Conservation of brick
- A. Elena Charola
- and Inge Rörig-Dalgaard
Good-quality bricks are among the most durable building materials available. Nonetheless, as they weather over time they will eventually return to the clay from which they were made. Deterioration is caused by several factors such as the presence of soluble salts, air-pollution, freeze-thaw cycling, biological action, and incompatible mortars used in their setting. The following discussion deals specifically with the conservation of exposed brick masonry and does not include brick veneer.
The presence of soluble salts, such as sodium chloride (NaCl, halite), either in the brick, mortar, or the ground where the wall foundation is set, as well as deicing salts, can cause rapid deterioration in a very short time when alternating wet–dry cycles occur. Water dissolves salt crystals, carrying their ions into porous bodies of bricks. When dry conditions prevail, the water migrates back to the surface and evaporates, causing the salts to recrystallize causing the damage. The degree of this damage is related to the amount of salt that recrystallizes, the conditions under which the recrystallization occurs, the number of wetting and drying cycles to which the masonry is exposed, as well as changes in temperature (T) and relative humidity (RH). Some bricks, if not fired properly, may inherently contain salts, such as sodium sulfate which can crystallize as an anhydrous salt (Na2SO4, thenardite) or a decahydrate (Na2SO4.10H2O, mirabilite). Transitions between these two phases take place with changes in T and RH, and this secondary crystallization process can be far more destructive than the simple crystallization of a non-hydrating salt such as halite.
Usually several salts are present at once and a distribution pattern can be observed in the masonry: the most soluble salts will tend to migrate higher up the wall, while the less soluble ones will crystallize closer to the ground. The relative porosity of brick and mortar—which is proportional to the material strength—will determine which material will deteriorate first, since crystallization will take place preferentially in the more porous material.
Brick walls were traditionally laid with lime mortar that is generally more porous than brick and therefore requires periodic repointing of the walls. It was believed that the advent of Portland cement in the mid-18th century would reduce the need for repointing, but what was not taken into account was the fact that Portland cement, being less porous, would shift much of the deterioration to the more porous brick. Both lime and Portland cement mortars contain free calcium hydroxide (Ca(OH)2, portlandite), the latter retaining more free portlandite that fails to fully carbonate and may leach out over the bricks. Portland cement may also contain soluble salts such as gypsum and thenardite. Given that lime-water is highly alkaline (pH ~12), it may slowly dissolve the glassy phase in the brick. Although this deterioration may be less aggressive than that caused by soluble salts, it should be taken into account when laying brick as the porosity of the mortar should be adjusted to suit the porosity of the brick being used.
The deterioration produced by frost is induced by the growth of ice crystals in a process similar to that caused by salt crystallization. Deterioration may also result if liquid water is trapped between a frozen core and an advancing freezing front. The susceptibility of brick to frost damage will depend upon its porosity and pore-size distribution. The extent of the deterioration also will depend on the degree of water saturation, the number of freeze–thaw cycles, the length of the freezing period, and the rate at which temperature changes take place.
Air pollution from the burning of fossil fuels is acidic due to the presence of sulfur oxides that form sulfuric acid with atmospheric moisture. This acidic solution does not cause a direct deterioration of bricks, though it may affect those with a glassy matrix through an ion exchange mechanism in which hydrogen ions replace sodium ions in the glass matrix, inducing shrinkage and crizzling. However, the main cause for deterioration due to acid is its reaction with the calcium carbonate (CaCO3, calcite) present in both lime mortars and Portland cement. This reaction leads to the formation of calcium sulfate that crystallizes as the dihydrate form known as gypsum (CaSO4.2H2O). Although gypsum is not a very soluble salt, it is about a hundred times more soluble than calcite. Air pollution from traffic can also include nitrogen oxides (NOx), in which case the salt resulting from this reaction will be sodium nitrate (NaNO3, nitratite), which is highly soluble. Soluble salts are also hygroscopic and will absorb atmospheric water until they go into solution when the RH goes above a certain level, known as the deliquescent RH (DRH), that is specific for each salt. In the case of nitrate, the DRH is 75% RH at 20ºC. Therefore, when the ambient RH is above 75%, this salt will be able to migrate higher on a wall and keep it damp, which may enhance biological colonization. While the mechanism by which growing microorganisms induce damage is slow, the presence of higher-order plants, such as shrubs and trees, are more deleterious by the mechanical action they exert especially when they grow between the brick and the mortar.
The conservation of brick cannot be undertaken independently of the masonry context in which it is being used. When bricks are replaced or an old masonry structure is repointed, care must be taken to ensure that the porosity of the new materials matches those of the old and that the bond is good, thus diminishing the chance of water penetration. In fact, as salts enter due to the action of water, the main conservation concern is that of keeping the masonry dry. All sources of water infiltration have to be identified and stopped before any water-repellent agents can be applied to the masonry. In addition, the design of the wall and how the water will flow over its detailing has to be taken into account to ensure that water is not trapped behind the wall that could result in ice damage.
The most successful hydrophobizating (i.e. water-repellent) agents are those based on oligo- or polysiloxanes. If applied correctly, these agents will also greatly diminish the occurrence of biocolonization. However, they should not be applied to bricks that already contain soluble salts as flaking will occur when the RH falls and salts crystallize behind the hydrophobized layer. In this case, it would be necessary to extract the salts (e.g. by poulticing), a procedure that may not always be successful, and provided that no new salts have access to it and regular maintenance is implemented. Unfortunately, attempts at pre-treating bricks with these agents before they are used in construction have not been successful since they interfere with the brick–mortar bonding.
Once the brick has lost its mechanical resistance, in most cases, it can be replaced by a similar brick, which should be compatible with the remaining ones. However, there are cases in which consolidation can be used to help preserve a historic fabric. For this purpose, the best solution is the application of silicate ester based products, since they are compatible with brick. It is important that only the bricks be consolidated. Once the treatment is applied, the joints around the consolidated bricks should then be repointed with a compatible mortar.
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