itself is a manufactured powder, not a natural one. The Kansas Geological Survey describes crushed limestone and clay or shale mixed together and baked in kilns, then ground, with gypsum often added as a retarding agent to stop it setting too quickly. FHWA says portland cement is made by crushing, milling and blending raw materials with suitable proportions of lime, iron, silica and alumina, and that it typically makes up about 15 percent of a paving concrete mix by weight. So cement is the glue, and concrete is the finished mixture. Water, crushed stone, sand and gravel are added to the cement to make concrete, in the Survey's wording.

02What aggregate does in the mix

Aggregate is the bulk of the material. FHWA says the coarse and fine aggregates in paving concrete make up about 80 to 85 percent of the mix by mass, or 60 to 75 percent by volume. Cement paste normally accounts for about 25 to 40 percent of the volume, and air occupies a further 1 to 3 percent, or 5 to 8 percent in air-entrained concrete. Within the paste, cementing materials make up roughly 7 to 15 percent of the absolute volume of the mix and water 14 to 21 percent. These shares come from highway pavement concrete, so other kinds of concrete will differ.

A second FHWA document, its Highway Materials Engineering Course, says aggregates occupy the largest volume of any component in concrete and have a great influence on strength and deformation, durability, dimensional stability and economy (FHWA Highway Materials Engineering Course, Lesson 10). The coarse fraction is defined there as material predominantly retained on a 4.75 mm sieve. More on where that material comes from is in what aggregates are and how they are made, and the fine fraction is covered under sand types and uses.

The hardened paste is what holds everything together. FHWA adds that for ordinary pavement concrete the strength of the aggregate particles is rarely tested, as it is usually much greater than the strength of the paste or of the bond between paste and aggregate. For high-strength mixes, particle strength becomes an important factor.

03Why clean, well-graded aggregate matters

Grading is the mix of particle sizes. FHWA says the size distribution affects the proportions of cementing materials and water, workability, pumpability, economy, porosity, shrinkage and durability, and that the distribution should be a combination of sizes that results in a minimum of void spaces. In other words, small grains fill the gaps between large ones, so less paste is needed to fill what is left. The idea is developed in aggregate sizes and grading.

Cleanliness has the same kind of effect. Fines, the very small particles that pass the No. 200 sieve, cling to larger pieces and are often removed only by washing, according to the FHWA course. Because small particles have a large surface area for their weight, fines raise the water demand of a mix. If water is added to keep the mix workable, the course says the hardened concrete loses strength and durability and drying shrinkage increases. It reports that most agencies limit the combined fines in fine and coarse aggregate to 3 to 5 percent.

Some contaminants are worse than fines. The same material lists clay lumps, shale and other soft or friable particles as deleterious materials, meaning materials that can harm the concrete. Clay lumps can break down during mixing and raise water demand, while vegetable matter can act as a retarder or reduce the entrained air content, and in hardened concrete it can rot and leave voids. Problems like these are one reason washing and screening are part of producing aggregate for concrete.

Shape counts too. According to FHWA, rough, angular or elongated particles require more water for workable concrete than smooth, rounded ones, so they need more cement to hold the same water-to-cement ratio. Hardened strength generally increases with more angular coarse aggregate, and flat or elongated coarse pieces are best avoided.

04Water and admixtures

Water has its own balancing act. The Illinois guide calls it the key ingredient: too much water results in weak concrete, and too little results in a mix that cannot be worked. Moisture in the sand counts as part of that water, since the guide warns that wet sand can carry a considerable amount. FHWA's course adds that a wet fine-aggregate stockpile can contribute a substantial amount of free water to a batch.

An admixture is defined by FHWA as a material other than portland cement, water and aggregate that is added during mixing to modify the fresh or hardened concrete. Chemical admixtures fall into three basic groups: water-reducing agents, which cut the water a mix needs while keeping workability and strength; air-entraining agents, which raise resistance to freezing and thawing and to scaling from deicing chemicals; and setting agents, which retard or accelerate the set. FHWA also describes supplementary cementitious materials, such as coal fly ash, ground granulated blast furnace slag and silica fume, that can be used to modify concrete properties.

05Concrete and mortar compared

Mortar is the material that bonds bricks or concrete blocks together, in the Kansas Geological Survey's description. It is made with finer grains of sand instead of gravel, which produces a smoother finish. The practical difference is the aggregate: concrete contains stone or gravel as well as sand, while the mortar in the Illinois teaching guide is built from cement, sand and water alone.

Teaching guides give rough proportions to show the contrast. The Illinois guide suggests a basic classroom mortar of 1 part water, 2 parts cement and 3 parts sand by volume, and an old rule of thumb for concrete of 1 part cement, 2 parts sand and 3 parts gravel by volume, with water added until workable. It notes that either may need adjusting for the aggregate used. These are demonstration figures, not specifications. Real mixes are designed to a standard, and the local standard or a qualified engineer or mason has the final word.

Where crushed old concrete and asphalt fit into the picture is covered under recycled aggregates. And when an order arrives by the truckload or bag, reading a delivery ticket and measuring volume explains how quantities are stated.

06The short version

Concrete is cement paste wrapped around a large share of sand and stone, with water and sometimes admixtures to adjust its behavior, and mortar swaps the stone for finer sand to bond masonry. Aggregate takes up most of the volume, so its grading, shape and cleanliness shape how the whole mix performs. Specifications differ by country and by project, and local standards are the final reference.