I would agree on a straight cement mix. A ready mix company will not send a straight cement mix unless he orders it as such. Portland cement is about .06 per lb. To increase profit, especially on COD residential orders, plants substitute up to 20%_30% of Portland with slag, fly ash or C-ash. These products are about .01 per lb.

These products only become cementitious when in contact with Portland cement. The strength curve between 7-56 days looks far different on one of these mixes than that of a straight Portland mix. A straight Portland mix is expected to achieve 70% of 28 day design strength in 7 days. In that case, a 3000psi 28 day design should break at no less than 2200psi on day 7. It is at its most rapid rate of strength gain in the first 7 days. It gradually slows down as it moves to day 28.

On the flip side, a mix with 25% or more of slag, fly ash or c-ash is slow to gain strength in 7 days. Not uncommon to only achieve 50%-60% in 7 days. It really accelerates at around days 12-14 and picks up steam moving to the 28 day design goal. After 28 days, these blended mixes far outperform straight cement mixes as they continue to gain strength more rapidly than a straight Portland mix.

Pros of each

Straight cement:
1. Earlier 7 day strength gains.

Blend:
1. Less plastic shrinkage issues
2. More attractive finished product....especially a slag mix.
3. Better reflectivity of artificial lighting

If you ever wondered why some year old concrete driveway or sidewalk appears to be so much "whiter" than another it is because it was a blended mix. Straight cement mixes are more gray looking. Fly ash or c ash blends are a bit lighter. Slag is more white. A porch or patio, driveways, walks that will be under lighting at night always look best with a blended mix.....especially if your local plant has a slag silo.

I was a commercial sales manager for a very large ready mix company for 15 years, mainly selling to GCs. I dealt primarily with more complex commercial designs with design strengths up to 12,000 psi and a multitude of performance enhancing admixtures. But there was a lot of carry over similarities to residential applications.

I would listen to these guys who actually work in the field building residential construction. Their know best as they have practical experience. But I would advise you to know what they are shipping you. Don't just order "3000 psi mix" and be done as it might not be the best for your job. Order specific. If it is concrete slab that will be covered up and you want the assurance of early 7 day strength gain....order a straight Portland cement mix. If it is something you will not cover up and will reflect light at night, order slag/Portland mix or Fly Ash if they don't have slag.

The ready mix company will have many options if you specify your own design requirement. You can specify how much slag or ash you want to be included torwards the total cementitious ratio up to about a 60/40 blend.(60 being Portland) I would personally do a 70/30 for an exposed patio or driveway.