Impact of Accelerators and Retarders on the Hydration of PC

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Impact of Accelerators and Retarders on the Hydration ofPortland Cement

Denise Silva



July 29 th, 2009 International Summit on Cement Hydration Kinetics, Quebec, 27-29 July 2009 2

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

Overview




• Point of view of a formulator

• Development of products for cement plants: dosage constraints (~0.03 - 0.3%) andspecific 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 agiven 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 structureof individual anhydrous phases; presence of SCM; alkali and sulfate contents; etc.

1. Introduction




• Retarders

• Water soluble salts: sodium metaborate, sodium tetraborate, stannous sulfate, leadacetate, monobasic calcium phosphate.

• Salts of lignosulfonic acid (Ca, Na, NH 4)

• Salts of hydroxylated carboxylic acids (Na, NH 4)

• 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

1. Introduction




• Kinetic Parameters: QENS work by Peterson & Juenger (2006) with 2% CaCl 2 (C 3S wt)

2. Mechanisms of Acceleration with Calcium Chloride

Length of inductionperiod

Rate of formation ofhydrated phases (BWI)

Length of ‘nucleationand growth’ period

Degree of hydration atearly ages

Diffusioncoefficient

More permeable (higher SSA) hydrates

V.K. Peterson, M.C.G. Juenger. Chem. Mater. 2006, 18, 5798-5804




2. Mechanisms of Acceleration with Calcium Chloride (cntd)

• More permeable C-S-H with CaCl 2:

• Juenger et al, 1995 : Ability of CaCl 2 to flocculate hydrophilic colloids, resulting in a morepermeable C-S-H surface layer, through which water and ions can diffuse faster (higherhydration rate during first stages of diffusion-controlled period)

M.C.G. Juenger et al. Cem. Conc. Res. 2005, 35, 19-25





• Interaction with aluminate phases:

• Chlorides participate of aluminates reactions, forming chloroaluminate phases mostly whensulfate available is not enough to react with C 3 A. Ettringite will not convert to monosulfate iffree chlorides are available (Tenoutasse, 1980).

Uptake of CaCl 2 by C3A (no sulfatespresent)

V. Dodson. Concrete Admixtures, 1990.

Source: Dodson, 1990




• CaCl 2 dosage effect


Limi t fo rre inforced

concrete

Non l inear dose xperformance for vas tmajority of systems…





• Impact of chlorides on blended cements

• NaCl x CaCl 2

• SCN x ClBlank

0.42%NaCl 0.40%CaCl 2

0.02% Na-Gluconate

70% slag cement

0.00E+00

5.00E-01

1.00E+00

1.50E+00

2.00E+00

2.50E+00

3.00E+00

3.50E+00

4.00E+00

1 3 5 7 9 11 13 15 17

Times [Hours]

P o w

e r [ m W / g ]

Blank

Amine:CaCl2

Amine:NaCl

Amine:NaSCN

Am i ne

50% slag cem ent

600pp m Cl -

700pp m SCN -

Same impact onmortar s t rength

regardless the typeof sal t





• SCMs, alkalis, sulfates,additives…

• Light colors: 600ppmCaCl 2 (0.06%)

• SCM content andcharacteristics play keyrole in the interaction withchemicals

• Fly ash presents a hugechallenge on its own

• Particle to particlevariation

• Presence of contaminants(e.g. carbon particles)

Low alkal i cement

High alkal i cement

30% C ash

OPC

30% slag

30% F ash

30% C ash

OPC

30% slag

30% F ash




3. Mechanisms of Retardation with Sucrose

Impact on kinetics Mechanisms of interaction withC3S

Mechanisms ofinteraction with C 3A

QENS of C 3S with 0.01 and 0.05% sucrose:Longer induction period; increased rate offormation of hydrated phases; longernucleation and growth period, resulting inhigher degree of hydration after this period.Higher diffusion coefficient.

V.K. Peterson, M.C.G. Juenger. Chem. Mater. 2006,18, 5798-5804

Chelation of Ca 2+ & adsorption onto C-S-H and CH nuclei (growth poisoning).More nuclei form. Heterogeneous growthafter sucrose depletion: “delayedaccelerator”.

M.C.G. Juenger, H.M. Jennings, Cem. Conc.Res. 2002, 32, 393-399.

Accelerates ettringiteformation due toconsumption of Ca 2+ fromgypsum.

Formation of interlayercomplexes with hydratedaluminate phases.

frutosea- glucose

http://en.wikipedia.org/wiki/File:Sucrose.gif




3. Mechanisms of Retardation with Lignosulfonate

Mechanisms ofinteraction with

C3S

• Strong retardation of C 3Shydration.

• Adsorption of sulfonate and OHgroups onto C-S-H and CH,possibly incorporating into the C-S-H gel layer. Possible formationof a more impermeable hydratedlayer on cement grains (diffusionbarrier to hydration)

• Chelation of Ca 2+ by thepolymer.

Bishop and Barron, 2006

Mechanisms ofinteraction with

C3A

• Strongly adsorbs on AFt and AFm phases (C 3 A is said to be a“sink” for LS). Molecules canenter the layers of aluminatehydrates (intercalation).

• Delayed addition of LS reducesadsorption onto aluminatephases: more LS to retard C 3S

M.R. Rixom, N.P. Mailvaganam, 1999

Lignosulfonates may contain up to 30% sugars/sugar acids

Adsorpt ion of LS onOP C




3. Mechanisms of Retardation with Na-gluconate• Impact of delayed addition (3 minutes) of Na-gluconate in two different cements

30% C ash




• Controversies

• Different starting materials (C 3A, C 3S) 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 notagreed upon

• Timing factor: delayed addition of chemicals

• More than one admixture in the same system: synergistic effects?

4. Knowledge gaps




• J.F. Young. A review of the mechanism of set-retardation in portland cement pastes containing organicadmixtures. 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. 7 th 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, 2675-2681.

• 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 ofcement 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 theeffects 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: analyticaland 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.

Bibliography

Impact of Accelerators and Retarders on the Hydration of PC

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