Application of Shotcrete Linings under Sulfate Attack Environments

Jianqin Ma1, a 1 College of Highway, Chang'an University, Xi’an, Shaanxi, 710064, China

amajq@gl.chd.edu.cn

Keywords: Sulfate attack, Aggressive ground water, Shotcrete lining, Interaction, Application, Tunnel.

Abstract. Shotcrete lining has been increasingly used in tunnel and underground structure since 1960.

Cement in shotcrete is the most vulnerable to aggressive sulphate ions from the environment in the

presence of moisture. Sulfate attack can change the chemical composition of the cement matrix.The

sulfate attack can be caused by alkali sulfates or due to thaumasite form of sulfate attack (TSA). Based

on the mechanism of sulfate attack in conrete, the features of the interaction of the sulfate bearing

ground water with shotcrete linings are presented in this paper. Under sulfate attack environment,

shotcrete durability and the specification of materials should be highlighted in design. The basic

requirements to the shotcrete are high strength, low permeability and good homogeneity, which can

be produced by well controlled wet process.

Introduction

Tunnels, which are parts of an infrastructure system, should have enough durability. Shotcrete linings

in tunnels have been popularized since 1960. Of a modern tunnel, shotcrete linings are primary or

even dominate support system. In China, shotcrete, with or without reinforcement, has been popularly

used as primary tunnel lining since 1990.

Being most reactive materials in shotcrete, cement is vulnerable to the aggressive ions from the

environment. Sulphate ions in the presence of moisture can change the chemical composition of the

cement matrix. For example, the shotcrete contacting with ground water in long term is often affected

by leaching and simultaneously by the formation of sulfate minerals [1]. Mainly due to the structure

feature of the shotcrete linings, small amounts of shotcrete alteration at the interface between the

lining and wallrocks can lead to significant reduction of adhesion and corresponding risk of

detachment. The deterioration of the shotcrete linings can increase the costs of repair and even lead to

failure of a tunnel.

Concrete deterioration by sulphate attack at tunnel sites has been increasingly reported over the

past years [2-3]. The formation of ettringite due to sulphate attack can be significantly reduced using

sulphate-resisting Portland cements with low C3A content. Thaumasite however appears in concretes

with negligible availability of Al but higher levels of limestone filler [4] by consuming C-S-H phases

and therefore resulting in a significant decrease of the concrete stability [5].

Based on the mechanism of sulfate attack in conrete, the features of the interaction of corrosive

water with shotcrete linings are presented. Under sulfate attack environment, shotcrete durability and

the specification of materials should be highlighted in design, and the basic requirements to the

application of shotcrete are discussed in terms of the state of art of shotcrete technology.

Mechanisms of Sulfate Attack in Concretes

Attack in Concrete. Conventional sulfate attack in mortars and concretes is referenced to the

formation of expansive sulfate phases like ettringite and gypsum. The formation of thaumasite can

occur if the cements and concrete contain a source of carbonate [6-7].

According to the source of the sulfate, there are two types of sulfate attack: external or internal

sulfate attack. External sulfate attack (ESA) usually occurs when environmental sulfate penetrates

into a concrete structure. Internal sulfate attack (ISA) occurs in a sulfate free-environment by the late

Advanced Materials Research Vols. 233-235 (2011) pp 2061-2067Online available since 2011/May/12 at www.scientific.net© (2011) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.233-235.2061

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sulfate ions release from either cement or gypsum-contaminated aggregates. Here only the ESA in

shotcrete will be discussed.

Of the ESA in concrete, the deterioration of concrete is the result of the penetration of aggressive

agents into the concrete and their chemical reaction with cement matrix. The main reactions [8-9] are:

(1) the degradation of C-S-H, and (2) the decalcification of cement paste hydrates upon sulfate ingress

or sulfate attack on C-S-H and CH in the presence of carbonate ions to form thaumasite. The ettringite

formation is related to the conversion of hydrated calcium aluminate to calcium sulphoaluminate. The

gypsum formation is related to the conversion of the calcium hydroxide to calcium sulfate and to

volumetric expansion.

