I have made an modified concrete with sewage sludge powder(M35), the partial replacement was done with fine aggregates 5 and 10% only. Water requirements was more W/C (0.47) and superplasticizer (1.5%), to achieve the slump 100 mm. sample casted and cured for 28 days. After 28 days i tried to take compressive strength but, the sample has not at all set, it was broken easily. What may the problem?
The delay in setting time can be attributed to the action of sulphate. Thus, an alternative action to reverse this behavior is the use of admixtures that could work as set accelerators. Beware of the chloride-based admixtures if your concrete is designed to receive steel reinforcement. There are a number of these admixtures and should be to ascertain the best compatibility between admixture and cement. Another way to change the behavior is to use increased the temperature of the concrete´s curing . Thus, thermal cures can reverse this problem of delayed setting time.
Hello Ramachandran, may I know what kind of starting binder did you chose? If isn't an R cement I think that ,without accelerators, as suggested by Antonio, your binder isn't able to reach its heat of hydration correctly, so hardening doesn't occur and you obtain weak performance. If you couldn't change your binder I suggest to use some pozzolanic materials natural or artificial to improve early and final strength ,but is mandatory to use accelerators of setting and of hardening (organic or inorganic, alkaline or alkali free) that increase speed of pozzolanic effect and your concrete will improve mechanical properties .
Best regards
Alex Reggiani
Dear all,
I have used OPC as binder, Sulphonated naphthalene formaldehyde (SNF) as Superplasticizer. But i didn't added any accelerators. Please suggest me some accelerators suitable for this modified modified concrete.
Dear Ramachandran,
In Italy, I use liquid or powder accelerators, you could test aluminium sulphate hydrate but take care of your hands is alkali free, so acid , but works well and finally you will obtain very good strength development. On the net you will find technical sheet with dosages suggested (but depends on your binder the final amount).
If strength development is your goal you could use more efficient superplasticizer (w/c about or under 0,4) so acrylic one instead of NSF, you will reduce also VOC content at the same time and you should use microsilica to have pozzolanic effect and final strong matrix.
Other accelerators are sodium silicate (liquid better than powder) that is the best at early ageing but loses a little bit of performance after 28days (values however comparables with aluminium sulphate) , sodium carbonate (cheap powder), lithium carbonate (very expensive unfortunaly) or lithium sulphate (also expensive).
You have only to test and chose the best solution for your recipe.
Regards
Alex
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