Set-Retarding Admixtures for Concrete: Chemistry, Dosage, Hot Weather and Long Hauls

Set-retarding admixtures delay the initial and final set of concrete: under EN 934-2:2009+A1:2012 Table 8, a conforming retarder delays initial set by at least 90 minutes and final set by no more than 360 minutes against a control mix. Which chemistry, how much and which class does a hot or long-haul job need?

The same products are sold as retarders, set retarders, retarding admixtures and cement retarders, and the European water-reducing version is the retarding plasticizer. Surface retarders sprayed on fresh concrete for exposed-aggregate finishes and evaporation retarders are surface-applied products and are not covered here. ACI PRC-212.3-16 sorts concrete admixtures into 12 groups, and set-retarding and extended set-control admixtures are two of them.

Retarders slow set by adsorbing on early hydration products, and they come from 4 chemistry groups: lignosulfonates, hydroxycarboxylic acids such as sodium gluconate, sugars and the phosphorus-containing salts of extended set-control products. Data sheets give typical product ranges of 130 to 650 mL/100 kg (2 to 10 fl oz/cwt) of cementitious material for normal-range Type A and D products and 0.15 to 2.0 % by mass of cement for retarding and extended-set products, and EN 934-2 caps any admixture at 5 % by mass of the cement. ASTM C494/C494M-24 classes them as Types B, D and G, with Type D at most 95 % and Type G at most 88 % of the control water (C494/C494M-17 and -19e1 Table 1). In practice set can be delayed 12 hours or more (BRMCA, July 2020), while ACI SPEC-301-20 still caps concrete as delivered at 35 °C (95 °F).

Each value is traced to its source: standards and codes to their issuing body, product values to the data sheet named with them, prices to the dated source beside them (methodology and fact-checking).

What Is a Concrete Retarder?#

A concrete retarder is a set-retarding admixture that keeps concrete plastic longer by delaying its initial and final set. ACI PRC-212.3-16 Table 1.2 lists set-retarding admixtures under ASTM C494/C494M Type D and AASHTO M 194 as reducing water content by at least 5 % while delaying set, with the same materials as water-reducing admixtures.

The 76-page report (2016) keeps extended set-control (hydration control) admixtures, which stop hydration rather than slow it, in a group of their own. Set retarding is far older: ACI E4-12 records that the Mayans used bark extracts and other substances as set retarders for stucco.

Retarder, retarding admixture and cement retarder: names by region#

Retarder, set retarder, retarding admixture and cement retarder name the same admixture class in concrete. The regional names follow the specification that classifies the product, as the three markets below show.

• US: retarder, Type B, D or G (ASTM C494; AASHTO M 194).
• UK and EU: set retarding admixture (EN 934-2 Table 8), retarding plasticiser (Table 10), retarding superplasticiser (Tables 11.1/11.2).
• India: retarding admixture (IS 9103).

What can you add to concrete to make it set slower?#

A set-retarding admixture that conforms to ASTM C494 Type B, D or G or to EN 934-2 Table 8 or 10 is the controlled way to slow the set of concrete. NRMCA's 2026 guide specification (2PE004-26, March 2026, 2.14) assigns Type B or D to retarders and extended set retarders, so a specification can name the class instead of a product.

Sugar also slows set, but not in a controlled way. Sugar contamination, for example from sugar bags or molasses containers, can delay or prevent setting, and in a cement paste study the response to the sugar dose was not monotonic (Al Khafaji and Ruddock, Liverpool John Moores University, 2018). Commercial carbohydrate retarders are formulated products tested to ASTM C494.

How Do Retarders Delay the Set of Concrete?#

Retarders delay set by adsorbing on early hydration products, notably the hydrates of tricalcium aluminate (C3A), and on calcium hydroxide and C-S-H nuclei, which slows their growth and lengthens the dormant period (ACI E4-12; FHWA HIF-18-017). The dormant period is the quiet stage before the main heat of hydration, and a longer dormant period moves both initial and final set later.

Adsorption on early hydration products#

Adsorption is the working step: the retarder attaches to the first hydrates and nuclei and poisons their growth (ACI E4-12; FHWA HIF-18-017).

Sugars are the studied case of this poisoning. Sugars adsorb onto and poison hydration-product nuclei, and the sugar molecule bonded to calcium fits lattice sites in Ca(OH)2 or C-S-H (Thomas and Birchall, Cement and Concrete Research 13, 1983).

Why cement C3A content and addition timing change the effect#

Cements with more C3A need more retarder, and adding the retarder after the other materials are mixed increases retardation (ACI E4-12).

ACI E4-12 also notes that high doses can cause rapid stiffening with some cements, the opposite of the intended delay, so a new cement source calls for a trial batch before the dose is raised.