These chemical reactions can lead to cracking of concrete, and/or the loss of strength and elastic

properties of concrete. The form and extent of damage to concrete will depend on the sulfate

concentration, the type of cations in the sulfate solution, the pH of the solution and the microstructure

of the hardened cement matrix. For example, the thaumasite formation may occur with every type of

sulfate salts, especially under humid atmospheres and low temperature (<10°C). The processes of

both diffusion and reaction may control the sulfate attack mechanism [8].

Of the basic requirements for the sulfate attack in concrete, the sources of SO42-

ions, CO32-

ions,

CaO, H2O are important. For the formation of thaumasite, cold temperatures are usually required. For

the sulfate attack in shotcrete linings in tunnels, the sources of CO32-

ions, CaO and H2O are usually

abundant. Source of CO32-

ions includes internal sources, e.g., limestone used as aggregate or cement

filler, external sources, e.g., carbonate or bicarbonate dissolved in the sulfate-bearing water, and

atmospheric CO2 through carbonation of the paste. The source of CaO is abundant in Portland cement

concrete. The source of H2O is from wallrocks.

Sources of SO42-

ions include groundwater in sulfate or sulfide bearing ground, contaminated

aggregates. The sulfide-bearing wall rocks are referred to any wall rocks containing one or more

sulfides with concentration more than 0.5 wt% around tunnel or underground engineering project.

Sulfide is one of the most common minerals in rocks. The most common sorts are pyrite, pyrrhotite

chalcopyrite, sphalerite, galenite, etc. The sulfides may occur in any kind of rocks or soils [10]. For

the sulfides bearing ground, the SO42-

ions come from the oxidation of the sulfides.

Formation of Corrosive Water from Sulfide Bearing Wallrocks. In a theoretical sense, the

formation of corrosive groundwater in sulfides bearing wallrocks is attributed to the dissolution of the

oxidized products of the sulfides in wallrocks. The sulfides in wallrocks would change their states

from reductive forms to oxidized ones, only if they are exposed to an environment with high oxygen

fugacity. This process can be explained by the oxidizing procedure of pyrite, with the most frequent

occurrence and highest concentration in wallrocks. The following equations (1-3) can present the

procedure of the formation of corrosive groundwater due to pyrite oxidizing.

2FeS2(s) + 7O2 + 2H2O = 2Fe2+

+ 4SO42-

+ 4H+. (1)

4Fe2+

+ O2 + 4H+ = 4Fe

3+ + 2H2O. (2)

Fe3+

+ 3H2O = Fe(OH)3(s) + 3H+. (3)

If the pH value of the groundwater is below 3, the pyrite could be oxidized by ferric iron to ferrous

iron as the below equation (4).

FeS2(s) + 14Fe3+

+ 8H2O = 15Fe2+

+ 2SO42-

+ 16H+. (4)

The oxidation of the sulfides not only produces corrosive groundwater, but also causes pH of

environmental medium declining and turns the environment from reductive state to oxidized state.

The Fe2+

is dissolvable in acid groundwater and can be migrated to a distance place [11-12]. And

therefore, another corrosive place may be produced or the corrosive environment is expanded.

Sulfate Diffusivity. There are limited data available on the measurement of sulfate diffusivity in

cement-based materials. Tixier and Mobasher [8] reviewed that, some sulfate attack models have

2062 Fundamental of Chemical Engineering

arbitrarily chosen diffusivities between 0.75×10-12

and 9×10-12

m2/s; Following the values of the

intrinsic sulfate diffusivity in concrete, the sulfate diffusivities are about 1.0×10-11

to 3.5×10-11

m2/s

for water-cement ratio (w/c), w/c=0.45 to 0.65. As the effect of sulfate attack on the value of the

porosity of the cement paste is considered, it is estimated that the diffusivity in a paste can be

increased by at least an order of magnitude [8].

Another indirect approach is relating intrinsic ionic diffusivity to the water permeability

coefficient, and sulfate diffusivities are in the range of 2×10-12

to 20×10-12

m2/s [8].

