Question: Admixtures

Dear Ben, I work for a company that that manufactures concrete pavers and block. When I google “Concrete Admixtures” I mostly see info about ready mix concrete which does not apply to our business. Can you give me a quick summary of how admixtures and dry cast concrete interact?

Trying to find good dry cast information amongst a deluge of Ready Mix sites is a very common problem in our (niche) segment of the concrete market!  Here’s a few pointers:

Ready mix concrete is also known as slump or wet cast concrete. While many of the raw materials are the same or similar to what we use in the dry cast concrete products industry, the manufacturing process is quite different. As those of you who have ready mix concrete experience know, in wet cast concrete we are usually trying to take water out of the mix (that’s why you see all the “Water Reducer” admixtures for ready mix). By contrast, in dry cast concrete, to get the highest performance in our products, we are usually trying to add as much water as possible to our mix without causing it to slump. 

Bottom line is that the admixtures for ready mix and dry cast are quite different. That’s why we typically  don’t use admixtures designed for ready mix in a dry cast application.

So , let’s look at what admixtures do in dry cast concrete. Admixtures have three general functions:  

1. Give us properties that we can’t get any other way
   1. Make units water repellent so that they can be specified for water repellent masonry structures
   2. Reduce efflorescence by reducing movement of water through the units and binding efflorescence producing components
2. Improve product quality and manufacturing efficiency 
   1. Improve product quality: for example by increasing density and lowering absorption
   2. Reduce the cost of production: for example by reducing the amount of cement needed to achieve a desired strength
   3. Improve production efficiency: for example by reducing the amount of time and vibration needed to properly fill and compact a mold
   4. Reduce waste: for example by reducing chips and cracks in units 
3. Allow us to make concrete products in wider temperature conditions
   1. Allow concrete production in hot weather conditions: for example using a retarding admixture to slow down hydration
   2. Allow concrete production in cold weather conditions: for example using an accelerating admixture to speed up hydration

Now let’s look at the specific dry cast admixture classifications: You can also check out our Dry Cast Admixtures page.

1. Water Repellent Admixtures – These admixtures are a key ingredient in the manufacture of Architectural CMU, the concrete block units used to manufacture single wythe masonry walls. Water repellent admixtures reduce the amount of moisture absorbed into the surface of the concrete unit and limit moisture wicking through the face shell. 
2. Efflorescence Control Admixtures (ECAs) – Efflorescence is a white deposit that sometimes occurs on the surface of concrete units. Unfortunately, any product that contains cement has the potential to effloresce. ECAs help to control the four factors that produce efflorescence. Simplified, this means that ECAs work by making the product more hydrophobic which reduces the amount of water that can move into and out of the concrete. This minimizes the amount of efflorescence that can migrate out of the products.  ECAs can also include densification packages that further minimize efflorescence by increasing the products’ density, lowering their absorption levels, and reducing migration pathways.  
3. Plasticizers – As the name suggests, plasticizers aim to make the mix more flowable. They act as a lubricant and production aid to allow the dry mixture to fill and compact in the molds better, resulting in a more consistent product with higher strengths and sharper edges and corners. Plasticizers also make production more efficient by reducing the amount of time to fill and compact the molds. These reduced cycle times increase production throughput while reducing vibration and wear and tear on machines. Finally, depending on the chemistry used in the plasticizers they can affect the surface appearance of the CMU giving either a swipe or open texture, as desired.
4. Densifiers – Densifiers aim to help disperse the cement, improve compaction in the mix, and make the products denser. Increased density means fewer voids or gaps between the particles which lowers the absorption and increases the strength of the products. It also follows that fewer voids means a tighter surface and fewer pathways for moisture to be absorbed into and out of the unit, thus reducing potential efflorescence.
5. Retarders – Sometimes we need more time to work with concrete mixes due to hot temperatures  or when we make multiple batches at the same time to use in blended color products. Retarders delay the hydration of cement particles and extend the time before the concrete losses its workability .
6. Accelerators – Sometimes we need to speed up cement hydration, particularly during colder weather. Accelerators can help ‘kick start’ the cement hydration process and can be especially helpful in curing systems in which no heat is added to the kiln.