The physical properties of interstellar dust grains at high-redshifts are still poorly known, and for simplicity dust is usually assumed to have the same properties as Milky-Way dust. However this assumption remains completely untested, leading to potential systematics in the way we correct for the effects of dust attenuation in observations of distant galaxies, and to inconsistencies with models of dust formation and evolution in early galaxies. One of the most poorly-understood parameters is the dust emissivity. In this talk, I will present new measurements of the dust emissivity index in z~2.5 galaxies enabled by sensitive observations at 2mm with the Atacama Large Millimetre Array (ALMA). I will demonstrate that low-frequency observations are crucial to constrain this parameter and I will discuss our results in the context of dust evolution models.