Carbonation kinetics of a bed of recycled concrete ...Thiery M. (Ifsttar) Carbonation of RCA May 30,...

Carbonation kinetics of a bed of recycled concrete

aggregates: a laboratory study on model materials

WASCON 2012, 30 May-1 June 2012, Gothenburg, SWEDEN

Thiery M.∗, Dangla P.∗⋄, Belin P.∗, Habert G.∗, Roussel N.∗

∗Ifsttar, Materials Departement, Paris, France⋄Navier, Ecole des Ponts/CNRS/Ifsttar, Champs-sur-Marne, France

May 30, 2012

Thiery M. (Ifsttar) Carbonation of RCA May 30, 2012 1 / 27

CO2 uptake

CO2 uptake

Possible CO2 uptake during a stockpiling period of RCA (after demolition and crushing,before the secondary use in road construction)

Nordic research [Engelsen, Jonsson, Kjellsen, Lagerblad, 2005] [Nilsson and Fridh, 2010]

CO2 absorption in concrete thanks to carbonation can be compared to CO2 absorption intrees by photosynthesis

Why not taking advantage of carbonation to partially re-absorb CO2 which was initiallyemitted during limestone decalcification?


Outline

1 Carbonation: deleterious and positive aspects

2 Materials and experimental program

3 A model for CO2 uptake through a packing of CPA

4 Investigation of the carbonation kinetics of a granular bed of CPA


Neutralization of the cementitious system

Consequences on durability of RC

CO2 diffusion through the concrete cover - dissolution in the pore water film

The reduction in the buffering capacity (CH and C-S-H) - Formation of CC

Depassivation - corrosion of rebars







Fig. 1: CH macrocrystals (SEM-SEobservations).

Fig. 2: CH macrocrystal surrounded by CC(SEM-SE).







Fig. 1: Corrosion of rebars (Ifsttar).

0 5 10 15−1

−0.5

0

0.5

1

1.5

pH

E (

V) Fe(OH)

3

Fe2+

Fe(OH)2

HFeO2−

Fe3+

Fe

O2

H2O

H2O

H2

Valeur usuelle du pH du béton

Carbonata

tion

Fig. 2: Pourbaix’ diagram.


Clogging effect

Porosity changes

Clogging of the porosity (formation of CC)

Reduction in porosity is both related to CH and C-S-H carbonation[Thiery, 2007-2011] [Castellote, 2008-2009]

0.25 0.35 0.45 0.50 0.600

0.02

0.04

0.06

0.08

0.1

0.12

0.14

W/C ratio

∆φ(−)

∆φ (H2O)

∆φ(Hg)

∆φ(mod. CH +C−S−H)

∆φ(mod. CH)

Fig. 3: Decrease of porosity observed on fully carbonated cement pastes exposed to CO2 = 50%.


Improvement of RCA quality

Improvement of the quality of Recycled Concrete Aggregates (RCA)

Lower absorption coefficient (cf. water demand issue)

Improved quality of the attached mortar phase (denser interface between the adheringmortar phase and the new cement paste) [de Juan, 2009] [Abbas, 2009]

Higher density, lower Los Angeles abrasion cœfficient

Higher chemical stability


Materials

Materials

Cement pastes made of OPC (CEM I 52.5): w/c = 0.25− 0.8

Preparation in plastic bottles (use of a rotating system)

Sealed curing conditions during 18 months (in plastic bottles)

Crushed pieces (1.5-2.5 mm size) → Cement Paste Aggregates (CPA)


Experimental protocol

Carbonation procedure

Drying pretreatment: RH = 53% until mass stabilization

Carbonation: CO2 = 10% and RH = 53% (incubator)


Experimental protocol

A beaker (7 cm diameter and 9 cm height) was filled with sieved aggregates of cement paste(1.5 - 2.5 mm) up to a height of 7 cm.

The beaker was put inside a CO2 incubator (CO2 = 10% and RH = 53%). For each filledcentimetre inside the beaker, a plastic grid of 1 mm mesh was added. The role of these grids isto easily separate the different layers of cement paste grains for TGA-MS measurements inorder to determine carbonation profiles.


