SELF HEALING PHENOMENA IN CONCRETES AND MASONRY MORTARS: A MICROSCOPIC STUDY

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1 SELF HEALING PHENOMENA IN CONCRETES AND MASONRY MORTARS: A MICROSCOPIC STUDY Timo G. Nijland*, Joe A. Larbi*, Rob P.J. van Hees*,, Barbara Lubelli and Mario de Rooij*, * TNO Built Environment and Geosciences, Delft, The Netherlands, timo.nijland@tno.nl Faculty of Architecture, Delft University of Technology, The Netherlands Microlab, Faculty of Civil Engineering & Geosciences, Delft University of Technology The Netherlands A microscopic survey of over 1000 of samples of concrete and masonry mortars from structures in the Netherlands shows that, in practice, self healing occurs in historic lime and lime puzzolana mortars, in contrast to modern cement bound concretes and mortars. Self healing may be effected by the formation of either new Cacarbonate, CaCO3, or portlandite, Ca(OH)2. The latter may represent a precursor stage to final self healing producing CaCO3, which has a larger molar volume than portlandite; transition from the possibly initially formed CaCO3-polymorph vaterite will, however, involve a slight decrease in volume. In the paper, compositions and external conditions of samples clearly showing self healing will be evaluated. In addition, it is discussed whether the observed self healing phenomena should be desired, i.e. whether internal mass transfer in the material resulting in the healing does not imply weakening of deeper zones of the material. Keywords: Self healing, masonry, historic mortars, concrete, portlandite, Ca-carbonate 1 Introduction Self healing phenomena, in the sense of healing of cracks, are commonly observed in mortars and, to a lesser extent, concretes. This holds in particular for historic, lime and lime-pozzolana bound mortars. In the current paper, an overview will be given of these phenomena, as well as criteria to distinguish them from other newly formed secondary phases that result from deleterious reactions in mortars and concretes, such as formation of ettringite, thaumasite and gypsum in either mortars or concretes, or alkali-silica reaction (ASR) in concrete. Understanding of naturally occuring self healing phenomena may serve as a starting point for the development of purpose-made self healing lime or cement bound materials. 2 Secondary phases in mortars and concretes Both mortars and concretes may develop a range of reactions resulting in the formation of secondary phases that, in many cases, result in damage in the sense of desintegration of the binder matrix or crack formation. 1 Springer 2007

2 In all cases, the presence of water, as a transport agent and, in most reactions, as a reacting compound, is an essential requirement for these reactions to proceed. Excluding more exotic ones, these reactions include: 1 Ca-sulfate-bearing phases, such as ettringite, Ca6Al2(OH)12(SO4)3 26H2O, (Fig. 1), thaumasite, Ca3Si(OH)6(CO3)(SO4) 12H2O (Fig. 2), or gypsum, Ca2SO4 2H2O (Fig. 3), in response to either an internal Ca-sulfate source (for example excess Ca-sulphate added to control setting of cement or from the breakdown of ettringite during steam curing), or an external source (such as SO4-bearing solutions); these may occur both in mortars and concretes 2 Alkali-aggregate reactions (AAR), i.e. reactions between aggregates in concrete and the alkaline pore solution resulting from Portland cement hydration. In the most common form, alkali-silica reaction (ASR), a Ca-Na (K)-SiO2-H2O gel is formed, which, by uptake of water, causes swelling and damage. 3 Crystallization of water-soluble salts in masonry mortars. These may include a wide range of salts, common ones being halite, NaCl, thenardite, Na2SO4, nitratine, NaNO3, gypsum, Ca2SO4 2H2O, and syngenite, K2Ca(SO4)2. 4 Formation of brucite, Mg(OH)2, in concretes under marine conditions. In the case of sulfate-bearing phases, ettringite in particular is encountered in almost all concretes and many mortars with cement as a binder constituent. It is, however, in most cases only filling part of air voids, and, in that case, not deleterious (Fig. 1). Occasionally, preexisting cracks may be filled with ettringite. When not enough empty space is available, formation of additional ettringite will, however, result in cracking; in addition, the formation of excessive secondary ettringite at the cement paste aggregate interface may damage the material. For this reason, and because ettringite s instability upon carbonation, filling of cracks by ettringite is not considered self healing. Thaumasite tends to desintegrate the binder matrix itself (Fig. 2). Figure 1: Microphotograph showing an irregular void due to inadequate compaction, partly filled by ettringte (View 1.4 x 0.9 mm, plane polarized light) Figure 2: Microphotograph showing an example of deleterious secondary phases in mortar: Formation of thaumasite throughout the cement paste in a 2 years old, Portland cement bound pointing mortar (View 1.4 x 0.9 mm, cross polarized light) 2 Springer 2007

