Alkali reactivity

Other parameters that are indirectly related to the composition of edible oils include iodine value and saponification value. The iodine value is a simple chemical constant for a fat or oil. It measures unsaturated or the average number of double bonds in fats and oils. Iodine value is defined as the number of grams of iodine that could be added to 100 g of oil, which is measured with the AOCS Method cd 1-25 (22). Meanwhile, saponification value is a measure of the alkali-reactive groups in fats and oils and is defined as the mg of KOH needed to saponify 1 g of oil. Shorter chain fatty acids give higher saponification values than do longer chain fatty acids. 

The first account of the preparation of an arsonium ylide (1) (see Section I) involved what is known as the salt method , namely the reaction of a halogeno compound with triphenylarsine to form an arsonium salt, which is then treated with a suitable base to provide the ylide (see Scheme 1). Stable ylides are frequently made and isolated by using aqueous alkali. Reactive ylides need anhydrous conditions and the use of a suitable strong base, and are used in situ. Thus triphenylarsonium methylide (25) has been prepared in solution " " it was isolated by using as base sodium amide in tetrahydrofuran under an atmosphere of nitrogenTrimethylarsonium methylide (26) has been made indirectly, by desilylation of the trimethylsilyl methylide (27) with trimethylsilanoP. Ylide... 

The durability of concrete depends on both the long-term stability of the aggregate and on the quality of the cement paste system. The main aspects of aggregate durability are soundness, alkali reactivity and frost susceptibility. 

Alkali reactivity results from sodium and potassium hydroxides present in cement reacting with silica, silicates, or carbonates in aggregates. Under certain conditions, the reaction can result in expansion of the aggregate and cracking of the concrete. This can lead to mis-alignment of components within structures and corrosion of reinforcement. 

Limestones generally do not contain sufficient reactive silica or silicates to cause expansion, and damaging alkali-carbonate reaction has rarely been reported. The reactions involving carbonate rocks can be either expansive or non-expansive and are more likely to occur when the limestone contains appreciable quantities of dolomite and clay minerals [8.1]. ASTM C586 [8.8] gives a test method for determining the potential alkali reactivity of carbonate rock aggregates. 

ASTM C586-92 Test method for potential alkali-reactivity of carbonate rocks for concrete aggregates (rock cylinder method) , 1992. 

Three conditions are essential to ASR development alkalis, reactive silica and sufficient moisture. If one of these conditions is not present, the reaction will not occur. Therefore, it is very important to evaluate the reactivity of these rocks before using them as aggregates, by taking preventive measures such as the use of puzzolanic materials. 

ASTM C227. 2010. Standard Test Method for Potential Alkali Reactivity of Cement-Aggregate Combinations. (Mortar-Bar Method). Annual Book of ASTM Standards (04.02) 6 pp. 

Marfil, S. Maiza, R Bengochea, A. Sota, J. Batic, O. 1998. Relationships between SiOj, AljOj, FejOj, CaO, KjO and expansion in the determination of the alkali reactivity of basaltic rocks. Cement and Concrete Research. Ed. Elsevier. USA 28 (2) 189-196. 

Shayan, A. Quick, G. 1988. An alkali— reactive basalt from Queensland, Australia. International Journal of Cement Composites and lightweight Concrete 10(4) 209-214. 

In order to determine the potential alkali reactivity of aggregates to undergo ASR, mortar bar tests as per ASTM-C-1260 were carried out and their results indicated that their expansions were well below 0.10% thereby indicating their innocuous behaviour. 

ASTM International. C1260-05a. Standard test method for potential alkali reactivity of aggregates (mortar-bar method). ASTM International, West Conshohocken (PA), 2005. 

ASTM C289-07 Standard Test Method for Potential Alkali Reactivity of Aggregates (Chemical Method). 

Acid value is defined as the number of mg of potassium hydroxide required to reneutralize the alkali-reactive groups (in this case epoxy groups) in 1 g of material under standard test conditions. Thus the conversion of epoxy to ester groups is reflected in a decrease in acid value. 

Saponification Value n A measure of the alkali reactive groups in oils and fatty acids which is expressed as the number of milligrams of potassium hydroxide that react with 1 g of sample. (See ASTM, www.astm.org) Also known as Koettstorfer Number. 

When the carbonated front reaches sufficient depth, the steel reinforcement corrodes due to loss of passivity. The process of re-alkalization is, therefore, employed to recreate the alkaline conditions on the reinforcement by attracting Ca " ions and Na+ ions (see below) which keeps it in a passive state. The vicinity of reinforcement is re-alkalized and a new passive layer is thus regenerated. The re-alkalization process is a consequence of the electrochemical chloride removal process. The anodes used are the same as the one used for chloride removal (titanium or steel). Sodium carbonate is used as an electrolyte. The function of Na2C03 is to introduce alkali metal ions from an external source, however it would not be necessary to use Na2C03 if alkali reactive a egates are present in the concrete in which case water is used as an electrolyte. The introduction of alkaline metal ions prevents the occurrence of re-carbonation. At the anode oxidation takes place according to... 

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