Calcium Nitrate as a Multifunctional Concrete Admixture
by Prof. Harald Justnes, SINTEF Technology and Society, Concrete, Trondheim, Norway Abstract
Industrial quality calcium nitrate can be utilized as concrete admixture in many ways. Examples are given for applications as 1) Set accelerator, 2) Counteraction of retardation by plasticizers while maintaining rheology, 3) Long term strength enhancer, 4) Anti-freeze admixture or winter concreting admixture and 5) Inhibitor against chloride induced corrosion of steel. The first two points may achieved by small dosages (in the range 0.2-1% dry admixture by mass of cement), point 3 and 4 at moderate dosages (1-3%), while the last point require dosages in the higher end (3-4%). Calcium nitrate as a set accelerator, and side effects of that, is well proven in practice over many years, while the corrosion inhibitor effect was only discovered in the later years. Unlike more well known inhibitors like calcium nitrite, calcium nitrate is harmless for the environment and more cost-effective. Key-words: Calcium Nitrate, Accelerator, winter concreting, corrosion inhibitor INTRODUCTION
SINTEF has been working with calcium nitrate as concrete admixture since 1992 and confirmed known applications (e.g. accelerator) as well as finding new ones (e.g. corrosion inhibitor). Thus, the following chapters are divided into the main applications/properties; set acceleration, counteraction of plasticizer retardation, long term strength improvement, winter concreting admixture and corrosion inhibitor. However, due to limited number of pages, each topic is only briefly discussed, but with emphasis on the main application set acceleration. SET ACCELERATION
In the past, a concrete accelerator was synonymous with an admixture increasing the 1 day compressive strength. It is only in the later years, with the implementation of the European standards that the industry distinguishes between setting and hardening accelerators, realizing practical utilizations of both both admixture effects independently. independently. According to the European admixture standard EN 934-2 of 2001, a setting accelerator must give at least 30 min initial setting time at 20°C, and maximum 60% of the initial setting time of the reference at 5 °C measured on mortar with equal flow. A hardening accelerator should give minimum 120% compressive strength compared to the reference after 1 day at 20°C, and minimum 130% compressive strength compared to the reference after 2 days at 5°C, as measured on concrete of equal flow. Requirements are set to long term strength and air also. Note that the European standard uses the practical approach of equal flow of mixes allowing variations in water-to-cement ratio (w/c). However, variations in w/c may mask the chemical effects of admixtures, so more fundamental studies searching for active accelerators tend to use constant w/c and cement content. Concrete is a relative low priced construction material, which means that any high prices chemical admixture must enhance its performance at a relative small dosage, or alternatively a
cheaper chemical can be allowed at moderate dosage. Hence, accelerators should be sought among industrial bulk chemicals. Calcium chloride, CaCl2, was an ideal accelerator being a combined setting and hardening accelerator, in addition to being an industrial bulk product. However, in 1960’s awareness on corrosiveness of chlorides on embedded reinforcement arose and today chloride containing admixtures is prohibited for steel reinforced concrete (limits are set to < 0.4% Cl- of cement mass, or < 0.1% for structures serving in chloride containing environment). In the search for a relatively cheap chloride-free accelerator, calcium nitrate, Ca(NO 3)2, has arisen as an alternative setting accelerator, but