Accelerating admixtures speed up cement hydration so concrete sets earlier or gains early strength faster, and they are classed as ASTM C494/C494M-24 Type C or E in the US and as set or hardening accelerators in EN 934-2:2009+A1:2012. Which chemistry, how much, and how much chloride does a mix allow?
The same products are sold as concrete accelerators, cement accelerators, set accelerators and hardening accelerators, and IS 9103:1999 in India calls them accelerating admixtures. ACI PRC-212.3-16 sorts concrete admixtures into 12 groups, and accelerating admixtures are one of them.
Calcium chloride is the reference chemistry: it is dosed at up to 2 % by mass of cementitious materials, and the UK restricts it to unreinforced concrete (Concrete Society). Non-chloride blends serve reinforced work, and ASTM C1622/C1622M-24 systems cover concrete placed as cold as -5.0 °C (23.0 °F). ACI CODE-318-25 caps water-soluble chloride at 0.06 % by mass of cementitious materials in prestressed concrete, and EN 934-2 Table 7 asks a hardening accelerator for at least 120 % of control strength at 24 hours.
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 an Accelerating Admixture?#
An accelerating admixture is a chemical admixture that accelerates setting and early strength development of concrete, and ACI PRC-212.3-16 Table 1.2 pairs it with ASTM C494/C494M Type C or E and AASHTO M 194. The same ACI table names the materials behind the class: calcium chloride (ASTM D98, AASHTO M 144), triethanolamine, sodium thiocyanate, sodium and calcium formate, sodium and calcium nitrite, calcium nitrate, aluminates and silicates.
A "concrete curing accelerator", in the sense of faster strength gain, is this admixture: it is dosed into the batch and acts on cement hydration. Curing compounds and other products applied to the surface of fresh concrete are outside the scope of this page.
Set accelerators vs hardening accelerators#
EN 934-2 separates set accelerating admixtures (Table 6) from hardening accelerating admixtures (Table 7), while ASTM C494 Type C covers both effects. EN 934-2:2009+A1:2012 defines a hardening accelerating admixture in clause 3.2.7 as one that increases the rate of early strength development, with or without affecting the setting time.
A set accelerator is tested on initial setting time at 20 °C (68 °F) and 5 °C (41 °F), a hardening accelerator on compressive strength at 24 and 48 hours. ASTM C494/C494M-24 has no separate hardening class: Type C and E products are compared with a reference concrete for both setting time and strength.
What is an accelerator used for in concrete?#
Accelerating admixtures serve 4 uses named in FHWA, NRMCA and SINTEF documents. The 4 uses are listed below.
- Earlier set in cool weather: calcium chloride compensates a temperature drop of about 15 °C (27 °F) and calcium nitrate about 10 °C (18 °F) (Rixom, in SINTEF 2007).
- Higher early strength: EN 934-2 Table 7 asks for at least 120 % of control strength at 24 hours.
- Earlier pavement opening: accelerated pavements opened within a day, and even in 4 to 6 hours (FHWA HIF-18-017).
- Faster finishing in hot weather: accelerators expedite finishing and help avoid plastic shrinkage cracking (NRMCA CIP 12).
How Do Accelerating Admixtures Work?#
Accelerating admixtures shorten the dormant period of cement hydration by speeding up the reaction of tricalcium silicate (C3S) or of tricalcium aluminate (C3A) (FHWA HIF-18-017). Inorganic salts such as chlorides, nitrates, nitrites, thiocyanates and formates act on C3S, triethanolamine acts on C3A and ettringite formation, and C-S-H seed nanoparticles add nucleation sites.
Inorganic salts and the dormant period#
Inorganic salts accelerate C3S hydration in a fixed order of efficiency (SINTEF 2007). For anions the order is Br ≈ Cl > SCN > I > NO3 > ClO4, and for cations Ca > Sr > Ba > Li > K > Na ≈ Cs > Rb. Calcium chloride pairs chloride with calcium, and both ions stand at the top of their series. Non-chloride products therefore combine salts: calcium nitrate with sodium thiocyanate gives 1-day strengths near calcium chloride levels (Justnes, in SINTEF 2007).
