Anti-settling agents are important, yet often over-looked, additives for the design of stable cement slurries.
Two of the biopolymers the most frequently used as anti-settling agents are Welan and Diutan gums. The structure of Welan gum (shown above) consists of a main chain with 4 units (glucose -glucuronic acid-glucose -rhamnose) with a side chain of either l-rhamnose (shown) or l-mannose. Diutan gum has a similar structure except that there are two l-rhamnose units on the side chain. The side chains prevent interactions between the carboxylate group on the main chain and the surface of the cement and cement hydrates thus preventing the polymer from adsorbing on the cement.
These polymers function by viscosifying the aqueous phase of the cement slurry. In solution the polymers behave as rigid rods. At rest they entangle giving rise to a yield stress that helps suspend the cement particles. Under shear the polymers chains align giving a shear thinning behaviour.
The polymers have a high molecular weight and are used at low concentrations (<0.1% bwoc in 1890 mg/m3 [15.8 lbm/gal] density cement slurries). The low concentration combined with the non-adsorbing behaviour of these polymers means that they have little or no effect on the hydration of cement.
However, there are sometimes observations that addition of these types of anti-settling agents are detrimental to compressive strength development. This is most likely an artifact due to instabilities in the cement slurry before addition of the anti-settling. Compressive strength measurements on cubes of set cement made from slurries that show free fluid and sedimentation will have higher compressive strength than expected as discussed here.
E. Üzer, J. Plank, Impact of welan gum stabilizer on the dispersing performance of polycarboxylate superplasticisers, Cement and Concrete Research, 82, 2016, 100-106.
The structure of Welan gum was drawn using PubChem Sketcher.
W.D. Ihlenfeldt, E.E. Bolton, S.H. Bryant SH. The PubChem chemical structure sketcher. J Cheminform. 2009 Dec 17;1(1):20. doi: 10.1186/1758-2946-1-20. PMID: 20298522; PMCID: PMC2820498