Carbonation monitoring

Gamma-ray attenuation and TGA-MS✄

✂

�

✁Carbonation conditions: CO2 = 10% and RH = 53%


Carbonatable amounts of CH and C-S-H

Investigation of the CO2 binding capacity of CH and C-S-H by TGA-MS

TGA coupled with Mass Spectrometry (MS): enhancement of the assessment of the CO2binding capacity of each hydration product ⇒ more accurate assessment of thetemperature range of CC decomposition (CO2 emission)

Necessity to carry out a fitting procedure for quantification

TGA-MS on carbonated CP80

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

200 400 600 800 1000 12000

0.2

0.4

0.6

0.8

1

DTG[%.min-1]

Ioncurrent[Ax10-9]

T [oC]

DTGICO2IH2O

Overlap

CO2 - H2O

Carbo. hcp

w/c = 0.8

MS as a tool of CO2 assessment

Carbo. hcp

w/c = 0.8

aCO2 = 7.3x10-10 %.min-1.A-1

aH2O = 25.5x10-10 %.min-1.A-1

0

0.2

0.4

0.6

0.8

1

1.2

1.4

200 400 600 800 1000 12000

0.2

0.4

0.6

0.8

1

-DTG[%.min-1]

Ioncurrent[Ax10-9]

T [oC]

MS: αCO2xICO2

+ αH2OxIH2ODTG


Carbonation model

Main assumptions

Spherically-shaped CPA

Dual scale modelling: CO2diffusion through the granularpacking and carbonation ofeach CPA

Each CPA is supposed atequilibrium with thesurrounding RH (the moisturetransfers are neglected)


Carbonation model

Main assumptions

The carbonation mechanism of each CPA

is governed by CO2 diffusion through thecementitious matrix within each CPA[Castellote et al., 2008][Muntean et al., 2008]

Chemical reactions are instantaneous withrespect to CO2 diffusion

Propagation of a sharp carbonation front

through each CPA

Stationary CO2 diffusion (Fick’s law) withinthe fully carbonated region

RC-δ

RC

R

nCaCO3 [mol/L (material)]

Partia

lly r

eacte

d a

rea

Fu

lly r

eacte

d a

rea

RCR

δ

CO2

CO2

CO2

n0CaCO3


Set of equations

Mass balance equation

Φ∂Cg

∂t+ (1− Φ)ξ =

∂

∂xD∂Cg

∂x

where the sink term ξ represents the carbonation rate (per unit volume of CPA). ξ is provided by:

ξ =[

nCHCC

+ nCSHCC

] dη

dt=

3DeffCg

R2

[1− η]1/3

1− [1− η]1/3if η ≤ 1

Variables

Cg (x , t): CO2 concentration within the gaseous phase of the granular packing (at depth x)

η(x , t): carbonation degree (at depth x)

η = 1−R3c

R3

where Rc is the radius of the carbonated concentric layer and R is the CPA radius


Carbonation model

Φ∂Cg

∂t+ (1− Φ)ξ =

∂

∂xD∂Cg

∂x

ξ =[

nCHCC

+ nCSHCC

]dη

dt=

3DeffCg

R2

[1− η]1/3

1− [1− η]1/3if η ≤ 1

Parameters

Φ: porosity of the packing of CPA (≃ 0.5− 0.6)

nCHCC

+ nCSHCC

: CO2 binding capacity (per unit volume of CPA)

D: effective CO2 diffusion coefficient through the granular bed

D = D0Φ4/3(1− Sl )

10/3 ≃ D0Φ4/3

Deff: effective CO2 diffusion coefficient through each CPA

Deff = D0φa(1− sl )

b

where φ stands for the CPA porosity and sl represents the CPA liquid-water saturation. slis inferred from the surrounding RH according to the desorption isotherm of the cementpaste [Baroghel-Bouny, 2007]


Boundary and initial conditions

Boundary and initial conditions are formulated as follows

Cg (x = 0, ∀t > 0) = C0g

Cg (∀x ≥ 0, t = 0) = 0

η(∀x ≥ 0, t = 0) = 0

where C0g is the CO2 concentration prevailing in the atmosphere outside the granular bed



0

1

2

3

4

5

6

0 10 20 30 40 50 60 70

nC

O2 [

mo

l.L

-1]

x [mm]

C80

gamma t=6hgamma t=1dgamma t=2dgamma t=3d

modelTGA t=3d

Fig. 4: Penetration of carbonation through abed of CPA made of C80 (w/c = 0.8).

CH

decomposition CaCO3decomposition

C80

400 500 600 700 800 900

DTG

T [oC]

45-55 mm

35-45 mm

25-35 mm

15-25 mm

5-15 mm

0-5 mm

Fig. 5: DTG diagrams performed on variouslayers of cement paste (C80, w/c = 0.8)taken from the granular bed (after 3 days ofexposure).



