Hydration resistance

Another means of providing a hydration-resistant surfaee is its treatment with a hydration inhibitor [41]. Fig. 9 shows wedge tests results for a Forest Produet Laboratory (FPL) bond [43], an FPL bond pretreated with nitrilotrismethylenephos-phonie (NTMP) aeid [42,44,45], and a PAA bond. The monolayer eoverage of NTMP stabilizes the FPL surfaee against hydration and provides wedge test bond performanee similar to that of PAA-treated adherends. 

In addition to the four compounds discussed above, the final Portland cement may contain gypsum, alkali sulfates, magnesia, free lime and other components. These do not significantly affect the properties of the set cement, but they can influence rates of hydration, resistance to chemical attack and slurry properties. 

Hydration Resistance. Visual examination of a series of Inhibitor-treated FPL-prepared 7075-T6 A1 coupons exposed to high humidity conditions for specified time Intervals Indicated good short-term hydration resistance for several phosphonate and silane compounds (no visible discoloration) The most effective silane compound tested contained the mercapto (-SH) functional group. 

The hydration resistance of the organosilane compounds was reflected by the wedge test performances of our silane-... 

The ionic phosphonates like NTMP are effective hydration inhibitors because they can form an insoluble complex with the oxide surface. They are useful as epoxy adhesive couplers in cases where the adhesive and its curing cycle are compatible with the adsorbed phosphonate molecule. (14) Wedge test results indicate that in two epoxy-aluminum systems studied, certain organosilanes tend to both increase the epoxy-metal bond durability and maintain hydration resistance. The results of anodic polarization experiments further suggest that these silane films are effective against localized pitting. 

One very beneficial chemical pretreatment treatment for aluminum substrates is phosphoric acid anodization (PAA), which provides an oxide coating that is inherently hydration-resistant. Its stability is due to a layer of phosphate incorporated into the outer A1203 surface during anodization. 

S.L.C. da Silva, Improvement of the hydration resistance of magnesia and doloma using organosilicon compounds, Ph.D. Thesis, UMR, Ceramic Engineering Dept., 2000. 

Even after sintering, dolomite is susceptible to hydration and should be protected from undue exposure to atmospheric moisture. Hydration resistance is increased by impregnating the bricks with pitch, resin or tar. 1 to 3 % of zirconia is sometimes added to increase the resistance to thermal shock. 

This hydration resistance is amply demonstrated by the resistance of the H2P0 anodized surface to sealing after... 

Tempered Dolomite Block. A tarred dolomite block that has been preheated at about 400-500°C to eliminate volatile material from the tar this prevents the blocks from slumping while they are being heated up to the operating temperature of the furnace or steel converter in which they are being used. The process is also claimed to improve the hydration resistance. 

Based, in part, on the results of the previous section, we decided to investigate the use of several organic hydration inhibitors on PAA and Forest Products Laboratory (FPL)(92) sodium dichromate-sulfuric acid etched surfaces.(3.5,i3,9i,93) One of the inhibitors that is very successful is nitrilotris methylene phosphonic acid NTMP, N[CH2P(0)(OH)2]3. It provides hydration resistance for FPL surfaces in a manner similar to anodization in phosphoric acid, i.e., it forms a protective molecular layer that does not allow the aluminum oxide substrate to hydrate until the inhibitor complex dissolves. 

Bonds made with PAA adherends exhibit greater inherent durability than those made with FPL adherends(i7,44) due to both the better hydration resistance of PAA surfaces(22) and the evolved microroughness and subsequent interlocking that occurs between the oxide and the adhesive.(14) Typical crack propagation data for PAA adherends (with and without primers) are shown in Figure 11. Although slower than for FPL surfaces, hydration of PAA surfaces can... 

Tests for hydration resistance can be done under pressure or without pressure application. Under pressure, tests can be conducted in an autoclave or in a pressure cooker. A humidity chamber can be used for testing without pressure. 

The increased usage of lime as a flux in steel making led to the replacement of doloma with magnesia, because poorly calcined magnesia was both more refractory- and more hydration-resistant than the low-purity doloma. Additionally, the doloma was no longer needed as a lime source for steel production. At this time, doloma was mainly used as a low-purity repair material for magnesia hearths. 

Silica, iron oxide, and alumina are the most common impurities in high-purity doloma. Since the impurity levels are low and the melting points of the liquid phases are so high, the dolomite can be difficult to calcine. Temperatures in excess of 1850°C are usually required in order to achieve satisfactory density and hydration resistance. The microstructure of a typical high-purity doloma as shown in Fig. 1 consists of magnesia crystallites (3-5 microns in size) embedded in a matrix of lime. 

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