Materials and Structures, 1992, 25, 429-435

Microstructural investigation of natural deterioration of building materials in Gothenburg, Sweden

S. L . S A R K A R

Department of Civil Engineering, Universitk de Sherbrooke, Sherbrooke (QuObec), Canada J I K IR1

S. C H A N D R A , M . R O D H E

Division of Buildin9 Materials, Chalmers University of Technology, Gothenburg, Sweden

Site investigations of several concrete and masonry structures in Gothenburg, Sweden, revealed carbonation to have affected a number of concrete structures. At times, synergistic freeze-thaw action was found to have accelerated the deterioration process. Some of the masonry structures have developed internal and external efflorescence (the latter much more severe) in the form of alkali sulphate salts. The mechanisms involved in these processes and the influence of microclimatic conditions are discussed. The products formed that were mainly investigated by microstructural techniques (SEM with energy-dispersive X-ray analysis, X-ray diffraction analysis) are also described.

1. I N T R O D U C T I O N

Cases of natural deterioration of building materials such as concrete and masonry are frequently reported from different parts of the world. Deterioration can often reach a level at which the structure may have to be thoroughly renovated or completely replaced. It is therefore impera- tive that the condition of the structure is inspected on a regular basis, and repair work carried out wherever necessary to prevent further damage.

Building materials are not immune to the climatic conditions to which they are exposed. Naturally, deterio- ration occurs with time. Incorrect selection of materials and improper concreting procedures (with respect to the environment) are often alleged to be the cause of deterioration [1]. However, with improved construction techniques, good workmanship and careful selection of materials, the life-span of a structure can undoubtedly be increased.

A number of mechanisms are proposed for such deteriorative processes. Well known among them are chemical attack [2], freeze-thaw action [31 the effect of de-icer salts [4], carbonation [5], rehar corrosion [6] and alkali-aggregate reaction [-7].

The structures that exhibited signs of natural deteriora- tion included a concrete ramp from the street level to the second storey of a four-storey building built in 1968, a concrete parapet (of the 1930s) along the side of a hilly road, a concrete wall of the same period, an exposed brick column of 1990, and the brick-lined interior (1925-1930) of an ancient church tower.

The main purpose of this investigation was to determine the nature of damage to these structures, study the deteriorative mechanisms involved, and identify the products of deterioration. SEM with energy-dispersive X-ray analysis (EDXA) was used to study the micro-

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structure of the affected parts, and X-ray diffraction analysis (XRDA) for precise mineralogical identification of the products. These two microanalytical techniques are known to provide valuable information from which important inferences can be drawn regarding the mechanism of deterioration.

2. DESCRIPTION OF AFFECTED STRUCTURES

The ramp, 10 m in length, serves as a connector for a building from the street level. Though examination of the sides of the ramp showed no deterioration, the underside had developed extensive whitish incrustation, with stalactite formation (Fig. 1). This obviously indicates that the process has been in progress for a considerable period of time.

The parapet, on the other hand, exhibited an entirely different mode of disruptive feature. Massive chunks of concrete, 10-50mm thick, had peeled off from the external surface in some parts of the structure, while in others an external layer was still loosely bonded to the main body of the concrete (Fig. 2).

The concrete wall was observed to have developed several elongated irregular cracks with a whitish infilling, and a thin incrustation (of the same colour) spread a few millimetres across the width of these cracks, which gave the appearance of exudation from the interior of the concrete.

Some of the bricks in the exposed columnar brick structure had formed patches of white powdery deposit on their surface (Fig. 3), typical of efflorescence, whereas the interior of the church tower inspected by the authors contained profuse whitish masses on a large number of bricks as well as on the rendering on these bricks (Fig. 4). Closer examination of these masses showed the

430 Sarkar, Chandra and Rodhe

Fig. 1 Stalactite formation on the underside of the concrete ramp.

Fig. 4 Deteriorated masonry in the interior of the church tower showing profuse salt deposition.