it must be combined with other components to function as a hardening accelerator. Unless otherwise stated the calcium nitrate (abbreviated CN) referred to here is of granulated, technical quality with formula xNH4 NO3⋅yCa(NO3)2⋅zH2O where x = 0.092, y = 0.500 and z = 0.826, or in other words composed of 19.00 % Ca2+, 1.57 % NH4+, 64.68 % NO3- and 14.10 % H2O. When testing accelerators, it is important to know that outcome may strongly depend on the composition of the Portland cement used. Justnes and Nygaard [1] published the set accelerating efficiency of calcium nitrate (CN) at 5-7°C on pastes (w/c = 0.40) based on 5 different Portland cements with a C3A content ranging from 7.4 to 1.0 %. The accelerating efficiency ranged from very strong to slight and there was no correlation between set accelerating efficiency and C 3A as initially assumed, but rather surprisingly with the belite, C2S, content as plotted in Fig. 1. The correlation between belite content and set accelerating efficiency of CN was confirmed by Justnes and Nygaard [2] in a study of five other cement pastes at 5 °C. Justnes and Nygaard [3] discussed the reason for acceleration efficiency differences among Portland cements by analyses of the liquid of cement pastes prior to setting. They found a linear correlation between the alkali content of the fluid of 9 cement pastes in the fresh state with the reduction in initial setting time when 1.55 % calcium nitrate was added, as reproduced in Fig. 2. In order to find out whether setting acceleration is dominated by the calcium cation or nitrate anion in Ca(NO3)2, Justnes [4] tested the efficiency of both calcium nitrate (65% NO 3due to some crystal water) and sodium nitrate (73 % NO3-) as set accelerators for 4 different Portland cements pastes at 5 °C by Vicat needle. The dosages were 0.00, 0.25, 0.50, 0.75 and 1.00 % calcium nitrate of cement mass, while sodium nitrate was dosed to give correspondingly equimolar nitrate content. The influence on setting time is listed in Table 1. From this limited study, calcium nitrate seems in general to be a substantially better set accelerator than sodium nitrate for Portland cements. Specifically, Ca(NO 3)2 gave shorter setting time relative to reference than NaNO 3 in 10 out of 12 comparative tests and about twice or more reduction in setting time in 6 of the 12 cases. It seems like Ca 2+ dominates setting, while NO3- may have an effect as well, depending on cement type. Temperature evolution profiles in insulated concrete (i.e. semi-adiabatic) and early compressive strengths for concrete cubes cured at 20 °C have been measured for different additions of Ca(NO3)2 [5]. The concrete composition corresponded to w/(c+s) = 0.45 and 4 % silica fume replacement of cement for both CEMI 52,5R-LA and CEM I 42,5R. The accelerating effect of Ca(NO3)2 was also compared to additions of calcium acetate, Ca(CH 3COO)2, and formate, Ca(HCOO)2, at equimolar concentrations of Ca 2+ for 3.5 % Ca(NO3)2 added to the CEM I 42,5R concrete. Calcium acetate and formate gave about the same acceleration according to the temperature profiles in Fig. 3, while Ca(NO3)2 showed greater accelerating effect in spite of soluble calcium ions dominating the set accelerating effect. The reason for the lesser efficiency of calcium salts of organic acids (i.e. formate, acetate etc) may be due to partial complex formation with one of the anions (e.g. CaOOCCH3+), meaning that the overall chemical equilibrium of the paste fluid does not experience the same effective concentration of Ca 2+ ions as for Ca(NO3)2. The temperature profiles in Fig. 3 reveal that setting time is accelerated (criterion is 2°C above base line) and not the early strength development rate (i.e. temperature
