Special additive for water-permeable concrete and preparation method of special additive

The invention relates to a special additive for water-permeable concrete. The special additive is prepared by carrying out aqueous solution polymerization on three monomers of methyl-allyl polyoxyethylene ether, N,N-dimethylaminethyl acrylate and unsaturated carboxylic acid, a thickening agent and the like under the initiation of a redox system. A preparation method of the special additive comprises the following steps of: (1) adding the methyl-allyl polyoxyethylene ether and the N,N-dimethylaminethyl acrylate into a reactor, heating to 50 DEG C, stirring to enable the methyl-allyl polyoxyethylene ether and the N,N-dimethylaminethyl Acrylate to be dissolved in water, heating to 60 DEG C, and then adding an oxidant solution; (2) dissolving a reducing agent and the unsaturated carboxylic acid in the water to prepare a solution A, adding a chain transfer agent to water to prepare a solution B, and respectively and uniformly dropwise adding the solution A and the solution B to the reactor to ensure that the solution A is dropwise added for 3h and the solution B is dropwise added for 3.5h; (3) maintaining the temperature at 60 DEG C for 1h, adding the thickening agent, and continuously maintaining the temperature and curing for 1h; and (4) cooling to room temperature, adding water and caustic soda liquid, and adjusting the pH value to 5-6, wherein the solid content is 40%. The preparation process of the special additive is simple to operate, short in production period, easy to control on a reaction process and environment-friendly.