Cement alkalis, soluble

The replacement of Portland cement by fly ash class F (ASTM C 618) has been found to reduce the rate of slump loss in a prolonged mixed concrete, and the extent of the reduction is greater with increased cement replacement (Fig. 7.37). Fly ash also was found to be beneficial in reducing slump loss in concretes with conventional water-reducing and retarding admixtures [95], The effect of fly ash on reducing slump loss can be attributed to chemical and physical factors. It was found that the surface of fly ash particles may be partly covered with a vapor-deposited alkali sulfate that is readily soluble [103, 104], Thus the early hydration process of Portland cement is effected because sulfate ions have a retarding effect on the formation of the aluminates. Indeed, fly ash was found to be a more effective retarder than an... 

The uncontrolled variation in the gypsum-hemihydrate or soluble anhydrite ratio (G-H or A) that occurs during production, storage and transportation of cement can cause compatibility problems with certain superplasticizers, particularly a reduction in workability. The extent of the reduction on workability properties is dependent on the G-H ratio (ranging from 80 20% to 20 80%), level of C3A and alkalis present in the cement and the fineness of... 

Water-soluble alkalis in Portland cement or fly ash can have a harmful effect on the characteristics of the air-void stmcture (AVS), particularly the stability of the spacing factor. Because of the more restrictive environmental... 

Although previous investigations [132, 134, 135] have shown that increased soluble alkalis could have an important influence on the AVS, more recent work [136, 137], however, indicates that soluble alkalis per se do not influence the production of the AVS. It is only when the sulfate level (inherent or by extraneous addition) is high that the AVS production is difficult. An increase in the soluble-alkali content of cement has been found to improve significantly the stability of AVS, particularly in concrete mixtures to which a superplasticizer (SP) is added [138]. 

An interesting study has been conducted in this connection about the reduction of soluble components in concrete standing in water, providing support to the statements made here not even the concentration of alkali ions, which are the most soluble components of concrete, was massively reduced H.A. El-Sayed, Cement and Concrete Research, 11 (1981), pp. 351-362. 

The uncontrolled variation in the gypsum-hemihydrate or soluble anhydrite ratio (G-H or A) that occurs during production, storage and transportation of cement can cause compatibility problems with certain superplasticizers, particularly a reduction in workability. The extent of the reduction on workability properties is dependent on the G-H ratio (ranging from 80 20% to 20 80%), level of C A and alkalis present in the cement and the fineness of the cement. For the most reactive type of cement with a high content of both C A and alkalis, a reduced G-H or A ratio affects the yield stress, while piastre viscosity is not much affected. These effects are much less pronounced for a less reactive cement, but with the lowest G-H or A ratio, false set can occur [130-132], An increased fineness of cement increases the effect of the G-H or A ratio. The effect has been found to be most pronounced with melamine-based superplasticizers [130, 133],... 

Taylor (T37) described a method for predicting the concentrations al any desired age after I day from the w/c ratio and the contents of total Na,0. total K2O, water-soluble Na20 and water-soluble K2O in the cement. It was assumed that the amount of each alkali cation taken up by the products is proportional to its concentration in the solution and to the quantity of products (C-S-H and AFm phase) taking it up. This led to the equation... 

AIPO4 is isoelectronic with silica and, as such, readily forms glasses and Si02-like crystalline materials. As well, framework stmctmes similar to zeolites may be prepared by the use of amines as templates. Like zeolites, these are active in catalytic reactions such as methanol conversion to hydrocarbons (seeZeolites) As a ceramic material, AIPO4 is an infusible material that is insoluble in water but is soluble in alkali hydroxides. It is often used with calcium sulfate and sodium silicate for dental cements. AIPO4 is also used as a white pigment that also acts as a corrosion inhibitor. 

Earth materials with abundant soluble alkali components (such as cement or concrete) or acidic... 

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