Accelerator efficiency can also be stated as a temperature offset. Rixom, as cited in SINTEF 2007, puts the compensation of calcium chloride at a temperature drop of about 15 °C (27 °F) and that of calcium nitrate at about 10 °C (18 °F).
Triethanolamine and C-S-H seeds#
Triethanolamine accelerates only inside a narrow window of 0.010 to 0.025 % by mass of cement; at 0.25 % it retards and at about 1 % it causes flash set (Dodson, in SINTEF 2007). That window makes triethanolamine (TEA) a component, not a stand-alone accelerator: it works inside accelerator blends and inside water reducers, where it offsets their retardation. Its target is the aluminate phase, since TEA accelerates C3A hydration and ettringite formation (FHWA HIF-18-017).
C-S-H seeds follow a third route. Nanoparticles of calcium silicate hydrate act as nucleation sites for hydration products (FHWA HIF-18-017), so they accelerate without adding a chloride, nitrate or thiocyanate anion.
Types of Accelerating Admixtures by Chemistry#
Accelerating admixtures fall into 4 types by chemistry and use: chloride-based, non-chloride, shotcrete accelerators dosed at the nozzle, and antifreeze systems for concrete as cold as -5.0 °C (23.0 °F). The table below sorts the chemistries behind those types by role, product class and hazard line.
| Chemistry group | Substances | Role in the mix | Product class | Hazard line |
|---|---|---|---|---|
| Chloride salts | calcium chloride (CAS 10043-52-4) | top of the efficiency series; setting and hardening | ASTM C494 Type C or E; EN 934-2 Table 6 or 7 | Harmonised CLP Annex VI (index 017-013-00-2): Eye Irrit. 2, H319 |
| Nitrates | calcium nitrate accelerator | acts on setting (SINTEF 2007); compensates about 10 °C (18 °F) | ASTM C494 Type C or E; EN 934-2 Table 6 or 7 | No Annex VI entry; no hazard statement in a majority of ECHA C&L notifications |
| Thiocyanates | sodium thiocyanate in non-chloride accelerators | acts on hardening (SINTEF 2007); below 1 % by mass of cement | ASTM C494 Type C or E; EN 934-2 Table 6 or 7 | Harmonised CLP Annex VI group entries 615-004-00-3 and 615-030-00-5: Acute Tox. 4* H302, H312, H332; Aquatic Chronic 3 H412 |
| Formates | calcium formate accelerator | powder; works best in undersulfated cements (C3A/SO3 above 4.0) | ASTM C494 Type C or E; EN 934-2 Table 6 or 7 | No Annex VI entry; ECHA C&L notified classification: H318 |
| Nitrites | calcium nitrite | Type C accelerator that also inhibits corrosion | ASTM C494 Type C | see the product safety data sheet |
| Alkanolamines | triethanolamine (TEA) in concrete admixtures | C3A accelerator at 0.010 to 0.025 % by mass of cement | component of Type C and E blends and of water reducers | No Annex VI entry; majority of ECHA C&L notifications: not classified |
| Aluminum sulfate (alkali-free, pH 2 to 3) | aluminum sulfate in alkali-free shotcrete accelerators | forms ettringite and Al(OH)3 at the nozzle | EN 934-5:2007 | No Annex VI entry; ECHA C&L notified classification: H318 |
| Aluminates and silicates (alkaline) | sodium aluminate (CAS 1302-42-7), sodium silicate | flash-setting action at the nozzle | EN 934-5:2007 | sodium aluminate: No Annex VI entry; ECHA C&L notified classification: H290, H314, H318 |
| C-S-H seeds | nanoparticle calcium silicate hydrate | nucleation sites | n/a | see the product safety data sheet |
Chloride-based accelerators are built on calcium chloride, the first material in the ACI PRC-212.3-16 accelerator list. NRMCA 2PE004-26 accepts chloride-based admixtures in plain concrete such as slabs on ground.
