Welan is produced by bacteria from the Alcaligenes species (ATCC-31555) in an
aerobic, submerged fermentation. The medium contains D-glucose as the carbon
source, phosphate as a buffering agent, ammonium nitrate and soy peptone as
sources of nitrogen, and trace elements.
The welan repeat unit shows a single sugar sidechain containing either L
rhamnose or L-mannose substituted on C3 of every (1→4) linked glucose unit. The welan side chain shields the carboxylate group in the double helix.
This renders welan insusceptible to cation-mediated bridging. The welan molecule
is rather stiff, whereby it becomes relatively insensitive to temperature and pH
variation. This and its tolerance for high calcium ion concentrations make it ideal
for applications in concrete, mortars, cement grouts and high temperature drilling.
Among the different welan applications in construction, two stand out and will be
discussed in the following: The use in concrete as a stabilizer, and the use in oil
well construction for spacer fluids.
As discussed, highly fluid concrete such as SCC with a slump in
excess of 270 mm has a tendency to disintegrate. Of all microbial biopolymers,
welan gum has been found the best to stabilize this concrete and to prevent surface
bleeding. With concrete requiring liquid admixtures rather than dry powders for
ease of homogeneous mixing, considerable effort was devoted to develop a liquid,
yet highly concentrated colloidal welan suspension.
Dial et al. discovered and patented an aqueous suspension containing 10% welan
gum, 40% of a sulfonated melamine formaldehyde condensate, 0.75% ultra fine
cellulose fibers for suspension stability, and the remainder water.
Published test data suggest that in concrete the welan gum suspen
sion effectively prevents the formation of surface bleeding water and the sag of
large aggregate particles. It does however impede early strength development to
some extent when used at higher concentrations. Still, welan gum suspensions are
the most widespread admixture used to stabilize highly fluid concrete.
The liquid welan-superplasticizer suspension also found use in cementitious injec
tion grouts. These are used, for example, for crack repair, for sealing of sub
merged anchorages and for filling post-tensioned ducts. In these applications, like
in SCC, it is essential to control bleeding of mix water and sedimentation of sus
pended cementitious materials.
Among all applications, probably the largest volume of welan is used in spacer
fluids for well construction. In this application, welan has
several advantages over any other biopolymer on the market, those being:
(1) rapid development of viscosity in solution
(2) high viscosity efficiency at concentrations as low as 0.1% in solution
(3) temperature stability up to 150°C
(4) relative stability to divalent cations such as calcium
This combination of properties turned the market almost exclusively to welan. As described earlier, spacer fluids are pumped and circulated through the bore hole to separate the drilling fluid from the cement slurry. While previous spacers typically contained dispersing and abrasive agents and were pumped under turbulent flow at high annular velocity to achieve complete removal of drilling fluid and mud cake, it is generally accepted now that spacer fluids showing a shear-thinning type of viscosity imparted by a microbial biopolymer and pumped under plug flow conditions are more efficient for the job.
Typically, 25 kg of the above dry spacer formulation is mixed with 1 m³ of water and circulated through the hole prior to cementing the casing. When surfacing after circulation, the spacer fluid either is blended into the drilling fluid to be used later on, or is disposed off.