Types of Concrete Retarders by Chemistry#

Concrete retarders come from 4 chemistry groups: lignosulfonates, hydroxycarboxylic acids such as sodium gluconate, carbohydrates such as sugars, and the carboxylic acids and phosphorus-containing organic acid salts of extended set-control products. ACI PRC-212.3-16 Table 1.2 refers set-retarding admixtures to the water-reducing materials, which include lignosulfonic acids and their salts, hydroxylated carboxylic acids and their salts and polysaccharides, and it names carboxylic acids and phosphorus-containing organic acid salts for extended set control.

Table 1 compares the 4 chemistry groups by example, ACI group, role in concrete and substance hazard line.

Formulated retarders: see the product safety data sheet. The lines above belong to the substances, not to any product.

Lignosulfonate and hydroxycarboxylic acid retarders#

Lignosulfonates and hydroxycarboxylic acids both retard set and reduce water, and they serve in Type B retarders and in Type D water-reducing and retarding admixtures (EN 934-2 Table 8 or 10). The water reduction of lignin-based retarders is measured: in HRB Bulletin 310 (1962), lignosulfonate retarders reduced water by 2.8 to 11.8 %.

Sodium gluconate is a hydroxycarboxylic set retarder and a water-reducing component in Type B/D and hydration-control blends. Both chemistries reach the plant as diluted solutions: the EFCA model environmental product declaration gives maximum solids in the product of 40 % for lignosulphonate and 35 % for Na-gluconate.

Carbohydrates and long-acting retarders that stop hydration#

Sugars retard strongly at low doses, and extended set-control admixtures go further: they stop or severely retard cement hydration (ACI PRC-212.3-16 Table 1.2). In the Liverpool John Moores paste study (Al Khafaji and Ruddock, 2018, w/c 0.35), sugar at 0.5 and 1.5 % by mass of cement retarded final set, while 2.5 and 5 % of cane sugars accelerated it and beet sugar still retarded. Sugar contamination can delay or prevent setting, which is why sugar in concrete is treated as its own topic.

Extended set-control (hydration control) admixtures are declared as ASTM C494 Type B or D and are used in wash water, in returned concrete and for long hauls (ACI PRC-212.3-16 Table 1.2). Stabilized wash water then qualifies as mixing water under ASTM C1602/C1602M-22 when its 7-day strength reaches at least 90 % of the control and set falls no more than 1:00 earlier or 1:30 later.

What Do Retarders Do to Fresh and Hardened Concrete?#

A retarder delays initial set by at least 90 minutes and final set by no more than 360 minutes against a control mix under EN 934-2:2009+A1:2012 Table 8. It also trades some strength in the first 1 to 3 days for later strength that is often higher (FHWA HIF-18-017).

Table 2 lists each effect with its value and the type of source behind it.

How far a retarder delays initial and final set#

The European conformity window sets both a minimum delay (90 minutes for initial set) and a maximum one (360 minutes for final set), measured on mortar to EN 480-2:2006.

Field delays depend on dose, cement and temperature, and in practice set can be delayed 12 hours or more with retarding admixtures (BRMCA, July 2020). Concrete set is measured by penetration resistance under ASTM C403/C403M-23, and testing of initial and final set is covered on its own page.

Early strength and later strength#

Retarded concrete gains strength more slowly in the first 1 to 3 days, and its later strength is often higher (FHWA HIF-18-017).

EN 934-2:2009+A1:2012 puts a floor under the loss: at least 80 % of control strength at 7 days and 90 % at 28 days for a Table 8 retarder, and at least 100 % at 28 days for a Table 10 retarding plasticizer.

Water reduction and entrained air in lignin-based retarders#

Lignosulfonate retarders reduce water and can entrain air: in HRB Bulletin 310 (1962) they reduced water by 2.8 to 11.8 %, and 7 of 19 entrained more than 5 % air without any air-entraining agent. EN 934-2:2009+A1:2012 Table 8 limits a conforming retarder to no more than 2 % air by volume above the control, and controlled air voids are the job of air-entraining admixtures.

Retarder Dosage: Ranges, Basis and a Worked Batch#

Retarders are dosed by volume per mass of cementitious material or by percentage of cement mass, as supplied. Two US data sheets give a typical product range of 130 to 650 mL/100 kg (2 to 10 fl oz/cwt) of cementitious material for normal-range Type A and D products, and two data sheets give 0.15 to 2.0 % by mass of cement for retarding and extended-set products. EN 934-2:2009+A1:2012 (3.2.1) caps any admixture at not more than 5 % by mass of the cement content.

Table 3 gives each value with its basis and value type.

