Can calcium nitrate be used as a concrete accelerator or antifreeze admixture?
Yes. Calcium nitrate is a chloride-free set accelerator for Portland cement and is used in cold-weather concreting.[1][4][15] In a Lithuanian study on CEM I 42.5 R concrete, doses of 0.5–3% by cement mass shortened initial setting time at +20 °C from 149 minutes to 120 minutes (1%), 105 minutes (2%) and 45 minutes (3%).[1] The same study found the best 2-day strength gains at 1% (+34% at +5 °C) and, at −5 °C, 3% gave about 2.5 times the 28-day strength of untreated concrete cured the same way.[1] It is mainly a set accelerator, not a strong hardening accelerator at 1 day and 20 °C.[4] It does not prevent freezing and the optimum dose must be found by trial batches.[1][19]
Key facts at a glance
| Item | Value / answer |
|---|---|
| Admixture class | ASTM C494 Type C (accelerating) or Type E (water-reducing and accelerating); ACI 212.3R lists calcium nitrate among accelerating materials[10][17] |
| Tested dose range (lab) | 0.5–3% calcium nitrate by weight of Portland cement in pastes and concrete[1] |
| Set time at +20 °C (paste) | Initial set 149 min (none), 120 min (1%), 105 min (2%), 45 min (3%)[1] |
| Set time at +5 °C (paste) | Initial set 285 min with no admixture; reduced 21% (1%), 58% (2%), 81% (3%)[1] |
| Set time at 0 °C (paste) | Initial set 318 min with no admixture; reduced 34% (1%), 56% (2%), 79% (3%)[1] |
| 2-day strength gain, +5 °C curing | +34.1% at 1% dose; only +11.4% at 28 days[1] |
| Strength gain at +20 °C (1% dose) | +8.6% at 2 days, +12.9% at 7 days, +6.2% at 28 days[1] |
| Not a 1-day hardening accelerator at 20 °C | Described as a set accelerator; very early strength rise is a side effect of earlier hydration[4] |
| Lowest tested temperature where it worked alone | At −10 °C no dose (0.5–3%) reached the 3.5 MPa frost-safe strength in 2 days[1] |
| Frost-safe strength reference | Protect from freezing until about 500 psi (3.5 MPa), roughly two days after placement[19] |
| Dose cited for corrosion inhibition | 3–4% by cement mass (long-term tests); evidence is mixed[5][6] |
| Chloride-free | Calcium chloride is discouraged or prohibited in many countries because chloride causes pitting corrosion of rebar[21] |
| Compound identity | Ca(NO3)2[22] |
How does calcium nitrate accelerate cement hydration?
Cement set is controlled mainly by two clinker phases. Alite (C3S) dissolves and releases Ca2+ and OH−; when the pore solution becomes supersaturated in calcium hydroxide (portlandite) it crystallises and triggers a burst of renewed alite hydration. The aluminate phase (C3A) reacts with gypsum to form ettringite and later converts to monosulfate (AFm).[4]
- C3S side. Calcium nitrate dissolves quickly and adds Ca2+ to the mixing water, so less alite has to dissolve before calcium hydroxide supersaturation and crystallisation begin. This shortens the induction period and the setting time.[4] A calorimetry study measured a control paste induction period of about 3 hours, falling to about 2 hours (1%), 1 hour (2%) and almost none (3%).[1]
- C3A side. Nitrate ions react with aluminate to form nitrate-AFm in addition to ettringite at early age, and the extra hydrates fill pores.[3] A cement containing C3A is significantly affected by calcium nitrate.[1]
- Side effect on heat. In an insulated paste test the peak hydration temperature fell from 91.9 °C (control) to 67.9 °C at 3% dose, so reaction starts earlier but peaks lower.[1]
What dosage of calcium nitrate is used in concrete (% by cement mass)?
