DEF is one of the most common deterioration phenomena of concreate. DEF occurs by the combination of three conditions as follows: a high temperature at early age, a sufficient moisture supply, and a large amount of sulfate in concrete. This phenomenon causes cracking and reducing the strength of the hardened cement mortars. The most dangerous point of DEF expansion is that the amount of expansion is very large. Comparing the amount of expansion with that of ASR, which has a similar appearance of deterioration, the expansion rate of ASR is 0.1%, on the other hand, that of DEF is more than 1%. In this way, DEF expansion is extremely dangerous. However, the reason why DEF expansion occurs is not explained. In previous studies, replacement of Ground Granulated Blast Furnace Slag with Ordinary Portland Cement between 0 ~ 70% was found to suppress DEF expansion when more GGBS was replaced. The relationship between the rate of expansion and the amount of ettringite formation was confirmed. The mortar with GGBS showed less AFt formation than before expansion. It is considered that DEF expansion occurs when AFt disappears once and is produced again. Therefore, we considered that the expansion would not occur if there was a space, where AFt could be reproduced. In this study, mortars with different amount of space were prepared by AE agent. It was confirmed that a larger amount of space would suppress DEF expansion. The porosity of the mortar was measured to confirm whether the total porosity was affected by the amount of space. The expansion rate of the mortar with different amount of space was suppressed when the amount of space was increased, and the expansion rate of the mortar with high GGBS replacement rate was suppressed.