Fig. 2 Showing the deteriorated parapet. Fig. 5 Micrograph of two different morphologies of calcite crystals in the stalactite.

Fig. 3 Efflorescence on the brick column.

presence of very delicate whiskery crystals. The render- ing in places had been completely destroyed, and the exposed bricks severely affected, giving a strongly pitted appearance.

3. RESULTS

Carbonation was observed to be the main agent of deterioration in all the affected concrete structures. The mineralogical composition of the carbonate was calcite, confirmed from XRDA. A substantial portion of the stalactite structure under the ramp (observed under the SEM) was composed of different crystalline shapes and sizes (Fig. 5). The smaller grains (1-2 gm) were randomly distributed, whereas the larger crystals (up to 10 gm in length) displayed a stacking formation. Certain internal parts of the stalactite comprised globular features (100 gm diameter) as seen in Fig. 6. A highly magnified view of these globules revealed the presence of less crystalline calcite mineral agglomeration (Fig. 7). Similar morphology of calcite crystals constituted the incrusta- tion and the infilling of the fissured concrete wall.

The deteriorated parapet concrete, however, consisted of a thick carbonated (calcite) layer with islands also containing calcite crystals, though of less regular morphology (Fig. 8). The formation of variable mor- phology for the same crystal is interesting, and its

Mater ia ls and Structures 431

Fig. 6 Globular crystallization of calcite in the stalactite. Fig. 9 Porous paste of the parapet concrete, also showing fissures in the paste.

Fig. 7 Magnified view of a globule showing the nature of calcite deposition.

Fig. 8 Carbonated film surrounding some irregular calcite crystals in the parapet concrete.

importance in terms of physical factors is discussed in a later section. Though carbonate minerals covered most of the concrete surface, a few observable exposed areas showed a porous matrix and fissures running through the paste (Fig. 9). A 100 mm long core section of this concrete which was sliced longitudinally shows that

(b)

Fig. 10(a) Concrete core showing segregation and debonding zone: E = exposed face, I = interior, D = debonded zone. (b) Optical micrograph of this concrete showing bright carbonation rims.

the first 40 mm (from the exposed face) contains very little coarse aggregates (Fig. 10a), whereas strong segregation of coarse aggregates in the interior part of the concrete is evident. A zone of distinct debonding is also visible. Petrographic examination also revealed profuse carbonation rims (Fig. 10b), which correspond to the carbonated layer observed under the SEM.

The composition of the whitish deposit on the exposed

432 Sarkar, Chandra and Rodhe

C H U R C H R E N D E R I N G

2 . ' + ' 5 1 A

1 7 1 1 9 S

1 5 . 2 Q . 2 5 .

7

2 5

1 "

w

' ~ 0 .

2

2

" ~ . , 4 0 .

�9 t ,==;

C H U R C H B R ~ C K

I . + . 4

2 1 .o-1.-.4

:L -t.- 2

2 0 . = = ' 5 " ~ 0 �9 3 ~ . 4 0 .

Fig. 11 XRD trace of salt deposits on church rendering and brick: (1) mirabelite, (2) thenardite, (3) gypsum, (4) hydroglauberite, (5) calcite, (6) quartz, (7) albite, (8) microcline, (9) anhydrite.

bricks in the column was clearly identified from XRDA as thenardite (NS)* and apthitalite (N, K)S. The whitish mass on the church bricks, however, comprised several sulphate minerals, namely mirabelite (NS" 10H), with some thenardite, hydroglauberite (CNS-6H), gypsum (CS-2H) and calcite (CC), whereas the rendering on the brick was mainly constituted of thenardite, with a small amount of anhydrite (CS), mirabelite, and calcite (Fig. 11). Quartz and feldspar which appear in the XRD pattern represent the aggregates, though the cementitious components could not be positively identified owing to the profusion of salt deposition.

