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PLAIN LIMESTONE & MAGNESIAN LIMESTONE (top)  
     
  Plain limestones have CaCO³ content between about 80% and 96%. This is attacked by sulphur acids as described above. Magnesium limestones are different, containing as they do a considerable amount of magnesium carbonate, MgCO³, replacing calcium carbonate. Many British limestones are magnesium; e.g., Anston stone containing 55% CaCO³ and 43% MgCO³. The effect of sulphur acids on these is different. The CaCO³ forms CaSO with sulphuric acid as before, but the MgCO³ forms MgSO, magnesium sulphate (Epsom salt, when crystallised), which is even more disruptive than calcium sulphate. It crystallises as sharp acicular (needle-like) crystals, causing expansion as it forms, taking in seven molecules of water of crystallisation to one of the salt (MgSO . 7H²O). If not rain-washed, this salt is even more energetic than gypsum in causing exfoliation. Anston stone was used in the structure of the Houses of Parliament. This and St Pauls Cathedral are forever having their stonework renewed in some part or other.
  
     

SANDSTONES (top)  
     
 

Calcareous sandstones may fail in a similar way to limestones as the sulphuric acid slowly attacks the CaCo matrix of the stone. The silica of the sand grains is acid-proof, but sand grains have no strength without a good matrix. Sandstones with an argillaceous (clay-ey) matrix are not suitable for a very wet situation, as water softens any clay. The best sandstones are siliceous, i.e.; their binding matrix is fine silica. Many British sandstones fall onto this class.

  
     

CONCRETE (top)  
     
  The effect of atmospheric sulphur gases on concrete is not generally regarded as serious, but the action of the sulphates within concrete is a different matter. Atmospheric H² SO may slowly attack the carbonated lime on the surface and remove some of it, but well made concrete should stand weathering and also abrasion (e.g., on street pavings) as the aggregate becomes exposed. New pavings look cement coloured, not showing aggregate: old and weathered ones show exposed aggregate, which resists further wear.  
     

METALS (top)  
     
  Metals used for roof coverings are attacked by ordinary pure country air as well as by sulphur gases. The important consideration here is whether the corrosion is progressive or not. Iron or steel without protection by paint, etc., may rust right away. With non-ferrous metals used for roof coverings, the case is different.  
  Copper in the open air is slowly corroded on the surface by water and carbon dioxide to form a thin coating or patina, which is green in colour. This is basic copper carbonate, Cu(OH)² . CuCO³. In normal circumstances this is protective after a thin coating has been formed and further corrosion ceases. Where the atmosphere is polluted with sulphur gases some patinas may also contain basic copper sulphate, Cu(OH)², CuSO . This is also protective and further corrosion ceases.  
  Where lead is laid on a slope and rainwashed one can see a white deposit. This is basic lead carbonate, Pb(OH)². PbCO³, similar to white lead. It is the normal coating that lead acquires in the open, and is protective.  
  Both lead and copper roofs last for centuries.  
  Zinc corrodes to form basic zinc carbonate, Zn(OH)², ZnCO³ , which cannot be said to be as protective as the corrosion products of copper and lead. Zinc roofs tend to pinhole as the result of pitting corrosion. A small scratch or pit or lack of uniformity starts a corrosion cell, corrosion progressing at the bottom of the pit and proceeding deeper until the roof pinholes. Underneath the white corrosion product is quite visible. A roof may last 25 years, but rarely longer, without trouble. Zinc soakers also corrode and crack at the fold. Stress may aid this particular corrosion.  
  Aluminium is a comparative newcomer to the field of roof coverings. It does appear to last rather better in roofs than in some other uses. All aluminium is alloyed, and the compositions vary. In general it can be said that aluminium very rapidly forms an extremely thin layer of its oxide, alumina Al² O³, over its surface. This covering is only partially protective against three corrosive agents – acid and alkali, which both dissolve aluminium and salt, which causes pitting corrosion of some aluminium alloys and is destructive to aluminium parts of vehicles or bicycles, as it is picked up from the road in winter de-icing.  
  The run-off from metals like copper or bronze exposed to weathering for a length of time is the familiar green patina. When it stays in-situ on a roof we are not concerned, but the run-off from a copper lighting conductor onto white masonry disfigures it. Even more distressing is the run-off from bronze statuary onto a limestone base, which quite spoils the otherwise aesthetic effect. It requires careful deign to avoid such marring.  
     