Interaction of Ground Water with Shotcrete Linings

As the above mentioned that the sulfate attack in shotcrete occurs as the percolating of sulfate solution

in the structure. In general, tunnel sites provide a great potential for concrete damaging processes as

different ground water and temperature may occur. Shotcrete is often placed together with drainage

elements. Shotcrete linings in tunnels are often in contact to percolating ground water. However, the

linings are typically saturated with water only in few areas and water is penetrating the shotcrete

locally. These areas are often associated with pathways (joints, cracks) through the tunnel lining. On

the other hand, water moves along local and permeable inhomogeneities through the shotcrete lining

[1-2]. Cement paste dissolution and erosion occur due to acid attack, sulphate attack, and lead to loss

of adhesion between shotcrete and rocks. The back side of shotcrete linings is more prone to chemical

interaction.

Fig. 1(a) shows schematically locations within the cross section where concrete degradation due to

interaction with ground water percolating. The degradation generally varies in their intensities over

time and even within short distance. The groundwater is usually related to the structures of the

wallrocks, indicating as the arrows in Fig. 1(a).

The sulfate attack in shotcrete linings in case histories showed that the attack is triggered by

permeable inhomogeneities which serve as pathways for the percolating water, and the zonations of

leaching and thaumasite formation occur along the pathway [1, 13-14]. The alteration and interaction

zonations are mainly along the contact to the shotcrete and lesser along the layer contacts within the

concrete [1]. The alteration increases from the unchanged shotcrete to the above mentioned contacts.

The alteration may include several distinct zones, as shown in Fig. 1(b). For example, the zone of

leached cement paste is followed by the lack of portlandite and there is a general grain size reduction

of cement phases [1]. The corrosion zone is characterized by complete dissolution of cement paste

and more or less intensive formation of secondary phases resulting in a highly porous and

mechanically weak material. The transitions between the regions of different state of alteration are

usually clear.

Interaction

area

(a) Groundwater water percolating (b) Interaction and formation of alteration zone

Fig. 1 Shotcrete lining with local water percolating and the formation of alteration zones

The stability of hydration products in hardened shotcrete depends on the chemical composition of

the pore solution which itself is chemically buffered by the hydration products [7-8]. The interaction

with an aqueous environment usually result in the formation of a zonation pattern, which indicates a

moving boundary behavior in the opposite direction of the diffusion flux [15]. A typical corrosive

Advanced Materials Research Vols. 233-235 2063

process include dissolution of cement phases, transportation of dissolved chemical species and

(re-)precipitation of secondary minerals. The water flow through a crack is always leading to leaching

of the crack walls, as shown in Fig. 1(b).

Sulphate attack to hydraulic concrete involves a series of possible physicochemical processes that

are evidenced through deterioration of the material [1, 16]. The sulfate attack in shotcrete can be

generally considered as sulphate attack by alkali sulfates and thaumasite form of sulfate attack in

terms of chemical reaction and the formation of secondary minerals.

Sulphate Attack by Alkali Sulfates (ASA). As an alkali sulfate solution (e.g., Na2SO4, K2SO4)

percolating into shotcrete, once the Ca(OH)2 in the shotcrete has been consumed, the Ca2+

ions

required may be provided by the decomposition of the C-S-H phase in it. This results in a gradual

lowering of the CaO/SiO2 ratio of the C-S-H and a gradual loss of its bonding properties. If the highly

soluble sodium (or potassium) hydroxide accumulates an equilibrium is reached and the reactions

cease. Otherwise, the reactions will proceed in flowing waters with a constant supply of Na2SO4.

The ASA can be presented as: sulfate solution → ettringite formation → gypsum formation and

reduced Ca(OH)2 → gypsum formation and decalcification of C-S-H, as shown in Fig.2(a). In this

process, the main secondary minerals are gypsum. As a result, a physical process of salt crystallization

and expansion by repeated wetting and drying will make the deterioration worse.

−2

4SO −2

4SO

−2

4SO −2

4SO

(a) Sulphate attack by alkali sulfates (ASA) (b) Thaumasite form of sulfate attack (TSA)

Fig. 2 Percolating of groundwater in shotcrete and the formation of secondary minerals

Thaumasite Form of Sulfate Attack (TSA). The sulfate attack due to the formation of

thaumasite is similar to that of ettringite. Thaumasite appears in concrete with negligible availability

of Al but higher levels of limestone filler by consuming C-S-H phases [4]. Where the ground water is

enriched in sulphate, the shotcrete lining may be intensively attacked due to the formation of

thaumasite. Field investigations [1, 17] verified that the damage of shotcrete was partially or fully

related with the thaumasite formed by sulfate attack. The sulfate attack due to the formation of

thaumasite can be presented as the following equation (5).

