ACCIONA Infrastructure carries out the pH correction technique using CO2 in place of hydrochloric acid. This practice is currently in use in the construction of the Pajares tunnel for the high speed railway line to Galicia.

The above-mentioned project initially corrected the pH at the concrete plant and in the tanks of the Folledo tunnel with hydrochloric acid (HCl).

This system has the following principal disadvantages:

• HCl is a strong acid.
• It is supplied in 100 litre tanks with the consequent risk of spillage and discharge during handling.
• The pH curve of strong acids has a steep slope in the equivalence zone, and therefore it is very difficult to control the final pH of the correction, leading to values which may fall with respect to the limits permitted for the discharge, encouraging the appearance of precipitates as the pH falls below 6.
• Given the low temperatures in the region, the correctors are continually breaking down and on occasions the pumps freeze.

In view of these disadvantages, the project management proposed the use of the pH correction technique using CO2 instead of acid which offers the following advantages over HCl:

• It is a weak acid which in the event of overdoses never leads to a pH of less than 5.8, and therefore in the event of a leak as a result of breakdown, the discharge will always be within legal limits.
• It is supplied in bulk in standardised tanks, thus enabling a high level of independence and safety during transport.
• The pH curve for CO2, in addition to correcting pH, provided the treatment is carried out in various stages, also permits the correction of hardness and the precipitation of aluminates.
• The system is based on the injection of gas, therefore by placing a bath evaporator in water it is totally independent of the outside temperature.
• The cost of correction with CO2 is considerably lower than with HCl.