If aluminum pipe is so bad for pumping because it generates so much gas in the concrete, and if aluminum powder is purposely used to produce gas in some grout, why is it permissible to use aluminum forms? Don't they cause trouble by making gas bubbles on the surface?
Apparently they don't cause trouble. Often the surface of the aluminum form has been made relatively inert by being anodized by exposure to the atmosphere. But even if it does react with the lime and alkalies in concrete to produce hydrogen it becomes so quickly covered with other reaction products that gas generation is quickly choked off. In a pipeline, on the other hand, these reaction products are quickly abraded away by the moving concrete, so fresh metal surface is continually being exposed and gas continues to be generated. The aluminum powder in grouts has such a high surface area in relation to its volume that there is great opportunity to react and produce a considerable volume of gas.