increase slope not steeper than reference) which is of importance avoiding thermal cracks in massive structures. Increased 8 h strength for HSC (CEMI 52,5R-LA) and OPC (CEM I 42,5R) concretes with increasing CN dosage is due to increased maturity at this early age due to the set acceleration. Another important parameter to control when testing accelerators is the temperature. Justnes et al [6] measured reductions in initial and final set at 5°C, 13°C and 23°C for cement pastes with 1.55% calcium nitrate and equimolar Ca dosage of calcium chloride hexahydrate, and compared them with neat cement pastes (two different Portland cements). The results revealed that the efficiency of calcium nitrate as set accelerator is higher at lower temperatures. COUNTERACTING RETARDATION BY PLASTICIZERS
Practical utilizations of set accelerators are often to start concrete hardening earlier to avoid cooling of concrete in winter concreting or to counteract the retarding effect of plasticizers. Rettvin and Masdal [7] showed that additions of 50 % technical calcium nitrate solution (ammonium free) to concrete gave a set acceleration proportional with the dosage up to 0.50 % of the cement weight. They also described the utilization of calcium nitrate (0.25% dosage) to secure the slip forming rate during the construction of the shafts for the Troll Gravity Base Structure (369 m height) in the North Sea, since calcium nitrate (CN) counteracted the retarding effect of the plasticizer sodium lignosulphonate used. Justnes and Petersen [8, 9] showed that CN can partly counteract retardation of plasticizers while maintaining rheology as exemplified by plastic viscosity and yield point according to the Bingham model for mortars with different lignosulphonates and CN in Table 2. Calcium chloride, CaCl2, and sodium thiocyanate, NaSCN, is also used for comparison. The 1 day strength of these mortars given in Table 3 is an indication of the counteraction of retardation. LONG TERM STRENGTH INCREASE
If pure calcium nitrate is used as set accelerator, very early strength (e.g. 8 h in Fig. 4) is increased, but not 1 day strength. However, often the long term strength (from 28 days an onwards) is increased in spite of equal porosity as illustrated after 220 days in Fig. 5 (same mixes as in Fig. 4). Justnes [10] discussed potential reasons for long term strength increase. WINTER CONCRETING ADMIXTURE
Calcium nitrate (CN) is approved as an anti-freeze admixture for winter concreting in Poland with a typical dosage of 0.5-1% dry CN of cement mass. However, there is no standard test for anti-freeze and the test often involves a period of freezing in the fresh state and thawing before testing strength (i.e. does the concrete stand early freezing?). Thus, the effect may be a result of set acceleration. The experiment described in Fig. 6 with temperature profiles of concrete in insulated moulds placed in a cabinet at -10 °C for 1 day followed by another day at +20°C illustrates that earlier setting may result in slower cooling down, less minimum temperature and a faster warming up. All this translates into 2 day strength of concrete with 1% CN of 20.6±0.3 MPa while the reference only reached 15.6 ±2.3 MPa (4 parallel cubes). CORROSION INHIBITOR
Justnes [11] recently gave a short review of inhibitors against chloride induced corrosion of steel reinforcement in concrete, and specifically compared the performance of calcium nitrate with the more common inhibitor calcium nitrite. Among other evidence, a simple test of casting rebars in the centre of concrete cylinders where chloride has been mixed into the recipe has been made. The cylinders are stored at 38 °C and 90% RH with air access over the years and inspected for cracks periodically as a result of expanding rust formation. When the
