Portland cement deterioration

Sulfur concretes are used in many specialty areas where Portland cement concretes are not completely satisfactory. Because SC can be formulated to resist deterioration and failure from mineral acid and salt solutions, it is used for construction of tanks, electrolytic cells, thickeners, industrial flooring, pipe, and others. In addition, SC is under investigation for many other prospective uses (58,59) (see CEMENT). 

With portland cement concretes, deterioration takes the form of horizontal cracks, pop-outs, D-cracking, spalling and scaling. Salts used as deicing agents compound the problem. Early theories attributed the mechanism of failure to the 9 per cent volume increase when water converts to ice. "Critical saturation" -moisture filling more than 91 per cent of the voids was considered important. 

Unlike portland cement, sulfur can be stored in the open for indefinite periods without deterioration. 

Alkaline and Neutbal Solutions. The instability of sulfur-infiltrated concrete in sulfatic soils has been long known (16), but the reason for deterioration is still poorly understood. Degradation of untreated Portland cement concrete in such environments often appears related to... 

Figure 8. (left) Deteriorating portland cement concrete footings beneath evaporators in a sodium sulfate plant in Saskatchewan, (right) Fractured surface of sulfur-infiltrated concrete specimen placed in same area shows no deterioration after 9 months exposure. 

The inherent instability of sulfur-infiltrated concrete in aqueous media illustrated in this study may be the most important factor in utilization, because it will affect long-term durability of the concrete in many natural settings. The Ca(OH)2 produced by the hydration of portland cement is a principal reactant in the leaching process, and while it remains sulfur could be extracted, leaving the matrix vulnerable to other destructive processes. The removal rate of sulfur will vary greatly, depending mostly upon the pH of the immersion medium thus, the concrete deteriorates in alkaline sulfatic soils but is relatively stable in the corrosive neutral sulfatic solutions from the sodium sulfate plant. 

A typical thermal expansion curve for PC is shown in Fig. 9. The thermal expansion is important when PC is used in conjunction with other materials such as steel or portland cement concrete since the coefficient of thermal expansions of PC is at least twice as high as those corresponding to steel or cement concrete. Hence, changes in temperature in the composite structure will create shear stresses at the interface between the two materials that may eventually cause deterioration in the structure. 

The adhesion of asphalt to the mineral aggregate is a fundamental property of road asphalt. Once the adhesion deteriorates, the surface becomes unstable and unusable. There is a test method (ASTM D-1191) designed for use on crack and joint sealers that is used primarily to determine whether a jointing material possesses an arbitrary amount of bonding strength at low temperatures where portland cement concrete is being used. 

One very important niche application for calcium aluminate (cements) is as refractory castables. Key to the success of calcium aluminates in this application are their refractory properties that contrast with those of Portland cements. Although Portland cement maintains good strength when heated, reactive components (CaO) are liberated and can absorb moisture from the atmosphere when cooled, causing expansion and deterioration of, for example, kiln linings. CACs are not much susceptible and can be used to form monolithic castables and refractory cements [28, 29],... 

In reality the mechanism of concrete deterioration as a consequence of acid corrosion, if it is sulphuric acid, is the same as in case of sulphate corrosion. For this reason in both environments the matrix based on Portland cement with reduced Cj A content is more resistant. This example shows the imperfection of kind of corrosion classification, presented above. 

S. Brown deserves special recognition for his observational skills and interpretive acumen. Brown worked for Lone Star Research Laboratory in Hudson, New York, in the 1930s and in 1940 joined the research staff at the Portland Cement Association, where he spent approximately 25 years in cement and concrete investigations. Most of his scientific efforts were dedicated to the microscopical interpretation of clinker burning, cement hydration, and concrete deterioration. An unpublished report (Brown, 1936) contains the following interesting observations ... 

Reduced concrete deterioration due to alkali-silica reaction in mixes in which Portland cement has been partially replaced by fly ash has been widely reported (Hobbs, 1986, 1989 Meland, 1986 Shayan et ai, 1996). Fly ash seems to act mainly as an alkali diluter, lowering the amount of available alkalis in the system. The capability to reduce the alkali-aggregate expansion may vary in different ashes, and depends on their own alkali content and fineness. 

Corrosion is the destructive attack of a metal by chemical or electrochemical reaction with its environment. Deterioration by physical causes is not called corrosion, but is described as erosion, galling, or wear. In some instances, chemical attack accompanies physical deterioration, as described by the following terms corrosion-erosion, corrosive wear, or fretting corrosion. Nonmetals are not included in this definition of corrosion. Plastics may swell or crack, wood may split or decay, granite may erode, and Portland cement may leach away, but the term corrosion, in this book, is restricted to chemical attack of metals. 

The heightened compounds of hydrated silica (up to 0.3 g/dm ) and aluminum (up to 0.038 g/dm ) are often recognized in specimens of deteriorated reinforced concrete due to leaching of portland cement matrix. In many cases, it was detected a significant concentrations of sulfate (up to 34.5 g/dm ) that may be caused by deterioration and leaching of hydrated calcium sulfoaluminate of lining backfill. 

Patching and Repair Very high bond strength of PPCC makes it a suitable material for patching and repair of portland cement concrete. The deteriorated or unsound concrete must be removed properly before PPCC application. 

Sulfates will also cause portland cement to deteriorate. In addition to being able to produce sulfuric acid, which is highly corrosive to portland cement, sulfates are also reactive with some additives used in the formulations. Refer to Table 17.2. 

As discussed in Chapter 17 under portland cement coating, when water is added to portland cement a chemical reaction takes place during the hardening. This reaction produces calcium hydroxide and tricalcium silicate hydrate. The alkalinity of concrete is provided by the presence of calcium oxide from the cement. Consequently, concrete attack can be due to chemicals that react with the Portland cement binder and form conditions that physically deteriorate the material. Any material that will cause the calcium oxide or hydroxide to be removed, lowering the pH of the cement mix, will cause instability and solution of the cement hydrates. 

Fibres may also prevent the occurrence of large cracks, thus preventing percolation of water and contaminants into ceramic materials such as Portland cement mortars and concretes. So, corrosion of steel reinforcement or potential deterioration of concrete may be reduced with the addition of a variety of fibres to the Portland cement matrices. In addition, other enhanced... 

Mortar Deterioration. Mortar may decay from the formation of calcium sulfoaluminate (which causes expansion and loss of mortar strength) and by the attack of pollutants in the atmosphere. Portland cement contains tricalcium aluminate, which reacts with sulfates in solution to form calcium sulfoaluminate. Exhaust gases from automobiles contain sulfur dioxide, sulfur trioxide, and nitrous oxides. These oxides react with moisture in the atmosphere to form sulfurous acid, sulfuric acid, and nitric acid, which are the attacking agents. As attack continues over the years, the mortar joints may crack, the surface of the joint may spall off, and the mortar may become softer and more crumbly. 

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