Non-chloride accelerators are formulations of formates, nitrates, nitrites and thiocyanates, and ACI E4-12 notes that some are less effective than calcium chloride. Nitrate acts mainly on setting and thiocyanate mainly on hardening (SINTEF 2007), and the Concrete Society records calcium formate in UK use since the mid-1960s and thiocyanates since the 1970s.
Shotcrete accelerators enter at the nozzle: alkali-free types are weakly acidic solutions (pH 2 to 3) mainly of aluminum sulfate that form ettringite and Al(OH)3, while alkaline types are sodium silicate or alkali aluminates with flash-setting action (SINTEF 2007). ASTM C1141/C1141M-15, the US specification for shotcrete admixtures, was withdrawn in 2024 without replacement.
Antifreeze admixtures, called cold-weather admixture systems (CWAS) in ASTM C1622/C1622M-24, combine a high-dose non-chloride accelerator with freezing-point depressants such as nitrites and nitrates and with water reducers (ACI E4-12). An FHWA demonstration protected concrete to an internal temperature of -5 °C (23 °F) and extended the construction season by up to 2 months in a northern Minnesota climate.
What Do Accelerators Do to Concrete?#
Accelerating admixtures bring setting forward and raise early strength, so accelerated pavements can open within a day and even in 4 to 6 hours (FHWA HIF-18-017). Table 2 lists each effect with its value and the kind of source behind it.
| Property | Effect | Value | Source (type) |
|---|---|---|---|
| Initial set | earlier | 1 to 2 h, depending on type, dose and temperature | Concrete Society (guidance) |
| Early strength | higher | at least 120 % of control at 20 °C (68 °F) and 24 h | EN 934-2:2009+A1:2012 Table 7 (standard) |
| Pavement opening | sooner | within a day, even 4 to 6 h | FHWA HIF-18-017 (agency) |
| Set at high dose | can extend | organic accelerators and calcium nitrate | FHWA HIF-18-017 (agency) |
| Air-void system | coarser | larger bubbles, higher spacing factor | FHWA HIF-18-017 (agency) |
| Drying shrinkage and curling, calcium chloride | higher | about +100 % early age, about +40 % later age | ACI 224R-01 citing ACI 212.3R (guidance) |
| ASTM C1202 charge passed (rapid chloride permeability test) | higher, permeability unchanged | ionic accelerators | GCP TB-0100; FHWA-HRT-05-056 (bulletin, agency) |
| Embedded aluminum | corrosion | calcium chloride at 1 % or more | ACI E4-12 (guidance) |
Earlier set and faster early strength#
Accelerators bring setting forward and lift strength in the first 2 days, the two effects that EN 934-2 tests in Tables 6 and 7. Concrete Society guidance puts the setting advance at 1 to 2 hours, depending on type, dose and temperature. The difference between initial and final set, and how each is measured, is covered under setting time of concrete.
Early strength is the measured half of the effect. EN 934-2:2009+A1:2012 Table 7 requires a hardening accelerator to reach at least 120 % of control strength at 20 °C (68 °F) and 24 hours and at least 130 % at 5 °C (41 °F) and 48 hours. A higher dose does not always mean a faster mix: high doses of organic accelerators and of calcium nitrate can extend set (FHWA HIF-18-017).
Air voids, shrinkage and test readings#
Accelerators can increase bubble size and spacing factor, so FHWA HIF-18-017 advises ASTM C457 checks where concrete is exposed to freezing. A fresh air-content test does not show that change, because spacing factor is measured on hardened concrete.
Calcium chloride raises early-age drying shrinkage by about 100 % and later-age shrinkage by about 40 % (ACI 224R-01, citing ACI 212.3R). Ionic admixtures, among them calcium chloride, calcium nitrate, calcium nitrite and sodium thiocyanate, raise the ASTM C1202-25 charge passed without changing permeability (GCP TB-0100; FHWA-HRT-05-056), so a high coulomb reading on an accelerated mix does not by itself show a permeable concrete. Air-void targets by exposure class are set under freeze-thaw damage and air voids.
