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(^1) Assistant proffesor,Faculty of Technical Sciences of Novi Sad,Trg Dositeja Obradovića 6, Novi Sad
,[email protected]
Vesna Bulatović^1
EXTERNAL SULFATE ATTACK ON CONCRETE
Abstract: Durability of concrete may be defined in different ways one of them is that it is the ability of concrete to resist weathering action, chemical attack, and abrasion while maintaining its desired engineering properties. Many factors determine the durability of concrete and its life such as: concrete ingredients, their proportioning, interactions between them, placing and curing practices, and the service environment. Concrete is resistant to most natural environments and many chemicals. However, concrete is sometimes exposed to substances that can attack and cause deterioration. For example, sulfates can attack concrete by reacting with hydrated compounds in the hardened cement paste. Sulfate attack on concrete structures in site is a very complex process due to its complex behaviour and the overlapping of several phenomena simultaneously
Кey words : concrete, durability, chemical process, sulfate attack, Na 2 SO 4 , MgSO 4
The European Commission support for the production of this publication does not constitute an endorsement of the contents which reflects the views only of the authors, and the Commission cannot be held responsible for any use which may be made of the information contained therein.
1. DURABILITY OF CONCRETE
Concrete durability can be defined as the ability of concrete to resist to the attack of environmental, physical and chemical aggressive conditions, through the time. A concrete structure should be able to maintain the expected performance during its service life [1].
The durability and service life of a concrete structure is determined by the interaction between the structure and its environment. Depending of the type and importance of the structure, its service life may be between 10 and 100 years and more ( Table 1) [2].
Table 1- Design service life of structures [2] Design service life tSL [years] Examples
10
Temporary structures (structures of parts of structures that can be dismantles with a view to being re-used should not be considered as temporary) 10 - 25 Replaceable structural parts, e.g. gantry girders, bearings 15 - 30 Agricultural and similar structures 50 Building structures and other common structures
100 Monumental building structures, bridges, and other civil engineering structures
Concrete structures designed and built in accordance with the current codes of practice with regard to materials characteristics, architectural and structural design, processes of execution, inspection and maintenance procedures, normally do not deteriorate prematurely. However, some older and some new concrete structures show (or will show) decreased durability due to insufficient cover to the reinforcement, porous concrete, reactive aggregates or faulty design [2]. If the aggressiveness of the structure’s environment has not been adequately identified and dealt with during the design and construction process, premature deterioration may render the structure unfit to serve its purpose [2].
It has become evident that all deterioration mechanisms depend upon an aggressive substance penetrating from the surrounding environment into the outer layer of concrete – the cover, so it is the essential about concrete durability is related to the intrinsic properties of this material. Knowing and understanding the transport mechanisms of liquid and gaseous substances into and within concrete structures is, therefore, the most important element in ensuring sufficiently durable concrete structures. This is also the fundamental basis for quantifying durability in the form of service life performance [2].
- The propagation phase. During this phase an active deterioration develops and loss of function is observed. A number of deterioration mechanisms develop with time at an increasing rate.
For the majority of ordinary structures placed in aggressive environments a conscious choice of cement type, concrete mix (especially w/c ratio), concrete cover, curing (moisture and temperature control) and geometry of the exposed parts of the structure, will normally result in a satisfactory service life [2].
The deterioration of concrete structures includes mechanical, chemical, physical process or combination of them (Figure 3) [2].
DETERIORATION OF CONCRETE
Mechanical Chemical Physical
· Impact · Overload · Movement (e.g.settlement) · Explosion · Vibration
· Alkali-aggregate reaction · Aggressive agents e.g.sulfates,soft water,salts · Biological activities
· Freeze/thaw · Thermal · Salt crystallization · Erosion · Wear
Figure 3 – Common causes of deterioration of concrete structures [2] In this paper the emphasis is on the chemical attack on concrete structures.
2. CHEMICAL PROCESSES
Well-designed and constructed concrete will perform satisfactorily when exposed to most atmospheric conditions, most soils and many kinds of chemicals. However, in some chemical environments the useful life of even the best concrete will be shortened unless specific measures are taken. For significant attack to occur, the chemical must be in solution and sufficiently concentrated or reach a critical concentration after evaporation of the solution.
The most important the chemical reactions which lead to a decrease in quality and to increased deterioration of the concrete are [2]:
· Reduction of the pH value of the concrete due to carbonation; · Reaction of alkalis with reactive aggregates in the concrete; · Reaction of sulfates with the aluminates in the cement; · Reaction of acids, ammonium salts, magnesium salts and soft water with hardened cement; · Effects of biological activities.
According to EN 206-1 the environmental actions are classified as exposure classes and for chemical attack designed classes are XA1, XA2 and XA3 (Table 2). Another table in EN 206-1 (Table 3) shows limiting values for each of these classes (XA1, XA2 or XA3).
