| Summary: | The suitability of applying shrinkage reducing additives in alkali activated coal gangue-slag composites is discussed in this study. The effect of sulphoaluminate cement (SAC), high performance concrete expansion agent (HCSA) and U-type expansion agent (UEA) on the reaction process, shrinkage behavior, phase composition, microstructure and mechanical properties are evaluated. The results show that the addition of SAC slightly mitigates the early stage reaction process, while HCSA and UEA can either accelerate or inhibit the reaction depending on their dosage. The addition of SAC presents an ideal balance between drying shrinkage reduction and strength increment. As for HCSA and UEA, the shrinkage and mechanical properties are sensitive to their replacement level; excessive dosage would result in remarkable strength reduction and expansion. The specific surface area and average pore size of the hardened matrix are found to be closely related with shrinkage behavior. SAC addition introduces additional hydrotalcite phases within the reaction products, while HCSA and UEA mainly result in the formation of CaCO<sub>3</sub> and Ca(OH)<sub>2</sub>. It is concluded that applying expansive additives can be an effective approach in reducing the drying shrinkage of alkali activated coal gangue-slag mixtures, while their type and dosage must be carefully handled.
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