The maximum solid phase concentration in geophysical turbulent gas‐particle mixtures is essential for understanding the flow dynamics but is poorly known. We present laboratory experiments on turbulent mixtures of air and ceramic particles generated in a vertical pipe. The mixtures had maximum bulk concentrations Cmax = 0.7–2.4 vol. % set by the onset of clustering and that increased with the degree of turbulence. Comparison with results of similar experiments with less dense (glass) particles reveals that Cmax increases with the particle Reynolds number according to Cmax = 0.78 × Rep0.17. Published results of experiments at specific Rep in different configurations are consistent with our data, suggesting that the model for Cmax may be generally applicable to turbulent mixtures. From our empirical laws we infer that natural turbulent gas‐particle flows with Rep ~ 102–105 have maximum solid concentrations of ~2–5 vol. %.