Table 2- Classes for chemical attack according to EN 206- 1
Chemical attack
XA1 (^) Slightly aggressive chemical environment Concrete exposed to natural soil and ground water according to Table 2
XA2 (^) Moderately aggressive chemical environment Concrete exposed to natural soil and ground water according to Table 2
XA3 (^) Highly aggressive chemical environment Concrete exposed to natural soil and ground water according to Table 2 Table 3- Limiting values for exposure classes for chemical attack from natural soil and ground water Chemical characteristic
Reference test method XA1^ XA2^ XA Ground water SO 42 –^ mg/l EN 196- 2 ≥ 200 and ≤ 600 > 600 and ≤ 3 000 > 3 000 and ≤ 6 000 pH ISO 4316 ≤ 6,5 and ≥ 5,5 5,5 and ≥ 4,5 < 4,5 and ≥ 4, CO 2 mg/l aggressive EN 13577^ ≥ 15 and ≤ 40^ > 40 and ≤ 100^
100 up to saturation NH 4 +^ mg/l ISO 7150- 1 ≥ 15 and ≤ 30 > 30 and ≤ 60 > 60 and ≤ 100 Mg2+^ mg/l EN ISO 7980 ≥ 300 and ≤ 1 000 > 1 000 and ≤ 3 000 > 3000 up to saturation Soil SO 42 –^ mg/kga total EN 196-^2
b (^) ≥ 2 000 and ≤ 3 000c > 3 000c and ≤ 12 000
12000 and ≤ 24000 Acidity according to Baumann Gully ml/kg
prEN 16502 > 200 Not encountered in practice
a Clay soils with a permeability below 10−5 m/s may be moved into a lower class. b The test method prescribes the extraction of SO4 2– by hydrochloric acid; alternatively, water extraction may be used, if experience is available in the place of use of the concrete. c The 3 000 mg/kg limit shall be reduced to 2 000 mg/kg, where there is a risk of accumulation of sulfate ions in the concrete due to drying and wetting cycles or capillary suction.
The most common forms of chemical attack of concrete stone are:
· Sulfate attack · Clorides attack · Acids attack · Other substances
At this point accent is on the sulfate attack.
atmospheric SO 3 , or from liquid industrial wastes. In literature, the greatest attention has been paid to this type of attack.
Depending on the quality of the concrete and the environmental conditions, a complex mechanism of sulfate action can cause various chemical and physical changes in the concrete. Some of chemical changes include [13]:
· Removal of Ca2+^ from certain hydration products (decomposition of portlandite/calcium-hydroxide (CH) and C-S-H) – leaching; · Changes in the ionic composition of the pore liquid phase; · Formation of hydrous silica (silica-gel); · Decomposition of still unhydrated clinker components; · Decomposition of previously formed hydration products; · Formation of gypsum, ettringite, thaumasite; · Formation of magnesium compounds, such as magnesium hydroxide (Mg(OH) 2 )- brucite and magnesium silico-hydrate (M-S-H); · Penetration into concrete of sulfate anions and subsequent formation and repeated recrystallization of sulfate salts (for example NaCl, K 2 SO 4 , MgSO 4 ). These chemical changes are also associated with physical changes:
· Complete alteration of the pore structure and microstructure of the solid phase; · Increase in porosity and permeability; · Volume expansion and the appearance of micro-cracks; · Surface swelling, spalling, exfoliation; · Softening of paste; · Formation of salt deposits on the surface and exfoliation in cracks; · Loss of strength; · Reduction of modulus of elasticity. Generally, the basic mechanisms for the detrioration of concrete due to the action of sulfate may be classified as:
- Volume changes Expansion as a result of the formation of new products under the sulfate attack is only possible if the sulfates are in the dissolved state. The reactions form a significantly larger volume of solid phases which cannot be accommodated by the capacity of the pore system. Internal stresses increase and the concrete swells and then cracks. The expansion does not have to cause deterioration if the concrete has the capacity to withstand these changes. Also, the expansion depends on how many water molecules are absorbed by the new products.
- Decomposition of compounds The sulfate attack may cause the decomposition of cement hydration products such as C-S-H and CH as they become unstable due to the decrease in pH.
- Salt exfoliation Deposits of different salts can occur not only on the surface but also in the inside of the concrete element, which is exposed to sulfate. The solubility of salts such as Na 2 SO 4 and MgSO 4 increases significantly with increasing temperature. The pores in concrete can be saturated with salt solution when the temperature drops, e.g. during the night, salt crystals can also grow rapidly and cause pressure on the interior of the concrete. Also, the salt solution may suddenly crystallize due to the vibration, e.g. passing traffic.
The most commonly occurring salts for sulfate resistance tests are sodium sulfate (Na 2 SO 4 ) and magnesium sulfate (MgSO 4 ), as these salts most commonly occur in the water and soil that surround the concrete structure. These cation types (Na+^ or Mg2+) related to sulfate ions define the mechanism of attack and significantly influence concrete deterioration [6].
