Qingke Nie1

Changjun Zhou2

Huawei Li1

Xiang Shu3

Baoshan Huang3

3Hebei Research Inst. of Construction & Geotechnical Investigation Co., Ltd.

3Harbin Institute of Technology

3The University of Tennessee, Knoxville

At

International Symposium on Systematic Approaches to Environmental

Engineering in Transportation

Sulfate Attack

Soils in Xinjiang Autonomous Province,

China

Mechanism of Sulfate Attack

HSAC 16H HS2C HSAC

(aq) 2OH HSC (aq)SO CH

32362124

-

2

-2

4

• Improve strength, workability, durability;

• Generally less expansive than cement;

• Green. Many are industrial by-products, like

fly ash and silica fume.

SCM’s Advantages

Laboratory Tests

Part 1

Laboratory Tests

Objectives: to find an optimal concrete

Resisting the sulfate attack;

Meeting compressive strength, workability,

etc;

Utilizing the locally available materials

adequately, especially the SCMs.

Laboratory Tests

• Chemical Analysis

• XRD

• Calorimetry test

• Cube compressive strength test

• Mortar bar expansion exposed to a sulfate

solution

• Chloride ion penetration test

Chemical Analysis

SiO221.34% Al2O3

4.06%

Fe2O3(T)5.45%

MnO0.30%

MgO2.20%

CaO63.22%

Others 3.43%

Sulfate Resisting Cement

SiO223.14%

Al2O36.62%

Fe2O3(T)3.93%

MnO0.12%

MgO1.94%

CaO60.63%

Others 3.63%

Portand Cement

SiO260.09%

Al2O319.51%

Fe2O3(T)6.69%

MnO0.09%

MgO2.23%

CaO4.88%

Others 6.52%

Fly Ash 1

SiO250.52%

Al2O334.46%

Fe2O3(T)4.23%

MnO0.04%

MgO0.56%

CaO2.83%

Others 7.36%

GN Admixture

FAII

XRD

Quartz

58%

Diaoyudaoite

15%

Mullite 15%

Na2Al22O34·2H2O

6%

Tobermorite 4% Other 2%

C3S 52%

C2S 27%

C3A 5%

C4AF 12%

Other 4%

SRC

Calorimetry test

OPC OPC: FAI OPC:FAII OPC: S75 OPC: S95 OPC: SF

1 0.7:0.3 0.7:0.3 0.7:0.3 0.7:0.3 0.9:0.1

OPC:

CM OPC: GN

OPC: FAI:

CM OPC: FAII OPC: FAII

OPC: FAII:

S95

1:0.1 1:0.03 1:0.11:0.1 0.55:0.45 0.6:0.4 0.6:0.2:0.2

SRC SRC: CM SRC: GN SRC: FAI:

CM

Type I

Cement

Type I

Cement: FAI

1 1:0.1 1:0.03 1:0.11:0.1 1 0.7:0.3

w/cm=0.4

OPC

70%OPC+30%F

A I

70%OPC+30%FA II

70%OPC+30%

S75

70%OPC+30%

S95

Time since started (h)

Cube Compressive Strength & Mortar Bar Expansion

Rapid Chloride Permeability Test

Conclusions based on Tests

• SCMs decreased hydration rate and C3A content of cementitious materials;

• Compressive strength of combined cement mortar with SCMs met requirement;

• OPC+SCMs mortar and SRC mortar with/without CM admixture performed better than OPC mortar under sulfate environment;

Conclusions based on Tests

• According to chemical analysis, GN admixture

is similar to fly ash, while CM admixture is

similar to slag.

• OPC+fly ash concrete is a better choice than

SRC concrete in an environment enriching

both sulfate and chloride in soil.

• The recommended percentage of fly ash added

into concrete would be 25-35%.

Part 2

Numerical Simulation of

Sulfate Attack on Concrete

Acknowledgement

o Dr. Barzin Mobasher from Arizona State University

o Dr. Kimberly E. Kurtis from Georgia Technology of

Institute

o U.S. Bureau of Reclamation (USBR)

Chemical

components

change in cement

paste

Volume

expansion Cracks

Service

Life

Decrease

Sulfate

Diffusion

Accelerate Repeat Steps

Process of Sulfate Attack on Concrete

𝑁𝑢𝑚𝑒𝑟𝑖𝑎𝑙 𝑠𝑖𝑚𝑢𝑙𝑎𝑡𝑖𝑜𝑛 𝑤𝑖𝑙𝑙 𝑓𝑜𝑙𝑙𝑜𝑤 𝑡ℎ𝑖𝑠 𝑓𝑙𝑜𝑤 𝑐ℎ𝑎𝑟𝑡.

