Over the years I have designed and fabricated many different types of equipment and forms for the precast concrete industry. I have often had the question asked if you can make concrete with just sand and cement, so can you?
You cannot make concrete with only sand and cement because it requires a coarse aggregate like gravel. The stone component is the most critical, as that is what gives it its durability and strength. When mixing merely sand, cement, and water, you get a material closer to mortar.
You can purchase concrete with a high sand content or even a no-fines concrete where they exclude entirely sand in favor of coarse aggregates. This article will take a close look at the different methods of mixing concrete, what they contain, and their applications.
Why Concrete Needs a Coarse Aggregate
Concrete needs to be strong and durable, and this is achieved by carefully mixing the necessary ingredients in the correct proportions. The reason concrete cannot be made simply by mixing sand with cement is that it is a composite material that needs a coarse aggregate to achieve this strength.
In essence, concrete is a mixture of Portland cement, water, sand, and rocks. The cement or paste coats and binds the sand and rocks. Then, a chemical reaction called hydration, which is initiated by the water, takes place. The paste will harden and strengthen to form the solid mass known as concrete.
Within this chemical process lies the key to a remarkable trait of concrete: it’s plastic and malleable when newly mixed, and yet strong and durable when hardened. It is such qualities that help explain why one material, concrete, can be used to construct everything from skyscrapers to bridges and sidewalks, superhighways, and dams (source).
If you were only to mix sand and cement with water, you would get a mixture closer to mortar. Mortar is also often used in building construction as this thick mixture serves as a glue for holding materials like bricks together and in place.
The water-to-cement ratio in mortar is higher than that in concrete, which makes it thicker and a better bonding element.
Adding the element of lime or gypsum to the mix creates a material called plaster. Plaster and mortar are often confused as they are so similar.
While workers use mortar to glue together the bricks make up a building, they use plaster to finish off the interiors and exteriors of building walls. Plaster is generally finer than mortar, which makes for a better finish.
Concrete is a composite material and the next level up from plaster as it incorporates an aggregate to make it stronger. Of the total mass making up a concrete mix, the aggregate or stone will account for 60 percent on average, and it can even go right up to 80 percent in certain situations.
The aggregate is what makes concrete so unique, and the cement and water merely hold it together. While concrete doesn’t need a high water-to-cement ratio, it is quite thin when freshly mixed, and that is why it’s not used as a bonding element. It is most effective in structural projects and for support.
Concrete on its own is strong and durable but, for added structural integrity, steel rebar is often used to reinforce the concrete (source).
One interesting variation of concrete is known as no-fines concrete. This type of concrete eliminates the fine aggregate of sand completely, meaning that it is merely made with cement, water, and stone. The cement and water mixture will only be enough to cover the aggregate to bond it.
No-fines concrete is much lighter than ordinary concrete, but it is also weaker and, therefore, not used in places where structural integrity or support counts. You will find no-fines concrete mostly used for flooring where heavy concentrated loads are not present.
The Effect of Aggregate Types
The aggregate is one of the essential components of concrete and is the material the cement coats and binds together to make concrete. We can understand aggregates as the solid bodies bonded by the cement, and they can come in an abundance of sizes, forms, and materials.
The aggregate is also one of the cheapest ingredients in concrete, which is why it generally accounts for 60 to 80 percent of the total volume of the concrete mix.
The type of aggregate used in the concrete is usually informed by the desired characteristics that the concrete should have when hardened.
If you use a soft and porous aggregate, the resultant concrete will be weaker with low wear resistance. If you use a hard and solid aggregate, the concrete will be durable and highly resistant to wear and tear.
Ideally, you want a hard, durable, and clean aggregate when mixing concrete. It should be washed before mixing in, as impurities can hinder the bonding reaction once it comes into contact with the cement mix.
You also ideally want aggregates of uniform size in one specific concrete mix, so aggregates for construction will generally be sorted by size.
The composition, shape, and size of the aggregate all have a significant impact on the malleability of the fresh product. This will also affect the weight and shrinkage of the concrete as a whole. The most common aggregates are sand, gravel, and crushed stone (source).
Aggregates and Concrete Weight
When planning a concrete project, there are several factors to take into consideration. However, the most important factor to consider will take into account the purpose of the concrete in question as this will affect which aggregate you will need.
Concrete is typically either ultra-lightweight, lightweight, normal-weight concrete, or heavyweight concrete.
Lightweight and Ultra-Lightweight Concrete
The two lightweight concrete types, ultra-lightweight concrete, and lightweight concrete are both used for their insulation properties. Ultra-lightweight concrete can often be sawed or nailed and can be used in the bases of prefab buildings.
Common aggregates used in lightweight concrete include vermiculite, ceramic spheres, and perlite.
Lightweight concrete is often used for domestic and office flooring, and aggregates for this type of concrete include expanded clay, shale or slate, and crushed bricks. If you were making concrete without gravel, this would be the result.
Normal-weight concrete is the most commonly used concrete, and it is very strong and durable as well as resistant to shock and severe vibration. It is often used for heavy-duty floors, watertight walls, roads, and precast units.
Because the finished product needs to be strong, hard, and durable, the aggregate needs to be of similar standards. That is why gravel, crushed recycled concrete, and crushed limestone are often used as the aggregate in this type.
Heavyweight concrete is as tough as they come and used for lintels, beams, and floor-bearers. It is also used in the construction of bridges, precast and prestressed concrete slabs, and pipes.
In this type of application, ordinary concrete will not suffice, and that is why heavyweight concrete is often reinforced with steel, and the aggregate is also steel or iron shot or pellets.
Applications for Sand and Cement Mixes
Just as you can make no-fines concrete without sand, you can create a concrete mix that incorporates a high proportion of sand.
However, concrete that incorporates higher levels of sand compared to coarser aggregates will be weaker overall and cannot be used for structural applications or where support or load-bearing abilities are required.
A product with a higher sand ratio will resemble the properties of mortar or plaster rather than concrete. Either concrete or mortar that has a high sand content is challenging to work with as it will be sloppy when freshly mixed, and it can be brittle under pressure once hardened.
Applications for high-sand concrete or mortar range from repairing cracks and filling holes in existing concrete or masonry, decorative concrete overlays, filling masonry block cores, filling paver joints, or using it as dry pack underneath ceramic tiles or shower floors.
Both Quickrete and Sakrete make a sand mix for topping and bedding, which is primarily sand and cement. You can use such mixes to repair concrete that is no greater than two inches thick and apply it in layers of ½-inch.
When using these for a mortar bed, you typically apply them in layers around two to three inches for ceramic tile, for example.
Concrete is a mixture of Portland cement, water, sand, and stone. The stone element, also known as the aggregate, is why concrete cannot be made simply by mixing sand and cement. Sand is much too fine of an element to provide the compressive strength and durability that concrete needs. There are even variations of concrete that leave out the sand component altogether.
A mix of sand and cement will function more as a mortar or bedding for things like ceramic tile. While this has its uses, it is technically not concrete.