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For lightweight chemical foaming mixtures, it is a technical challenge to control the stability while reducing its density. In this study, cenospheres (CP, a lightweight component) was used to replace part of fly ash (FA) to prepare ultra-lightweight foamed geopolymer concrete (UFGC), and the relationship between the fresh properties of the paste and the stability of UFGC mixture were discussed. Results show that incorporating CP to the slag-FA system can effectively reduce the reaction rate and heat release of the geopolymer paste, thereby effectively slowing down the decomposition rate of H 2 O 2. Different from prefabricated foams, the initial size of the foams decomposed by H 2 O 2 is also affected by the fresh properties of the paste. Meanwhile, the float of bubble and the drainage of the paste are controlled by the yield stress of the paste in the stable stage. As the CP substitution ratio increases, the initial yield stress of the paste decreases, which is beneficial to generate smaller initial bubbles. However, for bubbles of the same size, the low yield stress of the paste is detrimental to its long-term stability. Finally, the UFGC mixture with 50% CP has better stability and the best hardened pore structure. In addition, the calculated critical yield stress that prevents the foamed mixture from collapsing is compared with the initial yield stress of the geopolymer paste. [ABSTRACT FROM AUTHOR]