In our area, people are discussing the benefits or disadvantages of finishing exterior concrete containing ground granulated blast furnace (GGBF) slag. One group claims that since slag is finer than portland cement, a finisher brings more fines to the surface after straight edge and floating and thus creates a less durable surface. Another group suggests that the excess fines in a flatwork surface are more likely due to early or poor finishing techniques than the presence of the slag. Who is right?

We contacted a sales engineer at a GGBF slag supplier. He says there's a common misunderstanding on this issue. In today's market, GGBF particles are very similar to the size of Type III cement particles but smaller than normal Type I cement particles. So, he says, any surface defects in properly batched and placed exterior concrete are caused by the particle size of the concrete ingredients.

Most of the people with whom we spoke, however, attribute these surface problems to contractors unfamiliar with finishing properties of GGBF slag concrete.

A finisher used to working with a 100% portland cement mix must be warned that a mix containing GGBF slag may require a slightly longer set time and that there will be a little less bleeding. Our experts suggest that the change in bleeding is why it's more difficult to accurately judge when to get on the slab for the final finish. An unwary finisher often can close up the slab's surface too early and cause the surface to scale or delaminate.

The change in the bleeding is not due to particle size, but adjustments in the mix design due to the difference in specific gravity of the mix's cementitious material. While GGBF slag particles may be similar in size to cement particles, they have a specific gravity of about 2.85 to 2.9, compared with portland cement (around 3.15). If the producer replaces GGBF slag pound for pound for cement, the mix would overyield. Many adjust the yield by reducing the fine aggregate (sand). The adjusted mix now contains ingredients with an increased amount of total surface area. Studies show that as surface area increases, bleeding decreases.

A good source of information on this question is ACI 233, "Ground Granulated Blast Furnace Slag As a Cementitious Constituent in Concrete." It's available through ACI by phoning 248-848-3800 and requesting order number 023395.CT00.