Steel corrosion and embedded aluminum#
Chloride-based accelerators raise the corrosion risk of embedded steel, which is why the UK restricts calcium chloride to unreinforced concrete (Concrete Society) and NRMCA advises avoiding chloride-based admixtures in reinforced and prestressed concrete.
Embedded aluminum reacts at lower calcium chloride contents than the 2 % ceiling: 1 % or more calcium chloride may corrode uncoated aluminum conduit enough to collapse it or crack the concrete (ACI E4-12). Sodium thiocyanate at 0.75 to 1.0 % by mass of cement reduced or did not change the steel corrosion rate in the work of Nmai and Corbo, as summarized in SINTEF 2007.
Accelerating Admixture Dosage: Ranges and Dose Basis#
Accelerator dose depends on chemistry and always carries a cement or cementitious basis, from 0.010 to 0.025 % by mass of cement for triethanolamine to up to 2 % of cementitious materials for calcium chloride. NRMCA TIP 13 equates 2 % calcium chloride with about 1 % chloride by weight of cement. Liquid non-chloride products are dosed in mL/100 kg (fl oz/cwt) of cementitious material; one study mix with 474 kg/m³ (799 lb/yd³) of Type I cement used 2,086 mL/100 kg (32 fl oz/cwt) (FHWA-NJ-2001-015).
The table below compares dose values by chemistry, each with its basis and the type of source.
| Chemistry | Value | Basis | Value type (source) |
|---|---|---|---|
| Calcium chloride | up to 2 | % by mass of cementitious materials | guidance (ACI PRC-212.3-16 via FHWA HIF-18-017); NRMCA CIP 15: up to 2 % by weight of cement for non-reinforced concrete |
| Triethanolamine | 0.010 to 0.025 | % by mass of cement | literature (Dodson, in SINTEF 2007) |
| Sodium thiocyanate | 0.75 to 1.0 recommended maximum; practical maximum below 1 | % by mass of cement | literature (Nmai and Corbo, in SINTEF 2007) |
| Calcium formate | no significant extra 24 h effect above 2 | % by mass of cement | literature (Rixom, in SINTEF 2007) |
| Liquid accelerator in a high-early-strength mix | 2,086 mL/100 kg (32 fl oz/cwt) | of cement, as supplied | study (FHWA-NJ-2001-015) |
| Shotcrete accelerators | more than 5; 6 to 8 as the typical range | % by mass of cement | literature (SINTEF 2007) |
| Antifreeze blend, 3 parts NaNO3 to 1 part Na2SO4 | 6 to 8 | % by mass of cement, for strength gain at -5 °C (23 °F) | literature (Korhonen, in SINTEF 2007) |
1 fl oz/cwt = 65.2 mL/100 kg; trial batches set the final dose.
A worked example turns the calcium chloride ceiling into batch mass and chloride; its 4 steps are listed below.
- Input: cement content 356 kg/m³ (600 lb/yd³), calcium chloride at the 2 % maximum by mass of cement.
- Flake calcium chloride: 356 × 0.02 = 7.12 kg/m³ (12.0 lb/yd³).
- Chloride at about 1 % by weight of cement (NRMCA TIP 13): about 3.56 kg/m³ (6.0 lb/yd³).
- ACI CODE-318-25 cap for Exposure Class C1, 0.30 % water-soluble chloride by mass of cementitious materials: 0.30 % × 356 = 1.07 kg/m³ (1.80 lb/yd³) on the same binder.
The NRMCA figure is chloride added by the admixture and the ACI cap is water-soluble chloride in the concrete, so the two numbers are not a direct pass or fail comparison. Other doses and unit conversions run in the admixture dosage calculator.
Batching and dispensing accelerators#
ACI E4-12 adds accelerators with the mixing water and never mixes them with the air-entraining admixture. Its dispensing tolerance is 3 % of the required volume or the minimum recommended dose per unit of cement, whichever is greater.
