If you are interested in concrete polishing or simply wish to know more about the concrete that you walk on everyday, you should know that the composition of concrete can actually be quite varied. Aggregates, cement, chemical admixtures, reinforcement, mineral admixtures, and the finish all play a critical role in the formulation of concrete. The relative proportion of these materials, as well as the length of the cementitious and aggregate phases, allows concrete to be tailored specifically for the purpose it is intended for. The final product’s strength, density, chemical resistance, and thermal resistance may all be adjusted using the above methods.
Let’s take a closer look at what each of the above components are and how they affect the final concrete product.
The aggregates are the large chunks of material in a concrete mixture. Coarse gravel, crushed granite, and crushed limestone are all very common choices for this purpose. Finer materials such as sand may also be used to fill in any holes left by the larger particles above.
The relative size of aggregate particles determines how much binding agent will be necessary to keep the mixture together. If all of the aggregates are roughly the same size, the mix will have more gaps that the binding agent will need to seal. The binding agent is usually the most expensive component of concrete, so requiring more of it will significantly increase the cost of the job at hand.
By contrast, adding a smaller aggregate to fill in gaps left by larger aggregate particles fills many of the gaps without relying on the bonding agent. The aggregate materials are almost always stronger than the binding agent too, giving the final concrete mixture more durability.
The aggregates may end up redistributed after a compression event due to the resulting vibrations, creating strength gradients that compromise the homogeneous strength of whatever is being constructed.
Some landscape designers also use “exposed aggregates,” such as crushed glass, quartzite, or small river stones to lend concrete polishing a decorative finish. These extra components may boost the robustness of the final product in addition to improving its visual appeal.
Cement is the binding agent mentioned above, responsible for holding the mixture together and helping it flow more freely while it is being poured. The word cement is commonly associated with concrete as a synonym, but a range of materials may perform its function. Asphalt cement, fly ash, and slag cement are currently popular alternatives to classic cement.
To produce concrete, cement is usually mixed in with water and the aggregates in a dry powder form. This produces a semi-liquid slurry that may be shaped to the needs of the task at hand, usually by pouring it into the desired form. A series of chemical processes called hydration solidifies and hardens the mixture, creating the stone-like material we know as concrete. Note that this process does not apply to asphalt-based concrete mixtures.
According to Abrams’s Law, increasing the cement to water ratio in the initial mixture will increase the strength and durability of the resulting product. Decreasing the amount of cement relative to water will help it flow more freely with a higher slump. Either way, the water used must be pure to avoid unwanted interference with the settling process that may result in the premature failure of the structure being built.
3. Chemical Admixtures
Chemical admixtures are chemicals mixed into the concrete with the aggregates and the cement in order to give a broad range of additional properties to the finished product. They may be in powdered or liquid form when added to the concrete mixture. Chemical admixtures typically make up less than five percent of a concrete mixture by mass.
Increased water resistance, entrapment of air, and both speeding up and slowing down the hydration process are common functions of chemical admixtures. Obviously, different materials are used to achieve each of these functions. Here are some of the most common ones.
First, accelerators speed up the hydration process so that the concrete hardens faster. These are especially useful in colder climates. CaCl2, Ca(NO3)2, and NaNO3 are the most common compounds used for this purpose. Chlorides may cause any steel in the concrete mix to prematurely corrode, so nitrates are preferred for formulations including steel. Government regulations may also determine what chemical admixtures are usable in any given area.
Retarders are the opposite of accelerators in that they make the hydration process take longer than it otherwise would. This is especially necessary if you have a complicated, time-consuming pour that you do not want the mixture hardening over the course of. Common retarding agents include sugar, sucrose, sodium gluconate, glucose, citric acid, and tartaric acid.
Air entraining agents allow small air bubbles to form in the concrete. These bubbles increase concrete’s resistance to the freeze-thaw cycle commonly faced in colder climates, but compromises the finished product’s compression resistance by approximately five percent for each percentage point of air by mass in the mixture. If too much air is accidentally introduced to the mixture, special defoamers may be used to encourage air bubbles to rise and disperse before the concrete hardens.
Plasticizers are chemical admixtures that allow “plastic,” or fresh, concrete to be worked more easily. Put another way, these chemical admixtures mimic the function of water in the concrete mix without compromising strength. They are sometimes referred to as water-reducers for this reason. Lignosulfonate is a commonly used water-reducer.
There are also superplasticizers, or high-range water-reducers, that make the concrete mixture more malleable than would be possible with just water or vanilla plasticizers. Common superplasticizers include sulfonated naphthalene formaldehyde condensate, sulfonated melamine formaldehyde condensate, acetone formaldehyde condensate and polycarboxylate ethers.
The above list details just a few of the chemical admixtures in use today. Corrosion inhibitors help protect any steel in the mix from corrosion, bonding agents are used to bind new and old sections of concrete into a cohesive whole, pumping aids make the concrete easier to pump, and pigments may be used to change the color of the concrete mix for aesthetic purposes. If you need concrete to have a chemical property, there is probably a chemical admixture that can provide it.
Concrete naturally has a very high compression resistance, but its tensile strength is relatively lacking because the cement can crack somewhat easily. For this reason, concrete is often reinforced by materials offering a strong tensile resistance (steel reinforcing bars, glass fibers, plastic fibers, steel fibers) to increase its overall durability.
5. Mineral Admixtures
Mineral admixtures have risen in popularity recently as a cost-effective way to produce concrete in a more environmentally friendly manner. Waste products from other industries are used as a partial replacement for cement, giving concrete the properties it needs to have for a fraction of the typical cost.
For example, fly ash from coal-fired power plants, ground granulated blast furnace slag from the steelmaking industry, and silica fume from industrial-strength electric arc furnaces can all substitute for some of the cement in a concrete mixture. They may be added directly to cement to create a pre-blended product or combined with all of the other components when concrete is manufactured.
These industrial waste products are difficult to dispose of otherwise, so the concrete industry is doing the environment a favor by using them. Likewise, creating cement is taxing on both the environment and the manufacturer’s bank account, so cheaper substitutes are a welcome addition to the process.
6. Surface Finishes
Raw concrete is porous with an unappealing appearance, so finishes are often added to increase its durability while packing a visual punch. Staining, water penetration, and freezing can all be mitigated with the right finish. Therefore, the finish is a vital component of any concrete structure.
For visual appeal, the process of concrete polishing uses diamond adhesives to polish the concrete before sealing it with polymers to lock in the resulting shiny appearance. Patterns may be introduced by stamping the concrete before it dries, with popular designs including the illusion of bricks or cobblestones. Designs may also be chiseled in once the concrete has dried, painted, or otherwise produced by covering the concrete with decorative materials.
Most people take concrete for granted, but there is actually a lot of science behind the most utilized man-made building material on Earth. Perhaps you’ll spare a thought on the sidewalk’s chemical composition the next time you go for a stroll!