MASS-TRANSFER SIMILARITY OF MIXERS FOR CARBONATION OF CONCRETE MIXTURES

Abstract

The paper provides a critical engineering synthesis of how laboratory carbon-dioxide injection regimes for fresh cementitious systems can be transferred to industrial concrete mixers. Existing studies show that carbon mixing can enhance early strength or permit binder reduction under selected conditions, yet the reported outcomes remain sensitive to injection stage, dosage, pressure, mixing sequence, water-to-binder ratio, and the method used to quantify bound CO₂. The central argument is that geometric similarity, rotational speed, blade-tip speed, or constant specific power cannot independently preserve the coupled phenomena that control the process. A carbonation mixer is therefore represented as a batch gas–liquid–solid reactor containing a dense, thixotropic and evolving medium, with intermittent renewal of wetted reactive surfaces. Three implementation routes are compared: carbonation of mixing water, carbonation of an aqueous cement suspension, and direct injection into the complete concrete mixture. For each route, the review identifies the likely rate-limiting resistance, the scale-sensitive hardware features, and the measurements required for qualification. A hierarchy of scaling descriptors is proposed that combines specific mechanical energy, power number, generalized Reynolds number, blade-tip speed, Froude number, gas-flow number, tracer mixing time, volumetric mass-transfer coefficient, injected dose, and verified CO₂ fixation efficiency. Instead of treating the quantities as interchangeable, product-critical invariants, equipment constraints, and adjustable compensators are separated explicitly. An illustrative scale transition from a 0.05 m³ laboratory batch to a 2.0 m³ industrial batch demonstrates that maintaining constant power per unit volume raises blade-tip speed and changes the gas-flow number when the CO₂ dose and injection time are held constant. Conversely, preserving gas-flow similarity requires a longer injection window or a redesigned distributed injection system. The numerical example is a screening calculation rather than a validated equipment prescription. An original vector cross-section of a sealed twin-shaft working chamber is presented, incorporating overlapping circulation zones, distributed CO₂ nozzles, off-gas sampling, torque monitoring, temperature and pressure sensors, and a discharge gate. The scientific contribution is a mass-transfer-centred scale-up framework that distinguishes the amount of CO₂ supplied from the rate and spatial uniformity of its delivery. The practical result is a staged verification procedure in which power and torque signatures, tracer homogenisation, off-gas balance, bound-carbon measurements, fresh-concrete properties and properties of hardened concrete are evaluated at successive acceptance stages. This approach reduces the risk of declaring scale-up successful on the basis of a single hydrodynamic criterion or nominal CO₂ dosage.