Presets and playback
Presets
Process
Clay properties
Layers
Model checks
The science
What activation does
Natural calcium bentonite swells little because divalent Ca2+ holds the montmorillonite layers together. Adding soda ash (Na2CO3) swaps Na+ onto the exchange sites while Ca2+ leaves as insoluble CaCO3, which pulls the exchange forward. Sodium ions are monovalent and strongly hydrated, so the layers can move apart and the clay swells and gels.
Equivalent bookkeeping
1 wt % soda ash supplies 2000/105.99 = 18.9 meq Na per 100 g clay. With cation exchange capacity CEC and initial Na fraction X0 the free sites are CEC(1 − X0); the equilibrium Na fraction is Xeq = min(1, X0 + η · dose · 18.9 / CEC). Surplus Na stays in the water as salt. The stoichiometric dose is where the free sites are just filled.
Kinetics
Exchange approaches equilibrium as a first-order process, dX/dt = k(T)(Xeq − X), with the Arrhenius rate k(T) = kref exp[−(Ea/R)(1/T − 1/Tref)], Ea = 45 kJ/mol. The page integrates it with RK4 and compares with the exact exponential.
Why overdosing hurts
Dissolved salt raises the ionic strength I and shortens the electrical double layer, whose thickness is the Debye length λD = 0.304 nm / √I (1:1 electrolyte, 25 C). A thinner double layer means less osmotic swelling, so free swell peaks near the stoichiometric dose and then declines.
Honest simplifications
The swelling law V = 2 + 26 X2(1 − e−λ/1.6 nm) mL per 2 g is a semi-empirical curve chosen to resemble typical free-swell tests; it is not a molecular simulation. Magnesium, pH, carbonate speciation, particle size and drying are ignored, and the drawing is schematic (layers are not to scale).
What to try
Start with Raw calcium bentonite (small swell), then Optimal dose and watch sodium ions replace calcium over time. Compare Overdosed: the Debye length collapses and the swell drops. Try Hot activation to see the Arrhenius speed-up in the half-time readout.