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Calcium Nitrate as an Accelerator of Cementitious Systems
Cecilia Vong, Tandré Oey, Julyan Stoian, Jialin Li, Gaurav Sant Department of Civil and Environmental Engineering, University of California-Los Angeles
• Isothermal Calorimeter • Measures the heat flow and cumulative heat of a paste sample • Shows the effect of additions on the rate of reaction
• Compressive Strength • Measures the strength of a given cementitious mixture
• Vicat Needle • Penetrates needle into a paste sample to determine set time • Initial set: Needle penetrates 25mm (percolation) • Final set: Needle can no longer penetrate the paste
• Type I/II and II/V Ordinary Portland Cement • Accelerator Dosage(% as mass of cement): 0% to 4% • Water to solid ratio(w/s): 0.485 • Sand volume(%): 30% • Mixture Composition:
• Plain Type I/II Cement and with 15% replacement (by mass) of Limestone, Fly Ash, and Slag
• Type II/V Cement
Isothermal Calorimeter
Compressive Strength Tester
Automatic Vicat Needle Tester
unhydrated cement
water
C-S-H precipitating
Percolation has occurred when there is a solid chain through the whole paste
capillary pores
Dissolution: Calcium ions and hydroxyl ions, among others, are released from the surface of each cement particle.
Nucleation and Growth Analogy: When the system reaches supersaturation with respect to key reaction products, C-S-H(calcium silicate hydrate) and other reaction products precipitate at the surface of the cement particles and grow to form a layer around the cement particle. C-S-H: variable composition phase; main contributor to strength development in concrete.
Diffusion: As the C-S-H layer grows thicker, the reaction becomes diffusion controlled, reacting slower as the barrier gets thicker.
Abstract
Background
Materials and Methods
Results and Discussion
Acknowledgements
References
Conclusion and Future Research
Ordinary portland cement is a fine powder which, when mixed with water, undergoes a hydration reaction to produce reaction products with desirable mechanical properties. As a consequence, cement and water, i.e., cement paste serves as a primary binding component in concrete. Accelerating admixtures are commonly used to speed up the setting time and strength gain of cement paste to reduce the amount of time needed for construction. This study investigates the use of calcium nitrate as an accelerating admixture, which is also known to be helpful in preventing the corrosion of steel embedded in reinforced concrete. A series of evaluations encompassing heat flow (i.e., reaction rate), compressive strength, and setting time are measured on cement paste or mortar formulations. The results highlight the unsystematic behavior of calcium nitrate—for most systems, 2% was an optimum dosage for reducing the set time, while lower than expected strengths were measured. The heat flow curves denoted an acceleration in reaction rates, however at only early ages (<24 hours), after which the rate of reactions was noted to be slightly lower than the control (plain cement) formulations. The results suggest that while calcium nitrate does exert acceleratory effects, further work is needed to better understand the nature of its interactions with cementing systems.
Figure 1: • Cement: Finer particles increase the rate of reaction because more surface area will be available • Fillers: An increase in the surface area provides more area for precipitation of reaction products [2]
• Heat flow curves of fly ash and slag are similar to that of plain cement • Limestone has an earlier and higher peak because of its acceleratory properties, but its cumulative
heat is lower than that of slag and fly ash • Higher cumulative heat of slag and fly ash are due to pozzolanic reactions; Limestone’s heat is from
dilution effect
• Cement: fine hydraulic powder that sets and hardens on reaction with water • Accelerators: reduce the setting time and speed up the hardening process when
cement reacts with water • Set accelerators: reduce the time at which a paste sample sets
• Calcium nitrate: set accelerator that can help prevent corrosion
Figure 2: • Curves of calcium nitrate systems shift to the left, indicative of a set accelerator, not hardening accelerator • Shapes of the curves suggest that calcium nitrate affects the dissolution and the onset of nucleation and growth • For Type I/II systems, this acceleration is usually accompanied by a slight peak depression • Induction period is higher at low dosages, returning to normal at 4% dosage for Type I/II systems • Systems containing SCM’s behave similar to the plain system • Type II/V cement affected differently by the calcium nitrate accelerator due to its different phase composition
Figure 3: • Slope, heat peak, inverse time to peak, and induction heat parameters quantify effects observed from calorimetry curves • Heat peak values relatively unchanged with dosage • Inverse time to peak values show similar trends for Type I/II systems, limestone being an exception due to its acceleratory effect • Induction heat values have a maximum for all systems at 2% dosage
Figure 4: • 2% dosage is the minimum for most systems, indicating that 2% is the optimum dosage level for reducing set time • In general, with a higher minimum induction heat, the set time decreases • Rough correlation between set time and minimum induction heat suggests that the raised induction period plays a role in
speeding up the set time
The Cement Reaction[1]
Chloride ions contribute to corrosion in steel reinforcements, which can weaken the steel and cause parts of the concrete to fail. Calcium nitrate is known to be helpful in preventing corrosion by forming a passivation layer on steel reinforcements. However, since it is less studied, the effect of different dosages of calcium nitrate on cement behavior is unknown. Because of its ability to prevent corrosion, calcium nitrate is an attractive alternative to chloride based accelerators.
Figure 5: • Despite 2% dosage being optimal for set, strength shows a
minimum value at 2% dosage • Dotted lines show the strength of a plain Type I/II system
for reference • Calcium nitrate has more impact on early strength (1 day) • 28 day strengths are comparable for most systems at
dosages other than 2%
Many thanks to Mr. Herrera for coordinating this program, Harsha Kittur for his poster critiques, and my lab members for their guidance and allowing me to participate in their research.