3Ca2+

+ SiO32-

+ CO32-

+ SO42-

+ 15H2O = CaSiO3·CaCO3·CaSO4·15H2O (5)

The formation of thaumasite is favoured by lower temperatures (e.g., <10°C) as well as by high

sulfate concentrations (e.g., 10-20wt% SO42-

by weight cement paste) and high pH values in the

presence of calcite. It was said that pH values above 12 enhance thaumasite formation whereas pH

below 11 led to gypsum formation during sulfate attack [18].

The TSA requires a source of calcium silicate, sulfate and carbonate ions, excess of humidity and

low temperatures [19-20]. Limestone can be a source of carbonate necessary to form thaumasite. In

cement systems with 5 wt% limestone addition the calcite is the limiting factor whereas in the case of

25 wt% limestone addition sulfate limits the amount of thaumasite formed [21].

Case histories [1] showed that, in the damaged zones due to intense sulphate attack in shotcrete,

the cement matrix of the shotcrete can be completely replaced by thaumasite. The thaumasite attack

can also be found in concretes already damaged by other deterioration processes. The ASA can be a

precursor to TSA, as shown in Fig.2(b).

The formation of thaumasite can be presented by a process with two stages [21]. In the first stage

carbonation of the shotcrete surface occurs and the paste becomes rich in calcite and in the second

2064 Fundamental of Chemical Engineering

stage sulphates attack the C-S-H of the cement in the carbonated zone to form thaumasite. The

process is controlled by the diffusion of carbonates and sulphates in the shotcrete and is also

controlled by the solubility of gas CO2 in water.

The result of the TSA in shotcrete can lead to safety problem since the thaumasite formation can

significantly damage to the paste matrix. The C-S-H is replaced by thaumasite. As thaumasite does

not possess binding ability, the paste is transformed into an incoherent mass. The above mentioned

sulfate attack in shotcrete indicates that the formation of thaumasite in shotcrete lining is strongly

related to a preceding or simultaneous leaching process. The sulfate content in ground water samples

at sites showing thaumasite formation in shotcrete was low and hardly aggressive corresponding to

current standards [1]. More attention should be paid to the TSA in shotcrete.

Basic Requirements to Cope With the Sulfate Attack in Shotcrete

For the ESA in shotcrete, the ground water is natural and the engineers always have no ability to

change it. When we want to use shotcrete linings in sulphate attack environment, the engineers have

to design proper shotcrete. The physical parameters like permeability and inhomogeneity are as

important as chemical parameters like the composition of the concrete paste, pH-values and

composition of the interacting water.

The aggressive waterproof is one of the main factors related to both operation reliability and

durability of tunnels. Adequate shotcrete is one of the most important elements to achieve adequate

levels of both attributes. For this purpose, the wet process with appropriate workability time of a low

w/c ratio concrete mix [22] is a technology direction. The high-performance alkali-free accelerator

admixtures increasingly substitute the alkali-rich accelerator, since alkali-free accelerator (AFA) has

beneficial potentials, including long-term strength development, working safety and less

environmental impact.

Basic Requirements of the Sulfate Attack Resisting Shotcrete. Sulfate resistance of the

concrete is mainly related to its physical and chemical resistance to penetrating sulfate ions [9]. Good

physical resistance of the concrete is directly related to w/c ratio and cement content; good chemical

resistance is related to the resistance of the cement matrix to the sulfate reactions.

In general, the standards that regulate the use of Portland cement in aggressive sulphates ambient,

limit the content of aluminates in cements in order to avoid formation of expansive ettringite. Indeed,

no prevention is considered regarding the destructive action on concrete due to thaumasite formation.

Sulfate-resisting (SR) concrete has traditionally been specified prescriptively by the maximum w/c

ratio and a specific type of SR cement. To ensure good physical and chemical resistance of the

concrete to limit the penetrating sulfate ions, concrete should have a coefficient of water permeability

(K) no greater than 1.8×10-12

m/s or have a sulfate permeability less than 1750 Coulombs [9].

