Alite reactivity

C. Nital is perhaps the most common etchant and stain for silicates and improves with age. Nital is 1.5 mL of nitric acid (HNO ) in 100 mL of ethyl, methyl, isopropyl, or amyl alcohol. The author routinely uses asolution of 1 mL of HNOj and 99 mL of anhydrous isopropyl alcohol. The solution quickly reacts in 6 to 10 seconds with alite and belite. At a 0.05% dilution the reaction time is 20 to 40 seconds. Ono (1995) relates alite reactivity to color produced with 0.2% nital. Depending on the relative reactivity of silicates, alite normally turns blue to green, belite is brown to blue—both silicates showing details of internal structure. Nital superimposed on a 20-second potassium hydroxide etch turns C3A light brown and colors the silicates. 

The writer has found that etching and exami nation with CDTA at successive 15 second inter vals reveals information about relative rates of alite reactivity, for example, when comparing clinkers from different production periods. The polished sections are bound together with a rubber band and etched simultaneously, or the clinkers can be en capsulated in a multichambered container. 

Minor or trace components derived from raw materials, fuel, refractories or other plant materials, or added deliberately, can affect the reactions of clinker formation, or the properties of the product, or both. Their effects can be beneficial or harmful. Beneficial effects include acceleration of the clinkering reactions or lowering of the temperature at which they occur, or increase in the reactivity of the product leading to more rapid strength development. Harmful effects include decrease in alite content, volatilization in the kiln with consequent formation of kiln rings or other deposits, decrease in the durability of concrete made with the cement, or the introduction of poisonous elements. Some elements have beneficial effects at low concentrations and harmful ones at higher concentrations. Bucchi (B33,B28) reviewed some of the effects on the manufacturing process. 

Locher (LSI) reviewed ways of decreasing the energy consumption in cement manufacture to values substantially below those characteristic of modern Portland cement production. They included the use of fluxes to lower the burning temperature, the production of cements containing more reactive forms of belite or based on belite together with highly reactive phases, replacement of alite by the chloride-containing phase, alinite, and the use of composite cements. 

Amino-5-hydroxy-3,6-bis((4-((2-(sulfooxy)ethyi)-sulfon-yl)phenyl)azo)-2,7-naphthalenedisulfonic acid tetra-sodium salt. 4-Amino-5-hydroxy-3,6-bis((4-((2-(sulfooxy)ethyl)-sulfon-yl)phenyl)azo)- C.l. 20505 C.l. Reactive Black 5 Cav-alite Black B Celmazol Black B DIamira Black B Dri-marene Black R/K 3B EINECS 241-164-5 Intraoron Black VS-B Levafix Black E-B Primazin Black BN Reactive Black 5 Remazol Black B Remazol Black GF Sumifix Black B. 

Portland cement clinker is composed of fora principal phases alite, which is close to tricalcium orthosilicate Ca3[Si04]0, belite close to dicalcium orthosilicate Ca2[SiOJ, tricalcium aluminate Ca3[Al20g] and ferrite C2(A, F). Chemical composition of these phases is complicated by solid solutions, which have the decisive influence on their reactivity with water. For the elements from which chrrker phases are composed isomorphism is typical and very developed phenomenon. The highest concentration of isomorphic elements is encountered in tricalcium aluminate (about 12-13%), and then in alumino-ferrites (about 10-11%), belite (about 6%), and the lowest in alite (about 4 %). 

Relatively large alite and belite crystals, narrow range of size variation, lower than average reactivity Burning with natural gas (Hawthorne, Richey, and Demoulian, 1981)... 

Efch reactivity variations (specify etch and length of time). Crystal morphology crystal surface characteristics (depth of alteration) internal structure tendency for coarse cannibalism (masses greater than 5 times the average alite crystal size) inclusions. Size and abundance variations within and between clinkers. 

Effects of common minor and trace elements derived from recycling waste materials in fuels and as raw materials for clinker production, as well as cement hydration, are summarized by Uchikawa and Hanehara (1997). Crystal size and optical property variations in clinker phases (alite, belite, aluminates, and ferrite), and their hydraulic reactivities, are shown to be related to concentrations of sulphm, magnesium, phosphorous, fluorine, chlorine, chromium, manganese, zinc, and many other elements. The cement industry is based in crystal chemistry. 

Owing to the lower reactivity of belite, the overall rate of hydration, and along with it the strength development up to about 90-180 days, is slowed down with increasing belite and decreasing alite contents in the cement (Bei and Ludwig, 1990). At the same time the final strength of a Portland cement with an elevated C2S content may exceed that of an ordinary Portland cement, because more C-S-H and less portlandite is formed in the hydration of dicalcium than of tricalcium silicate. 

Garbacik (1997) reported the possibihty of producing a clinker with an elevated belite content, which exhibits favorable strength properties, by burning at maximum temperatures as low as 1250-1300 °C a raw mix containing chalky and marly starting constituents and very fine ciystalhne reactive forms of silicates. The alite and belite phases formed imder these conditions exhibit very small dimensions of their crystals, and are exceptionally reactive. 

The clinker produced itsually has a low alkali content, which is due to the more intensive alkali volatihzation caused by the higher burning temperature employed. It also may contain increased amoimts of free lime, owing to the relatively poor bumability of the raw meal. The reactivities of the alite and behte phases may be relatively low, owing to the reduced amoimts of foreign ions incorporated into their crystalline lattices. 

Another dopant that may be considered for increasing the reactivity of belite cements is S04 ions (Gies and Knofel, 1987 Stark et al, 1987). Under these conditions the final clinker contains the rather than the a -CjS phase, even if high cooling rates have been employed however, the SO -doped form of yff-dicalcium silicate is much more reactive than its SO -free counterpart. The doped C S typically contains about 3% each of SO3 and AI2O3 in its crystalline lattice. If an SO3-doped belitic clinker that also contains some alite is to be produced, the amoimt of SO3 in the raw meal must not exceed... 

The average phase composition of Portland cement (ASTM Type I, CSA 10) is -50% alite, C3S -25% belite, P-C2S -12% tricalcium aluminate, C3A -8% ferrite, C4AF > and -3.5% gypsum (Mindness, 1983). The structural order and crystallinity of the cement phases-and hence their hydraulic reactivities-are heavily dependent on the rate of cooling of the clinker ... 

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