The right screed mixture

For centuries, people have mixed sand, cement, and water, following certain mixing techniques, in order to get the right screeds for their projects. Roman aqueducts, the huge dome of the Pantheon, and the Baths of Caracalla are excellent examples of structures made from durable screed and concrete.

But is the regular 4:1 aggregate-cement ratio able to deliver the right screed for your project, durable enough to stand the test of time? Should you replace water with another substance? Should you add more aggregate, more cement, or additives?

Typically, the 4:1 mix gives constructors the same result every time. While professional screeders know that a series of variables, such as relative humidity, residual moisture, type of aggregate, and additional materials used, can greatly affect the end result, DIY enthusiasts have no idea that all these factors must be taken into account when trying to get the right screeding mix.

Cement
Cement delivers a fine mineral powder obtained from ground limestone and clay. Then, the mixture is heated at 1,450 degrees Celsius and combined with different aggregates for a variety of applications, including concrete, screeds, mortars, renders, and reconstituted stone products. When mixed with water and aggregates, cement transforms into a paste, which binds to almost any surface. Besides limestone and clay, cement may contain a series of additives, strength enhancers, workability enhancers, and accelerators, which alter initial cement properties for the sole purpose of enabling constructors to address and solve specific challenges.

Aggregates
Defined as granular materials of mineral origin, aggregates are actually sand, gravel, and crushed rock. In construction, these materials are combined with cement and water to give stability, volume, and resistance to erosion to the end product. For a good screed mix, aggregates need to be clean, hard, and free of any chemicals or coatings that could negatively impact screed quality. While fine aggregates, with particles under 1cm are appropriate for most screeds, coarse aggregates, including particles between 1cm and 3.5cm, are typically used in heavy duty screeds.

Consistency
In its simplest form, screed is a mixture of cement, aggregate, and water. Through hydration, a chemical reaction that takes place after the cement, aggregates, and water have been combined, the mixture hardens, gaining more strength as it dries out to finally form the rock-like mass, known as screed.

The secret to achieving a strong, durable screed consists in proportioning the right ingredients and mixing them thoroughly. Although smoother and more workable than other options, a screed with too much cement is expensive and can crack easily. Conversely, a screed mixture with not enough cement won’t only fail to hold the aggregate together, but also deliver a porous end product, which can crack or absorb residual moisture, potentially leading to moisture-related problems.

For more information on how to get the right screed mixture, contact us at HomeScreed. Our experts are ready to answer any questions you may have about cement, aggregates, and screeds.