From Fig. 11 it is clear that a significantly higher amount of hydrated salt has formed on the brick than on the rendering. This obviously represents increased absorption of moisture in the brick. Larsen and Nielsen [8] also observed the higher sorption capacity of damaged bricks. Mild carbonation on the brick and the rendering due to dissolution of the lime also appears to have occurred. This is evident from the calcite peak in the XRD patterns.

From SEM/EDXA examination a thick matted struc- ture of prismatic NS crystals (Fig. 12), spike-like growths of NS and CNS crystals (Fig. 13), together with microglobular crystals of NS (Fig. 14) were observed.

Though the acicular and the globular crystals both

* C e m e n t t echno log i s t s ' no t a t ion : C = C a O , N = N a 2 0 , K = K 2 0 , g = SO3, H = H 2 0 , C = C O 3 .

Fig. 12 Matted structure comprising acicular sodium sulphate crystals.

yield Na, S and 0 2 o n EDX spectra, the hollow-shell structure and internal desiccation cracks in some of the larger subhedral crystals of NS (Fig. 15) suggest the globular crystals to be the hygroscopic variety of NS, or mirabelite, whereas the acicular type must be the anhydrous thenardite.

Interestingly enough there seems to be no evidence of the classical type of sulphate attack on either the brick or the rendering; the characteristic needle structure of ettringite or thaumasite is totally absent, although a number of workers, including Collepardi [-2] and Ludwig

Materials and Structures 433

Table 1 Chemical analysis of total and soluble alkali sulphate in brick and rendering

Material Total (wt %) Soluble (wt %)

Na20 K20 SO 3 Na20 K20 SO 3

Brick 3.4 4.6 0.21 0.07 0.04 0.13 Rendering 4.0 3.6 0.85 0.57 0.09 0.73

Fig. 13 Sodium and sodium--calcium sulphate crystals on the church brick.

for this may be the proximity of the Danish churches to the sea, which promoted the deposition of halite from the sea, whereas the church structure investigated by the authors, though also located in the Nordic region, is further inland.

Chemical analysis of total and soluble alkali sulphates (Table 1) shows lower amounts of soluble alkali sulphate in the brick. This suggests that a substantial portion of alkalis and sulphate in the brick has already been consumed in salt crystallization. XRDA of increased salification on the brick also supports this view.

Another noteworthy observation is the high level of K20 retention in the brick and the rendering, which precluded the formation of potassium-bearing sulphate salts. It undoubtedly suggests a greater solubility of sodium salts in comparison to potassium.

Fig. 14 Microglobular sodium sulphate hydrate crystals on the church brick.

Fig. I5 Large subhedral crystals of sodium sulphate hydrate showing internal desiccation cracks.

and Meher [9] reported the destruction of historical buildings due to the formation of these minerals. Halite (NaCI) crystals, observed by Larsen and Nielsen [8], as being the cause of decay of bricks in medieval churches in the coastal region of Denmark, are also absent in the church tower at Gothenburg. One possible explanation

4. DISCUSSION

In the case of the concrete structures, it has clearly been a dissolution process, whereby the CH in the concrete dissolved in the presence of CO2 to make it weak and porous [5]. The solution later exuded to the surface through cracks and interconnected pores to crystallize as calcite. The cracks, probably mechanical in nature and generated by repeated freeze-thaw action, obviously act as the ideal site for the passage of the solution. According to Charola [10] deterioration induced by mechanical action can be very rapid, and within a short period of time can do more damage than natural weathering spread over a longer span of time. The extent of damage thus may not become apparent for a number of years, but suddenly the structure may appear to be weak.

This is valid for the parapet concrete, where freeze- thaw action played an important role in the deterioration process. The location of the parapet makes it much more vulnerable to this kind of damage than the other structures reported, because of the higher amount of snow accumulation along the side of the road, followed by thawing action. This led to the destruction of internal cohesion in the .concrete with subsequent carbonation in the disjointed region. The destructive process also involved leaching action of some of the constituents of concrete, such as C-S-H, CH and aggregates [4, 11], thus making the concrete extremely porous. Biczok [12] has cited two examples of strength loss due to the leaching of lime.

