The answer is: Yes, definitely, no problem at all. 

However, you need to be aware that blocks are manufactured for different uses and will have different compressive strengths according to their specified use. You should ALWAYS check with the block manufacturer before making your purchase that the blocks you are ordering are suitable for rendering, and more importantly WHICH type of render they recommend. Obtain a datasheet which states this in writing from both render manufacturer and block manufacturer. 

Most premixed render manufacturers WILL NOT GAURANTEE their product on top of lightweight blocks (recommending at least medium density). This is because the render upon curing is actually stonger than the lightweight blocks themselves. The subsequent result would be that once the render is fully cured, the blocks beneath would crack, and then obviously the render too.

DURAREND WILL GAURANTEE ITS MONOCOUCHE RENDER ON TOP OF ANY LIGHT WEIGHT BLOCKS (3.5N)

As already explained, different manufacturers produce blocks with slightly different characteristics. Celcon, for example, produce a range of blocks, some of which are described below.
     
Standard blocks are available in a range of thicknesses from 75m to 230mm. Thermal conductivity is 0.15W/mK and compressive strength is 3.5N/mm2. When used in modern cavity walls with a brick external leaf they will usually require additional insulation to achieve the requirements of the Building Regulations. These blocks can be used in internal and external leaves of cavity walls, solid walls, party walls, partitions, beam and block floors, and foundations.

A Solar block is also available in thicknesses of 100 to 250mm. Its strengths is 2.8N/mm2 and its thermal conductivity is 0.11 W/mK.  Both these blocks are available in face sizes of 440x215, 610x215, 610x140, and 610x270mm.  These blocks are suitable for cavity walls, solid walls, partitions and foundations
   
Hi-seven blocks are 75 to 230mm thick, thermal conductivity is 0.19W/mK and strength is 7.0N/mm2. There is a black stripe on one end to facilitate recognition on site. They are suitable in the same situations as Standard blocks.

Hi-ten blocks are 100 to 200mm thick, thermal conductivity is 0.19W/mK and strength is 8.4N/mm2. There is a red stripe on one end to facilitate recognition on site. Both these blocks are used where greater strength is required, for example in houses of 3 storeys or more. Mortar mixes should not be too strong. Mixes of cement sand 1:3 are unsuitable because they will not accommodate movement in the blockwork. Celcon, for example, recommend the following mortars above ground level:

• Cement:lime:sand 1:1:6 
• Cement sand with plasticiser 1:6 
• Masonry cement:sand 1:5 
• Cement:lime:sand 1:2:9 

Below ground level a slightly stronger mortar can be used, say 1:0.5:4, cement:lime:sand.

Rendering application methods may vary slightly according to which type of blockwork is used or the exact substrate, but as a general rule, all 6:1:1 renders will be suitable for 'correctly specified' lightweight block masonry - although in most cases specified by the render manufacturer to contain mesh reinforcement in the basecoat. This is additionally suitable for most normal aggregate concrete, expanded clay or expanded aggregate concrete, coarse block or brick masonry, hollow concrete masonry, lightweight cellular fired clay block masonry including thin joint glued construction and stone masonry.

If you need specific advice about rendering on top of lightweight blocks, then please feel free to contact us for personal assistance.