Neutralising Alkaline Construction Water with Carbon Dioxide: Chemistry, Dose, Contact Time and Permits

Why the water is so alkaline, and why that matters for the dose

Portland cement releases calcium hydroxide as it hydrates. Water in contact with fresh concrete, washout or crushed concrete is saturated with Ca(OH)₂, which fixes the pH near 12.4 at 20 °C, and the sodium and potassium hydroxide released from the clinker push it higher still, to 12.5 to 13. Two features of this water drive the design:

• The hydroxide is a reservoir, not just a concentration. Undissolved lime and cement fines in the water keep dissolving as hydroxide is consumed, so the water re-alkalinises after dosing stops. Titrating a filtered sample understates the demand of the real, solids-laden stream.
• Calcium is high, hundreds to over a thousand mg/L, which means the first product of CO₂ dosing is solid calcium carbonate, not dissolved bicarbonate.

Both facts mean the dose from pH alone is a lower bound and that solids handling is part of the neutralisation design.

The carbonate chemistry: why CO2 cannot overshoot far

Dissolved CO₂ reacts with hydroxide to form bicarbonate, and at high pH on to carbonate:

Three stages therefore follow as gas is added: above about 11.8 the water is in the carbonate stage, between 11.8 and 8.3 it is the bicarbonate stage where most of the pH fall happens, and below 8.3 the curve flattens because the buffer is now bicarbonate and dissolved CO₂, a weak acid. An excess of CO₂ can only take the water toward a carbonic acid solution, which at atmospheric partial pressure sits at about pH 5.6 and in practice levels out near 6 once some bicarbonate is present. That floor is what makes CO₂ safer than sulphuric or hydrochloric acid, whose titration curves are near-vertical through the pH 6 to 9 window and overshoot on any control lag. At elevated gas pressure in a pressurised contactor the floor is not a guarantee, so confirm it by bench titration with the site water at the working pressure.

Estimating the dose

To the bicarbonate end point (pH about 8 to 8.5) the stoichiometry is one mole of CO₂ per equivalent of base. For unbuffered hydroxide:

• pH 11: [OH⁻] = 10⁻³ mol/L → 0.044 kg CO₂ per m³
• pH 12: 10⁻² mol/L → 0.44 kg/m³
• pH 12.5: 3.2 × 10⁻² mol/L → 1.4 kg/m³
• pH 13: 10⁻¹ mol/L → 4.4 kg/m³

The same equivalents as sulphuric acid (96 percent) weigh about 15 percent more and as hydrochloric acid (30 percent) nearly three times more, which is why the gas is competitive on cost despite its price per tonne. Those figures are the minimum. Real concrete water carries undissolved lime and cement fines that keep releasing hydroxide, and its high calcium content consumes CO₂ as carbonate before any bicarbonate forms. Field demands of two to four times the pH-based figure are normal. A reliable number comes only from a titration of an unfiltered, stirred sample of the actual water with CO₂ gas, run to the target pH and held for 30 minutes to see whether the pH rebounds as solids re-dissolve.

Reaction kinetics: fast at pH 12, slow below 10

Dissolved CO₂ reacts with hydroxide at a second-order rate constant of roughly 8,400 L/mol/s at 25 °C (about 2,600 at 10 °C). At pH 12 the pseudo-first-order rate is therefore around 84 s⁻¹, so the reaction is instantaneous relative to gas transfer and the process is mass-transfer limited: every bubble's CO₂ is consumed as fast as it dissolves, which is also why transfer efficiencies of 85 to 98 percent are reported for well-designed contactors at high pH.

As the pH falls below about 10 the hydroxide route slows a hundred-fold and the direct hydration CO₂ + H₂O → H₂CO₃, with a rate constant of only 0.037 s⁻¹ at 25 °C (0.008 at 10 °C), takes over. Dissolved CO₂ now has a half-life of tens of seconds to minutes before it registers as a pH change. A controller watching a probe immediately downstream of the dose point sees the water still alkaline, keeps dosing, and the pH then continues falling for several minutes after the gas stops. Practical rules that follow:

• Place the control probe after a mixed contact volume of at least 15 to 20 minutes, not at the injection point.
• Use proportional or staged control with a conservative set point (8.0 to 8.5) rather than on-off at the limit.
• Expect the last pH unit, from 9 to 8, to take most of the contact time.

