Introduction
Retarders are widely used in various industries, and construction is one of the most common application fields. This article will introduce the common types of retarders used in concrete, cement mortar, and gypsum-based materials. We will also share some practical tips on the selection and application of retarder varieties.
What Is a Retarder?
A retarder is an additive that reduces the hydration rate and heat of hydration of cement or gypsum and prolongs the setting time.
In concrete, cement mortars, and gypsum-based construction materials, you can find the applications of retarders. The working principles of retarders for different materials are not all the same.
In concrete and cement mortars, retarders interact with cement particles directly. So it can delay the reaction between water and cement minerals, slowing down the hydration process.
In gypsum-based products, retarders can control the dissolution and crystallization process of gypsum. Thus, retarders can decrease the speed of crystal formation and extend the setting time.
By controlling the setting process, retarders have multifunctions during transportation, storage, and construction:
- Improve workability
- Extend application time
- Provide better flexibility
Classification of Retarders
Retarders can be classified according to various criteria. The most common method is based on chemical composition. We will categorize them separately based on their specific material applications.
Types of Retarders in Concrete & Cement-Based Materials
For cement-based materials, according to chemical composition, retarders can be divided into two categories: organic retarders and inorganic salt retarders.
1. Organic Retarders
The organic retarder is a more widely used category of retarder. Common varieties include hydroxyl carboxylic acids and their salts (such as tartaric acid, sodium tartrate, potassium tartrate, citric acid, etc., of which natural tartaric acid has the best retarding effects), polyhydric alcohols and their derivatives, sugars (glucose, calcium, gluconates, etc.), and other carbohydrates.
Most organic retarders typically contain hydrophilic active groups and also provide water-reducing effects. Therefore, they are also known as retarding water-reducing agents.
Advantages:
- Good compatibility with cement-based systems
- Effective retarding performance at a relatively low dosage
- Some organic retarders can also provide water-reducing effects, improving workability
Limitations:
- Overdosage may excessively delay setting and affect early strength development
- The retarding effect may vary depending on cement type and formulation conditions
2. Inorganic Salt Retarders
Inorganic salt retarders include borax, zinc chloride, zinc carbonate, and sulfates, phosphates, and metaphosphates of iron, copper, and zinc.
Advantages:
- Stable retarding performance
- Effective setting time control
- Suitable for specific cement systems requiring strong retarding effects
Limitations:
- Compatibility with other admixtures may require careful evaluation
- Excessive dosage may negatively affect early strength or final performance
As concrete belongs to cement-based materials, the above common varieties also apply to concrete retarders.
Gypsum Retarder Types
Gypsum retarders work by a different mechanism than cement retarders. They can be divided into several common types as follows:
1. Organic Acid and Salt Retarders
Organic acids and their salts are among the most commonly used gypsum retarders.
Typical examples include:
- Citric acid
- Sodium citrate
- Tartaric acid
- Potassium tartrate
They mainly work by interacting with calcium ions and gypsum particles, slowing down crystal formation and extending setting time.
Advantages:
- Strong retarding effect
- Effective at low dosage
- Good setting time adjustment
However, excessive dosage may negatively affect gypsum strength development.
2. Protein-Based Retarders
Protein-based retarders include:
- Animal glue
- Gelatin
- Modified protein derivatives
These retarders work mainly by adsorbing onto gypsum crystal surfaces and limiting crystal growth.
Advantages:
- Stable retarding performance
- Relatively small impact on final strength
- Good compatibility with gypsum formulations
They are commonly used in gypsum plaster and other building gypsum products.
3. Amino Acid-Based Retarders
Amino acid-based retarders are considered a higher-performance type of gypsum retarder.
They combine the effects of organic complexation and surface adsorption, allowing more precise control of gypsum setting behavior.
Advantages:
- Accurate setting time adjustment
- Lower impact on strength
- Good formulation flexibility
The main limitation is the relatively higher cost compared with conventional retarders.
4. Phosphate-Based Retarders
Common examples of Phosphate compounds are:
- Sodium hexametaphosphate
- Polyphosphates
They delay gypsum crystal growth by interacting with calcium ions during the setting process.
Advantages:
- Stable retarding effect
- Good setting control
However, excessive use may affect early strength development.
5. Sugar-Based Retarders
Sugar-based compounds, such as glucose and sucrose, can also be used to control gypsum setting.
