The Role of Accelerated Carbonation in the Accelerated Ageing of

Stabilised/Solidified Waste Forms

Ramesh Perera and Abir Al-Tabbaa

International ConferenceInternational ConferenceStabilisation/Solidification Treatment and RemediationStabilisation/Solidification Treatment and Remediation12 12 –– 13 April 200513 April 2005Cambridge, UKCambridge, UK Department of EngineeringDepartment of Engineering

Outline of the presentationOutline of the presentation

• Background

• Accelerated carbonation

• Details of mixes

• Results

• Concluding remarks

BackgroundBackground

Validation of S/S treated materials in the long-term is limited

There is a need for more longThere is a need for more long--term real time dataterm real time data

There is also a need to develop accelerated ageing There is also a need to develop accelerated ageing techniquestechniques

Suitable methods: elevated temperature, chemical accelerators and accelerated carbonation

Accelerated ageingAccelerated ageing

Elevated temperature: accelerates hydration reactions, temperature used up to 80oC, higher temperature changes microstructure

Chemical accelerators: accelerates hydration reactions, but mainly at the beginning, introduces an additional variable

Accelerated carbonation: speeds up carbonation which is a secondary reaction to hydration

Accelerated carbonationAccelerated carbonation

Carbonation is a slow naturally occurring process in cementitious material – mainly hydration products

Ca+2 + CO2 CaCO3

Accelerated carbonation involves increasing the rate at which carbonation occurs

Overall process quite complex and involves both the principal phases and the hydration products

Factors influencing rate of carbonation

• Level of CO2 concentration

• CO2 rate

• Permeability/porosity of material

• Internal surface area

• Presence of certain ions

• Temperature and relative humidity

• Binder system

Model contaminated soil

Sand & gravel - based on West Drayton site soilGravel Sand Silt Kaolin Bentonite Water 45.45 33.64 5.91 5.27 0.64 9.09

Contaminant loading (mg/kg soil)

Mix formulations

3 grouts used : cement & PFAcement, PFA & limecement & bentonite

Mix B:Cement : PFA = 3 : 8

Water : Dry grout = 0.42 : 1

Soil : Grout = 5 : 1

Both contaminated grouts and soil-grouts examined

Curing conditions & durations

Carbonated samples cured at ~ 100% CO2 concentration for duration of curing

Control samples cured at atmospheric CO2 concentration for duration of curing

Both were at room temperature and high relative humidity

Samples were tested at 28, 60 and 90 days

UCS of mix B

grout control grout carbonatedsoil-grout control soil-grout carbonated

0.0

0.2

0.4

0.6

0.8

1.0

20 40 60 80 100Time (days)

UC

S (M

Pa)

Bgcon BgcarbBsgcon Bsgcarb

Carbonated depth

Mix 28 day(mm)

60 day(mm)

90 day(mm)

B grout control 0.0 <0.1 <0.1B grout carbonated 2.7 3.5 4.5

B soil-grout control <0.1 5.0 7.2B soil-grout carbonated full full full

Sample radius 25 mm

Thermo-gravimetry was adopted to quantify carbonate content

NRA leachate pH

28 day 60 day 90 dayouter core outer core outer core

B grout control 12.1 12.2 - - 11.9 12.1B grout carb. 11.1 11.8 11.1 12.0 11.1 12.0

B soil-grout control 11.8 12.1 11.1 12.0 11.2 11.8B soil-grout carb. 8.0 8.0 7.9 7.9 7.9 7.9

Mix

NRA leachate concentrations of Pb

0

5

10

15

20

20 40 60 80 100Time (days)

Con

cent

ratio

n (m

g/l)

Bgcono BgconcBgcarbo Bgcarbc

0

1

2

3

4

5

20 40 60 80 100Time (days)

Con

cent

ratio

n (m

g/l) Bsgcono Bsgconc

Bsgcarbo Bsgcarbc

control outer control corecarbonated outer carbonated core

SEM images at 28 daysGrout

Soil-grout

(a) control outer(b) control core(c) carb outer(d) carb core

E – ettringiteP – PFACC – CaCO3H – hydration products

(a) (b)

(c) (d)

E

PP

PP

CC

CC

CC

H

E

(c) (d)

(a) (b)

E

P

P

CC

H H

H

P

E

Conclusions

Rate of carbonation of soil-grouts greater than grouts due to higher porosity

UCS of grout B was not a very good parameter for use under these conditions

Soil-grout B showed strength gain at early ages, but this was inhibited once the samples were fully carbonated

SEM images was an useful visual aid to changes to microstructure due to carbonation

Carbonated depth, NRA leachate pH and Pb leaching results were good indicators of the degree of carbonation

Thank you