Can I fill in decorative concrete blocks to create garden seating base?

your best bet is to get someone/a trades man to come down and take look at the ground to see what would be best to use most mortar bases are the best to use and to use a concentrated mix ratio of 6/1 ballast and cement. you can use the membrane and hard core over the top of it to use and a solid base then lay over the concrete and level it out with a mix that's not To dry and not to wet let it set for a good ten mins and the use a float to screed it out. if you fell this is bait to complex then seek advice with someone to come down and to take a look and what you need help with doing.

A lot of unkowns with this one: Type of concrete block Soil and support type Drainage runs and water flow above and below soil. When I build these structures from scratch, things I consider include: - Footings, if soil is clay in an area that is subject to wet and very dry conditions, such as bottom of a slope in a sun trap, it will undergo the largest changes in moisture, and so can swell, shrink and heave amazingly. How to overcome it. - Weight of retention/slump calcs (google 'angle of repose'), weight of wet soil vs dry soil - drainage, weep holes, waterproofing the rear of the blocks The mortar/mix is always designed to be weaker than the blocks or bricks, so it fails but doesn't crack the bricks (look at old pre- victorian houses, they experimented with 'new' concrete, mixed it too strong for old tudor walling bricks, and created big issues that way. Personally, I would throw old away and start afresh, it can be more economical to do it right from the beginning

Please, don't do it. As a professional tradesman, fill with hardcore, building regulations.

Hi, it sounds like you're talking about leaf block or similar. I think it'll be a real pain to get concrete into the gaps, and that stuff isn't very strong. your best bet is to dig out 100mm minimum below your desired finished height, shutter the area with timber and smash that leaf block into bits- make sure there is 50mm min between the core and the top of the shutter Then screed a wet 5:1 concrete mix (20mm to dust) over the the core. The runny mix will bind into the core and make a solid base. Use a decent piece of 4×2 to screed, and if the concrete is going to be the finished surface, add sand to the mix (4 ballast:1 sand: 1 cement) knocking back with a rake or spazzle, then screed. Dont forget to add febs, or your 'crete might crack!