The massive incrustation and stalactite formation

434 Sarkar, Chandra and Rodhe

under the ramp can be considered to be a cumulative process. The longitudinal dimension of these stalactites indicates the process of carbonation to have occurred over a number of years. It is likely that the concrete was made with a high w/c ratio which resulted in a higher porosity. This enabled continual percolation of water from the deck to the underside. In the absence of records on the composition of the original concrete, this phenomenon, however, could not be firmly established. The segregation observed in the parapet concrete, how- ever, demonstrates poor workmanship during placing.

Another factor which needs to be taken into account is the variable morphology of calcite crystals observed in these carbonated zones. It reflects the influence of microclimatic conditions on the crystallization behaviour of CaCO 3 that has been well documented by Goodbrake [13]. Congenial atmospheric conditions led to the formation of large rhombohedrat crystals, whereas under less favourable climatic conditions either smaller, less geometric, irregular crystals or a carbonated layer was formed. The globular masses observed only in the stalactite represent calcite crystallization around droplets of water. Impurities determine the crystallization of other polymorphs of CaCO 3 (vaterite and aragonite), while pure water forms calcite [14].

It is well known that the most commonly occurring salts in bricks are different types of sulphates [151, formed by the combustion of coal (used to manufacture bricks) with the clayey constituents of the raw meal during burning. Most of these sulphates are readily soluble salts. Gradual migration of these salts from the interior to the surface can cause efflorescence, or a whitish deposit on the exposed surface. As the absorbed water inside the brick dries out by evaporation, the soluble salts crystallize on the surface. External efflorescence, like that observed in the brick column, though not very aesthetic in appearance, very rarely causes serious damage. Internal efflorescence, as in the church, however, can be highly problematic, particularly when rendering is applied on bricks. The salts tend to crystallize at the brick-rendering interface, which can lead to delamination.

From the chemical analysis (Table 1) it is obvious that it is not the quantity, but the type of salt that is important. Potassium and sodium salts are known to be more soluble, and thus readily transferable. The present study shows that potassium tends to be retained inside the brick and the rendering.

Moisture due to high humidity (97%) was absorbed in the bricks and the surface rendering. Subsequent dissolution of alkali salts from inside the bricks led to their precipitation on the surface of the bricks and the rendering. Similar observations were also made by Ragsdale and Raynham [15]. In outlining the chemical reactions involved in sulphate attack, Mehta [16] proposed the equation

Na2SO4 + Ca(OH) z + 2H20

CaSO4.2H20 + 2NaOH (1)

This can explain the presence of gypsum on the church brick, but not on the rendering. The lime from the rendering was transported to the brick, which then reacted with some of the available sodium sulphate in it to transform into gypsum. Though no confirmatory proof could be obtained on the actual composition of the rendering, lime is known to have been used as a plastering compound in historical monuments [2]. The study by Regourd et al. [17] of mortars from ancient Egyptian pyramids has shown that conversion of anhydrite to gypsum can also produce 20% volume expansion.

According to Chatterjee and Jensen [18] some of the primary efflorescents are anhydrous and hydrated sodium sulphate. As seen from the X-ray diffraction results (Fig. 11), they can occur as a mixture. Arnold [19] has shown that the decomposition of the hydrated form to the anhydrous salt is a reversible process, temperature being the determining factor. Though anhydrous sodium sulphate is stable above 32.4~ it can also exist as a metastable form below this temperature. The mechanism also appears to be related to the saturation level of the solution and the dissolution capacity of these salts [18]. In any case, disintegration due to salt crystallization arises from a pressure differential [19]. Under thermodynamic equilibrium conditions, a pressure builds up in the saturated salt solution due to crystal growth [20]. The volume change associated with crystallization exerts pressure against the pore wail. Several other mechanisms have been proposed to explain the destructive mechanism. For example, Perander and Nieminen [21] explain this process in terms of chemical dissolution.

