Impact of Accelerators and Retarders on the Hydration of
Portland Cement
Denise Silva
Overview
1. Introduction
2. Mechanisms of Acceleration with Calcium Chloride
•
Latest theories about the mechanisms
•
Examples of performance
3. Mechanisms of Retardation with Sucrose and Lignosulfonate
•
Latest theories about the mechanisms
•
Examples of performance with retarders
4. Knowledge Gaps
July 29th, 2009
International Summit on Cement Hydration Kinetics, Quebec, 27-29 July 2009
2
1. Introduction
• Point of view of a formulator
• Development of products for cement plants: dosage constraints (~0.03 - 0.3%) and
specific technical targets
• Benefits of having a model
• In depth understanding of cement and SCMs hydration mechanisms and in depth
understanding of interactions mechanisms of cement x admixtures (molecular level)
• Ability to design molecules for specific responses
• A model would allow reduction of testing (different cements/SCMs respond differently to a
given chemical admixture) – Utopia?
However…
• Mechanisms of accelerators and retarders are not well understood
• Possible mechanisms are:
• Adsorption on the surface of particles
• Chelation of metal ions
• Poisoning of nucleation and growth
• Precipitation of insoluble salts
• Change in microstructure of hydrated phases
• Several variables involved:
• Chemical admixture composition; chemical admixture dosage.
• chemical composition, PSD, mineralogy of cement; impurities/inclusions and crystal structure
of individual anhydrous phases; presence of SCM; alkali and sulfate contents; etc.
July 29th, 2009
International Summit on Cement Hydration Kinetics, Quebec, 27-29 July 2009
3
1. Introduction
• Retarders
• Water soluble salts: sodium metaborate, sodium tetraborate, stannous sulfate, lead
acetate, monobasic calcium phosphate.
• Salts of lignosulfonic acid (Ca, Na, NH4)
• Salts of hydroxylated carboxylic acids (Na, NH4)
• Carbohydrates
• Accelerators
• Soluble inorganic salts (chlorides, bromides, fluorides, carbonates, thiocyanates, nitrites, nitrates,
thiosulfates, silicates, aluminates, alkali hydroxides).
• Soluble organic compounds (TEA, Ca formate, Ca acetate, Ca propionate, Ca butyrate)
• Admixtures for shotcrete (Na silicate, Na aluminate, Al chloride, Na fluoride, strong alkalis)
• Goal for this presentation
• Present a brief glance on the complexity of hydration of cement in the presence of admixtures
• Highlight some knowledge gaps preventing modeling
July 29th, 2009
International Summit on Cement Hydration Kinetics, Quebec, 27-29 July 2009
4
2. Mechanisms of Acceleration with Calcium Chloride
• Kinetic Parameters: QENS work by Peterson & Juenger (2006) with 2% CaCl2 (C3S wt)
V.K. Peterson, M.C.G. Juenger. Chem. Mater. 2006, 18, 5798-5804
Length of induction
period
Rate of formation of
hydrated phases (BWI)
Length of ‘nucleation
and growth’ period
Degree of hydration at
early ages
Diffusion
coefficient
More permeable (higher SSA) hydrates
July 29th, 2009
International Summit on Cement Hydration Kinetics, Quebec, 27-29 July 2009
5
2. Mechanisms of Acceleration with Calcium Chloride (cntd)
• More permeable C-S-H with CaCl2:
• Juenger et al, 1995: Ability of CaCl2 to flocculate hydrophilic colloids, resulting in a more
permeable C-S-H surface layer, through which water and ions can diffuse faster (higher
hydration rate during first stages of diffusion-controlled period)
M.C.G. Juenger et al. Cem. Conc. Res. 2005, 35, 19-25
July 29th, 2009
International Summit on Cement Hydration Kinetics, Quebec, 27-29 July 2009
6
2. Mechanisms of Acceleration with Calcium Chloride (cntd)
• Interaction with aluminate phases:
• Chlorides participate of aluminates reactions, forming chloroaluminate phases mostly when
sulfate available is not enough to react with C3A. Ettringite will not convert to monosulfate if
free chlorides are available (Tenoutasse, 1980).
