There is no such thing as 100% hydration. Optimally 90% - 95% hydration is all that is reached. This has been proven by curing and regrinding mortars and recurring again.
You may want to find a copy of this publication as a guide. American Concrete Institute. Controlled Low Strength Materials (CLSM). Report No. 229R-94, ACI Committee 229, Detroit, Michigan, July, 1994.
The ACI seminar I attended referred to flowable backfill as controlled low strength material.Typical CLSM mixture usually consists of 1.5% +/- of cement, 7-12% of flyash, 70-85% of sand and 7-17% of water, it has a flow spread of at least 8 in. You can type "controlled low strength material"...
Metakaolins are another pozzolan that can be used. Lime is the culprit as mkashi mentioned. Pozzolans react with the lime which takes it out of the equation. As a result of the pozzolan and lime react yielding a stronger mortar. Pozzolans are very fine particle sizes compared to cement and as a...