Abstract
Alkali-activated slag (AAS), synthesised through activating the ground granulated blast-furnace slag (GGBS) by the alkaline activator, has been recognised as a green binder possessing excellent mechanical properties and chemical resistance. Meanwhile, the AAS materials show higher autogenous shrinkage compared with ordinary Portland cement (OPC) materials, which increases the cracking tendency and limits their wide applications in the construction industry. It is well-known that the autogenous shrinkage of AAS materials can be effectively mitigated by internal curing using superabsorbent polymer (SAP). However, there are lack of studies focusing on the effects of SAP properties on the internal curing effectiveness of AAS materials and their mitigation mechanisms.This study first evaluates the effects of SAP dosage, particle size, addition method, and absorption status on the internal curing effectiveness and the microstructure evolution of AAS pastes. The SAP particles and additional activator are added into the AAS pastes to achieve an internal curing process. The macro-properties and microstructure of the mixtures are tested to analyse the effects of SAP on the AAS pastes. Incorporating SAP can significantly mitigate the autogenous shrinkage of AAS pastes, particularly for those with more and larger-size SAPs. The activator released from SAP promotes the geopolymerisation around SAP particles, improving the local micro-hardness and reducing the fraction of pores that affects the driving force of the autogenous shrinkage. The travel distance of the activator released from SAP is mainly affected by the SAP size. The large SAP particles achieve a farther travel distance, enhancing the internal curing effectiveness for AAS paste.
The pore structure modification of the AAS pastes with SAP also affects the internal curing effectiveness by altering the diffusion of SAP-desorbed activators. Therefore, this study then focuses on the modification of the pore structure of AAS pastes by adding limestone powder (LP). The incorporation of 5% to 15% LP in the AAS pastes with SAP increases the internal curing effectiveness and reduces the autogenous shrinkage. This is mainly caused by the reduced mesopore fraction due to the enhanced reaction and filler effect of LP, as well as the increased size of the minimum pores driving autogenous shrinkage due to the moisture supply from SAP.
The variation of the activator affects both the reaction and the diffusion of the activator in the paste, changing the internal curing effectiveness. Hence, this study also investigates the effects of alkali dosage (i.e., Na2O%) and silicate modulus (i.e., Ms) on the internal curing effectiveness of AAS pastes with SAP. The SAP-incorporated pastes prepared with 6% Na2O% and 1.0 Ms achieve the most effective internal curing process, as the large self-desiccation can be significantly mitigated by the activator released from SAP. However, the lower or excessive Na2O% or Ms reduces the internal curing effectiveness, as the low or high concentration of activator delays or accelerates the activator consumption at the early stage, respectively.
Overall, this study offers a guide on enhancing the internal curing effectiveness for AAS pastes with SAP. Specifically, this can be improved by optimising the properties of SAP, the pore structure, and the geopolymerisation degree of AAS pastes. The roles of the activator released from SAP on the internal curing effectiveness and microstructural evolution of the pastes are revealed. The study eventually contributes to reducing the autogenous shrinkage of AAS paste.
| Date of Award | Nov 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Bo Li (Supervisor), Yung-Tsang Chen (Supervisor) & Weizhuo Shi (Supervisor) |