Results and Discussion(cont’d)
• Calcium nitrate is a set accelerator, not a hardening accelerator. • Optimal dosage level at 2% for reducing set time, though it results in the lowest compressive strengths. • Correlation between set time and minimum induction heat suggests a relation between the raised induction period and
the set behavior. • Calcium nitrate results in acceptable late age strengths, and is a viable alternative to chloride based accelerators. • Future research may include using X-Ray Diffraction and Thermogravimetric Analysis to determine the influence of
calcium nitrate on phase composition. 0.01 0.1 1 10 100 1000 10000
Particle Diameter (µm)
0
10
20
30
40
50
60
70
80
90
100
Cum
ulat
ive
Pass
ing
(%)
Particle SizeType I/IIType II/V3µm LimestoneFly AshSlag
0 6 12 18 24 30 36Time (hours)
0
1
2
3
4
5
6
7
Hea
t Flo
w (m
W/g
cem
ent)
ReferenceType I/III/II, 15% LimestoneI/II, 15% Fly AshI/II, 15% SlagType II/V
0 24 48 72 96 120 144 168Time (hours)
0
100
200
300
400
500
Cum
ulat
ive
Hea
t (J/
g cem
ent)
ReferenceType I/III/II, 15% LimestoneI/II, 15% Fly AshI/II, 15% SlagType II/V
0 6 12 18 24 30 36Time (hours)
0
1
2
3
4
5
6
7
Hea
t Flo
w (m
W/g
cem
ent)
Type I/II, NitrateReference1% Dosage2% Dosage4% Dosage
0 6 12 18 24 30 36Time (hours)
0
1
2
3
4
5
6
7
Hea
t Flo
w (m
W/g
cem
ent)
Type II/V, NitrateReference1% Dosage2% Dosage4% Dosage
0 6 12 18 24 30 36Time (hours)
0
1
2
3
4
5
6
Hea
t Flo
w (m
W/g
cem
ent)
I/II, 15% Slag, NitrateReference1% Dosage2% Dosage4% Dosage
0 1 2 3 4Dosage(%)
0
0.4
0.8
1.2
1.6
2
2.4
Slop
e (m
W*g
-1*h
our-1
)
Slope (Nitrate)Type I/III/II, 15% LimestoneI/II, 15% Fly AshI/II, 15% SlagType II/V
0 6 12 18 24 30 36Time (hours)
0
1
2
3
4
5
6
7
Hea
t Flo
w (m
W/g
cem
ent)
I/II, 15% Limestone, NitrateReference1% Dosage2% Dosage4% Dosage
0 6 12 18 24 30 36Time (hours)
0
1
2
3
4
5
6
7
Hea
t Flo
w (m
W/g
c em
e nt)
I/II, 15% Fly Ash, NitrateReference1% Dosage2% Dosage4% Dosage
0 1 2 3 4Dosage(%)
0
1
2
3
4
5
6
7
Hea
t Pea
k (m
W*g
-1)
Heat Peak (Nitrate)Type I/III/II, 15% LimestoneI/II, 15% Fly AshI/II, 15% SlagType II/V
0 1 2 3 4Dosage(%)
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
Inve
rse
Tim
e to
Pea
k (h
our-1
)
Inverse Time to Peak (Nitrate)Type I/III/II, 15% LimestoneI/II, 15% Fly AshI/II, 15% SlagType II/V
0 1 2 3 4Dosage (%)
0
2
4
6
8
Min
imum
Indu
c ti o
n H
e at (
mW
/ gc e
me n
t) Induction HeatType I/III/II, 15% LimestoneI/II, 15% Fly AshI/II, 15% SlagType II/V
0 1 2 3 4Dosage(%)
0
1
2
3
4
5
Initi
al S
et T
ime(
hour
s)
NitrateType I/III/II, 15% LimestoneI/II, 15% Fly AshI/II, 15% SlagType II/V
0 1 2 3 4Dosage(%)
0
1
2
3
4
5
6
7Fi
nal S
et T
ime(
hour
s)
NitrateType I/III/II, 15% LimestoneI/II, 15% Fly AshI/II, 15% SlagType II/V
0 1 2 3 4 5Minimum Induction Heat (mW/gcement)
0
2
4
6
8
10
Initi
al S
et T
ime(
hour
s)
Initial Set (Nitrate)Type I/III/II, 15% LimestoneI/II, 15% Fly AshI/II, 15% SlagType II/V
R2 = 0.302
0 1 2 3 4 5Minimum Induction Heat (mW/gcement)
0
2
4
6
8
10
12
Fina
l Set
Tim
e(ho
urs)
Final Set (Nitrate)Type I/III/II, 15% LimestoneI/II, 15% Fly AshI/II, 15% SlagType II/V
R2 = 0.343
[1] Mindess, Sidney, Young, J. Francis, and Darwin, David. “Concrete.” 2nd ed. Prentice Hall, 2003. [2] A. Kumar, T. Oey, S. Kim, D. Thomas, S. Badran, J. Li, F. Fernandes, N. Neithalath, G. Sant, “Simple methods to estimate the influence of limestone fillers on reaction and property evolution in cementitious materials”, Cem. And Concr. Comp., 42 2013, pp. 20-29
http://www.worldoftest.com/vicatronic.htm
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