Cement paste, acid attacks

On mixing the cement paste, the calcium aluminosilicate glass is attacked by hydrogen ions from the poly(alkenoic acid) and decomposes with liberation of metal ions (aluminium and calcium), fluoride (if present) and silicic acid (which later condenses to form a silica gel). 

The third form of attack is a secondary effect of the first two forms. The reaction of acids with the various calcium compounds present in the cement paste or aggregate leaves a residue of soluble salts, which accumulates on or just beneath the surface. 

A few studies have tried to quantify acid formation at the anode. Early work used relatively high current densities for CP [37]. Erom tests up to one year it was found by spraying colour-pH indicators that acidification occurred, but only over distances of about one miUimetre from the anode. The pH was reduced from 13 down to 8 after seven months at current densities of about 100 mA/m (anode surface) and down to 4 at densities considerably higher (up to 2 A/m anode surface). The amount of acid formed by the anodic process can be calculated by Faraday s law. The amount of acid observed in these experiments can be estimated. It appears that only about 10 % of the acid theoretically produced actually attacks the cement paste. This is because most of the hydroxyl ions produced at the cathode migrate to the anode and neutralise the acid formed. Later work studied acid production in CP installations in the field and long-term laboratory tests. Samples were studied of conductive coatings that had served as anode material in several CP systems in... 

The chemistry of pitting is quite complex and is explained in most chemistry text books. However, the principle is fairly simple, especially where chlorides are present. At some suitable site on the steel surface (often thought to be a void in the cement paste or a sulphide inclusion in the steel) the passive layer is more vulnerable to attack and an electrochemical potential difference exists that attracts chloride ions. Corrosion is initiated and acids are formed, hydrogen sulphide from the MnS inclusion and HCl from... 

Carbonation is the result of the interaction of carbon dioxide gas in the atmosphere with the alkaline hydroxides in the concrete. Like many other gases carbon dioxide dissolves in water to form an acid. Unlike most other acids the carbonic acid does not attack the cement paste, but just neutralizes the alkalies in the pore water, mainly forming calcium carbonate that lines the pores ... 

The action of acids on hardened cements pastes is similar to that of plain water, but is significantly more intensive. Acids yielding products of limited solubihty, such as H2SO4 or H3PO4, are less aggressive than those whose reaction products are easily soluble, such as HNO3 or HCl. In addition to the cement paste, carbonate aggregates, snch as limestone or dolomite, are also attacked by the acid. 

Acid attack from air or water may produce the conversion of calcium in Portland cement paste into soluble salts. As a result, the binding capacity of hardened mortars and concretes is reduced. 

Concrete can be attacked by chemical reactions that dissolve the components of the cement paste (usually reactions with acids, soft tap water or sea water) or by chemical reactions that form insoluble corrosion products with large volume (e.g. reactions with sulfates, magnesium ions). The dissolving potential of diluted acids depends on their concentration and on the type of acid strong acids such as sulfuric, nitric or hydrochloric acids react with the free lime in the cement paste and form soluble salts. Due... 

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