8. Incrustation and Sinter

This aqueduct bridge of Perge (Turkey) was completely 'overgrown' by calcareous deposits because of leaks, the proof that the water source was very calcareous. Downstream of the aqueduct bridge Pont du Gard (near Nimes, France) massive layers of sinter (middle and right) were formed in the aqueduct channel.
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1. Introduction

Incrustation in lead and ceramic pipes and aqueduct channels, which occurred at varying rates according to the hardness of the water, was a problem. Polishing the cement in the channel served to alleviate this problem somewhat, but deposits of calcium carbonate and lime carbonate (also known as sinter) could choke the channel by as much as 50%. Pipes were an even bigger problem, as a pipe is likely to be full any layer of deposit reduces the cross-section by the square of the reduced diameter. Thus sinter had to be removed more often from pipes than from channels.
If the pipe consited of lead, this was easy. According to Fahlbusch, lead pipes could be cut open, the sinter broken out, and the pipes soldered closed again (Hodge, 1991:8). Fahlbusch also speculates that boiling vinegar might have been used to remove sinter(Hodge, 1991:9).

2. Chemical proces

Ca2+ (aq) + 2(HCO-3) (aq) <-> CaCO3 (s) + CO2 (aq) + H2O (l) -> CaCO3 (s) + CO2 (g) + H2O (l)

where:
aq = dissolved in water; g = gas; l = liquid; s = solid

In words:
Hard water means that it contains much lime in the form of Calciumbicarbonate Ca(HCO3)2; that is why there are much Calcium- (Ca2+) and Hydrocarbonate- (HCO3) ions present.
Under special circumstances the equilibrium turns more to the right, Calciumcarbonate (CaCO3) arises in solid state, plus water H2O and dissolved Carbondioxide CO2; the latter evaporates.

The solid Calciumcarbonate (CaCO3) is usually called boiler scale or after the German term in aqueduct studies, sinter. The whole proces is called inscrustation.


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