0

1

2

3

4

5

6

0 10 20 30 40 50 60 70

nC

O2 [

mo

l.L

-1]

x [mm]

C80

gamma t=6hgamma t=1dgamma t=2dgamma t=3d

modelTGA t=3d

Fig. 6: Penetration of carbonation through abed of CPA made of (w/c = 0.8).

0

1

2

3

4

5

6

0 10 20 30 40 50 60 70

nC

O2 [

mo

l.L

-1]

x [mm]

C55

gamma t=6hgamma t=1dgamma t=2d

modelTGA t=2d

Fig. 7: Penetration of carbonation through abed of CPA made of C55 (w/c = 0.55).



1

2

3

4

0 20 40 60

[CO

2]

[m

ol.m

-3]

x [mm]

model C55 2dmodel C80 2d

0

0.2

0.4

0.6

0.8

1

0 5 10

MC

O2/M

CO

2

ma

x [

-]t [d]

model C80model C55

gamma C80gamma C55C80 no diff.C50 no diff.

Fig. 8: a/ Profiles of CO2 through the packing of CPA (C55 and C80). b/ Fraction of CO2absorbed by the overall packing over time.

✄

✂

�

✁Non-uniform CO2 profile through the granular bed


Sensitivity analysis: influence of w/c (type-I cement)

✄

✂

�

✁Natural conditions of carbonation: CO2 = 0.04% and RH = 53%

w/c φ[

nCHCC

+ nCSHCC

]

sl Γ t1/2

[-] [-] [mol.L−1] [-] [-] [d]0.45 0.41 6.0 0.41 3.2 4660.55 0.49 5.6 0.28 11.9 3870.60 0.54 5.1 0.25 18.5 3390.80 0.61 4.1 0.21 31.9 273

Tab. 1: Main characteristics of the studied cement pastes. φ corresponds to theaccessible-to-water porosity. sl is the liquid-water saturation degree corresponding to RH =53%according to the desorption isotherm of each material

Ratio between the characteristic time of CO2 diffusion through the bed of CPA and thecharacteristic time of CO2 absorption:

Γ ≃τD

τab≃

3(1 −Φ)L2Deff

R2D


Sensitivity analysis: influence of R and sl

0

0.2

0.4

0.6

0.8

1

0 500 1000 1500 2000

MCO2/MCO2

max[-]

t [d]

0

0.2

0.4

0.6

0.8

1

0 500 1000 1500 2000

t [d]

R = 20 mm

10 mm

7 mm

5 mm

3 mm

0.1 / 0.5 / 1 / 2 mm

Instantenous

carbonation

Instantenous

carbonation

sl = 0.9

0.8

0.7

0.50.6

0.4 / 0.3 / 0.2

Fig. 9: a/ Time evolution of the CO2 fraction absorbed during natural carbonation of a CPAbed made of C55. Influence of the radius of the CPA. b/ Time evolution of the CO2 fractionabsorbed during natural carbonation of a CPA bed made of C60. Influence of the water contentwithin the cementitious matrix.

For R < 2 mm or sl < 0.4, the CO2 absorption rate is controlled by the CO2 diffusion throughthe granular packing.


Conclusion

Model material: bed of CPA

Proposal of a model to quantify the rate of CO2 uptake through a bed of CPA (verificationfor CO2 = 10%)

Sensitivity analysis for actual conditions of carbonation. Which values of sl and R canaccelerate CO2 absorption? sl < 0.4 and R < 2 mm

At least, one year of exposure to real conditions of carbonation is necessary to absorb 50%of the CO2 binding capacity


Conclusion

What about the carbonation kinetics of a heap of RCA?

Proposal of a model for CO2 absorption through a heap of RCA. What is differentbetween CPA and RCA?

- Presence of an inert mineral phase- It is not a concentric layer of mortar which is bound around the original aggregate,but rather local patches

- Poly-dispersed granular medium- CO2 binding capacity influenced by carbonation during service life and crushingintensity

One thing is certain: it is important to protect the heap of RCA from rainfalls to acceleratethe CO2 absorption kinetics


Carbonation: deleterious and positive aspectsMaterials and experimental programA model for CO2 uptake through a packing of CPAInvestigation of the carbonation kinetics of a granular bed of CPA

Carbonation kinetics of a bed of recycled concrete ...Thiery M. (Ifsttar) Carbonation of RCA May 30,...

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