Published laboratory work spans 0.5–3% by cement mass for set acceleration.[1][2] There is no single “standard” dose: the best value depends on cement, temperature and the target. In the Lithuanian concrete series (310 kg/m3 CEM I 42.5 R, w/c 0.55, polycarboxylate superplasticizer) the strength-optimal dose fell as temperature rose.[1]
| Initial curing temperature | Best dose for strength | Strength gain vs no admixture |
|---|---|---|
| +20 °C | 1% | +8.6% (2 d), +12.9% (7 d), +6.2% (28 d)[1] |
| +5 °C | 1% | +34.1% (2 d), +11.4% (28 d)[1] |
| 0 °C | 0.5% | +45.6% (2 d), +8.7% (28 d)[1] |
| −5 °C | 3% | about 2.8× (7 d), 2.5× (28 d) vs untreated frozen-then-cured specimens[1] |
| −10 °C | 3% (after water curing) | 1.8× at 28 d; 3.5 MPa not reached at 2 d[1] |
A separate white Portland cement study (0, 1, 2, 3% dose) found the 1% dose cut initial setting time to 76% of the 275-minute control and raised 28-day strength from 35 to 44 MPa, while higher doses lowered 28-day strength.[2] Two practical points follow: more is not always better, and the 1-day and 28-day optima differ. Regulatory-style performance limits, not a dose, define a commercial accelerator: EN 934-2 requires at least 120% of reference strength at 1 day (20 °C) and 130% at 2 days (5 °C), with initial set at least 30 minutes at 20 °C and at most 60% of the reference at 5 °C.[1][18]
How much does it change set time and strength at 1, 3, 7 and 28 days?
- 1 day. At 20 °C calcium nitrate is not a hardening accelerator. In slag-blended mortar it gave lower 1-day strength at 20 °C, though concrete cured at +5 °C had higher 1-day strength.[4]
- 2 days. Gains are largest here and largest at cool temperatures (+34% at +5 °C, +46% at 0 °C in the concrete series).[1]
- 3 days. I found no sourced Portland-cement concrete data for exactly 3 days; slag-blended mortar showed higher strength from day 3 onwards.[4]
- 7 days. +12.9% at 1% and 20 °C;[1] white-cement specimens rose from 29 MPa to about 37–38 MPa at 1–2%.[2]
- 28 days. Gains shrink (+6% to +11% in the concrete series) because calcium nitrate alone has little effect on long-term strength.[1] Some work reports a longer-term benefit for CEM I.[1][4]
How does calcium nitrate compare with other concrete accelerators?
| Accelerator | Chloride-free? | What the sources say |
|---|---|---|
| Calcium chloride (CaCl2) | No | Most efficient and cheapest; NRMCA caps it at 2% by cement weight and advises non-chloride accelerators for prestressed concrete or corrosion-sensitive work. 2% flake CaCl2 contributes about 0.98% chloride ion by cement mass against ACI 318-08 limits of 1.00% (dry), 0.30% (moist) and 0.06% (prestressed).[19][20][21] |
| Calcium nitrate | Yes | Mainly a set accelerator; less efficient than CaCl2. A paste study found it shortens the induction period by a similar amount but with a slower hydration rate afterwards.[1][15] |
| Calcium nitrite | Yes | Accelerator and corrosion inhibitor; an ACI abstract reports good set acceleration at 10 and 22 °C. In frozen early-age paste an optimum of 1.5% was found, effective only above −6 °C.[9][13] Sodium/calcium nitrite blends were tested down to −10 °C.[12] |
| Sodium thiocyanate | Yes | Common non-chloride salt, less efficient than CaCl2; many commercial admixtures blend nitrate and thiocyanate.[10][15] |
| Triethanolamine (TEA) | Yes | Strongly dose-sensitive: below 0.1% of cement mass it retards setting; at 0.2% or more it greatly accelerates aluminate reaction and can flash-set.[14] |
| Calcium formate | Yes | Shortens initial and final set and raises compressive strength in Portland and pozzolanic cement pastes; studied as a CaCl2 substitute.[16] |
Calcium nitrate, nitrite, formate, thiocyanate, triethanolamine and CaCl2 all appear in the ACI 212.3R accelerator list.[10] Wikipedia also lists calcium nitrate, nitrite and formate as the common chloride-free choices.[21]
Does calcium nitrate protect concrete from freezing?
Only partly. Fresh concrete freezes at about −4 °C and frozen early-age concrete can lose more than 50% of potential strength; it should be protected until it reaches about 3.5 MPa.[19] Calcium nitrate speeds the strength gain needed to reach that point and depresses the freezing point of mix water, but the effect has a limit: in the Lithuanian tests −10 °C behaved as a eutectic limit and no dose (0.5–3%) reached 3.5 MPa in 2 days without extra measures, while at −5 °C 0.5% reached 3.7 MPa.[1] NRMCA states plainly that accelerators do not prevent freezing and do not replace protection and curing.[19] Early antifreeze work favoured nitrite-based blends at −5 and −10 °C.[12] Another study notes that larger nitrite/nitrate amounts than usual are used below −10 °C.[3]
What does cold-weather practice look like?
