If you want to investigate the effect of cement-tailings ratio on the strength of CPB, the water content (by weight) should be kept the same with all the specimens.

 Thanks, Dr. Jiang. Looking forward to communicate with you in person.

Mineralogy of the used tailings is important to be considered when investigating the CPB's strength development. However, based on your question, if I realized correctly, you said when you reduced the amount of cement per constant amount of tailings, you got higher UCS value. Is that correct? 

Dear Babak, thanks! what you realized is correct, but I reduced the cement and water as the same. The CPB is only made of cement, water and super-fine unclassified tailings without other additives.

For example, there are two types of backfills in the test. Type A: cement 0.150kg, tailings 0.600kg, and water 0.403kg. Type B: cement 0.100kg, tailings 0.600kg, and water 0.300kg.

After curing 7 days, I found the UCS of B is higher than A.

I do not know whether it is normal or not, and I also have not found any papers talking about this question.

Kind regards.

The water to cement ratio in CPB type A is less than that of CPB type B. In my opinion, repeat your test if you got the same result we need to control the particle size distribution and mineralogy of the tailings you used. 

Dear Babak, thanks for your kind suggestions. I will repeat the test and verify it again. Do you think the present result is normal?

 Since the strength development originated (mainly) from binder (e.g., Portland cement), it is supposed that any increase in the amount of binder (if the other variables are constant) boosts the strength (e.g., UCS). Use the same source of tailings, even if they are from the same mine, check XRD and particle size again to be confident of similarity. 

Dear Babak, thanks! your advice is really significant. I will repeat the test as suggested by you. Kind regards.