While these terms are often used interchangeably, they are not, in fact, the same thing.   

Concrete and screed mixes are similar in that they contain the same basic ingredients — cement, water, sand and textured aggregate in the form of stone, glass or other lightweight fillers. 

The composition of each mix is determined by its intended use. Concrete, as it is used for structural purposes, consists of larger hard-core aggregates that contribute to its strength and durability. Screed is typically a smoother mix with considerably smaller aggregates and is applied on top of the concrete slab as a finishing layer to level the floor.

Cement then is merely an ingredient used in both concrete and screed mixes as a binding agent. 

Ready-mix concrete is placed in formwork of the desired shape and size, and levelled using a vibrating concrete screed or a straight edge tool.  

The final step is called floating which involves compacting the cement either through mechanical or wood floating, each of which delivers a different aesthetic. If a polished concrete floor with aggregate exposure is what you are after a non-vibrated, wood-floated slab will deliver the best result.  

The process of screeding concrete to obtain a level surface, must not be confused with screed as discussed in our question ‘What is the difference between concrete, cement and screed?’

Either surface can only be a candidate for polishing if:

• The thickness of the slab or screed exceeds 50mm 
• The compressive strength of the slab or screed >25Mpa

While a concrete floor can be ground down to achieve aggregate exposure, a similar finish will not be possible when polishing a screed floor. This is due to the different size aggregates used in each type of mix. 

While it would, in theory, be possible to polish such a floor provided it met the minimum standards as set out in our question ‘Can any concrete floor or concrete screed be polished?’, there are three reasons we would not consider this a viable option. 

1. Removing the tile glue from the surface to create a level floor that will be suitable for polishing will be time-consuming, labour intensive and may damage the floor ultimately rendering it unsuitable for polishing. 
2. As we have no idea of what constituted the concrete mix and how it was floated, there is absolutely no way to guarantee the final aesthetic of the floor.
3. The floor level after removing the tiles will cause a problem at all thresholds.

Instead, we would recommend stripping out the existing floor bed and replacing it with a suitably designed and floated concrete mix. The possible exception to this is suspended floors that will be subject to structural engineering approval. With a concrete mix designed specifically to achieve your aesthetic vision, we are better able to guarantee the results.

The final aesthetic of your polished concrete floor will depend on the design of your concrete mix, the colour of the aggregate used, the placement method, the floor flatness and the aggregate exposure and sheen values you choose. Remember not all concrete mixes are the same; speak to the professionals who can help you design a mix that will deliver the finish you want to achieve.

One of the pain points when polishing an existing slab is irregular aggregate exposure. Even if a slab was cast with the express intent to polish it, it is unlikely that uniform aggregate exposure will be achieved across the entirety of the slab and variations between the bulk of the floor and edges are not uncommon.

A straight edge test conducted over three meters will show if the areas of greater aggregate exposure coincide with the high points of the floor. If so, it is evident that floor placement rather than poor griding practices is to blame for patchy aggregate exposure results. 

The impact of high point/low point differences can be mitigated by casting the slab 3-5mm higher than required and grinding down to the lowest aggregated point. Consistency can further be improved by cross-grinding and overlapping grinder passes by 20 to 25%.