Impact of Accelerators and Retarders on the Hydration of...

Impact of Accelerators and Retarders on the Hydration of

Portland Cement

Denise Silva

July 29th, 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%) 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.

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

1. Introduction


• Kinetic Parameters: QENS work by Peterson & Juenger (2006) with 2% CaCl2 (C3S wt)

2. Mechanisms of Acceleration with Calcium Chloride

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

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 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



• 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.

Source: Dodson, 1990


• CaCl2 dosage effect


Limit for

reinforced

concrete

Non linear dose x

performance for vast

majority of systems…



• Impact of chlorides on blended cements

• NaCl x CaCl2

• SCN x ClBlank

0.42%NaCl 0.40%CaCl2

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]

Po

we

r [m

W/g

]

Blank

Amine:CaCl2

Amine:NaCl

Amine:NaSCN

Amine

50% slag cement

600ppm Cl-

700ppm SCN-

Same impact on

mortar strength

regardless the type

of salt



• SCMs, alkalis, sulfates,

additives…

• Light colors: 600ppm

CaCl2 (0.06%)

• SCM content and

characteristics play key

role in the interaction with

chemicals

• Fly ash presents a huge

challenge on its own

• Particle to particle

variation

• Presence of contaminants

(e.g. carbon particles)

Low alkali cement

High alkali 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 with

C3S

Mechanisms of

interaction with C3A

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.

V.K. Peterson, M.C.G. Juenger. Chem. Mater. 2006,

18, 5798-5804

Chelation of Ca2+ & adsorption onto C-S-

H and CH nuclei (growth poisoning).

More nuclei form. Heterogeneous growth

after sucrose depletion: “delayed

accelerator”.

M.C.G. Juenger, H.M. Jennings, Cem. Conc.

Res. 2002, 32, 393-399.

Accelerates ettringite

formation due to

consumption of Ca2+ from

gypsum.

Formation of interlayer

complexes with hydrated

aluminate phases.

frutosea-glucose

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


3. Mechanisms of Retardation with Lignosulfonate

Mechanisms of

interaction with

C3S

• Strong retardation of C3S

hydration.

• Adsorption of sulfonate and OH

groups onto C-S-H and CH,

possibly incorporating into the C-

S-H gel layer. Possible formation

of a more impermeable hydrated

layer on cement grains (diffusion

barrier to hydration)

• Chelation of Ca2+ by the

polymer.

Bishop and Barron, 2006

Mechanisms of

interaction with

C3A

• Strongly adsorbs on AFt and

AFm phases (C3A is said to be a

“sink” for LS). Molecules can

enter the layers of aluminate

hydrates (intercalation).

• Delayed addition of LS reduces

adsorption onto aluminate

phases: more LS to retard C3S

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

Lignosulfonates may contain up to 30% sugars/sugar acids

Adsorption of LS on

OPC


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 (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?

4. Knowledge gaps


• Impact of crystal structure

of anhydrous phases

4. Knowledge gaps

CUBIC C3A

ORTHOROMBIC C3A

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


• 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. 7th 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 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.

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