Reactions in harden cement paste causing expansion

𝐺𝑦𝑝𝑠𝑢𝑚

𝐶𝑎 𝑂𝐻 2 + 𝑁𝑎2𝑆𝑂4 ∙ 10𝐻2𝑂 → 𝐶𝑎2𝑆𝑂4 ∙ 2𝐻2𝑂 + 2𝑁𝑎𝑂𝐻 + 8𝐻2𝑂

𝐴𝐹𝑡 𝐶4𝐴𝐻13 + 3𝐶𝑆 𝐻2 + 14𝐻 → 𝐶6𝐴𝑆 3𝐻32 + 𝐶𝐻

𝐶4𝐴𝑆 𝐻12 + 2𝐶𝑆 𝐻2 + 16𝐻 → 𝐶6𝐴𝑆 3𝐻32

residual 𝐶3𝐴 + 3𝐶𝑆 𝐻2 + 26𝐻 → 𝐶6𝐴𝑆 3𝐻32

(𝑉𝑃+∆𝑉𝑃)/𝑉𝑃

2. 48

1.51

2.26

More complicated considerations

due to fly ash Pozzolanic reactions

3𝐶𝐻 + 2𝑆 = 𝐶3𝑆2𝐻3 3𝐶𝐻 + 𝐴 + 3𝐻 = 𝐶3𝐴𝐻6

𝐶𝑎(𝑂𝐻)2 𝑆𝑖𝑂2 𝐴𝑙2𝑂3 𝐻2𝑂

Good Things Bad Things

CH from cement hydration consumed

CaO in fly ash added

Makes concrete less permeable 𝐶3𝐴𝐻6 prone to sulfate attack

Concrete curing

Reactions in harden cement paste causing expansion

𝐴𝐹𝑡 𝐶4𝐴𝐻13 + 3𝐶𝑆 𝐻2 + 14𝐻 → 𝐶6𝐴𝑆 3𝐻32 + 𝐶𝐻

𝐶4𝐴𝑆 𝐻12 + 2𝐶𝑆 𝐻2 + 16𝐻 → 𝐶6𝐴𝑆 3𝐻32

residual 𝐶3𝐴 + 3𝐶𝑆 𝐻2 + 26𝐻 → 𝐶6𝐴𝑆 3𝐻32

𝐶𝐴 + 𝑞𝑆 → 𝐶6𝐴𝑆 3𝐻32

Extra Reactions due to Fly Ash under Sulfate Attack

Gypsum is from:

• residual gypsum after cement hydration;

• Sulfates reacts with CH.

𝐶3𝐴 + 3𝐶𝑆 𝐻2 + 26𝐻 → 𝐶6𝐴𝑆 3𝐻32

𝐶𝑎 𝑂𝐻 2 + 𝑁𝑎2𝑆𝑂4 ∙ 10𝐻2𝑂 → 𝐺𝑦𝑝𝑠𝑢𝑚 + 2𝑁𝑎𝑂𝐻 + 8𝐻2𝑂

𝐶4𝐴𝐻13 + 3𝐶𝑆 𝐻2 + 14𝐻 → 𝐶6𝐴𝑆 3𝐻32 + 𝐶𝐻

𝐶4𝐴𝑆 𝐻12 + 2𝐶𝑆 𝐻2 + 16𝐻 → 𝐶6𝐴𝑆 3𝐻32

residual 𝐶3𝐴 + 3𝐶𝑆 𝐻2 + 26𝐻 → 𝐶6𝐴𝑆 3𝐻32

Reactions in OPC concrete

As consumed by pozzolanic reactions from fly ash, the CH may be not enough to

produce enough gypsum to support the formation of ettringte, which can restrain

the expansion of concrete.

Available CH in Concrete?

Cement hydration produces CH

Fly ash contains CaO, which can be partially

converted into CH

Pozzolanic reactions consumes CH

𝐶𝐻𝑎𝑣𝑎𝑖 = 𝐶𝐻ℎ𝑦𝑑𝑟𝑎 + 𝐶𝐻𝑓𝑙𝑦𝑎𝑠ℎ − 𝐶𝐻𝑃𝑜𝑧𝑧

Diffusion of sulfate ions

a saturated concrete, unsteady state:

Molecular transport=convection+accumulation+reaction rate

combination of Fick’s diffusion, convection transport, and chemical reaction

𝐷∆2𝑐 = 𝑢𝛻𝑐 +𝜕𝑐

𝜕𝑡+ 𝑟

Where u is velocity; c is concentration; t is time; D is diffusion coefficient.

