Concretes are often used in fragrances that are not going to go into ethanol. Concretes tend to be cheaper and can be used in Soaps, Creams, Shower Gels and even detergent products. If you use them in an alcoholic product you will probably get a cloudy deposits forming. This is all the non alcohol soluble material found in the concrete. If you do get this, leave your alcoholic fragrance in a cold place for a few days, then filter it. You should be left with a clear liquid.
Lavender Absolutes and Concretes should increase the longevity of your Lavender fragrance. You could also try using Clary Sage oil and Absolute, some Geranium Oil, and some Musk.