Silica mortars

Silica Mortars Silica mortar is a strictly acid and heat resistant material, handling all acids except HF and acidic fluorides at pH 0-7 and thermally stable up to 2000°F. A relatively recent self-curing silica mortar contains only borosili-cate glass powder, silica sol and crushed silica with no metallic constituents. This avoids both the sulfation-hydration reaction of the sodium silicates and the alum formation problems of the potassium silicates. Like the silicate mortars, silica mortars also resist organic chemicals. 

The liquid component of a silica mortar, if frozen, can never be reclaimed and must be discarded. 

H.E.S. and HB Mortars differ from other siliceous, silicate, or silica mortars in resisting exposures between pH 0.0 and pH 8.0 with the exception of acid fluorides and HF acid. In addition, they can be used in glycol acetate, potassium sulfate or persulfate, sodium sulfide and trisodium phosphate, all of which will damage other siliceous mortars. 

The furnace and thermostatic mortar. For heating the tube packing, a small electric furnace N has been found to be more satisfactory than a row of gas burners. The type used consists of a silica tube (I s cm. in diameter and 25 cm. long) wound with nichrome wire and contained in an asbestos cylinder, the annular space being lagged the ends of the asbestos cylinder being closed by asbestos semi-circles built round the porcelain furnace tube. The furnace is controlled by a Simmerstat that has been calibrated at 680 against a bimetal pyrometer, and the furnace temperature is checked by this method from time to time. The furnace is equipped with a small steel bar attached to the asbestos and is thus mounted on an ordinary laboratory stand the Simmerstat may then be placed immediately underneath it on the baseplate of this stand, or alternatively the furnace may be built on to the top of the Simmerstat box. 

Hydraulic limes (84) may be used for mortar, stucco, or the scratch coat for plaster. They harden slowly under water, whereas high calcium limes, after slaking with water, harden in air to form the carbonate but not under water at ordinary temperatures. However, at elevated temperatures achieved with steam curing, lime—silica sand mixtures do react to produce durable products such as sand—lime bricks. 

Determination of Metal Precursor Mobilities During Pretreatment. Relative precursor mobilities were obtained by premixing the sllica-or alumina-supported metal catalysts with pure silica (Cab-O-Sll, grade M-5, Cabot Corp.) or pure alumina (Alon C, Cabot Corp.) In a 1 2 ratio prior to pretreatment. The catalyst and silica were ground together using a mortar and pestle for at least 0.5 hr. before they were placed in the Pyrex microreactor for pretreatment. 

Abdelrazig, Sharp El-Jazairi (1988, 1989) prepared a series of mortars based on a powder blend of MgO and ADP with a quartz sand filler. They were hydrated by mixing with water. A mortar I (MgO ADP silica water = 17T 12-9 70-0 12-5), with a water/solid ratio of 1 8, formed a workable paste which set in 7 minutes with evolution of ammonia. The main hydration product, struvite, was formed in appreciable amounts within 5 minutes and continued to increase. Schertelite also appeared, but only in minor amounts, within the first 5 minutes and persisted only during the first hour of the reaction. Dittmarite appeared in minor amounts after 15 minutes, and persisted. 

Potential volume change of cement—aggregate combinations Accelerated detection of potentially deleterious expansion of mortar bars due to alkali-silica reaction... 

Gels containing over thirty mols. of water to one of silica are relatively mobile, those containing twenty mols. are stiff, with less water the gel becomes harder and can finally, with five or six mols., be ground up in a mortar to a fine powder. 

The different types of admixtures, known to reduce alkali-aggregate reactions, can be divided into two groups those that are effective in reducing the expansion due to the alkali-silica reaction, and those that lower expansions resulting from the alkali-carbonate reaction. For the alkali-silica reaction, reductions in the expansion of mortar specimens have been obtained with soluble salts of lithium, barium and sodium, proteinaceous air-entraining agents, aluminum powder, CUSO4, sodium silicofluoride, alkyl alkoxy silane,... 

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

The mechanism of the inhibitive action of LiOH proposed by Stark et al. [7] is attributed to the formation of lithium silicate that dissolves at the surface of the aggregate without causing swelling [7], In the presence of KOH and NaOH the gel product incorporates Li ions and the amount of Li in this gel increases with its concentration. The threshold level of Na Li is 1 0.67 to 1 1 molar ratio at which expansion due to alkali-silica reaction is reduced to safe levels. Some workers [22] have found that when LiOH is added to mortar much more lithium is taken up by the cement hydration products than Na or K. This would indicate that small amounts of lithium are not very effective. It can therefore be concluded that a critical amount of lithium is needed to overcome the combined concentrations of KOH and NaOH to eliminate the expansive effect and that the product formed with Li is non-expansive. 

The basic mechanism of plasticizing effect on fresh cement mixes is explained by forming a temporarily stable double layer on cement particles. Since the formation of the double layer is also connected with the surface of particles, the increased demand for AEA, WRA and SP, in silica fume concrete and the decreased demand for AEA in the presence of a WRA or SP can be directly correlated to the specific surface increase of silica fume-cement blends and the dispersing action of WRAs and SPs in achieving a mortar consistency that enables the air-bubble-generating and stabilizing process [147, 149]. Concrete producers are now cognizant of the effect of these factors. Field silica fume concrete with a satisfactory, stable air-void system can therefore be produced consistently. 

Sharp silica sand is used as a filler in resinous cement mortars and to provide an abrasive surface in polymers. Reactive silica ash, produced by burning rice hulls, and a lamellar filler, novaculite, which is obtained from the novaculite uplift in Arkansas, are also used as silica fillers in polymers. 

A given volume of hydrofluoric acid, freed from silica, was divided into two equal portions, and one portion was exactly neutralized by purified potassium carbonate. The two portions were mixed, and thus converted into the acid fluoride. The salt itself was obtained by evaporating the soln. in a platinum basin, on a water-bath, at 100° and completely dried under reduced press, (say 20 mm.) over cone, sulphuric acid. Two or three sticks of caustic potash were exposed in the same evacuated vessel. The salt was powdered from time to time in an iron mortar so as to expose fresh surfaces. The sulphuric acid and potash were renewed every 24 hrs. for about 15 days. 

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