Calcium chloride comes as flake (dihydrate, at least 77 % CaCl2), pellets (anhydrous, at least 94 %) or a solution, commonly at 29 % (ACI PRC-212.3-16, as summarized in FHWA HIF-18-017), and it is added in solution with the mixing water. Calcium formate is a powder and is batched by mass (SINTEF 2007). Handling and exposure data for each product are in the product safety data sheet.
Selecting Accelerators by Application and Climate#
Accelerators are selected by the steel in the element and by placement temperature: chloride-based products suit plain concrete, non-chloride products suit reinforced and prestressed work, and placement below freezing calls for a cold-weather admixture system. The table below matches application to accelerator choice and the rule behind it.
| Application | Accelerator choice | Rule or source |
|---|---|---|
| Plain (unreinforced) concrete | calcium chloride up to 2 % by weight of cement | NRMCA CIP 15 |
| Reinforced and prestressed concrete | non-chloride; prestressed cap 0.06 % water-soluble chloride by mass of cementitious materials | ACI CODE-318-25 Table 19.3.2.1; NRMCA 2PE004-26 |
| Cold-weather concreting | hardening accelerators, tested at 5 °C (41 °F); CWAS below freezing | EN 934-2 Table 7; ASTM C1622/C1622M-24 |
| Hot-weather concreting | to expedite finishing and avoid plastic shrinkage cracking; watch for very rapid set | NRMCA CIP 12; FHWA HIF-18-017 |
| High-early-strength concrete | liquid accelerator with a superplasticizer, strength targeted in 6 to 9 h under insulated covers | study FHWA-NJ-2001-015 |
| Admixtures for concrete pavements | opening within a day, even 4 to 6 h; check air voids by ASTM C457 | FHWA HIF-18-017 |
| Admixtures for precast concrete | hardening accelerators for early strength; prestressed chloride cap 0.06 % | EN 934-2 Table 7; ACI CODE-318-25 |
| Shotcrete mixes | nozzle accelerator, alkali-free or alkaline | SINTEF 2007 |
| Sulfate attack on concrete, Exposure Classes S2 and S3 (sulfate) | no calcium chloride admixtures | ACI CODE-318-25 Table 19.3.2.1 |
How much chloride the codes allow from an accelerator#
Three rule sets cap chloride from admixtures: ACI CODE-318-25 by exposure class, the EN 206 chloride classes and the EN 934-1:2008 admixture declaration. ACI CODE-318-25 Table 19.3.2.1, as reproduced in NRMCA 2PE004-26, limits water-soluble chloride by mass of cementitious materials to 1.00 % in Exposure Class C0, 0.30 % in C1 and 0.15 % in C2 for nonprestressed concrete, and to 0.06 % for prestressed concrete in every class. The same table does not permit calcium chloride admixtures in Exposure Classes S2 and S3.
The table below sets the chloride caps side by side, each with its edition and basis.
| Document (edition, clause) | Class | Limit |
|---|---|---|
| ACI CODE-318-25 Table 19.3.2.1 (as reproduced in NRMCA 2PE004-26) | C0 / C1 / C2 nonprestressed; prestressed | 1.00 / 0.30 / 0.15 %; 0.06 % in every class; water-soluble chloride by mass of cementitious materials |
| ACI CODE-318-25 Table 19.3.2.1 | S2 and S3 (sulfate) | calcium chloride admixtures not permitted |
| EN 206:2013+A2:2021 (superseded by EN 206-1:2026, chloride in clause 6.2.8, Table 14) | Cl 1,0 / Cl 0,40 / Cl 0,20 / Cl 0,10 | maximum chloride, % by mass of cement; Cl 1,0 for concrete without steel reinforcement |
| EN 934-1:2008 Table 1 | admixture declaration | water-soluble chloride and total chlorine at most 0.10 % by mass or not above the stated value; "chloride free" only at 0.10 % or less |
| NRMCA 2PE004-26 (2026) | guidance | specify chloride for the concrete, not for the admixture; avoid chloride-based admixtures in reinforced and prestressed concrete |
European chloride classes use a different basis, percent chloride by mass of cement. EN 206-1:2026 (CEN approval 2026-01-19) supersedes EN 206:2013+A2:2021 and places chloride content in clause 6.2.8, Table 14; its 2026 values are not reproduced because that table is not public. At admixture level, only a product at 0.10 % chloride or less may be described as "chloride free" (EN 934-1:2008). Corrosion thresholds and exposure classes are covered under chloride ingress and rebar corrosion.