3 Figure 2: Microphotograph showing an example of deleterious secondary phases in mortar: Formation of thaumasite throughout the cement paste in a 2 years old, Portland cement bound pointing mortar (View 1.4 x 0.9 mm, cross polarized light). Figure 3: Microphotograph showing an example of deleterious secondary phases in mortar: the formation of gypsum in a Medieval lime mortar, behind a late 19th or early 20th century repair mortar (View 1.4 x 0.9 mm, left plane polarized light, right cross polarized light) It will be clear that all these reaction products may be present in air voids, newly formed cracks, or pre-existing cracks. Self healing of mortars or concrete implicates the filling of preexisting cracks (rather than voids), without initiation of new cracks by non deleterious products, either new hydrates (C-S-H gel) formed from residual Portland cement clinker (or supplementary cementing materials such as blast furnace slag or pulverized fly ash) or deposition of Ca-bearing compounds. Plain recrystallization of Ca-carbonate in lime mortars generally results in the formation of coarser crystals in and around voids or along cracks (Fig. 4), but not necessarily in self healing of those cracks. 3 Springer 2007

4 Figure 4: Microphotograph showing recrystallization of carbonated lime binder in 12th century mortar (View 1.4 x 0.9 mm, cross polarized light) 3 Examples of self healing from building practice 3.1 Lime and lime pozzolana mortars th (?) century lime trass mortar self healing by Ca-carbonate This case involves a 20th century (?) lime mortar, without trass as pozzolanic addition. The masonry mortar was applied as a repair on an older, possibly 12th century, lime mortar in tuffstone masonry, suffering from a high salt load (Fig. 5). The salts affecting this part of the masonry are halite, NaCl, and nitratine, NaNO3. Within the mortar, lumps of lime occur, due to inadequate mixing. The lime lumps show shrinkage cracks, as is commonly observed. Most of the cracks are entirely healed by fine grained Ca-carbonate (Fig. 6). Figure 5: Polished slab of masonry, stabilized by epoxy (yellow), showing the masonry mortar in between parts of the tuff stone blocks, with white efflorescence of salts remobilized during sample preparation visible on the margins and front side of the mortar Figure 6: Microphotographs showing lumps of lime in 20th (?) lime trass mortar, with the small shrinkage cracks in the lump being healed by fine grained Ca-carbonate (View 5.4 x 3.5 mm, left plane polarized light, right cross polarized light) 4 Springer 2007

5 Figure 6: Microphotographs showing lumps of lime in 20 th (?) lime trass mortar, with the small shrinkage cracks in the lump being healed by fine grained Ca-carbonate (View 5.4 x 3.5 mm, left plane polarized light, right cross polarized light) lime fired clay pozzolana mortar an arrested case of initial self healing This case involves an early 19th masonry mortar, with a binder of lime + ground fired clay, used in the construction of an inundation lock. This type of binder was patented in the Netherlands in the late 18th century, as so-called Amsterdam or Casiusz cement (Van der Kloes 1924); it is to some extent comparable to early Medieval lime mortars with ground fired clay brick as pozzolanic addition, that were used in Venice, Germany and the Netherlands, amongst others. The masonry will be very wet for considerable periods during the year, and mortars show severe frost damage. An overview of the mortar is given in figure 7. The wet conditions of the masonry evidently prevented carbonation of the mortar, with large, well developed portlandite crystals being present in many air voids (Fig. 8). In contrast to almost all other observed examples of self healing, this mortar shows healing of part of the cracks by portlandite (Fig. 9). Probably, this represents an arrested case of initial self healing. The permanent wet nature of the mortar preventing reaction of the portlandite to either metastable vaterite or stable calcite. Figure 7: Microphotograph with overview of early 19th century lime fired clay pozzolana mortar. Note the larger and smaller cracks (View 5.4 x 3.5 mm, plane polarized light) Figure 8: Microphotograph showing large, well developed portlandite crystals in air void in early 19th century lime fired clay pozzolana mortar (View 2.4 x 1.8 mm, cross polarized light) 5 Springer 2007

6 Figure 8: Microphotograph showing large, well developed portlandite crystals in air void in early 19 th century lime fired clay pozzolana mortar (View 2.4 x 1.8 mm, cross polarized light). Figure 9: Microphotograph with (left) overview of both empty cracks, and cracks entirely filled with portlandite in early 19th century lime fired clay pozzolana mortar (View 2.8 x 1.4 mm, cross polarized light), and (right) detail of coarse portlandite crystals in the same mortar (View 1.4 x 0.9 mm, cross polarized light) th century lime mortar repeated self healing This case involves a 19th century masonry mortar from a quay wall. The mortar shows severe cracking, amongst others due to frost action (Fig. 10). Several cracks in the mortar show self healing. In one case, the cracks are filled with Ca-carbonate, almost certainly calcite (Fig. 11). This type of self healing is common in lime mortars. Interestingly, there are also cracks filled by fine grained portlandite (Fig. 12). Probably, these represent non-carbonated precursors of the calcite-filled cracks, illustrating that the mechanism of self healing may be active repeatedly. 6 Springer 2007