reference cracks, the cylinders with CN intermixed are also split open and the rebars taken out for inspection. Fig. 7 shows such rebars after 3 years and that steel without CN present is badly corroded, sample with 2% CN only have spots of surface rust and sample with 4% rust have no sign of corrosion. It was concluded that calcium nitrate (CN) at least is equally efficient as calcium nitrite as inhibitor, and that it in addition is cheaper and less harmful. CONCLUSION
Calcium nitrate is a concrete admixture that can function as 1) Set accelerator, 2) Counteraction of retardation by plasticizers while maintaining rheology, 3) Long term strength enhancer, 4) Anti-freeze admixture or winter concreting admixture and 5) Inhibitor against chloride induced corrosion of steel. Thus, it is fair to denote it a multifunctional concrete admixture. REFERENCES
1. Justnes, H. and Nygaard, E.C., "Technical Nitrate as Set Accelerator for Cement at Low Temperatures", Cement and Concrete Research, Vol. 25, No. 8, 1995, pp. 1766-1774. 2. Justnes, H. and Nygaard, E.C., "Technical Nitrate as Set Accelerator for Cement at Low Temperatures", Advances in Cement Research, Vol. 8, No. 30, Apr. 1996, pp.1-9 3. Justnes, H. and Nygaard, E.C.: "The Mechanism of Calcium Nitrate as Set Accelerator for Cement", Proceedings of 10th International Congress on the Chemistry of Cement, Gothenburg, Sweden, 2-6 June 1997, paper 3iii012, 8 pp. 4. Justnes, H.: “Chloride-free Accelerators for Concrete Setting and Hardening”, Proc. IV Int. ACI/CANMET Conference on Quality of Concrete Structures and Recent Advances in Concrete Materials and Testing”, Recife, Brazil, 6-7 September 2005, 11pp. 5. Justnes, H. and Nygaard, E.C.: "Calcium Nitrate - A Multifunctional Concrete Admixture", Proc. Int. Conf. on High-Performance Concrete, and Performance and Quality of Concrete Structures", June 05-07 1996, Florianopolis, Brazil, pp. 514-525. 6. Justnes, H., Clemmens, F., Depuydt, P., Van Gemert, D. and Sellevold, E.J.: ”Setting Accelerators for Portland Cement”, Supplementary Paper of the Sixth International Conference on Superplasticizers and Other Chemical Admixtures in Concrete, Nice, France, 11-13 October 2000, CANMET/ACI SP-195, pp. 1- 16. 7. Rettvin, Å. and Masdal, T., "Use of Calcium Nitrate Solution as Set-Accelerating Admixture in Slip forming of High Strength Concrete", Proc. ERMCO'95, Istanbul, Turkey, 1995. 8. Justnes, H. and Petersen, B.G.: “Counteracting Retardation of Cement Setting by Other Admixtures with Calcium Nitrate”, Proc. 5th CANMET/ACI Int. Symp. Advances in Concrete Technology, July 29 – August 1, 2001, Singapore, ACI SP 200-3, pp. 39-49. 9. Justnes, H. and Petersen, B.G.: “Counteracting Plasticizer Retardation of Cement Setting with Calcium Nitrate”, Proc. Int. Conf. Innovations and Developments in Concrete Materials and Construction, Dundee, Scotland, 9-11 September, 2002, p. 259-267 10. Justnes, H.: “Explanation of Long-Term Compressive Strength of Concrete Cause by the Set Accelerator Calcium Nitrate”, Proceedings of the 11 th International Congress on the Chemistry of Cement (ICCC), 11-16 May, 2003, Durban, South Africa, pp. 475-484. 11. Justnes, H.: “Corrosion Inhibitors for Concrete”, Proceedings of the International Symposium on Durability of Concrete I Memory of Prof. Dr. Raymundo, Rivera”, 12-13 May, 2005, Monterrey, N.L. México, pp. 179-199.
Table 1 Initial setting time for different cement pastes (w/c = 0.40) at 5 °C with different dosages of calcium nitrate, Ca(NO3)2, and sodium nitrate, NaNO 3. Setting times are listed in hours with relative setting time (%) to reference in brackets. CEMENT TYPE Ca(NO3)2 (%) 0.00 0.25 0.50 0.75 1.00 NaNO3 (%) 0.25 0.50 1.00
CEM I 42,5R CEM II A-V 42,5R CEM I 52,5R-LA
-
8.10 (100) 5.50 (67.9) 5.25 (64.8) 5.33 (65.8) 4.30 (53.1)
6.75 (100) 5.41 (80.1) 4.41(65.3) 5.83 (86.4) 5.70 (84.4)
7.40 (100) 5.80 (78.4) 3.00 (40.5) 4.60 (62.2) 1.80 (24.3)
