Retarding admixtures are used in concrete practice to delay the setting times of cement paste, mortar and concrete. In hot weather concreting, delays in transport and handling between mixing and placing may result in early setting and loss of workability and in such instances incorporation of retarders becomes necessary. Retarders may be used in steam curing of concrete, to offset the long term lower strengths developed by the silicate phase and in the construction of large structural units, dams and the fabrication of exposed aggregate panels. Another important application of retarders is to maintain returned concrete from ready-mixed trucks in a workable condition overnight by completely freezing the hydration. Special types of retarders are capable of controlling the slump loss in superplasticised concrete. In oil well cementing operations where a temperature in the order of 900deg C or more is encountered, retarders are invariably added.
Phosphonates are suggested for use in high temperature oil and gas plugging operations, soil-cement mixtures, gypsum plasters and as set time extenders for cements. The ideal inhibitor for cementing would predictably delay the setting of cement, and then suddenly allow hydration to continue at a rapid rate. A wide range of compounds show set inhibition of the hydration of Portland cement. Some common examples include, tartaric acid, Sodium Gluconate, Lignosulfonate, and organic phosphonic acids.
- Sucrose is Ca binding, acts directly on silicates, and accelerates ettringite formation.
- Lignosulfonate accelerates ettringite formation, calcium becomes incorporated into the polymer matrix during hydration and forms a diffusion barrier.
- Tartaric acid acts via calcium complexation and calcium tartrate coating, and inhibits ettringite formation.
- Nitrilo-tris(methylene) phosphonic acid promotes Ca dissolution, forms [Ca(H6ntmp)]. Heterogeneous nucleation on aluminates creates a protective coating around the grain.
– Phosphonates have been termed “super retarders,” due to their increased effect on cement hydration relative to the effect of conventional retarders.