Soluble salts, concrete

A WBL can also be formed within the silicone phase but near the surface and caused by insufficiently crosslinked adhesive. This may result from an interference of the cure chemistry by species on the surface of substrate. An example where incompatibility between the substrate and the cure system can exist is the moisture cure condensation system. Acetic acid is released during the cure, and for substrates like concrete, the acid may form water-soluble salts at the interface. These salts create a weak boundary layer that will induce failure on exposure to rain. The CDT of polyolefins illustrates the direct effect of surface pretreatment and subsequent formation of a WBL by degradation of the polymer surface [72,73]. 

Common rock-forming mineral, although most Al-bearing silicates quite insoluble. Potentially soluble Al-hydroxides, hydroxysulfates form in lateric ore deposits, tropical soils, and precipitate in streams affected by acid-rock drainage. Al-rich soluble salts can occur in evaporative lake sediments, and in mine wastes. Potentially reactive forms in cement, concrete, smelter emissions, coal fly ash. 

Direct exposure to moisture may also have adverse effects on the performance of some sealants, either by its effects on the sealant or on the joint substrates." Absorption of water may cause softening of the sealant material, and some acrylic-based sealants are particularly affected in this way. If absorption is followed by prolonged near-freezing temperatures, the sealant may become stiffer, leading to an increase in internal stresses as the joint opens. These stresses tend to promote failures of adhesion at the interface of the sealant and the joint surfaces. The repeated absorption of moisture by components of porous materials, such as concrete, may cause a gradual migration of soluble salts towards the sealant interface, which could lead to adhesion failure. This effect can be reduced by the application of appropriate primers to the joint faces before application of the sealant. It is essential to follow the advice of the sealant manufacturer in this respect. 

Efflorescence - re-s n(t)s (1626) n. An encrustation of soluble salts, cormnonly white, deposited on the surfece of coatings, stone, brick, plaster, or mortar usually caused by salts or free alkalies leached from mortar or adjacent concrete as moisture moves through it, also known as laitance. G Ausbliihen n, F efflorescence f, S eflor-escencia f, I efflorescenza f. Weismantal GF (1981) Paint handbook. McGraw-Hill Corporation Inc., New York. 

The most common cause for the deterioration of concrete results from contact with inorganic and organic acids. Those that form soluble salts with calcium oxide or calcium hydroxide are the most aggressive. Typical compounds that can cause problems include sour milk, industrial wastes, fmit juices, some ultrapure waters, and organic materials that ferment and produce organic acids. Typical chemical families found in various types of chemical processing industry plants and their effect on concrete are shown in Table 18.1. 

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... 

Essentially, concrete is a hard sponge with a network of small conduits or capillaries allowing directional passage of water firom interior to exterior regions to cool hot contact surfaces. However, as moisture moves within the concrete, small amounts of soluble salts are in solution which deposit within the pores of the concrete as the water quickly evaporates during the fire event. As the salt deposit fills-in the voided areas, the concrete pores become blocked and do not allow further transfer of water to cool high-temperature contact surfaces. Therefore, the water accumulates behind this barrier and is phase changed from a liquid into a gas. 

Efflorescence refers to the deposit of water-soluble salts on the outside of painted masonry surfaces, such as stucco, concrete, volcanic-ash cinder block, and mortar between blocks. Water-soluble salts are leached out of the masonry when water penetrates the masomy from the inside. When water evaporates from the painted surface, it leaves a deposit of salts. Although the salts can be washed off with a stream of water, they will form again unless the source of water behind the paint is removed. Many times, the source is a leaking roof... 

Our efforts to concretely determine the relative stereochemistry of this dimer have been met by failure. We have made attempts to resolve several of the monomeric tetracyclic aminoaldehydes of type 100 by HPLC using chiral stationary phase, in order to know for sure the structure of the homodimer. The poor solubility of these compounds in typical HPLC solvents hampered these efforts to access enantiopure monomer. A few attempts at diastereomeric salt formation from compounds of type 101 using chiral carboxylic acids were also unsuccessful. Computational analysis corroborates the assumption that the homodimer should be formed preferentially. 

The range of anions that can occupy the X positions is very wide (A11,D20) even anions that form calcium salts of very low solubility, such as ffuoride, can be introduced using special techniques. We shall consider only those most relevant to cement chemistry. Tables 6.2 and 6.3 include data. C3A CaCl2 l0H2O or [Ca2Al(0H)g]Cl-2H20 (Friedel s salt) can be formed in concrete exposed to chloride solutions. Two polytypes are known p is the... 

Concrete made with calcium aluminate cement at a properly low w/c ratio is highly resistant to sulphate solutions, sea water or dilute acid solutions with pH > 4, including natural waters in which CO2 is the only significant solute. Resistance may extend to pH 3 if the salt formed is of sufficiently low solubility. Midgley (M96) showed that, for fully converted material exposed to a sulphate ground water for 18 years, penetration with formation of a substituted ettringite was limited to a depth of 5 mm. These properties are consistent with Lea s (L6) view that the resistance is due to the formation of a protective coating of alumina gel, coupled with the absence of CH. No fundamental studies, e.g. on microstructural effects, appear to have been reported. 

For artefacts recovered from marine sites, they are often covered in concretions. These are hard layers of calcareous deposit derived from decaying shells of aquatic animals (e.g. barnacles, mussels, etc.) or hardness salts present in seawater. The latter is present as soluble bicarbonate ions (HC( )3 ) in sea or fresh water. At cathodic sites on the metal surface, there is a rise in the local pH due to the production of OH- during the reduction of dissolved oxygen gas (see Equation (10) in corrosion section). This results in the precipitation of solid calcium carbonate (CaC03) scale on the cathodic sites according to the following reaction ... 

Waters with a low content of salts. The most important process is leaching of soluble constituents of material. As far as concrete is concerned, particularly the release of Ca(OH)2 during the hydration of cement should be borne in mind. 

Acid waters. Waters of this group contain mineral or organic acids which dissolve calcium components of concrete, which are thus removed from it. Acids whose calcium salts are quite soluble (HCl, HNO3) corrode concrete more quickly than those whose calcium salts are insoluble. The pH of water should not decrease below 6.7. 

---