Diffusion of sulfate ions

In dilute solution, if no pressure and temperature gradients exists:

𝐷∆2𝑐 = 𝑢𝛻𝑐 +𝜕𝑐

𝜕𝑡+ 𝑟

𝜕𝑈

𝜕𝑇= 𝐷

𝜕2𝑈

𝜕𝑋2− 𝑘𝑈𝐶

𝜕𝐶

𝜕𝑇= −

𝑘𝑈𝐶

𝑞

Define: 𝑍 = 𝑈 − 𝑞𝐶

𝜕𝑍

𝜕𝑇= 𝐷

𝜕2𝑍

𝜕𝑋2 Only one unique variable, Z

Boundary conditions

Boundary conditions

let L be the thickness of the slab, X=xL, T=L2t/D, u=U/U0, z=Z/U0, and c=C/U0

𝜕𝑧

𝜕𝑡=𝜕2𝑧

𝜕𝑥2

𝜕𝑍

𝜕𝑇= 𝐷

𝜕2𝑍

𝜕𝑋2 Written as

𝜕𝑢

𝜕𝑡=

𝜕2𝑢

𝜕𝑥2− 𝑟𝑢2 + 𝑟𝑢𝑧 𝑟 =

𝑘𝐿2𝑈0𝑞𝐷

where:

boundary and initial conditions: for all t, at x=0 and x=1: u=1; for t=0,

0<x<1: u=0.

Numerical solution of the diffusion-reaction equation

truncated Taylor series

𝑢𝑖,𝑗+

1

2

= 𝑢𝑖,𝑗 + ∆𝑋2 𝑢𝑖,𝑗 − 𝑟𝑢𝑖,𝑗

2 + 𝑟𝑢𝑖,𝑗𝑧𝑖,𝑗 (∆𝑡

2)

𝑢𝑖,𝑗+

1

2

= 𝑢𝑖,𝑗 + ∆𝑋2 𝑢𝑖,𝑗 − 𝑟𝑢𝑖,𝑗

2 + 𝑟𝑢𝑖,𝑗𝑧𝑖,𝑗 (∆𝑡

2)

∆𝑋2 𝑢𝑖,𝑗 =

𝜕2𝑢

𝜕𝑥2=𝑢𝑖+1,𝑗 − 2𝑢𝑖,𝑗 + 𝑢𝑖−1,𝑗

∆𝑥 2 where:

𝑢𝑖,𝑗+1 − 𝑢𝑖,𝑗

∆𝑡=1

2∆𝑋2 𝑢𝑖,𝑗 + 𝑢𝑖,𝑗+1 − 𝑟

𝑢𝑖,𝑗 + 𝑢𝑖,𝑗+1

2𝑢𝑖,𝑗+

12+ 𝑟𝑢

𝑖,𝑗+12𝑧𝑖,𝑗

Crank-Nicolson formula for 𝑢𝑖,𝑗+

1

2

:

Cracking affects diffusion

(D1)max/ D2=10

Numerical solution of the moving boundary diffusion-reaction equation

1. Solve the equation for the fixed boundary (composite medium);

2. Solve the moving boundary problem for the diffusion equation with no reaction (2nd Fick’s law with moving boundary), for the two cases: discontinuous and continuous diffusivity.

3. Apply the method devised for the previous step to the moving diffusion-reaction equation.

Crystallization pressure of ettringite

Riecke principle:

𝑃 =𝑅𝑇

𝑉𝑠𝐿𝑛(

𝐶

𝐶𝑠)

where R is the ideal gas constant, T is temperature, 𝑉𝑠 is molar volume, C is

actual concentration of the solute during concentration, and 𝐶𝑠 is saturation

concentration.

For ettringite, at temperature 25OC, with a molar weight of 1252g and a

specific gravity of 1.78g/cm3, P = 2.4 - 8.1 MPa for a degree of

supersaturation 𝐶

𝐶𝑠 of 2 - 10.

Effect of crystallization pressure of ettringite

When crystallization occurs in pores at a distance

comparable to the size of a pre-existing crack,

and if the crystallization pressure is high enough, this

crack can propagate.