Baden et al. [22] claim that the shape factor of crystals plays an important role in the destruction process. A prismatic crystal with sharp points will cause more damage than a cubic one, because of higher pressure. Thus, the acicular crystals on the church tower brick could have contributed towards the severity of the damage.

The amount of salification on the brick is positively higher, indicating the brick to be the principal source of salts, some of which transferred in solution form to the rendering during chemical reaction.

5. CONCLUSIONS

Microstructural investigation of deteriorated concrete and brick structures in Gothenburg, Sweden, clearly demonstrates carbonation to have affected the concrete structures. One structure in particular exhibits increased decomposition due to freeze-thaw action which has resulted in massive spalling and leaching, accompanied by severe carbonation. In the two other affected structures, fissures and the porous nature of the concrete led to different forms of carbonation, manifested in the form of exudation, incrustation and stalactite formation. Microclimatic changes have brought about differences in the crystallization phenomenon of calcite in these carbonated zones.

Mater ia l s and Structures 435

Qualitative assessment of damage to these structures suggests the parapet section concrete to have deteriorated most severely; parts of it are in need of immediate attention. Damage to the other two structures, however, is not as severe, and is not a cause of major concern at present, but needs to be monitored at regular intervals by qualified personnel to determine the rate of crack propagation.

The masonry structures, on the other hand, have undergone a different type of deteriorative mechanism. External sulphate efflorescence has affected the brick column but only at the expense of loss of aesthetic beauty, whereas the interior of the church has developed profuse sulphate salification resulting in delamination between the brick and rendering. At the time of writing this paper, the tower was undergoing extensive repair work; new plastering was being placed in the affected parts.

This study strongly points towards the important role that water plays in these destructive processes. Water is required for both the mechanisms to be activated. The synergistic role of freeze-thaw action cannot be over- looked. It also emphasizes the need for careful material selection, and proper concreting procedure with respect to the environmental conditions under which the materials are placed, because of the strong susceptibility of building materials to microclimatic variations. Other- wise, it can make the prediction of longevit3, of concrete and masonry structures much more uncertain.

The investigation also outlines the importance of microstructural study for defining more clearly the destructive process and the degradation products.

A C K N O W L E D G E M E N T S

The authors wish to thank the Swedish Council for Building Research and the Natural Sciences and Engineering Research Council of Canada for funding. SEM and XRDA facilities were provided by the Swedish Ceramic Institute.

REFERENCES

1. Mehta, P. K., 'Durability of concrete in marine environ- ment - a review', in 'Performance of Concrete in Marine Environment', ACI SP-65, edited by V. M. Malhotra (American Concrete Institute, 1980) pp. 1-19.

2. Collepardi, M., 'Degradation and restoration of masonry walls of historical buildings', Mater. Struct. 23 (1990) 81-102.

3. Idorn, G. M., 'Durability of concrete structures in Denmark', PhD thesis, Technical University of Den- mark, Copenhagen (1967).

4. Sarkar, S. L., Aitcin, P. C. and Lamothe, P., 'Concrete deterioration in three bridges in the Sherbrooke area', in Proceeding of 2nd CANMET/ACI International Conference on Durability of Concrete, Montreal, 1991, Supplementary Volume, pp. 219-234.

5. Regourd, M., 'Physico-chemical studies of cement pastes, mortars and concretes exposed to sea water', in 'Performance of Concrete in Marine Environment', ACI SP-65, edited by V. M. Malhotra (American Concrete Institute, 1980) pp. 63-82.

6. Hime, W. and Erlin, B., 'Some chemical and physical aspects of phenomena associated with chloride-induced corrosion' in 'Corrosion, Concrete and Chlorides', ACI SP-102, edited by F. W. Gibson (American Concrete Institute, 1987) pp. 1-12.

