2015 Volume 64 Issue 6 Pages 471-478
The relationship between strength development and microstructure was investigated using mortars in which various forms of calcium silicate hydrates were generated under long term autoclave curing. For the purpose of identifying the most stable form of calcium silicate hydrate for the highest strength, the mortar specimens were prepared under different conditions with the silica fume additive rate, autoclave curing time and temperature varied. All specimens cured for 10 to 100 hours exhibited the reduction of compressive strength and formation of a large amount of C-S-H. Those cured over 100 hours exhibited decreased compressive strength and crystallized calcium silicate hydrates. The results showed that the dominant factor for the strength development was the formation of C-S-H which was an amorphous calcium silicate hydrate filling the voids, and that coarsely crystallized tobermorite and tobermorite gel which were formed during the process of C-S-H generation had adverse effect on the strength development. However, no adverse effect was observed when the crystalline tobermorite and tobermorite gel were generated in a small and appropriate amount. The highest strength was achieved in the cases of a mixed presence of a large amount of C-S-H with a certain amount of crystalline tobermorite and tobermorite gel formed during the process. It was also found that the tobermorite was formed under long term autoclave curing even with the addition of silica fume. This suggests that the tobermorite may be formed even when an amorphous silica source is used.
