The composition of a concrete mix depends, to quite a large extent, on what you’re going to use it for. Putting in a garden fence pole requires a different specification of concrete than building the world’s tallest building, the Burj Khalifa, in Dubai.
You can mix a fast-setting concrete with regular concrete provided you are not overly concerned with the final properties of the mix. Adding fast-setting concrete mix to regular cement sacrifices compressive strength for drying time, and mixing the two makes drying time harder to gauge.
The strength of concrete depends primarily on the concrete mix design. We will see how various combinations of cement, aggregate, and water produce specific mix designs with different properties.
Factors Affecting Concrete Mix Design
Mixing different bags of concrete premix makes the design mix unpredictable. Since fast-setting concrete is a premix of cement, aggregates, and chemical accelerants, adding a bag or two of regular cement to this will alter the mix. We then have no way of really knowing how our mix will react to the added water. Start with a conservative amount of water and add to it until you get something that is easily workable.
The Normal DIY Formula
The common DIY formula for a concrete mix using regular (Portland) cement is one part cement to two parts dry sand to three parts dry stone, combined with half a part water. All measurements are by weight, not volume. Another common mix formula applies the “rule of six”. Six bags of cement per cubic yard of concrete, six gallons of water per bag of cement, plus a minimum of six days curing time. All the while, you must keep the concrete moist by covering it to prevent rapid evaporation or spray with a fine mist.
These formulas induce a chemical reaction that doesn’t account for the variations introduced by mixing regular cement with fast-setting cement mixes. Different formulas or mixes will produce different material outcomes.
In the US, concrete is graded for strength according to its various concrete mix proportions, as specified in IS456:2000 (source). These mix ratios are known as the Mix Design (M): M = Cement: Sand: Aggregate
The strength of the mixes can be from M5 to M25 for ordinary concrete. The number following the Mix Design ratio (M) stands for Characteristic Compressive Strength of the concrete. They measure this in N/mm2 at 28 days. 1 N/mm2 (Newton Per Square Millimeter) is a unit of pressure equivalent to 145.04 PSI (pounds per square inch). The British/European standards BS 8500-2 code these similarly as C10, C15, etc. to denote a Concrete Strength Class.
Beyond a concrete grade of M30, the structural design engineer will specify the concrete grade and design mix required to withstand the structural pressures of the structure on which they’re working. Ultra-High Performing Concretes have been designed in recent times to withstand compressive pressures beyond MPa250.
What is important for us to understand regarding a mix of fast-setting and regular concrete is that the rate of hardening and strength depends on the composition and fineness of the cement, mix proportions, and environmental factors such as ambient temperature and moisture (source).
For example, the concrete specified in Dubai’s Burj Khalifa was M80. This concrete also had to satisfy demanding flow characteristics. Because they had to pump it to a vertical height of over 600 meters.
Concrete mixes are typically premixed and batched according to fixed ratios up to M25. Augmenting your regular concrete mix with a fast-setting mix will, therefore, alter the chemistry taking place in both mixes.
The main chemical reaction characteristic of concrete is known as hydration. Which is how the concrete hardens and gains strength as it ages, a process that continues for some years.
Standard setters, however, have determined that the hardening and strengthening process mostly takes place during the first month. Hence the 28-day compressive strength standard.
There are also eight different types of Portland cement that, although similar, are each designed for specific applications. This regular cement variety is another indicator that not all cement — and, by extension, not all concrete — is the same. They will react differently or even unpredictably when combined (source).
Factors Affecting Setting Time
Remember that concrete is a chemical process involving the interaction of cement and water to form a paste. Cement itself is a chemically-formulated combination of a variety of substances that may include calcium, silicon, aluminum, iron, limestone, shells, and chalk, combined with shale, clay-slate, and a range of other ingredients. The cement/water paste combines with sand and other aggregates — crushed stone, sand, or gravel — to harden into the concrete structure. The mix ratio determines the strength of the concrete.
The water/cementitious material ratio (w/cm) also affects setting time. The higher the water ratio, the slower the setting time due to hydration. Faster hydration reduces setting time but may reduce the strength of the concrete.
