Is it possible to mix a high-alumina cement with an ordinary portland cement as an accelerator for rapid setting?
Addition of high-alumina cement to portland cement can accelerate setting time to a degree dependent on the proportions of the two cements. One manufacturer of high-alumina cement states that flash set will occur if the quantity of his firm's cement is from 38 to 85 percent of the mixture. Outside these limits considerable variations in setting time occur according to which cement predominates. Use of a high-alumina cement as an accelerator is not generally recommended because of the uncertainty of the reaction. Use of a commercial accelerator from a reliable manufacturer is preferred. Should such a product not be available and the need for rapid setting and high early strength be imperative (to stop water seepage promptly, for example) then a cement mixture can be used. It must be remembered that the cements must be very thoroughly mixed in powder form and that a trial mix should be made to determine their precise performance in the concrete.