ACI 306R covers cold-weather planning, protection, curing and accelerating or antifreeze admixtures.[11] NRMCA defines cold weather as more than 3 consecutive days with average temperature below 5 °C, recommends placement temperatures of 13 °C (sections under 300 mm), 10 °C (300–900 mm) and 7 °C (900–1800 mm), and notes a 10 °C drop roughly doubles setting time.[19] Saturated concrete that will face freeze-thaw cycles should reach about 24 MPa first.[19] Use air-entrained mixes for freeze-thaw exposure and avoid water curing when freezing is imminent.[19] Calcium nitrate does not change the air content: it stayed 4.6–5.2% at all doses tested, and it slightly raised initial slump.[1]
How do you trial calcium nitrate in a mix?
The steps below reflect the test design of the published studies, not a prescribed dose for your cement.
- Pick the cement and record its C3A, since C3A-containing cements are significantly affected.[1]
- Prepare Vicat pastes at 0.5%, 1%, 2% and 3% of cement mass at your site temperatures (+20, +5, 0 °C) and read initial and final set.[1]
- Cast concrete at those doses with your actual superplasticizer and check slump and air content.[1]
- Cure at the expected initial temperature and test strength at 2, 7 and 28 days.[1]
- Compare with the EN 934-2 criteria (120% at 1 day/20 °C, 130% at 2 days/5 °C) if you claim an accelerator class.[18]
- Continue moist curing after the cold phase, since further water curing was needed for strength gain.[1]
Does calcium nitrate protect rebar or cause alkali-silica problems?
The older HRSU page said nitrate “acts as a corrosion inhibitor”. That needs care. At 3–4% by cement mass, calcium nitrate inhibited chloride-induced rebar corrosion in long-term tests.[5] A 2001 Waterloo thesis, using synthetic pore solution and cement paste, found that none of the tested inhibitors, including commercial calcium nitrate products, delayed the onset of corrosion.[6] Typical accelerator doses are lower than 3–4%, so do not rely on set-acceleration dosing for corrosion protection. See calcium nitrate as a corrosion inhibitor. Separately, laboratory studies show dissolved calcium nitrate reduces alkali-silica reaction expansion by limiting aggregate dissolution; this is research-stage, not a standard ASR specification route.[7][8]
Is calcium nitrate compatible with other admixtures?
Evidence is limited to the mixes tested. The Lithuanian concrete used a polycarboxylate superplasticizer with no air-content penalty,[1] and ACI 212.3R recommends verifying multi-admixture systems by test placement.[10] Calcium nitrate also worked in 50% slag blends, with a 4% dose equalising 7- and 28-day mortar strength between two slags.[4] Always run your own trial with retarders, water reducers and air-entrainers.
When calcium nitrate is NOT the right choice
- Very low temperatures without protection. At −10 °C no tested dose of 0.5–3% reached the 3.5 MPa frost-safe strength in 2 days.[1]
- Relying on it for 1-day hardening at 20 °C or for long-term strength. It is chiefly a set accelerator with a small long-term effect.[1][4]
- Over-dosing. In one study higher doses lowered 28-day strength and flexural strength.[2]
- Counting on it as a rebar corrosion fix at accelerator doses. The 3–4% inhibitor dose is above normal accelerator use and results are mixed.[5][6]
- Treating fertilizer-grade powder as a certified admixture. EN 934-2 and ASTM C494 apply to the admixture as tested in concrete, so conformity needs your own testing.[17][18]
What HRSU supplies — and what it does not
HRSU Indore supplies fertilizer-grade calcium nitrate powder: 18.5–19% calcium, 15.5% minimum nitrogen, 100% water-soluble, 0% insoluble residue, in 25 kg and 50 kg bags with a batch Certificate of Analysis, bulk and export, and TDS/SDS available. HRSU does not claim conformity to EN 934-2, ASTM C494 or any concrete-admixture certification. Fertilizer-grade product contains nitrogen forms beyond nitrate, and historic concrete use of technical calcium nitrate has involved a calcium-ammonium nitrate double salt.[1] Share your admixture specification (chloride, alkali, ammonium limits) and request a sample and COA so you can validate it in your own paste and concrete trials.
Sample fully soluble calcium nitrate for your admixture trials
100% water-soluble · 0% insoluble residue · 25 kg & 50 kg bags · bulk/export available.
Buy online WhatsApp usFrequently asked questions
Does calcium nitrate accelerate concrete set time?
