Many theories have been put forward, but none can fully explain the retardation process. The chemical nature of cement retarders and their effect on the cement phase (silicate/aluminate), must both be taken into consideration. These are the main theories.
Cement Retarders Systems
Adsorption theory: The retarder is absorbed onto the surfaces of hydration items, which in turn inhibit contact with water.
Precipitation theory. When the retarder reacts with calcium or hydroxyl ions, or both, in the water phase, it forms an insoluble and impermeable coating around the cement grains.
Nucleation theory. The retarder adsorbs onto hydration products’ nuclei, stopping them from growing in the future.
Complexation theory. The retarder chelates the Calcium ions to stop the formation of nuclei.
Lignosulfonate Cement Retarders
The most widely used lignosulfonate cement retarder in well are the sodium salts. Lignosulfonates, polymers made from wood pulp, are unrefined. They also contain different amounts of saccharide compounds. The average molecular weight ranges between 20,000 and 30,000. Because purified lignosulfonates exhibit a lower retarding power, it is more common to attribute the retarding effect to impurities in the bulk material. Low-molecular-weight carbohydrates, such as pentoses and arabinose, hexoses like mannose, glucose, fructose, galactose, and aldonic acid (especially xylonic, gluconic, and glucose) are all examples of impurities.
They are added in concentrations of 0.1% to 1.5% BWOC. Depending on the lignosulfate compound and its chemical structure (e.g. molecular weight distribution, degree of sulfonation, and nature of cement), they can be effective up to 250°F (122°C) bottom hole circulating temperatures (BHCT). Mixing lignosulfonates with sodium borate will increase the temperature range to 600degF (315degC) BHCT.
Lignosulfonate hydrate retarders affect primarily the kinetics for C3S hydration, but their effects on C3A hydration can be significant (Stein, 1961b, Angstadt, and Hurley in 1963). It is believed that the lignosulfonates have a retardation mechanism that combines the effects of both the
Hydroxycarboxylic acids have hydroxyl and carbonoxyl groups in the molecular structures. Tartaric acid and glucoheptonate sodium are the most popular materials in this class. They can have a strong retarding effect and cause overretardation at lower BHCTs than 200degF [93degC]. Fig. 4 These materials are effective at temperatures close to 300°F [150°C].
Citric acid is another strong retarding hydroxycarboxylic acid. Citric acid, which is also an effective cement dispersant, is typically used at 0.1% to 0.3% BWOC.
Saccharide compounds (also known as sugars) make excellent Portland cement retarders. These compounds are rarely used in well cementing because they can cause retardation in small concentrations.
Saccharide compounds’ retarding effects are dependent on their susceptibility to alkaline hydrolysis degradation. The sugars are converted to saccharinic acids containing alpha-hydroxy carbonyl groups (HO-C-C=O), which adsorb strongly onto C-S-H phase surfaces (Taplin, 1960). Hydration is blocked when the C–S-H phase nucleation sites are inactivated (poisoned). (Milestone. 1979).
Cellulose Derivatives Cement Retarders
Cellulose polymers, also known as polysaccharides, are made from polysaccharides that have been derived from wood and other plants. They are stable within the alkaline environment created by cement slurry. When the polymer absorbs onto the cement surfaces, set retardation occurs. The ethylene-oxide link and the carboxyl groups are the active sites for adsorption.
You will need to achieve a 3- to 4-hour thickening time (using Class H and Class A cement).
Alkylenephosphonic acids and their salts became set-retarding agents for good cement in the 1980s. (Nelson (1984; Sutton et. al. 1985; Childs (2006); Nelson (1987). Effective phosphatemethylated compounds that contain quaternary groups of ammonium (Crump, Wilson, 1984) and N-phosphonomethyl imidinodiacetic acid (1995). They have excellent hydrolytic stability and depending on the molecular structure, can be used at circulating temperatures as high 450°F [232°C].
Well cementing is made easier by organophosphates cement slowers. This is due to their insensitivity towards subtle variations in the cement composition. Also, they tend to lower viscosity in high-density cement slurries. The mechanism of action is the adsorption on the nuclei and cement hydrates of phosphonate groups, which hinders their growth.