EFFECTS OF WETTING BUILDING STRUCTURES - FROST (top)  
     
  This is the worst agent of decay of bricks and it also attacks porous concrete, or cracked concrete or rendering. Water expands nearly 10% on freezing, and accommodation has to be found within a porous material for this expansion. It depends on the pore structure whether or not a material that is rigid like a brick will disrupt. There is no guaranteed laboratory test for a brick for frost resistance as yet. It is best to rely upon previous experience with a proposed type of brick. Engineering and some facing bricks are known to be good frost-resistant bricks as they stood the test of time. One facer is the well know London Yellow stock, which is sometimes used a second time in new work when recovered intact from demolition of old work.  
     

SULPHATE ATTACK (top)  
     
  This is a chemical attack upon one of the constituents of Portland cement, tricalcium aluminate. The three necessities for attack are:  
     
1
 Presence of a soluble sulphate, e.g., Na² SO , MgSO , CaSO.  
     
2
 Cement containing C³A (an abbreviation for tricalcium aluminate, a constituent of practically all varieties of Portland cement) – ordinary Portand cement contains about 12% of this, sulphate-resisting cement much less.  
     
3
 Water; present for a long time.  
     
  Note requirement (3). Attack occurs not only to matured concrete underground in wet soil, but also to mortar in well wetted brick walls (bricks often contain some of the sulphates mentioned). Rendering on walls is also attacked, particularly cracked rendering where driven rain can so easily enter, staying a long time before it dries out.  
     
  The substance formed is a white crystal, similar to a mineral call ettringite, calcium sulpho-aluminate, ScaO.A1². 3CaSO. 3IH² O or 32H² O. this can be seen to be a union of tricalcium silicate (3CaO.AI² O³), gypsum and rather a lot a water of crystallisation. Its formation is accompanied by expansion, and this disrupts concrete, lifts and spreads brickwork, disintegrates rendering, bends chimneys, etc. Note that water is the vital factor, often from rain or driving rain.  
     

EFFLORESCENCE (top)  
     
  To a chemist, this means the losing of water of crystallisation by a salt exposed to air so that it becomes powdery; to a builder it means merely the appearance of salts, usually white, on the surface of brickwork, etc., after a period of dry weather following a wet period. The salt brought to the surface in drying conditions is left there to crystallise. It usually consists of a sulphate or a mixture of sulphates, the most common being sodium sulphate, which tends to give a dense white appearance, or magnesium sulphate, which tends to show off longer crystals white fluffy appearance.  
Efflorescence is a disfigurement, but it is not usually harmful in itself. The salts, when inside the brickwork, may do no harm: they are the same salts that cause sulphate attack, and so may with a prolonged period in the wet state cause sulphate attack to the mortar. The salts do not usually attack bricks, but weak underburnt bricks are liable to attack.  
Salts form the worst disruptive effect upon limestones and sandstones. Washings from one stone should not tun onto another type below it or disruption is likely to take place. Stones, in other words, should not be mixed. Salts may get into stones from bricks backing or brick surrounds, and crystallise within the pores, gradually disrupting the stone.  
     

CONCLUSION (top)  
     
  It would be as well in conclusion to summarise or suggest precautions to prevent the effects of weather upon buildings.  
     

WIND (top)  
     
  Buildings, including free standing walls, parapets, roofs, chimneys, hoardings and the like, must be designed to stand up to the worst gales anticipated. Particularly vulnerable are light but large constructions, e.g., barns or other agricultural buildings and factories and large flat roofs of light construction. Adequate wind bracing must be used in large pitched roofs of light construction and end gable walls. Flat roofs or light construction must be tied down to the walls by straps or the like.  
  Shopping precincts, etc., should be designed so that no undue turbulence or wind-tunnel effect makes itself felt at ground level.  
     

RAIN & DRIVING RAIN (top)  
     
  Walls should be cavity walls in all areas subject to considerable driving rain. Greater precautions are needed in areas of severe exposure. Adequate eaves, gutters and downpipes are needed for these areas. Good detailing is required to deal with the run-offs and drip-offs generally. Chimneys require careful design to minimise the effect of severe rain.  
     

TEMPERATURE & MOISTURE EXPANSION (top)  
     
  Allowance should be made, according to the circumstances, for both kinds of expansion where long lengths are concerned. This may apply to long walls, especially freestanding ones, which should be allowed to move. It also applies to thermal expansion of metal and of PVC, and of course, to long lengths of concrete paving where both expansion and contraction joints are called for.  
     

 

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