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Nowadays, significant research effort is being dedicated to explore environmentally friendly block materials with high strength and high-temperature resistance. Ground-granulated blast-furnace slag (GGBFS) is produced during the iron smelting waste process and can be activated to form the environment friendly alkali-activated slag cementitious material (AASCM) when mixed with alkaline activator. The aggregate consists of particles of different sizes, which are crushed and are screened using the paste of AASCM. In this study, the compressive strengths of 234 block specimens during and after high-temperature treatment were investigated. The test results showed that the compressive strength of the blocks gradually decreased slowly when the temperature was lower than 600°C, and decreased rapidly when the temperature was above 600°C. The reduction coefficient of compressive strength of the blocks during and after 900°C exposure were 14.2% ~ 28.1% and 15.3% ~ 28.7% of the ambient temperature strength, respectively. The steel fibre reduced the compressive strength loss of the blocks during and after the high-temperature exposure. With the increase of temperature, the steel fibre lost its effect gradually. Moreover, the compressive strength of the block after the high-temperature treatment was higher than that during the treatment when the strength level and temperature remained constant, the ratio was between 0.99 and 1.14. The high-temperature strength loss of the alkali-activated slag crushed aggregate concrete block was lower than that of the alkali-activated slag ceramsite concrete block. The fitted equations for calculating the compressive strength during and after the high-temperature treatment provided a basis for evaluating the fire resistance of this new type of block. The microstructure and composition of the block were analysed through scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results provide theoretical basis and data support for the application of AASCM in masonry blocks in high-temperature environments. [ABSTRACT FROM AUTHOR]