Accelerators at low and high placement temperatures#
Accelerators do not stop concrete from freezing: calcium chloride at permitted doses lowers the freezing point by only a few degrees (NRMCA CIP 27). Anhydrous calcium chloride at 2 % lowers it by less than 1 °C (1.8 °F) (Dodson, in SINTEF 2007). Below freezing, protection needs a cold-weather admixture system to ASTM C1622/C1622M-24, which covers concrete as cold as -5.0 °C (23.0 °F) immediately after placement until initial set, with three levels of testing.
In hot weather the risk reverses. NRMCA CIP 12 allows accelerators in hot weather to expedite finishing and avoid plastic shrinkage cracking, while FHWA HIF-18-017 warns of very rapid set in hot weather. Accelerating compounds in some water reducers can speed the aluminate phase and cause false or early set, and FHWA notes that this is worse in hot weather and with Class C fly ash.
Which Standards Classify Accelerating Admixtures?#
Five documents classify accelerating admixtures: ASTM C494/C494M-24, EN 934-2:2009+A1:2012, IS 9103:1999, EN 934-5:2007 for sprayed concrete and ASTM C1622/C1622M-24 for cold-weather admixture systems. ASTM C494/C494M-24 is active and defines eight types, A to G and S, with Type C accelerating and Type E water-reducing and accelerating. Type E also cuts mixing water: the C494/C494M-17 and -19e1 editions set its water content at no more than 95 % of the control mix. Types C and E are not eligible for provisional compliance under ASTM C494/C494M-24.
EN 934-2:2009+A1:2012 is harmonised under CPR 305/2011, and its revision prEN 934-2 sits at CEN stage 30.98, a decision to reject. EN 934-5:2007 is cited in the Official Journal (EU summary list of 2026-02-09). IS 9103:1999, the Indian specification for concrete admixtures with five types, is active, was reaffirmed in 2018, carries two amendments and requires the manufacturer to declare chloride content (clause 1.3). NRMCA 2PE004-26 (2026) specifies accelerators as ASTM C494 Type C or E.
The table below lists each document with its edition, status and the accelerator classes it holds.
| Region | Document (edition, status) | Accelerator classes | Page |
|---|---|---|---|
| US | ASTM C494/C494M-24, active | Types C and E | ASTM C494 |
| US | AASHTO M 194 (listed with ASTM C494 in ACI PRC-212.3-16 Table 1.2) | DOT specification, Type C or E | n/a |
| US | ASTM C1622/C1622M-24, active | cold-weather admixture systems | n/a |
| US | ASTM C1141/C1141M-15 | withdrawn 2024 | n/a |
| US | ASTM D98-15(2021), active | calcium chloride, Type S solid and Type L liquid | n/a |
| EU | EN 934-2:2009+A1:2012, harmonised | Table 6 set accelerating; Table 7 hardening accelerating | EN 934-2 |
| EU | EN 934-5:2007, harmonised | admixtures for sprayed concrete | n/a |
| EU | EN 206-1:2026 | chloride content, clause 6.2.8 | EN 206 |
| India | IS 9103:1999, reaffirmed 2018, two amendments | accelerating admixture; chloride declared | IS 9103 |
| US | ACI PRC-212.3-16, 76 pages | guidance, no requirements | ACI 212.3R |
How accelerators are tested against a control mix#
EN 934-2:2009+A1:2012 tests each accelerator against a control mix and sets its limits as percentages of that control. Table 6 (set accelerating) requires an initial setting time of at least 30 minutes at 20 °C (68 °F) and at most 60 % of the control at 5 °C (41 °F), 28-day compressive strength of at least 80 % of the control, and 90-day strength at least equal to the test mix's 28-day value. Table 7 (hardening accelerating) requires at least 120 % of control strength at 20 °C (68 °F) and 24 hours, at least 90 % at 28 days and at least 130 % at 5 °C (41 °F) and 48 hours; both tables limit air content to the control plus 2 % by volume.