Uptake of CaCl2 by C3A (no sulfates
present)
V. Dodson. Concrete Admixtures, 1990.
July 29th, 2009
International Summit on Cement Hydration Kinetics, Quebec, 27-29 July 2009
Source: Dodson, 1990
7
2. Mechanisms of Acceleration with Calcium Chloride (cntd)
• CaCl2 dosage effect
Non linear dose x
performance for vast
majority of systems…
Limit for
reinforced
concrete
July 29th, 2009
International Summit on Cement Hydration Kinetics, Quebec, 27-29 July 2009
8
2. Mechanisms of Acceleration with Calcium Chloride (cntd)
• Impact of chlorides on blended cements
0.42%NaCl
70% slag cement
0.40%CaCl2
0.02% Na-Gluconate
• NaCl x CaCl2
• SCN x Cl
Blank
4.00E+00
50% slag cement
Amine:NaCl
3.50E+00
600ppm Cl-
Amine:CaCl2
Amine:NaSCN
3.00E+00
700ppm SCN-
Blank
Power [mW/g]
2.50E+00
Amine
2.00E+00
1.50E+00
Same impact on
mortar strength
regardless the type
of salt
1.00E+00
5.00E-01
0.00E+00
1
3
July 29th, 2009
5
7
9
11
13
15
17
International Summit on Cement Hydration Kinetics, Quebec, 27-29 July 2009
Times [Hours]
9
2. Mechanisms of Acceleration with Calcium Chloride (cntd)
OPC
• SCMs, alkalis, sulfates,
additives…
High alkali cement
30% C ash
30% slag
• Light colors: 600ppm
CaCl2 (0.06%)
30% F ash
• SCM content and
characteristics play key
role in the interaction with
chemicals
• Fly ash presents a huge
challenge on its own
Low alkali cement
• Particle to particle
variation
OPC
• Presence of contaminants
(e.g. carbon particles)
30% F ash
30% C ash
30% slag
July 29th, 2009
International Summit on Cement Hydration Kinetics, Quebec, 27-29 July 2009
10
3. Mechanisms of Retardation with Sucrose
Impact on kinetics
Mechanisms of interaction with
C3S
QENS of C3S with 0.01 and 0.05% sucrose:
Longer induction period; increased rate of
formation of hydrated phases; longer
nucleation and growth period, resulting in
higher degree of hydration after this period.
Higher diffusion coefficient.
Chelation of Ca2+ & adsorption onto C-SH and CH nuclei (growth poisoning).
More nuclei form. Heterogeneous growth
after sucrose depletion: “delayed
accelerator”.
Mechanisms of
interaction with C3A
Accelerates ettringite
formation due to
consumption of Ca2+ from
gypsum.
Formation of interlayer
complexes with hydrated
aluminate phases.
a-glucose
V.K. Peterson, M.C.G. Juenger. Chem. Mater. 2006,
18, 5798-5804
July 29th, 2009
frutose
M.C.G. Juenger, H.M. Jennings, Cem. Conc.
Res. 2002, 32, 393-399.
International Summit on Cement Hydration Kinetics, Quebec, 27-29 July 2009
11
3. Mechanisms of Retardation with Lignosulfonate
Lignosulfonates may contain up to 30% sugars/sugar acids
Mechanisms of • Strong retardation of C3S
hydration.
interaction with
C3S
• Adsorption of sulfonate and OH
groups onto C-S-H and CH,
possibly incorporating into the CS-H gel layer. Possible formation
of a more impermeable hydrated
layer on cement grains (diffusion
barrier to hydration)
Adsorption of LS on
OPC
• Chelation of Ca2+ by the
polymer.
Bishop and Barron, 2006
M.R. Rixom, N.P. Mailvaganam, 1999
Mechanisms of • Strongly adsorbs on AFt and
AFm phases (C3A is said to be a
interaction with
“sink” for LS). Molecules can
C3A
enter the layers of aluminate
hydrates (intercalation).
