SITE INVESTIGATION OF CHANGES IN CONVEYED WATER QUALITY AFTER LINING WATER MAINS WITH BLAST FURNACE SLAG CEMENT MORTAR - KIRK MERRINGTON FIELD TRIAL
Report No DWI0284

Nov 1988

SUMMARY

i) OBJECT

To investigate the effect on water quality of lining a dead end main with blast furnace slag cement mortar (BFS) in a moderately low alkalinity water and to compare the findings with those from a parallel trial using ordinary Portland cement (OPC).

ii) REASON

Cement mortar is widely used to internally renovate corroded iron water mains throughout the UK. In some cases the pH levels of the water passing through newly lined mains can be elevated above the EC directive maximum advisable limits. To alleviate this, WRc has developed a modified mortar replacing 65% of the OPC with BFS. The new BFS mortar, during laboratory and field trials has been found to reduce the pH in the conveyed water by up to one pH unit in comparison to OPC. The trial in Kirk Merrington has been conducted as part of a series of field trials to investigate the effect on pH of using BFS mortar in waters of varying alkalinity. This site was chosen as the alkalinity of the water was approximately 22 mg/l as CaCO3, a level not previously examined.

iii) CONCLUSIONS

  1. The pH of the conveyed water in contact with the blast furnace slag mortar and the ordinary Portland cement mortar linings remained above the EC directive MAV of 9.5 for a period of 10 to 11 weeks after return to service.

  2. The BFS mortar lined length produced conveyed water pH values approximately 0.4 units less than the comparable OPC length during the period of 1 to 6 weeks.

  3. The concentrations of aluminium, calcium, potassium and sodium within the conveyed water was initially high, but all metal concentrations for both linings were below their EC MAV's by the eighth day.

  4. The concentration values obtained for metals, and the alkalinity and conductivity throughout the duration of the trial were not significantly different between the BFS and OPC lined lengths.

5. RECOMMENDATIONS

  1. Although the use of blast furnace slag mortar shows little medium or long term benefit, BFS cement 'Mainline' should be used in waters with alkalinity of around 22 mg/l as CaCO3, as reductions in pH of the conveyed water are seen in the short term.

  2. Due to the high levels of pH and metals occurring within the conveyed water within the first few days after lining for both cement mortar types, a flushing programme should be initiated to alleviate any water quality problems during this period.

  3. Where dead end mains or-long rural lengths with low flows are lined, the flow on the main should be increased. This could be achieved by joining up dead ends to form a through main or by a controlled wastage at the dead end.

  4. WRc should further investigate the use of blast furnace slag cements by carrying further field trials in softer waters with an alkalinity of around 12-18 mg/l as CaCO3 as no information yet exists at this level.

  5. Further field measurements should be taken at Kirk Merrington, at 6 months and 1 year intervals after lining.
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