1 fl oz/cwt = 65.2 mL/100 kg. The final dose is set by trial batches with job materials at job temperature.

A worked example shows how the dose basis becomes a batch quantity. Its inputs are a mix with 350 kg/m³ (590 lb/yd³) of cementitious material and a dose of 196 mL/100 kg (3.0 fl oz/cwt), which sits inside the typical product range.

1. Retarder per cubic meter: 3.50 × 196 = 686 mL per m³ (5.90 cwt × 3.0 = 17.7 fl oz per yd³).
2. ACI E4-12 dispensing tolerance: 3 % of 686 mL = 21 mL per m³ (0.53 fl oz per yd³), unless the minimum recommended dose per unit of cement is larger.
3. Trial batches then move the dose until the set delay the job needs is reached.

What changes the retarder dose#

Four factors change the dose needed for a given delay. Each one comes from ACI E4-12 or FHWA HIF-18-017, as listed below.

• Cement C3A content: cements with more C3A need more retarder (ACI E4-12).
• Addition timing: adding the retarder after the other materials are mixed increases retardation (ACI E4-12).
• Other set-modifying admixtures: a Type A or D water reducer plus a separate retarder, or a high-range water reducer plus a retarder, compounds retardation (ACI E4-12).
• Fly ash and concrete temperature: lignosulfonate-based Type D water reducers with certain Class C fly ashes can cause early stiffening in hot weather (FHWA HIF-18-017).

Trial batches with job materials at job temperature fix the dose, and the arithmetic from mL/100 kg or fl oz/cwt to batch quantities for other doses and units runs in the admixture dosage calculator.

Batching and dispensing retarders#

ACI E4-12 sets a dispensing tolerance of 3 % of the required volume or the minimum recommended dose per unit of cement, whichever is greater. The same document adds accelerators with the mixing water and keeps them apart from the air-entraining admixture.

Liquid retarders ship in drums of 208 L (55 gal), IBC totes of 1,041 L (275 gal) and bulk. Formulated products carry their own classification, so the hazard information comes from the product safety data sheet, not from the substance lines in Table 1. ASTM C494/C494M adds a caveat for the concrete itself: fresh concrete is caustic and can cause chemical burns.

Selecting a Retarder by Application and Climate#

Retarders are used where concrete must stay plastic longer than the cement allows: at high concrete temperatures, on long hauls and delays, in large placements and to hold wash water and returned concrete.

Table 4 matches each application with the retarder's role and its source.

Retarders at high concrete temperatures#

A retarder offsets the faster set of hot concrete but does not lift the 35 °C (95 °F) delivery cap of ACI SPEC-301-20 4.2.2.5(b), and ACI 305R-20 remains the hot-weather guide. ACI 305R-20 is sold as ACI PRC-305-20 (28 pp., published September 2020), and the matching specification is ACI SPEC-305.1-14(20). Mix design gives a second lever beside the retarder: NRMCA CIP 12 states that fly ash and slag cement reduce hot-weather problems.

Hot weather also exposes one known incompatibility. FHWA HIF-18-017 reports that lignosulfonate-based Type D water reducers with certain Class C fly ashes can cause early stiffening in hot weather, and it lists 4 remedies: verify the material sources, change the admixture, cool the mix and delay the addition.

Long hauls, delays and returned loads#

Long hauls are a named use of retarders: ACI PRC-212.3-16 Table 1.2 lists extended set-control admixtures for long hauls, wash water and returned concrete. ACI E4-12 (4.3.2) adds that hydration-controlling admixtures stabilize truck wash water and returned plastic concrete, where an accelerator may restart hydration, and that stabilized wash water can be reused the next day or after a weekend without adverse effect on setting and strength.

NRMCA's guide to improving specifications advises against delivery time limits more restrictive than ASTM C94/C94M and points to water-reducing, workability-retaining and retarding admixtures and temperature reduction instead; ASTM C94/C94M-26c is the active edition. For self-compacting mixes, hot weather, long hauls and delays reduce flow (NRMCA CIP 37).

When a retarder is the wrong tool#

Three cases call for something other than more retarder: large placements where peak temperature is the problem, mixes that already contain a retarding water reducer, and mixes whose slump must be held at equal set. PCA EB547 states that retarders have little to no effect on peak temperature in large placements, and NRMCA CIP 12 names low-heat blended cements, fly ash and slag cement among the hot-weather levers instead. ACI E4-12 warns that a separate retarder added to a Type A or D water reducer compounds retardation. Slump retention without a set delay is the field of the workability-retaining admixtures that NRMCA's 2026 guide specification places under ASTM C494 Type S.

