Silica fume, 3.37

This densifies the material and probably accounts for the improved sulphate resistance. 

Latex can be used as a dispersant and a typical latex would be a styrene-butadiene copolymer e.g. (Dow 460NA) used at 20% w/w of cement. The latex must be used in conjunction with an anti-foam agent such as Dow Corning 2410. 

A number of workers have studied the improvements obtained by the addition of latex to mortars [11,16,17] and concrete [18,19]. 

Latex is expensive and an alternative dispersant is methyl cellulose (Dow Chemical Methocel A15-LV), used at 0.4% w/w of cement in conjunction with 0.13 vol % defoamer (Colloids 1010). The effect of methylcellulose admixture on the mechanical properties of cement has been determined [20]. 

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MPa) often involves the addition of ultrahne particles together with large proportions of organic admixtures [465], Silica fumes were found to be the most effective additive. 

Carbon black may serve as a low-cost additive for controlling the gas migration in cement slurries [303]. It is intended as a suitable substitute for polymer latex and silica fume and has been tested in field applications [304,1256]. The concentration of carbon black varies from 2 to 20 parts, based on the weight of the dry cement [1220]. The particle size varies from 10 to 200 nm. A surfactant is necessary for dispersion, for example, formaldehyde-condensed naphthalene sulfonate or sulfonated cumarone or indene resins. 

E. Grabowski and J. E. Gillott. Effect of replacement of silica flour with silica fume on engineering properties of the oilwell cements at normal and elevated temperatures and pressures. Cement Concrete Res, 19(3) 333-344, May 1989. 

D. T. Muller and R. L. Dillenbeck, III. The versatility of silica fume as an oilwell admixture. In Proceedings Volume, pages 529-536. SPE Prod Oper Symp (Oklahoma City, OK, 4/7-4Z9), 1991. 

J. W. Squyres and H. Lopez. Silica fume as a strength enhancer in low density slurries. In Proceedings Volume, pages 47-53. 37th Annu Southwestern Petrol Short Course Assoc et al Mtg (Lubbock, TX, 4/18 /19), 1990. 

C. Gervais, and S. K. Ouki, Performance study of cementitious systems containing zeolite and silica fume Effects of 4 metal nitrates on the setting time, strength and leaching characteristics. J. Hazard. Mater., 93,187-200, 2002. 

Toxieology. Amorphous silica fume exposure is associated with recurrent fever, similar to metal fume fever, and nonprogressive pulmonary changes. 

Adverse effects on the lungs of workers exposed to the fumes of ferrosilicon furnaces have been recognized since 1937. Subsequent clinical studies of workers exposed to amorphous silica fume in silicon and ferrosilicon plants reported pulmonary symptoms and X-ray findings difficult to differentiate from classic silicosis due to crystalline silica, especially because there is often concurrent exposure to quartz dust during furnace operations. 

The disease process in workers exposed to silica fume was originally described as silicosis or acute silicosis, but it is now recognized that the X-ray pattern and symptom complex are different from both, the severity of the symptoms is less, and there is apparently no progression. It has been postulated that heavy exposure to freshly formed silica fume causes an acute reaction similar to metal fume fever. Continued or repeated exposure causes the ferroalloy disease, which has been described. This is characterized by recurrent fever over a period of 3-12 weeks, with the appearance of X-ray markings similar to silicosis. The development of classic silicosis may be the result of long, continued exposure to amorphous silica fume, or possibly concurrent exposure to crystalline silica. 

More recently, the importance of silica fume particle size on toxicity has been noted. Specifically, particles of the ultrafine size range may be expected to have higher toxicity compared with particles of larger size. 

The 2003 ACGIH threshold limit valuetime-weighted average (TLV-TWA) for amorphous silica fume is 2 mg/m for the respirable fraction of dust. 

Cunningham EA, Todd JJ, Jablonski W Was there sufficient justification for the 10-fold increase in the TLV for silica fume A critical review. Am J Ind Med 33(3) 212-23, 1998... 

Commercially, only silica is used in oxide CMP slurries. The size of an abrasive particle is in the range of 500- 2000 A. There are two kinds of silica, fumed silica and colloidal silica. The fumed silica is formed by oxidizing... 

Although small amounts of silica fume are found to be effective at earlier periods [16, 17], they are somewhat ineffective in the long term [18, 19]. There... 

Table 6.3 Comparison of expansion of mortars containing silica fume and air-entraining agent (Ramachandran)...

Silica fume has been used as a cohesion inducing agent to improve adhesion and sag and washout resistance. Incorporation of silica fume in dry-mix... 

Durability most proprietary accelerating admixtures adversely affect the concrete s resistance to freezing and thawing [114, 118]. More recently, the widespread use of silica fume has enabled the use of considerably low dosages of accelerators and also contributed to the improvement of porosity values, thereby dramatically improving durability of such concretes. 

Permeability since the admixtures improve spraying conditions and minimize rebound, it is argued that permeability should be improved. Actual tests, however, show that the effect is marginal and the values obtained are similar to mixes with no admixture. Silica-fume-containing mixes, however, show improved permeability [118]. 

The addition of silica fume to dry-mix shotcrete in proportions of 10-15% by mass of Portland cement substantially improves the adhesive and cohesive properties of the freshly applied shotcrete. Silica fume creates a very dense and sticky mix with an almost complete lack of bleeding. These characteristics of the material are attributed to the extreme densification achieved by packing of the ultrafine silica fume particles between the cement particles in the plastic (fresh) shotcrete. Practical ramifications of these characteristics include the observations which follow [107, 117-119]. 

Adhesion Adhesion of the plastic dry-mix silica fume shotcrete, particularly in wet areas, is substantially improved. For example, adhesion in areas such as locks, dry docks, tunnels, and even leaking structures, can only be achieved through the use of high concentrations of shotcrete accelerators, to create a flash-setting condition, a process that is detrimental to the long-term durability of the hardened shotcrete. Silica fume has been found to promote excellent adhesion in such conditions with minimal or no accelerator addition. 

Rebound Because of marked improvements of adhesion and cohesion, the amount of rebound is significantly reduced. In controlled tests, rebound of a conventional dry-mix shotcrete, applied to a thickness of 50 mm (2 in) to the smooth overhead surface of concrete deck slabs, was measured at 40% by mass (the rebound was caught in tarpaulins and weighed). Rebound under the same circumstances was reduced to 25% by mass in dry-mix silica fume shotcrete. 

Compressive strength Tests conducted on cores extracted from test panels shot during routine quality control show that silica fume shotcrete produce consistently higher compressive strengths and lower permeability in the hardened shotcrete than shotcrete where accelerators were used. 

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