Low permeability concrete is the best defense. For the construction of sulfate attack resisting

shotcrete linings, the quality control factors are usually defined as: w/c ratio <0.45, impermeability

DIN 1048: penetration <20mm, compressive strength for rock support: >40MPa and for final lining:

>50MPa [9, 23].

Strength of shotcrete is one of the good indexes of shotcrete quality. When AFA is added 8%, the

objective strength (e.g., compressive strength over 40MPa and flexural strength over 4.5MPa at 28

days) can be acquired regardless of the amount of Silica fume [23]. The target specification of high

strength shotcrete, with compressive strength over 40MPa and flexural strength over 4.5MPa, was set

to satisfy that of C50 in Northern Europe with long-term durability [24].

Sulfate-resisting concrete can be achieved using a sufficient quantity of sulfate-resisting cement

and a low w/c ratio to obtain a concrete with low water permeability. The current shotcrete

technology, for the low hydration heal cement, additives for low porosity, K is about 10-14

m/s [25].

This means that the present wet process technology can meet the basic requirements of the sulfate

attack resisting if the process of shotcrete is well controlled.

Quality Control of Shotcrete Linings. The quality control of shotcrete must be reinforced at all

levels of management right through the operations to the nozzle-man at the face, since the sulfate

Advanced Materials Research Vols. 233-235 2065

attack usually occurs at the lower quality locations. The successful shotcrete in a tunnel requires all

the steps in the wet process being effective and working in unison. Any weak link will have

detrimental effects. In safe and rapid developments using shotcrete, all these factors, such as

specifications, mix design, testing, training, equipment, logistics, application, quality control, and

commitment must be well addressed [22]. The six main components of a wet shotcrete mix are:

aggregates, cement, water, additives, admixtures and accelerators. It is noted that there are variations

within the main components between tunnels. And therefore, site test is always needed.

The performance of shotcrete is significantly affected by the type of accelerator, environmental

condition, mixture and shotcreting equipment [22]. From sustainability point of view, it is important

for engineers to ensure longterm durability of shotcrete, and effective accelerator offers a promising

and reliable outlet to achieve this goal.

The latest AFA typically provide faster set times, but for every application, site specific trials

should be done to check out the cement-accelerator reaction times and subsequent strength

development over time. For example, fast initial set can be accompanied by very slow strength

development over the following 24 hours, which can create serious inadequacies with respect to the

ground support requirements and hence real safety issues [22]. As selecting AFA, a balance between

high early strength and constant ongoing strength development is always beneficial.

A few laboratory tests on the influence of AFA on sulfate resistance of mortar specimens [26]

showed that the mortar specimens with AFA were susceptible to sulfate deterioration. This suggests

that AFA can lead to a poor sulfate resistance. Therefore, a special care should be taken when the

shotcrete with AFA is applied under the sulfate-rich environments.

The accelerator which is added at the nozzle immediately knocks out any hydration control

admixture. The spraying procedure of adding the accelerator into the air stream just prior to entering

the nozzle and the shotcrete stream can provide optimum mixing and penetration [22]. Operator

experience and the spray action are always key factors in maintaining dose rates of accelerators with

high slump mixes.

Conclusions

The shotcrete linings in tunnels are vulnerable to sulfate attack. The deterioration of shotcrete linings

is triggered by permeable inhomogeneities which serve as pathways for the percolating water. Mainly

due to the structure feature of the shotcrete linings, small amounts of shotcrete alteration at the

interface between the lining and wallrocks can lead to significant reduction of adhesion and

corresponding risk of detachment.

The sulfate attack due to thaumasite formation can be the most serious sulfate attack in tunnel

shotcrete lingings, on the condition of sulfate content in ground water may be lower than the limit of

the current standards. The sulfate ion can be the results of the oxidation of sulfides bearing wallrocks.

In technology, the wet process can produce sulfate attack resisting shotcrete, in terms of sulfate

diffusivity as well water percolating. However, the quality control of the shotcrete, especially in

strength and homogeneity, depends on many factors. The process should be reinforced at all levels of

management right through the operations to the nozzle-man at the face.

For wet process, the type of accelerator and its dose are important to the performance of shotcrete

and special attention should be paid to the AFA application under sulfate-rich environments.

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Advanced Materials Research Vols. 233-235 2067

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