Tensile stress-strain response of concrete

𝐸 = 𝐸0

𝐸 = 𝐸0 1 − ω

𝐸 = 𝜎 ( 𝜀 − 𝜀0)

𝜀0 = 𝜀𝑝 − 𝑓𝑡 𝐸0

Modeling of expansion

𝑒 = 𝜎𝑟(1

𝐸𝑎𝑣𝑒,−1

𝐸0)

where 𝜎𝑟 is the residual stress in the specimen

before sulfate attack due to shrinkage; 𝐸𝑎𝑣𝑒, is

the average modulus over the cross-section.

Expansion:

Effect of porosity : 𝜀𝑉𝑐𝑜𝑟𝑟𝑒𝑐𝑡𝑒𝑑 = 𝜀𝑉 − 𝑓Φ

𝜀𝑉: volumetric strain

𝑓 is the fraction of capillary porosity being filled, and Φ is the capillary porosity

Reaction ∆𝑉𝑃/𝑉𝑃

AFm to AFt 0.51

C3A to AFt 1.26

C4AH13 to AFt 0.48

Validation on the extended model

A long term observation database on linear expansion of concrete

from US Reclamation Bureau was utilized to validate the extended

model.

Ordinary portland cement concrete and concrete with 25% cement

replaced with fly ash were selected.

Inputs

Parameters OPC concrete 75%OPC+25%FA concrete

L (m) 0.067 0.067

H (m) 0.152 0.152

D2 (m2/s) 4e-13 4e-13

D1/D2 (>1) 10 10

U0 (mol/m3) 9 9

Cement content (kg/m3) 360 270

MVC 3.12 3.12

wc 0.48 0.48

DRcement 0.9 0.9

phi_frac 0.45 0.40

CC3Ai 0.09 0.057

Gypsum 0.05 0.05

DRC3A 0.9 1

k (m3/mol·s) 1e-7 1e-7

E0 (MPa) 30000 30000

ft (MPa) 3 3

residual_s (MPa) 10 10

Fly ash dosage (kg/m3) 0 90

CaO content in fly ash (%) 0.14 0.14

Al2O3 content in fly ash (%) 0.19 0.19

SiO2 content in fly ash (%) 0.44 0.44

C3S content in cement (%) 0.433 0.433

C2S content in cement (%) 0.317 0.317

Should pozzolonic reactions considered or not?

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0 5 10 15 20 25 30 35 40

Lin

ear

Exp

ansi

on

(%

)

Time Since Sulfate Attack (Year)

Concrete Cylinders with 25%

Cement Replaced with Fly Ash

Tixer-mobasher Model

Extended Model

It is not enough to just consider the dilution effect and the permeability change in concrete.

Pozzolanic reactions are necessary to be considered in sulfate attack on concrete.

Porosity fraction can be filled by expansion products?

0

0.05

0.1

0.15

0.2

0.25

0.3

0 5 10 15 20 25 30 35 40

Lin

ear

Ex

pan

sio

n (

%)

Time Since Sulfate Attack (Year)

Concrete Cylinders with 25%

Cement Replaced with Fly Ash

Fraction of Porosity Filled by

Expansive Products=0.38

Fraction of Porosity Filled by

Expansive Products=0.40

Fraction of Porosity Filled by

Expansive Products=0.42

Penetration of sulfate ions in concretes

The dilution effect of fly ash makes the C3A concentration in concrete smaller,

thus postpone the transition from AFm to AFt, decrease the expansive products.

Penetration of sulfate ions in concretes

Since the same diffusion coefficient was utilized in the two concretes,

the penetration speeds were supposed to be the same in the two concretes

Conclusions

The addition of fly ash make concrete less permeable, therefore slows down

the penetration of sulfate ions in concrete.

Compared to OPC concrete, the fly ash concrete has better sulfate

resistance. The linear expansion of concrete with fly ash is greatly smaller than

the OPC concrete at the same moment.

The addition of fly ash dilutes the concentration of C3A and CH. the

pozzolanic reactions change the chemical components and their concentrations in

concrete, therefore slow down the transition from AFm to AFt.

Conclusions

The pozzolanic reactions due to the addition of fly ash into concrete should

be considered when numerical simulation methodologies are utilized to

investigate the sulfate resistance of concrete.

The proposed model was validated by the measured linear expansion of

concrete under sulfate attack by USBR. The model successfully reflects the

consumption of CH in concrete and gives reasonable prediction on the linear

expansion of concrete in 10 to 20 years.

Thank You!