My question is if a lens like a Tamron 90mm 2.8 could be used also for portrait. If not, why ? What are the difference between a specific portrait lens (like any 85mm 1.4) and a 90mm 2.8 ? Only the different aperture ?
Macro lenses are close focusing lenses. While most macro lenses can be used for photography that does not require close focusing, they are specifically designed for this ability. A typical 85mm f1.4 will not be designed to focus at close distances. The primary design of a macro lens is to be good for very close up work, whereas the primary design goal for a portrait lens is different. A macro lens might not be designed for good bokeh, for example, but it is typically very important in portrait lenses.
Typically a true macro lens will allow a magnification of 1:1 ( meaning the image size on the sensor or film will match the actual size of the object ), whereas a portrait lens has no need for such extreme magnification.
The Tamron 90mm f2.8 ( and long ago the f2.5 ) is perhaps the best known of macro lenses and have always been regarded as fine lenses.
Macro and portrait lenses are generally designed to do two different things that require different design characteristics.
Macro lenses are designed to focus at very close distances and they typically render a fairly flat field of focus. There are a few very specialized macro lenses that can only focus at the very close focus distances required by macro photography and would not be suitable for other types of photography. Most macro lenses, however, can also double as general purpose lenses. These can be used to focus at more typical focus distances and many photographers have a 90-100mm macro lens that they also use for portraits.
Other lenses specifically designed for portraiture often have a more spherical shape to their field of focus. The Canon EF 85mm f/1.2 L II is one such lens. They typically can not focus anywhere near as close as a macro lens can. There are reasons some photographers prefer to shoot portraits with a lens that has field curvature.
The field curvature that is a characteristic of many lenses purposely designed for portraiture would make most everything except the center of the frame extremely blurry due to the very shallow depth of field if used at the extremely close distances involved in macro photography.
Following on from Michael and Steven..... I think macro lenses have a different (fine) focus ring mechanism, to achieve absolute accuracy when dealing with a macro shot and the very small depth of field that is typical at the distances normally encountered with this type of shot. The fine ring is slightly slower in achieving focus vs a coarse pitch focus ring, the reason is simply because you have more thread to wind on the screw!
This is the reason why some macro lenses have a range limit switch, a feature also found on some high end telephoto lenses.
You can use a macro lens for portraits, although I don't shoot people often, my macro gives excellent results, but it is my most expensive lens..... With that being said, my cheapest lens also gives very good results! Even a telephoto lens (normally sold as/for wildlife) need not be used specifically for that reason, my tele lens gets used for everything but macro! It takes some nice landscape images, the effect of field flattening is quite nice. Finally, a wide angle lens, normally marketed for landscapes, takes good pictures of cars and aircraft in a museum.
Macro lenses also do not need to be high speed (large aperture). In fact, when shooting macro, you want a smaller aperture because the depth of field narrows as the focusing distance decreases.
The longer the focal length of the lenses, the smaller the aperture you will need, particularly if you are photographing a three-dimensional object (such as a flower or toy car, for example) and you want the entire object to be in focus.
That is why you often will see macro lenses in the 50-60mm range and 80-90mm range. Ideally, you own both lenses and select the one that works best for the situation.