They work by adsorbing onto gypsum particles and slowing down crystallization.
Advantages:
- Mild retarding effect
- Low cost
Because their effect can increase significantly at higher dosages, accurate dosage control is important.
Why Are Retarders Added into Wet-Mixed Mortar?
Retarders can delay the hydration and hardening of cement, allowing fresh mortar to maintain plasticity for a longer time and improving construction workability.
Wet-mixed mortar is transported to the designated site by mixing trucks after metering and mixing in a professional plant, and then stored in special containers for use when required.
At present, wet-mixed mortar is mostly supplied by concrete mixing stations. Compared with concrete, the quantity of mortar is relatively smaller. Mixing stations usually produce mortar in concentrated batches and transport it to the site in large volumes.
However, most building mortar construction is still manual and relatively slow. The mortar is not consumed immediately and needs to be stored for longer periods, sometimes even for one day and night. This requires the mortar to maintain workability without premature setting.
Therefore, retarders are commonly added to wet-mixed mortar to adjust setting time based on construction conditions, weather, transportation time, and other factors.
Why Special Retarders Are Used in Wet-Mixed Mortar?
In ordinary cement mortar or cement-lime mixed mortar, the cement is generally ordinary Portland cement. Its initial setting time is about 2 hours, and its final setting time is about 3–4 hours, so the overall workable period is around 3–8 hours.
Wet-mixed mortar is characterized by high batch production capacity. However, construction on site is usually manual and relatively slow. Therefore, the mortar cannot be used up quickly after delivery and must be stored in closed containers, where it will be gradually consumed within the required time frame.
Because of this, special retarders are needed to extend the operable time of wet-mixed mortar.
Depending on project requirements, the setting time can be adjusted to 8 hours, 12 hours, or 24 hours.
Practical Considerations in Retarder Selection
Retarder selection depends on the material system and application requirements. Extending setting time is only one consideration. The compatibility, workability, and final performance of the material should also be evaluated.
For Cement-Based Materials
Important factors include:
- Setting time requirement
- Compatibility with water-reducing admixtures
- Impact on bleeding and segregation
- Early strength development
- Storage and transportation stability
For Gypsum-Based Products
Important factors include:
- Required working time
- Setting consistency
- Surface finishing performance
- Compatibility with other additives, such as cellulose ethers
- Effect on final strength
In practice, stable, predictable performance is more important than maximum retarding strength.
Retarders Applications
Retarders are widely used in different construction materials. You can use them for the following applications.
Cement and Concrete Retarder Applications
- Ready-mix concrete
- Cement mortar
- Wet-mixed mortar
- Hot weather construction
Gypsum Retarder Applications
- Gypsum plaster
- Gypsum bonding compound
- Gypsum putty
- Gypsum precast
- Gypsum filler
- Gypsum modeling
- Gypsum-based decorative coating
FAQS
How Do I Choose Between Different Types of Cement Retarders?
Choose based on construction time, temperature, and cement compatibility.
Can Retarders Be Used with Water Reducers?
Yes, but compatibility testing is required before use.
What Happens If a Retarder Is Overdosed?
It may delay setting excessively and affect early strength development.
Does a Retarder Affect Final Strength?
At a proper dosage, final strength is not affected, but early strength development may slow.
Why Are Organic Retarders More Commonly Used?
Because they provide better compatibility and more stable performance in modern mortar systems.
Can Concrete Retarders Be Used in Gypsum Products?
No. You should choose retarders according to the binder system because cement and gypsum have different setting mechanisms.
Gypsum Retarder Supplier
Founded in 2005, Celotech gradually established a branch company and R&D centers, etc. Celotech set up the production site in Shandong, China, and is headquartered in Suzhou, Jiangsu province, a city very near Shanghai.
Our production annual capacity is
40,000 tons of CELOPRO® HPMC
5,000 tons of Polycarboxylate Superplasticizer
3,000 tons of Gypstar® Gypsum Retarder
15,000 tons of Prestar® Pregelatinized Starch, etc.
With over 20 years of industry experience, Celotech supplies many customers in the construction industry worldwide. Celotech can continuously accumulate professional knowledge and innovate and improve in technology, products, or services.
In the field of gypsum retarders, our brand, Gypstar® Gypsum Retarder, can significantly improve the efficiency while at the same time save cost and give flexibility to gypsum bonding agent or gypsum plaster by prolonging the open time.