Ambient temperature also affects setting time. Regular concrete sets in a day or two, and cures in five to seven days, depending on the outside temperature. To set properly, however, concrete should be kept moist until fully cured.
A third variable in setting time is the addition of admixtures to increase or decrease the setting time. This applies to our fast-setting formulations, of course, which is why interfering with these formulations by diluting them with regular concrete mixes may impact setting time in unforeseeable ways.
Cement technology continues to change and improve with the advent of polymer concretes and blended cements. The former uses polymers to bind the aggregate and is a type of fast-setting concrete capable of achieving a strength of 4000 PSI in only four hours. Regular concrete has a compressive strength of 1450-5800 PSI (source).
In summary, there are so-called nominal cement mixes for everyday small-scale applications and a wide range of fit-for-purpose mix designs that have become increasingly complex and specialized.
Pros and Cons of Fast-Setting vs. Regular Concrete
Fast-setting concrete is convenient because, well, it’s fast-setting. For those smaller jobs where you don’t want to be standing around for days waiting for the gatepost to set before hanging the gate, or when you’re not overly concerned about the compressive strength of the final set, fast-setting concrete does the job.
Still, there’s nothing wrong with the strength of fast-setting concrete unless you’re building a multi-story mansion. There are some cost considerations. Fast-setting tends to be more expensive, but for a smaller job, you should prepare yourself to pay a little extra for the convenience (source).
Regular concrete will be stronger and a bit cheaper, but will take longer to fully cure. Plus the extra strength is not super necessary because fast-setting concrete is strong enough for most applications. Once you start mixing the two, it can be anyone’s guess how it will turn out. If you are lucky, your mixture will be somewhere in the middle of fast setting and regular, but there are no guarantees.
Concrete and Cement
To understand what is meant by concrete mix design, and the implications of mixing fast-setting with regular concrete mix, we need to understand something about the nature and composition of concrete.
Cement is a fundamental component of concrete. It is the binding agent, or paste, that combines with aggregates (sand, crushed stone, or gravel) and water to form a rock-hard material. Construction workers have used it for building structures since at least Roman times.
The Romans improved on early attempts to make a building material by combining lime with pozzolana (volcanic ash). You can still see evidence of its durability today in structures like the dome of the Pantheon and the Colosseum in Rome. Lime continues to be used as an alternative foundation material to cement, mainly for its environmental and health benefits (source).
Regular cement is usually referred to by the generic term Portland cement. It is mostly crushed limestone and silica but may include shale, clay, and other crushed rock. These are placed in a large kiln and heated to nearly 300°F to form clinker. Which are small marble-sized pellets of silicate and aluminate compounds, which are finely ground to produce portland cement.
The term “Portland” was applied to cement by its English inventor, Joseph Aspdin when he patented the manufacturing process in 1824. He likened the hardened cement to Portland stone, a limestone from the Isle of Portland. In DIY language, cement, concrete, mortar, and grout share certain common characteristics as materials used in masonry that harden and bind together. But their individual characteristics and applications are quite distinct.
Cement is the binding component of concrete, mortar, and tile grout. Mortar combines cement, fine sands, and lime to form a binding material for building with brick, blocks, and stone. Grout is a type of mortar with a high water content allowing easy flow but without mortar’s lime additive. Which makes it a useful material to fill the gaps between tiles.
Fast-setting types of concrete have the advantages of convenience and time-saving. For smaller, non-safety-critical projects, they are fit-for-purpose but slightly more expensive than regular Portland cement. Adding a couple of bags of Portland cement to the fast-setting varieties to reduce costs may be an option if you are prepared for some guesswork regarding the mix design.
If you just happen to have components of both fast setting and normal concrete, you can mix them if you want. The mixture will not act the same as either of them, and its quality is tough to gauge. If you still need to buy everything to make concrete, I would suggest going with fast setting or regular concrete. Avoid mixing the two. This just makes sure you are going to get what you expect out of your efforts.