Concrete Corrigendum

The integrity of a building product manufacturer (and of its consultants) requires setting the record straight when it makes an error. I have written numerous articles and pieces of product literature with statements similar to the following:
"When portland cement hydrates, it yields calcium silicate hydrate (CSH) crystals that interlock to give concrete strength." (Chusid, Structural Engineer)
This is incorrect. CSH is a gel, not a crystal.
"Illustration of various steps in the digital-image-based cement hydration model showing, from bottom to top, initial cement particles in water (black), highlighting (white) of all cement particle surfaces in contact with water, generation of one-pixel diffusing species, and hydrated images at ~32% and 76% hydration, respectively (C 32 is red, C2 S is blue, C3A is bright green, C4AF is orange, gypsum is pale green, C-S-H is yellow, CH is dark blue, and aluminate hydration products (ettringite, monosulfoaluminate, and C3 AH6) are green)." (Bentz, Journal of the American Ceramic Society)
While conducting research prior to writing the various publications, I have seen hydrated cement paste described as both crystalline and gelatinous. It was easier for me to visualize the former because I am familiar with hard, dense, and strong crystals such as quartz and table salt. My mental image of a gel, however, was gelatine -- a substance too insubstantial, I thought, to explain concrete.

I now appreciate that, with respect to cement hydration, "The C-S-H gel is not only the most abundant reaction product, occupying about 50% of the paste volume, but it is also responsible for most of the engineering properties of cement paste. This is not because it is an intrinsically strong or stable phase (it isn't!) but because it forms a continuous layer that binds together the original cement particles into a cohesive whole." (Thomas and Jennings) Cement paste's properties as a gel help explain phenomena such as concrete creep (deformation over time) and swelling that occurs when alkali-silica reaction causes concrete to crack.

Perhaps only petrologists can fully appreciate the difference between a crystal and a gel, yet it is key to understanding concrete's performance or failure. It is also a crucial distinction for specifiers trying to interpret competing claims by producers of admixtures, supplementary cementitious materials, and concreting processes.

I thank Ward Malisch, PE, PhD, FACI, technical director for American Society of Concrete Contractors, for explaining this to me during a conversation at the recent World of Concrete tradeshow.

By the way, "corrigendum" has a similar meaning to "erratum" except that the former is best applied to an error by an author while the latter is an error in the production of a publication.