Gas transfer and contactor sizing

Three contacting methods are in use. Fine-bubble diffusers in a tank need depth, because transfer scales with bubble residence time; recarbonation practice in drinking water uses a minimum diffuser submergence of about 2.3 m and total detention of 20 minutes. Diffusers foul rapidly in calcium-rich water as carbonate precipitates in and on them, so allow for cleaning or use coarse, non-clogging diffusers. Pressurised side-stream injection dissolves the gas under pressure into a pumped fraction of the flow and returns it, which raises utilisation and removes the depth requirement; vendor comparisons report 20 to 60 percent less gas than diffusers for the same pH change. Jet-loop or venturi contactors entrain gas into a recirculating jet and give short contact per pass (a few seconds) but high turbulence; laboratory jet-loop work reached 98 percent CO₂ utilisation at an outlet pH of 8.4, with utilisation falling as the outlet pH was driven lower.

A workable site design for continuous flow is a contact tank of 20 minutes mean retention at the design flow, at least 1.5 m deep, with the dose point at the inlet and the control probe at the outlet. For 30 m³/h that is a 10 m³ tank. Batch treatment of washout pits (dose, recirculate, verify, release) is simpler and suits the intermittent nature of mixer washouts.

The calcium carbonate trap

In calcium-rich water the CO₂ first precipitates calcium carbonate, and the water clears as it does. If dosing continues to pH 8 and below, the carbonate begins to re-dissolve as bicarbonate: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂. What results is a treated water that meets pH but carries a high dissolved calcium and hardness load, and a sludge that shrinks as it is over-dosed. The robust sequence is the one found in the carbonation literature: dose to about pH 9 to 10 to precipitate the calcium, separate the solids (settlement or a lamella unit), then trim the clarified water to 8 to 8.5. Two stages also protect the diffusers in the trim stage, because the scaling calcium has already left the water.

Treated concrete water carries soluble chromium(VI) from the cement, which CO₂ does not remove. In England the regulatory position prohibits using treated concrete water for dust suppression for that reason; it may be reused for washing plant or batching, or discharged under a permit or consent that will usually include a chromium limit.

Regulatory position and specification checklist

In England the Environment Agency's regulatory position statement for water containing concrete allows filtration, settlement, pH correction by solutions or carbon dioxide, and solidification, and prohibits flocculants; storage is limited per location, with set-backs from watercourses and protected sites, and pH must be monitored and recorded. It covers treatment and reuse only: discharge to a watercourse or ground needs a permit and discharge to sewer needs the sewerage undertaker's consent. Scotland, Wales and Northern Ireland treat concrete-contact water as an effluent needing its own authorisation. Both regimes have review dates, so check the current text before relying on it.

A specification for a CO₂ neutralisation unit should therefore state: design flow and batch volume; influent pH and calcium from site titration; the two-stage sequence where calcium exceeds a few hundred mg/L; contact time and depth or the equivalent pressurised contactor duty; the control probe location, set point and alarm; gas storage and the carbon dioxide safety measures (ventilation, low-level detection, cylinder or bulk tank siting); solids separation and sludge disposal; and the pH logging the regulator will ask to see. Packaged units cover this duty well; the process check that turns a hired unit into a compliant installation is where packaged dosing skid suppliers and process designers earn their place. Wider context on silt and site water is in our construction site water hub and the related pH neutralisation guide.

How to specify CO2 neutralisation for concrete washout water

1. Characterise the water. Unfiltered, stirred sample: pH, calcium, suspended solids and a CO2 titration to the target pH with a 30-minute hold to reveal rebound.
2. Decide batch or continuous. Washout pits suit batch dose-verify-release; dewatering or site drainage flows need a continuous contactor sized at 20 minutes retention.
3. Stage the treatment where calcium is high. Precipitate calcium carbonate at pH 9 to 10, separate the solids, then trim the clear water to 8 to 8.5.
4. Place the control probe correctly. After the contact volume, not at the injection point; use a conservative set point and a high and low pH alarm.
5. Size the gas supply. From the titration demand and peak flow, with 50 percent margin; plan cylinder or bulk storage with the CO2 safety measures.
6. Close the permit loop. Confirm the discharge route and its pH and chromium conditions, log pH continuously and keep records for the required period.