Q. Is there a chemical or mineral that I can add to a concrete mix to stop it from hardening? I need to transport the concrete a long distance from the ready-mix plant to the jobsite.

A. Various chemical additives can be used to retard set-time. The most common commercially are lignosulfonates—dark, tarry byproducts of paper manufacturing. One of the large suppliers of this raw material is Borregaard LignoTech (www.ltus.com), which now owns what began as Marathon Corporation's Rothschild (Wis.) Pulp and Paper Works. Marathon developed the lignin market beginning in the late 1920s in an effort to develop commercial products from its waste materials.

All sorts of sugars also have some commercial use as set retarders. Most are used to retard the set of oil well cements that are pumped down the hole and then pushed up the side of the casing. If the cement sets up in the hole on the way down, you've got a big problem. These agents are generally used in low concentrations (less than 1% solutions), but it depends on how reactive the substrate is.

Regarding concrete, several admixture producers have products that control cementitious bond formation. WR Grace (www.graceconstruction.com) offers Recover, a hydration stabilizing admixture for concrete that delays setting for anywhere from a number of hours up to weeks. According to the manufacturer's literature, “organic chelating agents within the formulation seek and coat the hydrating cement grains. Water and ions needed for further hydration are blocked to completely suppress cement surface activity ... .” That halts the hydration process, which is what leads to concrete setting. Note that the stabilizer is eventually consumed and hydration resumes, so the set is not delayed indefinitely.

Another hydration control admixture that can delay the set of cementitious material for a selected period is Delvo from Degussa (www.degussaadmixtures.com). General Resource Technology (www.grtinc.com) provides its Polychem Renu admixture for similar applications.

While these three are all supplied in liquid form, dry packaged products are also available to accomplish the same thing. For example, Axim Italcementi Group (www.aximconcrete.com) offers Stop-Set to slow or stop cement hydration. The hydration process is resumed by applying one of the company's Catexol accelerators. General requirements for these admixtures are spelled out in ASTM C 494, “Standard Specification for Chemical Admixtures for Concrete.”