7 Figure 10: Microphotograph illustrating cracking in 19th century lime mortar (View 5.4 x 3.5 mm, cross polarized light) Figure 11: Microphotograph showing a crack in 19th century lime mortar, filled completely by calcite (View 0.7 x 0.45 mm, cross polarized light) Figure 12: Microphotograph showing a crack in 19th century lime mortar, filled completely by fine grained portlandite (View 0.7 x 0.45 mm, cross polarized light) 7 Springer 2007

8 4 Discussion and conclusion Self healing is frequently encountered in historic lime and lime pozzolana mortars. Although, over the years, we have investigated over thousand thin sections of cement bound mortars and concretes from all types of buildings and structures, no self healing has been encountered in samples with only cement as a binder (i.e. without additional lime), neither in those with CEM I, nor in CEM II/B-S, CEM II/B-V or CEM III/A or CEM III/B. Apparently, an source of free lime is required to enable self healing. In addition, mortars or concrete have to be moist for prolonged periods to enable transport of Ca. Carbonation (see below) probably implies that dry periods are also a requirement, as CO2 is most likely derived from the air. The cases discussed also show that, in mortars of suitable composition, self healing may repeatedly be active, and may even work under salt-loaded conditions. Generally, self healing in these mortars involves the precipitation of Ca-carbonate, either calcite or vaterite. However, two of the cases illustrated involve masonry that is wet for long periods, probably preventing full carbonation of the mortars. In these cases, self healing by portlandite is encountered. It is assumed that these represent arrested cases of initial self healing: The required Ca will first be transported in a dissolved form, precipitate as portlandite, and, after prolonged dry conditions with ingress of atmospheric CO2, be transformed into Ca-carbonate. Given the higher molar volumes of of all three polymorphs of CaCO3, viz. calcite (triclinic, cm3 mol-1), vaterite (hexagonal / orthorhombic, cm3 mol-1) and aragonite (orthorhombic, cm3 mol-1) relative to portlandite (33.06 cm3 mol-1) (Mincryst database), this will enhance the self healing effect. Alternatively, both vaterite and calcite may directly precipitate from solution (cf. Dupont et al. 1997). Though vaterite is metastable (Wolf et al. 2000), it is frequently encountered in carbonated concretes and mortars, as is calcite, but in contrast to aragonite (Lang 2003); vaterite is encountered in particular upon accelerated carbonation in the laboratory, indicating that kinetic processes (such as precipitation) might favour vaterite. Once formed, transformation of metastable vaterite to stable calcite will involve a decrease of volume of about 1 %, thus creating new porosity. However, though unstable, transformation of vaterite to calcite is hampered by reaction kinetics (activation energy of 250 kj mol-1 (Wolf & Günther 2001), whereas the presence of surfactants may stabilize vaterite (Dupont et al. 1997). A better understanding of the relationships between both processes, binder composition and external conditions may enhance possibilities of creating on purpose built-in self healing. The nature of self healing in these mortars, however, poses another important question, i.e. the source of the Ca. This will evidently be the mortar itself, implying that whereas at one location self healing occurs, at other locations new porosity will be generated. The effect of this process on the overall durability of the mortar is unclear. However, from a durability point of view, blocking of the cracks is likely to be the dominant aspect. The study clearly demonstrates the role of lime in self healing. In case of mortars or concretes with a low amount of free lime (portlandite), it may be expected that self healing will result in an increase in the porosity of the binder matrix itself, comparable to the way in which the porosity of ground granulated blast furnace slag cement concretes is increased by carbonation because C-S-H is attacked instead of free portlandite (e.g. Visser & Polder 2000). The absence of the self healing phenomena in ordinary portland cement bound materials indicates that even there, the amount of lime available, is not enough. The effect of addition of lime on self healing may be an interesting direction for future research. 8 Springer 2007

9 REFERENCES Dupont, L., Portemer, F. & Figlarz, M., Synthesis and study of a well crystallized CaCO3 vaterite showing a new habitus. J. Mat. Char. 7: Kloes, J.A. van der, Onze bouwmaterialen. Deel III. Mortels en beton. Van der Endt, Maasluis. Lang, E., Einfluss unterschiedlicher Karbonatphasen auf den Frost-Tausalzwiderstand Labor- und Praxisverhalten. Beton-Informationen (3): Mincryst database on Visser, J.H.M. & Polder, R.B., PRINDUCEB II-7: DuraCem. Phase 2: Basic investigation of the influence of the type of cement on the resistance of concrete against reinforcement corrosion. TNO, Delft, TNOreport 2000-BT-MK-R0198, 82 pp. Wolf, G. & Günther, C., Thermophysical investigations of the polymorphous phases of calcium carbonate. J. Therm. Anal. Calorim. 65: Wolf, G., Köningsberger, E., Schmidt, H.G., Köningsberger, L.C. & Gamsjager, H., Thermodynamic aspects of vaterite-calcite phase transition. J. Therm. Anal. Calorim. 60: Wolf, G., Lerchner, J., Schmidt, H., Gamsjager, H., Köningsberger, E. & Schmidt, P., Thermodynamics of CaCO3 phase transitions. J. Therm. Anal. 46: Springer 2007