Use of alternative ashes in concrete - Workability and hardening

The thesis is focused on the impact of phosphate on the early age compressive strength of cement mortar. Sewage sludge ash from Lynettefællesskabet I/S is used. The setting time decreased the more cement was substituted with ash. The reference mortar had the fastest setting time of 250 minutes, followed by the 5 % cement replacement mortar with 260 minutes and the 10 % sand replacement of 350 minutes. The ash contained 9 wt% phosphorous and that the compressive strength lowered concurrently with addition of sludge ash. Solutions with different concentrations of a readily soluble phosphate salt (ammonium dihydrogen phosphate NH4H2PO4) were mixed into the mortar substituting the pure water. After testing the compressive strength of the reference mortar and 5 % cement replacement mortar with concentrations of 0.10 M, 0.50 M and 1.00 M NH4H2PO4 and with DI water. There was a clear tendency for the 5-days compressive strength to decrease with increasing phosphate concentration. The setting of the 1.00 M NH4H2PO4 mortars for both the reference and 5 % cement replacement happened within the first 7 minutes. For comparison the 0.10 M mortar set after 400 minutes.

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