Sulfate Attack

Sulfate attack can produce profound damage to cement

paste and is among the most destructive processes

effecting concrete. When sulfate is allowed to react with the

hydrated aluminate phases within concrete paste the

expansive mineral ettringite is formed (Eq. 1). As ettringite

forms within the paste it causes internal stresses which

result in crack formation (Figure 4). The additional calcium

required to produce ettringite is supplied by calcium

hydroxide in the paste[5]. Sources of sulfate are generally

external to concrete and are supplied by seawater,

groundwater or available soil moisture; however, internal

sources of sulfate are not uncommon. Excessive addition of

gypsum or sulfate-rich aggregate are common sources of

internal sulfate contamination[5] (Figure 5).

Ca4Al2SO4 · 12H2O + 2Ca2+ + 2SO42− + 20H2O → Ca3Al2O3 · 3𝐶𝑎𝑆𝑂4 · 32H2O (1)

(Aluminate) (Ettringite)

Figure 4: The plane polarized light image (left) shows acicular needles of

ettringite formed within an air void. The scanning electron microscope image

(right) shows cracking caused by fibrous to acicular crystals of ettringite growing

at the interface between aggregate (center - top) and within the surrounding

cement paste (bottom of image). (Images courtesy of Saskatchewan Research

Council).

Figure 5: Although gypsum is added to cement mixes to control the rapid

hydration or “setting” of the aluminate clinker phases, internal sulfate

contamination is more likely to be the result of sulfate-rich aggregates. The

above image shows gypsum replacement in microfossils contained within a

micritic limestone aggregate. Petrographic pre-screening of aggregates can help

identify these potentially deleterious minerals (Image courtesy of Saskatchewan

Research Council).

J. Doxey I/II, K. Ansdell II and M. Boulfiza III

I Saskatchewan Research Council, II Department of Geological Sciences, Department of Civil and Geological Engineering

Composite Rocks

Concrete is a manmade composite “rock” consisting of a

carefully proportioned mix of natural aggregate, water and a

cementing agent. The cementing agent used in the

manufacture of concrete is known as ordinary Portland-type

cement (OPC) and is colloquially referred to as “paste”. When

the tiny clinker minerals (Figure 1) contained in OPC are

reacted with water a powerful exothermic reaction takes place.

This reaction results in the formation of a host of hydrated

mineral phases (Table 1) that are responsible for the strength

that is nearly synonymous with the word concrete.

Table 1: Chemical composition of cement clinker minerals and their associated

hydration products [5]. Concrete’s strength is derived from calcium silicate hydrates.

Petrographic Images

Concrete is known to suffer from a variety of deleterious

reactions that may take place between individual aggregates

and concrete pore fluids, external contact with aggressive ions

in solution and even freeze-thaw cycles. These processes can

result in deterioration that ranges from subtle to profound and

typically result in significant loss of both strength and

durability (the engineering equivalent for “weathering”). The

following sections illustrate how the petrographic microscope,

the scanning electron microscope and x-ray microanalysis can

be used to confirm deleterious processes such as alkali

aggregate reactions and sulfate attack in concrete.

Preamble

“There is also a kind of powder from which natural causes produce

astonishing results. This substance lends strength to buildings. I shall

begin with the concrete flooring observing that great pains and the

utmost precaution must be taken to ensure its durability. On this, lay

the nucleus, consisting of pounded tile mixed with lime in the

proportions of three parts to one, and forming a layer not less than

six digits thick.”

– Vitruvius, 1st Century BC [7]

Background

Petrography has been used by geologists for over a century to

classify and describe naturally occurring rocks, but did you

know that it can also be used to describe concrete and

cementitious materials? In this context, the word

“petrography” is used in a synonymous fashion with the term

“petrology” and the cement industry has an expectation that

petrographic studies will be complimented with mineralogical

and geochemical data relevant to the aggregate and

cementitious materials used in its manufacture. This poster

attempts to provide an overview of concrete as a composite

“rock” and investigating the various ways in which petrography

and petrology may be applied in the study of these “other”

rocks.

Clinker Minerals

Figure 1: Ordinary Portland-type cement clinker minerals are used to

make cement. The tiny clinker minerals are formed by heating

crushed calcium, aluminum and silica-bearing rocks to approximately

1500C in a rotating kiln. The photomicrograph (left) primarily shows

strongly zoned crystals of the clinker mineral alite (Ca3SiO5). The right

hand image shows alite (blue), belite (brown), aluminate (colourless)

and lath-form ferrite. Field of view in both images is approximately

200µm (Images are from Campbell, 1999 [2]).

Analogous to Stone

Concrete contains pore fluids within the matrix of the cement

paste (Table 2). These pore fluids may interact with aggregate

in the concrete in ways that are analogous to the rock-water

interactions observed between natural rocks and water.

Concrete is also frequently subjected to chemical and physical

weathering phenomenon in its service environment. For

example, the ingress of aggressive ions in solution can

influence the chemistry of cement paste in much the same

way that meteoric waters influence chemical weathering in

natural rocks and regolith. Physical weathering processes

which produce fractures or abrasion in natural rock are known

to have comparable effects on concrete. Comparisons such as

these are significant in that they suggest that the same

petrographic and petrologic techniques used to describe

natural rock can be readily applied to concrete.

