Preparation and microstructure of fly ash geopolymer paste backfill material

As a new paste backfill material, geopolymer cemented coal gangue-fly ash backfill is prepared from fly ash geopolymer and spontaneous combustion gangue and can be beneficial to the cleaner production of coal mines and coal-fired power plants. A new mixing technique, in which a small amount of fly ash was mixed with cement, slag and water and allowed to react for a period of time before an aggregate and residual fly ash were added, was proposed. The influences of the solid mass concentration, the fine gangue ratio and the fly ash content on the workability and strength of the paste backfill material were analyzed, and ratio optimization was carried out using the response surface method, multi-objective multi-verse optimization and the desirability function approach. The results show that the optimal mixture is obtained when the mass concentration is 79.65%, the fine gangue rate is 57.19%, and the fly ash content is 15.67% of the total mass. The new mixing technique makes the fly ash more active to stimulate the early formation of calcium silicate gel and calcium silicate hydrate gel that improve the aggregate formation strength. In the later stage, amorphous hydrated calcium aluminate gel-coated ettringite and calcium carbonate formed a flocculation structure between the fly ash particles that gradually formed a condensation-crystallization structure. The research results can make extensive use of fly ash and gangue to promote cleaner production. (C) 2019 Elsevier Ltd. All rights reserved.