12.25 (100) 4.7 (38.4) 7.5 (61.2) 3.8 (31.0) 4.0 (32.7)
5.60 (69.1) 5.55 (68.5) 6.00 (74.1)
5.15 (76.3) 6.55 (97.0) 7.52 (111.4)
6.10 (82.4) 5.40 (73.0) 4.80 (64.9)
8.33 (68.0) 6.33 (51.7) 5.00 (40.8)
Table 2 Viscosity and yield point of 1:3 mortars (w/c = 0.5) with different combination of admixtures 10 minutes after addition of water. Mix after 10 minutes
Viscosity µ p [Pa⋅s] 0.3% Borresperse (BSP) Ca 1.43 0.3% BSP Ca / 0.5% CN 1.47 0.3% BSP Ca / 1% CN 1.57 0.3% BSP Ca / 1% CN / 0.2% NaSCN 1.53 0.3% BSP Ca / 1% CaCl 2 1.55 0.3% Wafex P 1.26 0.3% Wafex P / 1% CN 1.51
Yield point τ0 [Pa] 56.5 58.8 67.2 53.1 65.0 49.5 52.2
Regression factor, r 2 0.9978 0.9949 0.9982 0.9968 0.9930 0.9976 0.9966
Table 3 Compressive strength (MPa) for 1:3 mortars (w/c = 0.5) with different admixture combinations as a function of age. Mix Reference (0%) 0.3% Borresperse1 (BSP) Ca 0.3% BSP Ca / 0.5% CN 0.3% BSP Ca / 1% CN 0.3% BSP Ca / 1% CN / 0.2% NaSCN 0.3% BSP Ca / 1% CaCl 2 0.3% Wafex P2 0.3% Wafex P / 1% CN 0.3% Wafex P / 1% CN / 0.2% NaSCN 0.3% Wafex P / 1% CaCl 2 0.3% SNF3 1
1 day 18.4±0.6 5.8±0.1 10.3±0.2 13.2±0.3 15.4±0.3 15±1 0.5±0.3 8.3±0.2 13.2±0.2 10.3±0.3 18.6±0.8
3 days 34±2 31±1 34±1 33±2 35±1 39±1 30±1 28±1 34±1 37±1 36±1
7 days 28 days 44±2 54±2 46±2 59±2 46±1 65±2 43±1 59±2 47±1 66±3 49±2 61±3 44±1 59±1 40±2 58±3 44±2 58±4 46±1 58±4 44±1 56±3
Sugar reduced lignosulphonate, 2Lignosulfonate containing natural sugars, 3 Naphtalene sulphonate-formaldehyde condensate
Effec t of belite content on CN acc elerator eff icienc y 75 ) %65 ( n o 55 i t c u d 45 e r t e 35 s l a i t 25 i n I
y = 4,6022x - 41,923 R2 = 0,9133
15 14
16
18
20
22
24
26
Belite conten t (%)
Fig. 1 The linear correlation between accelerating efficiency of 1.55 % CN additions of different cements and their belite content.
450
) n i m ( e m 350 i t g n i t t e s n 250 i n o i t c u d 150 e r
y = -661,44x + 447,11 R2 = 0,8196
50 0,1
0,2
0,3
0,4
0,5
0,6
Na2O-equ ivalent (%) in w ater
Fig. 2 The reduction in setting time for Portland cement pastes when 1.55 % calcium nitrate is added vs. the alkali content of the liquid phase 20 min after mixing.
Fig. 3 Temperature vs. hardening time curves for OPC concrete with equivalent dosages of soluble calcium from nitrate, acetate and formate compared with reference concrete.
8 h comp ressive strength
12
) e a 10 v P i s M 8 s ( e r h t 6 p g m n 4 o e r 2 C t s
HSC OPC
0
0
1
2 3 4 CN-dosage (%)
5
Fig. 4 Early compressive strength (8 h) for high strength Portland cement (HSC) and ordinary Portland cement (OPC) concrete with different dosages of calcium nitrate (CN). 220 days c ompr essive strength 100 h t g n 95 e r t s ) e a 90 v P i s M 85 s ( e r p 80 m o C 75
HSC OPC
0
1
2
3
4
5
CN-do sage (%)
Fig. 5 Compressive strength at 220 days for HSC and OPC concrete with different dosages of calcium nitrate (CN).
Concr ete in st yrofoam molds at -10/+20 C
24
Ref Styrofoam
Cabinet
1% CN Styrofoam Cabinet
18 ) 12 C ( e r u t a 6 r e p m e T 0
1% CN Ref 0
Ref
1% CN
8
16
24
32
40
48
-6
Cabinet -12 Time (h)
Fig. 6 Temperature evolution in 100 mm cube moulds consisting of 20 mm Styrofoam walls for insulation to simulate larger specimens (e.g. walls) placed in a -10 °C cabinet for 1 day followed by thawing at +20°C for another day before testing of compressive strength. Concrete with 1% calcium nitrate (CN) cooled down slower than concrete without due to earlier setting, did not reach the low temperature and thawed faster. The 2 day strength of concrete with 1% CN was 20.6 ±0.3 MPa while the reference only reached 15.6±2.3 MPa.
Fig. 7 Rebars removed from concrete cylinders with 3.2 % NaCl intermixed after 3 years storage at 38°C and 90% RH. Upper rebar from reference, middle rebar from concrete with 2 % CN and lower rebar from concrete with 4% CN.
