What Are Admixtures?

Question:
What are admixtures, and what role do they play in concrete countertop mixes?

Answer:
Admixtures are chemicals or minerals added to concrete to change or improve its properties, either while it is still fresh or after it has hardened. Chemical admixtures include water reducers, set accelerators, set retarders, air entrainers, shrinkage reducers, corrosion inhibitors and defoaming agents. Mineral admixtures generally refer to pozzolans, such as fly ash, silica fume, metakaolin and slag, but can also include nonreactive materials, such as pigments.

To explain each one of these admixtures and its use in concrete countertops would be a very long article. I will answer specific questions about specific admixtures in subsequent blog entries. In the meantime, for lots of good, readily understandable information about admixtures, visit The Concrete Network’s section on admixtures. Or read the Portland Cement Association publication Design and Control of Concrete Mixtures, a widely used textbook for engineers. Chapter 6 is all about admixtures.

You can also refer to ASTM C494, which specifies the requirements for seven chemical admixture types:

• Type A: Water-reducing admixtures


• Type B: Retarding admixtures


• Type C: Accelerating admixtures


• Type D: Water-reducing and retarding admixtures


• Type E: Water-reducing and accelerating admixtures


• Type F: Water-reducing, high range admixtures


• Type G: Water-reducing, high range, and retarding admixtures

It is noteworthy that some admixtures used in concrete, and especially in concrete countertops, do not fall under one of the ASTM C494 specifications.

In general, successfully using admixtures relies on an understanding of how concrete works and good batching practices. Admixture effectiveness often depends on a variety of factors, including temperature, cement content and type, water content, mixing time and even aggregate gradation in some cases. While most admixtures are compatible with other admixtures, not all are. Before using a cocktail of admixtures for a client’s project, I strongly recommend consulting with the admixture manufacturers and preparing a test batch.

Lastly, it is important to remember that admixtures cannot compensate for bad practice and low-quality materials. The mix design basics of low water-cement ratio, good aggregate gradation and proper curing are still essential for concrete countertop success.