3.1. SULFATE ATTACK WITH Na 2 SO 4
Sodium sulfate (i.e. sulfate ion) reacts with hydration products of cements, resulting in the formation of ettringite (3CaOAl 2 O 3 3CaSO 4 32H 2 O) and gypsum (CaSO 4 2H 2 O). The provision of Ca2+^ ions, necessary for ettringite and gypsum formation, is supplied by portlandite (CH) or by decalcification of C-S-H gel. Decalcification of C-S-H gel affects the loss of bonding and it is very dangerous. Gypsum is usually formed under mortar/concrete surface. In this region, aluminium ions are primarily consumed for the formation of ettringite and, therefore, sulfate ions can react only with the remained calcium ions [11] and form gypsum. The most common reactions that occur between cement components and Na 2 SO 4 are shown in the Figure 4.
Liquid Reaction zone Pore solution
Original cement paste
Na+
OH-
SO 4 2-
Na+ OH-
Ca2+
SO 4 2-
CaSO 4 ·2H 2 O
6CaO·Al 2 O 3 ·3SO 3 ·32H 2 O (ettringite) Al(OH)- 4
Ca(OH) 2
4CaO·Al 2 O 3 ·SO 3 ·12H 2 O (monosulfate)
Figure 4 - Reactions taking place between Portland cement components and sodium sulfate solution
measures other than those included in the official standards can be taken to increase the resistance of concrete to the attack of sulfate:
· Adequate thickness of the structural elements; · Proper installation and curing of concrete (for example short-term air exposure); · Application of the sulfate-resistant or mixed cement (less CH, aluminum bonding for mineral addmixtures and does not later participate in the formation of ettringite, different content of C-S-H, reduction of porosity); · A small increase in gypsum initially that can bind aluminum to itself at the onset of hydration. It is very important to consider the cation that is involved in the reaction when choosing the appropriate material, if possible.
Slag is recognized as an efficient Supplementary cementitious material (SCM) in improving the sulfate resistance, particularly at a higher level of cement replacement, as in the case of CEM III. Slag hydrated products are similar to Portland cement one [15]. The most abundant is C-S-H with modified morphology [16] and lower C/S ratio than in PC. Due to this C/S ratio, the degree to which aluminium replaces silicon is high. Additionally, hydrotalcite-like phase is formed. If the replacement level of Portland cement with slag in slag-blended cements increases, the amount of monosulfate, ettringite and CH decreases [17]. A lowered availability of these compounds can reduce the damage caused by sulfate attack due to a direct reduction in the quantity of ettringite and gypsum [9][18]. Their formation extent is limited due to the lack of Ca2+^ ion and due to the lack of aluminium ions forming monosulfate, the only one that can form ettringite. Namely, a part of aluminium ions are included into hydrotalcite or C-S-H gel structures, so the rest is enough to form only a small amount of monosulfate or ettringite. Also, slag can refine the pore size distribution and reduce the pore connectivity in mortar/concrete, which also contributes to the overall durability of concrete.
3.4. VISUAL APPEARANCE
The effects of sulfate attack on concrete structures in real conditions can be seen in the Figure 6 and Figure 7:
Figure 6 - Railroad tiles [6] Figure 7 - Concrete block [2]
The structure of concrete at the micro (Figure 8 and Figure 9) and macro levels (Figure 10 and Figure 11), after testing in the laboratory, is shown in the following figures.
Figure 8 - SEM image of samples exposed to Na 2 SO 4 [19]
Figure 9 - SEM image of samples exposed to Na 2 SO 4 [19]
Figure 10 - Appearance of samples exposed to Na 2 SO 4 Figure 11 - Appearance of samples exposed to MgSO 4
4. CONCLUSION
Sulfate attack on concrete structures in site is a very complex process due to the overlapping of several phenomena simultaneously. The environment never has the same conditions, which makes it difficult to examine this phenomenon and to choose appropriate methods to be standardized. Namely, in the field of testing concrete for sulfate attack, there are still no standard methods used to evaluate sulfate resistance. The methods most commonly used for laboratory testing of the sulfate resistance of concrete are mass change, strength change, length change, determination of porosity and supplemented by various microscopic methods. These methods are not standardized but have been refined and supplemented by researchers.
[16] Richardson, J.M., Biernacki, J.J., Stutzman, P.E., Bentz, D.P. (2002). Stoichiometry of slag hydration with calcium hydroxide. J Am CeramSoc 85 (4): 933-947.
[17] Whittaker, M., Zajec, M., Haha, M.B., Black, L. (2016). The impact of alumina availability on sulphate resistance. Construct Build Mater 119:356–369.
[18] Higgins, D.D. (2003). Increased sulfate resistance of ggbs concrete in the presence of carbonate. CemConcr Comp 25: 913–919.
[19] Schmidt, T., Lothenbach, B., Romer, M., Neuenschwander J., Scrivener, K. (2009). Physical and microstructural aspects of sulfate attack on ordinary and limestone blended Portland cements. CemConcr Res 39: 1111-1121.
