Calcium chloride has a strange reputation—it's probably the most used admixture for concrete and also the most controversial. On the positive side, CaCl is an excellent, inexpensive accelerator; its bad rap stems from its contribution to corrosion of steel reinforcement and metallic floor hardeners. However, moisture and other conditions in a slab have to be right (or wrong) in order for corrosion to occur. Manufacturers of nonchloride accelerators (NCA) have picked up on the corrosion issue and marketed their products as alternatives to calcium chloride. But what if you are constructing industrial concrete floors that don't have steel reinforcement (except for load transfer dowels across joints)? Can calcium chloride then be considered as an alternative to accelerate setting times? This is what owners of some “big-box” stores are currently wondering.
Today many businesses are increasingly aware of “green” issues and sustainability. For the concrete industry, this is often in response to the amount of greenhouses gases produced in cement manufacturing where one ton of portland cement produces one ton of carbon dioxide released to the atmosphere. The modernizing of production facilities changes that ratio somewhat, but the amount of carbon dioxide produced in cement manufacturing will always be high.
But a much larger contributor to greenhouse gas comes as a result of heating and cooling buildings and we can clearly show that concrete buildings save energy because of their thermal efficiency. The story gets even better if we make concrete more “green” by replacing some of the cement with supplementary cementitious materials, such as fly ash or ground granulated blast-furnace slag. The problem is that the SCMs usually slow the set time of concrete (especially when temperatures are low), adding to the cost of the finishing process. Calcium chloride represents an inexpensive and effective solution to this problem—but what about its bad image?
What we know about calcium chloride
The alkalinity of the concrete protects embedded steel from corrosion. Chlorides that move through concrete to the embedded steel reinforcing contribute to destroying the “passivating” layer that concrete forms around steel. Despite this fact, calcium chloride has been used successfully as an accelerator for concrete for more than 100 years, making it one of the oldest admixtures. It's possible that every ready-mix producer in the United States stocks it in either liquid or solid form to accelerate setting time or help manage surface crusting conditions. Contractors generally know how it performs and trust the results.
But aside from the corrosion issue, there isn't much known about the chemistry of its reaction with cement because little research has been done. Joe Althouse, a technical support specialist for Dow Chemical, Midland, Mich.—a major manufacturer of calcium chloride—says, “Much has been forgotten about the performance of this product when used as a concrete accelerator because much of the research was published prior to the early 1950s and the references are not readily accessible anymore.” He adds that the market for calcium chloride as an admixture for concrete is important to Dow, but not large enough to support specialized technical expertise in this application. The primary markets for Dow calcium chloride are deicing, dust control, and drilling fluid additives.
Cement chemists define four primary compounds that make up portland cement: tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A), and tetracalcium aluminoferrite (C4AF). C3S and C2S produce calcium silicate hydrate (C-S-H) and calcium hydroxide. C-S-H is responsible for initial set, early strength gain, and ultimate strength gain. C3A liberates large amounts of heat during early hydration, while C4AF produces the grey color of portland cement-based concrete. What we do know about calcium chloride is that it reacts with C3A to yield an early release of heat that Calcium chloride causes colored concrete finishes to be nonuniform and mottled. contributes to its role as an accelerator—the heat contributes to more rapid setting. What we know little about is how the other cement compounds are affected, although the ultimate strength of concrete accelerated by calcium chloride is approximately the same as concrete without.