SEM observations were used to explore the microstructure of the corresponding specimens. The denser internal
structures of the RWW specimens are formed as illustrated by SEM observations in Fig. 11. It may be due to pozzolanic
reaction between calcium hydroxide and reactive silica in RWW. The addition of RWW may cause smaller sizes of
capillary pores and RWW may decrease the pore interconnectivity because the fiber-shape RWW could bridge cracks
and arrest capillary pores. The SEM observations of 20 % RWW specimens at the age of 28 days and 91 days are
illustrated in Fig. 12. RWW in the composites helped to form a denser microstructure and cause improvement in
strength and permeability, which is consistent with the results of compressive strength and total charge-passed. In
addition, the inclusion of RWW in composites formed a denser microstructure when the PSAI value increases [13-14].
(From 82% at the age of 28 days to 103% at the age of 91 days) Thus, the addition of rock wool wastes leads to
reduction of permeable voids and improvement of the durability.