Calcium hydroxide reactivity additives

Suitable catalysts include the hydroxides of sodium (119), potassium (76,120), calcium (121—125), and barium (126—130). Many of these catalysts are susceptible to alkali dissolution by both acetone and DAA and yield a cmde product that contains acetone, DAA, and traces of catalyst. To stabilize DAA the solution is first neutralized with phosphoric acid (131) or dibasic acid (132). Recycled acetone can then be stripped overhead under vacuum conditions, and DAA further purified by vacuum topping and tailing. Commercial catalysts generally have a life of about one year and can be reactivated by washing with hot water and acetone (133). It is reported (134) that the addition of 0.2—2 wt % methanol, ethanol, or 2-propanol to a calcium hydroxide catalyst helps prevent catalyst aging. Research has reported the use of more mechanically stable anion-exchange resins as catalysts (135—137). The addition of trace methanol to the acetone feed is beneficial for the reaction over anion-exchange resins (138). 

Po22olans contain reactive siUca which reacts with cement and water by combining with the calcium hydroxide released by the hydration of the calcium siUcates to produce additional calcium sihcate hydrate. If sufficient siUca is added, about 30% of the weight of cement, the calcium hydroxide can... 

The amount of free calcium hydroxide in Portland cement-microsilica mixes increases initially, as its formation in the hydration of tricalcium silicate is faster than its consumption in the pozzolanic reaction with microsilica. Later on, however, the amount of free calcium hydroxide may start to decline, when the amount of it consumed in the pozzolanic reaction exceeds the rate by which it is formed in the hydration of tiicalcium and dicalcium silicate (Papadakis, 1999). This crossover point— that is, the time at which the rate of Ca(OH)2 consumption exceeds the rate of its formation— will depend on the amount and reactivity of the microsilica present, as well as on the reactivity of the clinker, and can occur after several hours or a few days of hydration, or not at all (especially at low microsilica additions). The amount of residual free calcium hydroxide in mature paste will generally decline with increasing amounts of nucrosilica in the original mix. It will also decline with decreasing watei/solid ratio, as under these eonditions the C/S ratio of the formed C-S-H phase tends to increase. 

A few additives have been tested in spray drying systems. Calcium chloride has proven effective in increasing the reactivity of limestone and Ca(0H)2 towards SO2 (10, 11). Adipic acid was also tested (12) with mixed results. Sodium sulfite, sodium hydroxide, Fe++ compounds, Ethylene-diamine-tetra-acetic-acetic acid and disodium salt (EDTA) have been used as additives during simultaneous SO2 and N0X removal (Niro Process) (13). The emphasis in the Niro process was to improve the removal of NOx. 

Sodium occurs widely as NaCl in seawater and as deposits of halite in dried up lakes etc. (2.6% of the lithosphere). The element is obtained commercially by electrolysis of NaCl melts in which the melting point is reduced by the addition of calcium chloride sodium is produced at the iron cathode (the Downs cell). The metal is extremely reactive. It reacts vigorously with the halogens, and also with water to give hydrogen and sodium hydroxide. The chemistry of sodium is very similar to that of the other members of group 1. 

Active metals are highly reactive metals. Oxides of active metals react with water to produce metal hydroxides. Calcium oxide, CaO, also known as lime or quicklime, is manufactured in large quantities. The addition of water to lime to produce Ca(OH)2, which is also known as slaked lime, is a crucial step in the setting of cement. 

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