Lignosulfonate retarder

Lignosulfonates retard the setting of cement. While this retardationcan occasionally be advantageous, under certain weather conditions (e.g. in cool weather) the setting time may be extended to an impractical length of time. Approximately a quarter of the total solids in spent liquor is sugars, which can cause a strong retarding effect. Therefore, the removal of some or all of the sugars is usually desirable. Different methods such as precipitation, alkaline heat treatment or ultrafiltration are used to reduce the sugar content in the spent sulfite liquor. This purification reduces the retarding effect. However, it does not disappear as sugar-free lignosulfonate shows retarding effects. In short, commercially available lignosulfonates show retardation, which is due to the lignosulfonates themselves, possibly but not always incremented by retardation from residual sugars.

Lignosulfonates are available as calcium, sodium and magnesium salts. In formulations with other polymers, sodium salts are preferred. Calcium would cause precipitation of gypsum with sulfate ions that may be present in other formulation constituents, e.g. polynaphthalene sulfonate (PNS) or polymelamine sulfonate (PMS). Lignosulfonates are sold either as a brown, aromatic-smelling powder or as brown 30–40% solutions.

Another important factor affecting the performance of lignosulfonates is their molecular weight. Tests with lignosulfonates with different molecular weights have shown that higher molecular weight fractions show better plasticizing effects (Reknes and Gustafsson, 2000). Although lignosulfonate is the most used material for the formulation of water-reducing products, it is hardly or never used in the design of high-performance concrete. However it is reported that lignosulfonates of high molecular weight, from which sugars and lower molecular weight compounds are removed in a special treatment, can be used for water reduction up to 20% and with moderate retardation (Reknes and Petersen, 2003; Banfillet al., 2007).

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