Which Standards Classify Set-Retarding Admixtures?#

Three product standards classify set-retarding admixtures: ASTM C494/C494M-24 in the US, EN 934-2:2009+A1:2012 in Europe and IS 9103:1999 in India, while ACI PRC-212.3-16 is a report that groups them.

IS 9103:1999 is the Indian specification for concrete admixtures, with five types: accelerating, retarding, water-reducing, air-entraining and superplasticizing admixtures. It is active, reaffirmed in 2018, with two amendments (BIS), and the chloride content is declared. Its retarding requirement values are not reproduced here because no verified copy of Table 1A is available. In the US, ASTM C1017/C1017M-13e1, the flowing-concrete specification with a Type II plasticizing and retarding class, was withdrawn in 2022 without replacement.

Table 5 sets the documents side by side with edition, status and retarder content.

Types B, D and G under the US specification#

ASTM C494/C494M-24 has 3 retarding types: Type B retarding, Type D water-reducing and retarding, and Type G water-reducing, high range and retarding. The standard is active (ASTM store, updated 2024-08-06) and covers eight types, A to G and S. Type D must cut water to at most 95 % of the reference mix and Type G to at most 88 % (ASTM C494/C494M-17 and -19e1 Table 1), the same 5 % and 12 % thresholds as ACI PRC-212.3-16.

Type B stands apart in one rule. Under C494/C494M-24 (1.3.1), only Types A, D, F and G may qualify provisionally, Type B may not, and provisional compliance is withdrawn if 6-month strength falls below 100 % of the reference. ACI PRC-212.3-16 Table 1.2 lists AASHTO M 194 with ASTM C494 for the set-retarding group; its edition is not stated there.

Retarder conformity tables in the European standard#

EN 934-2:2009+A1:2012 sets retarder conformity in Table 8 and the retarding plasticizer in Table 10, both against a control mix and with setting measured on mortar to EN 480-2:2006. Table 8 sets the 90-minute and 360-minute setting window, the 80 % (7 days) and 90 % (28 days) strength floors and the 2 % air cap described above. Table 10 keeps the same setting window and asks for at least 100 % of control strength at 28 days; Tables 11.1 and 11.2 cover the retarding superplasticizer, and their values are not reproduced because the tables are not in the public preview.

EN 934-2:2009+A1:2012 was published on 2012-06-12 and is harmonised under the Construction Products Regulation 305/2011. CEN lists it at stage 90.93 after its review, and the revision prEN 934-2 stopped at stage 30.98 and was not published. Retarders for ready-to-use mortar sit in a separate part, EN 934-3:2009+A1:2012 Table 3.

Complete List of Set-Retarding Admixture Types#

The 7 retarding admixture types below cover the 3 ASTM C494 retarding types, the extended set-control group of ACI PRC-212.3-16, 2 chemistry families with their own pages and the long-term retarding mortar admixture of EN 934-3:2009+A1:2012. Table 6 gives each type with its ASTM type, EN table and role.

Rows 1 to 4 and 7 are performance classes; rows 5 and 6 are chemistry families with their own pages.

Quotes compare cleanly when each offer names the declared ASTM C494 type or EN 934-2 table, the dose basis (mL/100 kg or % by mass of cement, as supplied) and the solids content.

What Are the Downsides of Using Retarders in Concrete?#

Retarders cost some early strength, can compound with other admixtures into excessive retardation and can stiffen some mixes early instead of delaying them. The sources are FHWA HIF-18-017 for the 1- to 3-day strength loss and the early stiffening with certain Class C fly ashes, and ACI E4-12 for compounding.

Trial batches with job materials at job temperature show these effects before the first pour. Retarders for oil-well cement slurries are a non-concrete use not covered here.

What happens if you put too much retarder in concrete?#

Too much retarder can do the opposite of what the dose was meant to do: high doses can cause rapid stiffening with some cements (ACI E4-12). Sugar shows the same reversal: in one paste study, 2.5 and 5 % of cane sugars by mass of cement accelerated final set while 0.5 and 1.5 % retarded it (Al Khafaji and Ruddock, 2018). BRMCA (July 2020) notes that with retarding admixtures set can be delayed 12 hours or more.

Can a retarder be combined with a superplasticizer?#

Yes, but the two compound: ACI E4-12 warns that a high-range water reducer plus a retarder can cause excessive retardation. ASTM C494/C494M-24 provides a combined Type G for water-reducing, high-range and retarding admixtures, which must cut water to at most 88 % of the reference mix (C494/C494M-17 and -19e1 Table 1). In Europe the equivalent class is the retarding superplasticizer of EN 934-2 Tables 11.1 and 11.2, whose values are not reproduced because the tables are not in the public preview.