Yes. At +20 °C a paste with no admixture set initially at 149 minutes versus 120, 105 and 45 minutes with 1%, 2% and 3% calcium nitrate by cement mass.[1]
What dosage of calcium nitrate is used in concrete?
Published lab tests use 0.5–3% by cement mass; the strength-optimal dose was 1% at +20 and +5 °C, 0.5% at 0 °C and 3% at −5 °C. Confirm with trial batches for your cement.[1]
Why use calcium nitrate instead of calcium chloride?
Can calcium nitrate be used for cold-weather concreting?
Is calcium nitrate a corrosion inhibitor in concrete?
Does calcium nitrate increase 28-day concrete strength?
How does calcium nitrate work chemically?
Can HRSU calcium nitrate be used as a concrete admixture?
HRSU supplies fertilizer-grade, 100% water-soluble powder with batch COAs and does not claim admixture certification. Test it in your own mix against your specification.[18]
Related guides
- Calcium nitrate as a corrosion inhibitor
- Calcium nitrate vs calcium chloride
- Calcium nitrate as a deicing agent
- Uses of calcium nitrate: the complete guide
References
- The effect of calcium nitrate on the properties of Portland cement pastes and concrete hardening at low temperatures (Kicaite, Skripkinas, Pundiene) — Ceramics-Silikaty 64(3), 263-270, 2020. Peer-reviewed
- Analyzing the effects of calcium nitrate over white Portland cement: a multi-scale approach (Rusu et al.) — Materials (MDPI), 2022, via PubMed Central. Peer-reviewed
- Effect of a nitrite/nitrate-based accelerator on the strength development and hydrate formation in cold-weather cementitious materials (Yoneyama et al.) — Materials 14, 1006, 2021. Peer-reviewed
- Performance of calcium nitrate as accelerator for cement blended with blast furnace slag (Justnes et al.), SP-362 — 12th ACI/RILEM International Conference, Toulouse, 2024. Conference paper
- Corrosion inhibitors for reinforced concrete (H. Justnes), ACI SP-234 — American Concrete Institute, 2006. Standards-body symposium paper
- Examination of the mechanism of corrosion inhibition by calcium nitrite and calcium nitrate-based admixtures in concrete (L. Mammoliti) — University of Waterloo, 2001. University thesis
- Calcium nitrate effectively mitigates alkali-silica reaction by surface passivation of reactive aggregates (Xiao et al.) — Journal of the American Ceramic Society 107(11), 2024. Peer-reviewed
- Calcium nitrate: a chemical admixture to inhibit aggregate dissolution and mitigate expansion caused by alkali-silica reaction (Oey et al.) — Cement and Concrete Composites 110, 103592, 2020. Peer-reviewed
- The effects of calcium nitrite on the mechanical properties and microstructure of early-age frozen cement paste (Wan et al.) — Materials (MDPI), 2024, via PubMed Central. Peer-reviewed
- ACI 212.3R-10 Report on chemical admixtures for concrete — American Concrete Institute. Standards body
- ACI 306R-10 Guide to cold weather concreting — American Concrete Institute. Standards body
- Antifreeze admixtures for cold weather concreting (Korhonen and Cortez) — Concrete International 13(3), 1991, American Concrete Institute. Professional journal
- Symposium paper by D. Chin on a calcium nitrite/nitrate-based accelerator and corrosion screening, ACI SP-102 — American Concrete Institute, 1987. Standards-body symposium paper
- Early performances of cement paste in the presence of triethanolamine: rheology, setting and microstructural development (Lu et al.) — Journal of Applied Polymer Science, 2021 (TU Berlin repository). Peer-reviewed
- Accelerating admixtures for concrete: state of the art (R. Myrdal) — SINTEF, 2007. Research institute report
- Effect of calcium formate as an accelerator on the physicochemical and mechanical properties of pozzolanic cement pastes — Transportation Research Board TRID record. Peer-reviewed (record)
- ASTM C494/C494M Standard specification for chemical admixtures for concrete — ASTM International. Standard
- EN 934-2:2009+A1:2012 Admixtures for concrete, mortar and grout, Part 2: concrete admixtures — CEN, via Estonian Centre for Standardisation (EVS). Standard
- CIP 27: Cold weather concreting — National Ready Mixed Concrete Association (NRMCA). Industry association
- Acceptable use of calcium chloride in concrete, Position Statement 31 — American Society of Concrete Contractors. Industry association
- Cement accelerator — Wikipedia. Encyclopedia
- Calcium nitrate — Wikipedia. Encyclopedia