Table 2: Concentration of ions present in pore fluids after 180 days (water to

cement ratio = 0.5) (modified after Taylor, 1997[5]).

Alkali Aggregate Reactions

Alkali aggregate reactions occur when certain rock types come

into contact with the highly alkaline (~ pH 13) pore fluids

within concrete. Alkali silica reactions (ASR) occur when

aggregates containing reactive forms of silica (e.g. opal,

chalcedony, micro- or crypto- crystalline quartz, etc.) interact

with the alkaline concrete pore fluids to produce potentially

damaging expansive alkali silica gel (Figure 2). Alkali

carbonate reactions (ACR) cause similar damage to that

associated with ACR and occur when alkaline pore fluids react

to breakdown dolomitic aggregate into calcite, the sodium

carbonate mineral trona and brucite. It is believed that the

mineral brucite may be responsible for the resulting expansive

damage caused by ACR (Figure 3). Both ASR and ACR are

known to cause varying degrees of internal stress and

degradation leading to structural cracking and loss of both

strength and durability.

Figure 2: Crack propagation caused by expansive ASR gel is confirmed by

petrographic or electron microscopy (images courtesy of ww.fhwa.dot.gov [6]).

Figure 3: Rounded aggregates of dolostone (left) have been partially dissolved

(right) upon contact with highly alkaline concrete pore fluids (image courtesy

of Saskatchewan Research Council).

The Pantheon – 200 AD

The Pantheon in Rome is the

largest unreinforced concrete

dome ever built. The art of

concrete was lost after the fall of

Rome and was not rediscovered

again until around 1678 [1].

www.ancient.eu/Pantheon/

Thaumasite form of Sulfate Attack

The thaumasite form of sulfate attack (TSA) is arguably the

most devastating of all forms of sulfate attack. Thaumasite

favors cool (< 20C), damp, alkaline (>pH 10.5) climates and

an abundance of silicate and carbonate ions[3,4]. When

environmental and chemical conditions are conducive and if

available aluminum from the aluminate phases (1) has been

depleted the formation of thaumasite (2) will be favored[3,4].

3Ca2+ + 2S𝑖32− +2CO3

2− + SO42− + 15H2O → 𝐶𝑎𝑆𝑖𝑂3 · 𝐶𝑎𝐶𝑂3 · 𝐶𝑎𝑆𝑂4 · 15H2O (2)

(Thaumasite)

Carbonate ions may be supplied to the system by way of

aggregates, limestone interground with cement, atmospheric

additions of carbon dioxide or groundwater. The required

silicate ions are derived from the calcium silicate hydrates

within the cement paste itself, resulting in a total loss of

strength. Thaumasite and ettringite have similar crystal habits

(Figure 7) and it is believed that the two minerals form a solid

solution[4]. If suspected, TSA can be confirmed by constructing

atomic ratio plots from data collected by x-ray microanalysis

(Figure 7).

Figure 7: Thaumasite may be intergrown with ettringite (left) and the two minerals

are believed to form a solid solution. Thaumasite can be inferred using atomic ratio

plots (right). Si/Ca ratios that are less than 0.48 suggest silica depletion in the

paste while S/Ca ratios will be effected by carbonate ion additions[3,4].

Ion in Solution Low Alkali Cement (mmol l-1) High Alkali Cement (mmol l-1)

Na+ 0.08 0.16

K+ 0.24 0.55

OH- 0.32 0.71

-0.2 0.0 0.2 0.4 0.6 0.8 1.0 0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

S/Ca

Si/Ca

Ettringite

Thaumasite

Gypsum

Concluding Remarks

The analogous comparisons between naturally occurring rocks and

“rocks” that we have made provide a sound basis for using petrography

and other petrologic techniques to understand their chemical, physical

and mineralogical properties. The petrographic microscope and the

scanning electron microscope are well suited to observing the delicate

mineralogical and microstructural characteristics found in concrete. The

petrologists understanding of mineralogy and geochemistry is

fundamental to understanding how concrete interacts with and is

effected by its service environment and tools such as x-ray

microanalysis are a useful companion for such studies.

References

[1] Ancient History Encyclopedia. 2015. Pantheon. [Online] Available at: http://www.ancient.eu/Pantheon/

[Accessed November 2015].

[2] Campbell, DH. (1999). Microscopical Examination and Interpretation of Portland Cement and Clinker, 2nd Ed.

Portland Cement Association.

[3] Freyburg, E., Berninger, A.M. (2003): Field experiences in concrete deterioration by thaumasite formation:

possibilities and problems in thaumasite analysis", Cement and Concrete Composites, 25, 1105-1110.

[4] Sibbick, R.G., Crammond, N.J, Metcalf, D. (2003): “The microscopical characterization of thaumasite”,

Cement and Concrete Composites, 25, pp. 831-837.

[5] Taylor, H. (1997). Cement Chemistry, 2nd edition. Heron Quay, London: Thomas Telford Publishing.

[6] US Department of Transportation Federal Highway Administration, 1025,:Concrete Pavements [Online]

Available at:: https://www.fhwa.dot.gov/pavement/concrete/pubs/hif09004/asr11.cfm. [Accessed

November 2015].

[7] Vitruvius. (1st Century BC). The Ten Books on Architecture. (M. H. Trans.) Harvard University Press, 1914.