• Delayed addition of LS reduces
adsorption onto aluminate
phases: more LS to retard C3S
July 29th, 2009
International Summit on Cement Hydration Kinetics, Quebec, 27-29 July 2009
12
3. Mechanisms of Retardation with Na-gluconate
• Impact of delayed addition (3 minutes) of Na-gluconate in two different cements
30% C ash
July 29th, 2009
International Summit on Cement Hydration Kinetics, Quebec, 27-29 July 2009
13
4. Knowledge gaps
• Controversies
• Different starting materials (C3A, C3S) with different reactivity
• Full analysis of sulfate source not provided
• Different mixing conditions
• Different contents of water
• Mechanisms of hydration with very high dosages of admixtures
• Non-linearity in dose x performance
• Structure for complexes formed between organic molecules and cement ions not
agreed upon
• Timing factor: delayed addition of chemicals
• More than one admixture in the same system: synergistic effects?
July 29th, 2009
International Summit on Cement Hydration Kinetics, Quebec, 27-29 July 2009
14
4. Knowledge gaps
• Impact of crystal structure
of anhydrous phases
ORTHOROMBIC C3A
CUBIC C3A
V.K. Peterson, M.C.G. Juenger. Chem. Mater. 2006, 18, 5798-5804
July 29th, 2009
International Summit on Cement Hydration Kinetics, Quebec, 27-29 July 2009
15
Bibliography
•
J.F. Young. A review of the mechanism of set-retardation in portland cement pastes containing organic
admixtures. Cem. Conc. Res. 1972, 2, 415-433.
•
N. Tenoutasse. The hydration mechanism of C3A and C3S in the presence of calcium chloride and calcium
sulphate. 7th ICCC, Paris, 1980. Supplementary paper II-118.
•
W.L. De Keyser, N. Tenoutasse. The hydration of the ferrite phase of cements. 7 th ICCC, Paris, 1980.
Supplementary paper II-120.
•
N.B. Singh, P.N. Ojha. Effect of CaCl2 on the hydration of tricalcium silicate. J. Mater. Sci. 1981, 16, 26752681.
•
N.L. Thomas, J.D. Birchall. The retarding action of sugars on cement hydration. Cem. Conc. Res. 1983, 13,
830-842.
•
V. Dodson. Concrete Admixtures. New York: Van Nostrand Reinhold, 1990.
•
V.S. Ramachandran (Ed.). Concrete admixtures handbook, Noyes Publications, New Jersey, 1995.
•
M.R. Rixom, N.P. Mailvaganam. Chemical admixtures for concrete, E&FN Spon Ltd, London, UK, 1999.
•
M.C.G. Juenger, H.M. Jennings. New insights into the effects of sugar on the hydration and microstructure of
cement pastes. Cem. Conc. Res. 2002, 32, 393-399.
•
M.C.G. Juenger, P.J.M. Monteiro, E.M.Gartner, G.P. Denbeaux. A soft X-ray microscope investigation into the
effects of calcium chloride on tricalcium silicate hydration. Cem. Conc. Res. 2005, 35, 19-25.
•
V.K. Peterson, M.C.G. Juenger. Hydration of tricalcium silicate: effects of CaCl2 and sucrose on reaction
kinetics and product formation. Chem. Mater. 2006, 18, 5798-5804.
•
V.K. Peterson, M.C.G. Juenger. Time-resolved quasielastic neutron scattering study of the hydration of
tricalcium silicate: Effects of CaCl2 and sucrose. Phys.B, 2006, 385-386, 222-224.
•
M. Bishop, A.R. Barron. Cement hydration inhibition with sucrose, tartaric acid, and lignosulfonate: analytical
and spectroscopic study. Ind. Eng. Chem, Res. 2006, 45, 7042-7049.
•
A.J. Allen, J.J. Thomas. Analysis of C-S-H gel and cement paste by small-angle neutron scattering. Cem.
Conc. Res. 2007, 37, 319-324.
July 29th, 2009
International Summit on Cement Hydration Kinetics, Quebec, 27-29 July 2009
16