Retarders for Concrete: Types, Mechanism, and Uses

Retarders for concrete are additives or admixtures that help retard or slow the process of hardening. Builders introduce concrete retarders in order to increase the workability of concrete to allow easier handling, placement, and finishing of concrete. This gives builders more control and flexibility to finish projects on a particular timeline. Concrete retarders are especially important where long-distance transportation or complex building processes are involved.

Types of Retarders for Concrete

Experts categorize retarders to concrete into two main groups: organic retarders and chemical retarders, though these classifications are not as distinct as they may seem as some retarders contain both chemical and organic compounds.

Organic Concrete Retarders

• Sugar-Based Retarders – Sugar-based retarders are made from sugars like sucrose and glucose. These sugars interfere with the hydration process thereby delaying concrete hardening. These are cost-effective, adaptable to different types of concrete, they are non-toxic and work in a wide range of temperatures.
• Lignosulfonates – Lignosulfonates are derived from the wood-pulping process. These are water-soluble retarders and are known for their effectiveness. They also help to improve the workability and flow of the concrete.
• Modified Lignosulfonates – Modified lignosulfonates are chemically-enhanced lignosulfonates. They have all the qualities of standard lignosulfonates, but they also improve dosage flexibility to give builders more control over the process and outcome.
• Organic Acid Retarders – Organic-based retarders form complexes with calcium ions in cement. These types of retarders are not dependent on temperature and are compatible with many concrete types and admixtures.

Chemical Concrete Retarders

• Phosphates – Phosphate retarders act by delaying the formation of hydration products in cement. These are effective in high-temperature settings and promote chemical stability and early strength development.
• Citric Acid – Citric acid, derived from citrus fruits, forms with calcium ions to slow down hydration and extend the setting time. The dosage rate for citric acid retarders is lower than other types, so they are more cost-effective, and they are non-toxic.
• Borates – Borate retarders, such as borax or calcium borate, are chemical additives that effectively slow down hydration and setting time. These are also low-dosage retarders and they reduce heat hydration, which is helpful in large-scale applications.

How Do Concrete Retarders Work?

All the distinct types of concrete retarders work by influencing the chemical reaction between cement and water, known as hydration. Yet, the specific mechanism for influencing hydration depends on the type of retarder.

• Adsorption – Some retarders, like lignosulfonates, work by adsorbing onto the surface of cement particles. This interferes with the contact between the water and cement and delays the initiation of the hydration process.
• Complex Formation – Other retarders, including organic acids like citric acids and tartaric acid, form complexes with the calcium ions in the cement. This complex inhibits the process of hydration.
• Surface Blocking – Certain retarders, such as sugar-based or modified lignosulfonate retarders, create a thin film on the surface of cement particles and slow down water penetration and delay hydration.
• Nucleation Delay – These retarders function by delaying nucleation, or the formation of crystalline hydration products in cement.

Effects of Retarders on Concrete Qualities

Retarders have several beneficial effects on concrete, which give them more flexibility over their construction timelines. It is vital to study the exact type and amount of retarder necessary for the job and type of concrete as the wrong kind or amount can have adverse effects on concrete.

Extended Workability

Extended workability is the main reason that most builders use concrete retarders. By slowing down the setting time, builders have a longer timeline to handle, place, and finish concrete. This effect is particularly beneficial when there is a long transport distance, complex formwork, or intricate construction processes.

Reduced Risk of Cracks

Retarders help reduce the risk of cracks appearing because they slow down hydration and heat generation. This controlled rise in temperature helps to minimize the potential for thermal cracking. This is particularly helpful in large concrete applications. Improper amounts of retarders can increase the risk of cracking and shrinking, so it is vital to follow expert advice for retarder amounts.

Enhanced Strength

Retarders help to increase the bond strength between existing and new concrete by extending the workability and allowing for increased interlocking between the aggregate particles. Retarders also increase the compressive strength of concrete mixed with retarders. It is important to the optimal strength development of concrete to use the correct type and amount of concrete retarder.

Bleeding

Retarders can impact the bleeding of concrete in different ways. Some retarders will reduce concrete bleeding and others will have the opposite effect. Retarders delay setting, allowing more time for water to rise to the surface of the concrete. Yet some retarders help the concrete mixture to retain more water to reduce the amount that will bleed to the surface.

Cohesion

Retarders also impact the cohesion of concrete in diverse ways. In some cases, the extended workability that retarders create will allow for better particle interaction and dispersion in cementitious material, aggregates, and other components. This can increase homogeneity within the overall mixture. Yet some retarders increase the water content which can decrease cohesion.

Concrete Applications That Benefit from the Use of Retarders

The use of concrete retarders specifically benefits certain concrete applications.

• Hot weather projects – High temperatures reduce the workability of concrete by speeding up water evaporation. Retarders help to extend the setting time of concrete, even in hot weather, to reduce the risk of premature stiffening.
• Large-scale construction – Retarders are beneficial in large-scale construction projects where concrete pouring, forming, and finishing take a longer time. Retarders extend the workability window for large projects.
• Pumped concrete – When concrete is pumped over large distances or to higher elevations, retarders help extend workability. This reduces the risk of blockages ensuring proper consolidation.
• Complex concrete or grouting – The prolonged set time of concrete with retarders gives builders more time for complex and intricate concrete projects. This helps in projects like concrete stamping or the application of color pigments.
• Precast concrete – Retarder use is common in precast concrete applications. They help to maintain the workability of concrete during the casting process enhancing cohesion, improving detailing, and reducing the risk of cold joints in precast elements.

Incorporating Retarders into a Concrete Mixture

Builders incorporate retarders into concrete during the mixing stage, but the specific method depends on the type of retarder and the form in which they are supplied either powder, liquid, or admixture. Here are the general steps for incorporating retarders into concrete.

• Determine the dosage – It is vital to consult the manufacturer’s guidelines and/or technical sheets to determine the correct amount of retarder you need for your project. Too little of the mixture will not have the desired effect, but too much can create a prolonged setting time and delay strength gain.
• Prepare the retarder – Retarders come in a variety of forms. Power retarders need to be dissolved in water before their addition to concrete. Liquid and admixture retarders can be used directly.
• Adjust the water content – Make sure to calculate the amount of water in powder retarders in the overall water content of the cement mix. You will need to adjust the water content to ensure that it is appropriate to the amount of concrete mix.
• Add the retarder – For small amounts of retarder, you can add it directly to the dry ingredients. For large-scale mixing, it is best to incorporate the retarders
• Mix thoroughly – Once you have added the retarder, you must mix all the ingredients to ensure uniform distribution. Follow standard concrete practice for mixing duration.