I am looking for a material that can cover an antenna to reduce electromagnetic wave. Cement reduces the wave but not a good choose in this case. Aluminium foil works too but it is too strong and blocks all waves at such a high frequency. It is better to be adjustable in thickness or number of layers. Is there any proper choose?
See: http://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.3.037001 for APS article on meta materials. Depending on what you want see commercial quality absorbers at http://tdkrfsolutions.com/products/absorbers Or the wiki small article on it, at https://en.wikipedia.org/wiki/Electromagnetic_absorbers
Bottom line, metals will reflect most RF, and you can try tuned band absorbers or some more broadband, depending on what you want. Lower freqs will be harder, you would need thicker absorbers.
Or you could disconnect the antenna, or place filters behind it (Google RF filters), or limiters.
So, not sure what you want this to cover the antenna for, maybe get smaller antenna, plenty of devices will lower the power in/out. If you say what you are trying to do maybe we can chime in on some answer.