Shi, S; (2017) Multi-stage Microwave Curing for Manufacturing Alkali-activated Fly Ash. Doctoral thesis , UCL (University College London).

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Abstract

Fly ash activated by NaOH solution shows comparable performance to Portland cement (PC) system and demonstrates its potential as a promising alternative cementitious material with a lower carbon footprint. Conventional thermal curing, typically at 85oC, is commonly applied to initiate and then to accelerate the chemical reaction. Although thermal curing can facilitate the strength development of alkali-activated fly ash (AAFA) products, it may result in greater energy consumption, offsetting its environmental benefits. In this study, microwave curing was investigated as an alternative thermal curing method for manufacturing AAFA. A custom-made microwave oven integrated with a fibre Bragg grating (FBG) sensor was employed to cure AAFA. Microwave power can be regulated automatically based on real time temperature feedback from an FBG sensor embedded inside the sample. A multi-stage curing regime consisting of curing temperatures at 65oC, 85oC, 105oC and 125oC was developed through a Central Composite Design (CCD) method. Optimal multi-stage microwave curing conditions were then identified by statistical modelling and analysis. The reaction products and microstructure of AAFA obtained by multi-stage microwave curing and conventional thermal curing were characterised and compared to identify the advantages and disadvantages of microwave curing as compared to conventional thermal oven curing. To address the possible industrial concerns over the insufficient microwave penetration into AAFA products with large dimension, the effect of sample size on the properties of AAFA under multi-stage microwave curing was studied, so that the findings from the current study can be scaled up in the future for industrial applications. The optimal multi-stage microwave curing regime obtained from statistical study at 2.45 GHz was: Pmax (maximum output power of microwave oven) at 400W, microwave curing duration at 65oC for 35 minutes, at 85oC for 35 minutes, at 105oC for 35 minutes and at 125oC for 85 minutes. The results showed that with accurate temperature control inside the sample, the AAFA under microwave curing initially gained equivalent strength with a much shorter curing duration (6 hours) than that under conventional thermal curing (24 hours). In addition, based on same thermal history (multi-stage curing for 4.5 hours), microwave curing favoured the higher strength gain and denser structure of AAFA with an Al-rich N-A-S-H gel, compared to AAFA with a Si-rich N-A-S-H gel under conventional thermal curing. One of the main issues that needs to be addressed in future studies is the loss of compressive strength of multi-stage microwave cured AAFA with subsequent room-temperature curing over time. With the sample size increased, the N-A-S-H formed in the region with lower temperature was more Al-rich compared to that formed in the region with higher temperature regardless of curing methods used. In general, multi-stage microwave curing could be a potential alternative curing method of manufacturing low carbon AAFA products with low energy consumption in the future.

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