ASTM C494/C494M-24 compares setting time and strength with a reference concrete. Its numeric Type C and Type E limits are not reproduced: the 2024 Table 1 has not been verified against the published text. The same applies to IS 9103 Table 1A (no verified copy), EN 934-5:2007 and ASTM C1622/C1622M-24 Table 1 (requirement text not obtained).
Complete List of Accelerating Admixture Types#
Accelerating admixtures come in 4 types, chloride-based, non-chloride, shotcrete and antifreeze, and each type has its own page with doses, standards and safety data. The table below lists the 4 types with chemistry, standard class and dose basis.
| # | Type | Chemistry | Standard class | Dose basis | Page |
|---|---|---|---|---|---|
| 1 | Chloride-based | calcium chloride flake, pellets or solution | ASTM C494 Type C or E; EN 934-2 Table 6 or 7 | up to 2 % by mass of cementitious materials | calcium chloride in concrete |
| 2 | Non-chloride | nitrates, nitrites, thiocyanates, formates, alkanolamines | ASTM C494 Type C or E; EN 934-2 Table 6 or 7 | mL/100 kg (fl oz/cwt) of cementitious material | non-chloride accelerators |
| 3 | Shotcrete | aluminum sulfate (alkali-free); aluminates, silicates (alkaline) | EN 934-5:2007; ASTM C1141 withdrawn 2024 | % by mass of cement at the nozzle | shotcrete accelerators |
| 4 | Antifreeze (CWAS) | high-dose non-chloride accelerator plus freezing-point depressants | ASTM C1622/C1622M-24 | % by mass of cement | antifreeze and cold-weather admixtures |
A Tennessee DOT research report lists an accelerator input price of USD 2.31 per L (USD 8.75 per gal), as of April 2025, for a UHPC mix study (TDOT RES2024-04, Table 4-16). Supplier quotes compare on the specification, the chloride declaration and the data sheet.
What Are the Disadvantages of Accelerators in Concrete?#
Accelerators carry 5 documented drawbacks: corrosion risk from chloride, higher drying shrinkage with calcium chloride, inflated ASTM C1202 readings, a coarser air-void system and very rapid set in hot weather. Calcium chloride also causes permanent mottled discoloration that increases with dose (FHWA HIF-18-017).
Triethanolamine dosed at the cement mill as a grinding aid and calcium chloride used for de-icing are other industries' uses and not covered here.
Accelerator vs retarder: what is the difference?#
ASTM C494 Type C accelerates and Type B retards; Type E adds water reduction to acceleration and Type D adds it to retardation. Hot-weather placements and long hauls are the main work of set-retarding admixtures.
What is the most commonly used accelerator in cement?#
Calcium chloride is the reference accelerator: chloride stands at the top of the anion efficiency series with bromide (SINTEF 2007), and ACI PRC-212.3-16 lists calcium chloride first among accelerator materials. Chloride caps move reinforced work to non-chloride products.
Is an accelerator an antifreeze?#
No: normal accelerator doses lower the freezing point by only a few degrees (NRMCA CIP 27), and protection to -5.0 °C (23.0 °F) needs an ASTM C1622/C1622M-24 cold-weather admixture system. Such systems run at high doses, such as 6 to 8 % by mass of cement for the Korhonen blend (SINTEF 2007).
Can accelerators be combined with water reducers?#
Yes: ASTM C494 Type E is a water-reducing and accelerating admixture, and triethanolamine is used in water reducers to offset their retardation. Normal, mid-range and high-range types are covered under water-reducing admixtures.