7. Grattan-Bellew, P. E. (Ed.), 'Concrete Alkali-Aggre- gate Reactions' (Noyes, New Jersey, 1987).

8. Larsen, E. S. and Nielsen, C. B., 'Decay of bricks due to salt', Mater. Struct. 23 (1990) 16-25.

9. Ludwig, U. and Meher, S., 'Destruction of historical buildings by the formation of ettringite or thaumasite', in Proceedings of 8th International Symposium on the Chemistry of Cement, Rio de Janeiro, Vol. 5 (1986) pp. 1-8.

10. Charola, A. E., 'Chemical-physical factors in stone deterioration', Durabil. Buildg Mater. 5 (I 988) 313-316.

11. Mehta, P. K., 'Concrete Structure, Properties and Mater- ials' (Prentice-Hall, Englewood Cliffs, 1986) pp. 135-136.

12. Biczok, I., 'Concrete Corrosion and Concrete Protection' (Chemical Publishing, New York, 1967) p. 291.

13. Goodbrake, C. J., 'Reaction of beta dicalcium silicate and tricalcium silicate with carbon dioxide and water', PhD thesis, University of Illinois (1978).

14. Teller, E. J. and Wray, J. L., 'Factors influencing the artificial precipitaton of calcium carbonate', Bull. Geol. Soc. Amer., 65 (1954) 1329-1330.

15. Ragsdale, L. A. and Raynham, E. A., 'Building Materials Technology' (Arnold, Bungay, 1972) pp. 102-154.

16. Mehta, P. K., 'Mechanism of sulphate attack on Portland ce- ment concrete', Cement Concr. Res. 13(3) (1983) 401-406.

17. Regourd, M., Kerisel, J., Deletie, P. and Haguenauer, B., 'Microstructure of mortars from three Egyptian pyra- mids', Cement Concr. Res. 18 (1988) 81-90.

18. Chatterjee, S. and Jensen, A. D., 'Efflorescence and breakdown of building materials', Nordic Concr. Res. No. 8 (1989) 56-61.

19. Arnold, A., 'Behaviour of some soluble salts in stone deterioration', in Proceedings of 2nd International Symposium on the Deterioration of Building Stone, Athens, 1976, pp. 27-36.

20. Kn6fel, D. K., Hoffman, D. and Nethlage, R., 'Physico- chemical weathering reactions as a formulatory for time-lapsing ageing tests', Mater. Struct. 20 (1987) 127-145.

21. Perander, T. and Nieminen, T., 'Neue Theorien der Ziegelverwittering', in 'Werkstoffwissenschaften und Bausanierung, edited by F. H. Wittman (Technische Akademie Esslingen, 1983) pp. 367-371.

22. Baden, B., Bacella, G. and Marchesini, L., 'Surface reactivity of marble and stone', in 'The Conservation of Stone', edited by I. Bologna and R. Rossi-Manaresi (Centro per la Conservazione delli Sculture all'Aperto, Venice, 1976) pp. 89-101.

436 Sarkar, Chandra and Rodhe

RESUME

L'~tude microstructurale de la d6t6rioration des mat6riaux de construction fi Gothenburg, Suede

L'~tude sur site de plusieurs b~tons et structures de mar gt Gothenburr en Sukde, a rdv~l~ qu'un certain nombre de structures en bdton ont ktk affectdes par une carbonatation. Le processus de dktbrioration a ktd accblkrb par le phdnomkne de 9el-dbgel. Certaines struc-

tures de mar ont d~veloppk une efflorescence interne et externe (cette derniOre est plus accentu~e) sous forme de sulfates alcalins.

Les mkcanismes dOveloppks dans ce processus, l'influence des conditions microctimatiques et les produits form,s ont Orb ktudibs par des techniques microstructurales (MEB/ EDAX, DRX) . L'effet de cette dktkrioration sur la durabilitk de ces structures a aussi btb kvaluk.