Effect of retarders on the properties of ultra-high strength alkali-activated concrete

Alkali-activated slag-fly ash-silica fume cement (AA-SFSC) is suitable for the production of clinkerless alkali-activated binder-based ultra-high strength concrete (AAB-UHSC). However, using a relatively low water-to-binder ratio and a high alkali dosage to prepare AAB-UHSC leads to rapid setting and a loss of flowability. Thus, the setting time of the AA-SFSC needs to be controlled. In our study, the effect of various retarders (single retarder borax and hybrid retarder a combination of zinc nitrate hexahydrate and sodium gluconate) on the AA-SFSC was investigated. The setting characteristics, reaction kinetics, mechanical performance, pore structure alteration, mineralogical and molecular transformations, and microstructure of the AA-SFSC were systematically examined. The results showed that both borax and a combination of zinc nitrate hexahydrate and sodium gluconate had a remarkable retarding effect on AA-SFSC with a low water-to-binder ratio and high alkali dosage. However, borax had a detrimental effect on the compressive strength, and the effect of adding zinc nitrate hexahydrate and sodium gluconate as the hybrid retarders on the compressive strength depended on the dosage; and a dosage of 1% zinc nitrate hexahydrate and 2% sodium gluconate was recommended for prolonging the setting time of AA-SFSC without compromising the mechanical performance. •The rapid setting problem of alkali-activated binder-based ultra-high strength concrete is successfully solved by adding retarders.•The addition of 1% zinc nitrate and 2% sodium gluconate dosage effectively prolongs setting time without compromising mechanical strength.•The retardation of adding borax or a combination of zinc nitrate and sodium gluconate is due to a blocking effect between the activator and precursors.