Flocculate with Expansive Clay

This entertaining topic comes up once again, this time in reference to a thread on Pat' Morrissey's LinkedIn Group, Means, Methods & Materials... in an ongoing discussion regarding rising damp in masonry.



Google books, "Clay Materials Used in Construction " edited by G.M. Reeves, I. Sims & J.C. Cripps

12.1.2. Bentonite

The name bentonite is popularly used for a range of natural clay minerals of the smectite group, principally potassium, calcium and sodium monnnorillonites derived from the weathering of feldspars. The name derives from the discovery of large deposits near Fort Benton in Wyoming. USA. Because of the chemistry and micro-structure of the clay particles they have a strong ability to absorb water and are able to hold up to ten times their dry volume by absorption of water. Montmorillonite (after Montmorillon, southwest of Paris) consists of very thin flat crystalline sheets of clay minerals which are negatively charged and are held together in 'stacks' by positively charged sodium or calcium ions in a layer of adsorbed water. In particular the soil particles comprising a stack of sheets of sodium montmorillonite form extremely small and thin platelets, being typically of the order of 1.0 pm or less in length and 0.001 um thick. The ability to absorb water comes from the relatively low bonding energy of the sheets, which allows water molecules to be adsorbed onto the internal and external sheet surfaces. Calcium ions provide a stronger bond than sodium, so that calcium mommorillonite swells less readily than sodium monnnorillonite. Potassium ions provide much stronger bonding between clay sheets as the potassium ion is of exactly the right diameter to fit between atoms in the sheet structure with negligible gap between the clay sheets. A similar material to mommorillonite but with potassium bonding is the non-swelling clay mineral known as illite. The substitution of sodium by calcium or potassium ions in monnnorillonite greatly reduces the ability of the clay structure to hold water.

The very small particle size of bentonite results in an extremely low hydraulic conductivity for intact clay, with a coefficient of permeability of typically less than 10-1" m/s. This allows the clay to be used to form 'impermeable' or 'waterproof layers and sustain high hydraulic gradients across thin layers with negligible water flow. The swelling property is also important in such applications, since should water permeate a layer of dry bentonite it will swell even against high pressures and tend to seal any crack or fault which might otherwise develop into a leakage path. The volumetric swelling of particles can be up to 13%. but that of an agglomeration of particles is somewhat less depending on their packing.

Many applications of bentonite involve the use of slurry. Mineral particles in a slurry generally carry electrical charges, the nature and intensity of which vary with the particle surface characteristics and the chemistry of the liquid phase. Polar water molecules may then be adsorbed on to the particle surface, forming a layer of 'bound' water surrounding each particle. The result of the two effects is to produce repulsive forces between particles, which are greater than attractive Van der Waal's forces except when the particles are very close together. The particles in a slurry therefore tend to keep apart from each other in a 'dispersed' condition (Fig. 12.1a). The effects are most noticeable with small particles (clay/silt rather than sand/gravel, and in practical terms only with finer clay particles) since the relative surface areas are much larger. and gravitational forces are much smaller. Under some conditions the plate-like particles of clay minerals may have different charges on the edges and faces of the particles, and are able to clump together in a 'flocculated' structure (Fig. 12.1b). The large flocs settle out of the slurry much more readily than the small individual particles.

Some slurries demonstrate the effect known as thixotropy, whereby they 'set' into a gel if left undisturbed, but revert to a viscous fluid (sol) when sheared. The alternation between sol and gel may take place any number of times. The phenomenon is well known in 'non-drip' paints. A gelled 'house-of-cards' type of structure with edge to face connections is illustrated in Figure 12.1c; gels of thin clay particles may contain only a few per cent of solid material. The gelled structure is also able to support larger soil particles and prevent them from settling out. Bentonite slurries are thixotropic and typically form a gel at concentrations of a few per cent by mass in water: this is an important property of bentonite slurries in many applications. For a more detailed discussion of the nature and properties of bentonite slurries see Jefferis (1992).

Bentonite clays occur, and are mined and processed commercially, in many parts of the world. Some natural deposits, notably those from Wyoming, have a high proportion of sodium. These tend to produce slurries with high viscosity but relatively low gel strength. The deposits mined in the UK, near Woburn, are mainly of the calcium form, and these are converted by ion exchange to the sodium form by ball-milling with sodium carbonate. These materials tend to be less dispersive and give lower viscosities for the same slurry density, but higher gel strengths. As natural products, bentonites vary widely around the world in quality and content of other minerals, even after commercial processing, and these variations must be taken account of in their specification and use.

Bentonite is available commercially in a variety of forms. but nearly always in a dry state, as powder (in bulk or bags. like cement), pellets or blocks. For applications in construction it will usually be hydrated, although in some waterproofing materials the hydration is allowed to occur in situ. For use as a slurry, the bentonite is mixed with water at a rate of a few per cent of solids by mass. The aim is normally to produce a slurry in which the bentonite particles are well dispersed and fully hydrated. For good mixing and rapid hydration, a high-shear colloidal mixer (shear rate >900/s) should be used, and the slurry then left to stand for some time while the clay particles hydrate. The quality of the slurry obtained depends on the hydrogen ion concentration (pH) of the water used in mixing; saline or acidic water or water containing impurities may cause the clay particles in the slurry to flocculate. This may initially cause the slurry to 'thicken', but there will then be a tendency for the flocculated particles to settle out of suspension and form a sludge. However there is not normally a practical problem with seawater coming into contact with a slurry, provided the slurry cannot mix freely with the seawater and has previously been fully hydrated with fresh water. Deliberate flocculation with flocculating agents may be used to help remove bentonite from suspension when the slurry is no longer required or has become too contaminated with cement, clay or silt. A combination of low hydraulic flow into the slurry (so long as hydraulic heads are low), and long diffusion times for salt compared with exposure times, usually causes few problems in the presence of seawater.

Bentonite is also used in combination with other materials, in particular other soil materials and Portland cement. At one extreme a small quantity of bentonite may be added to a concrete mix to produce highly plastic concrete able to undergo quite large deformations without cracking: while a small quantity of cement in a bentonite slurry can produce a hardening slurry with a small shear strength. Natural clay, silt and sand may be used as 'fillers' to produce cheaper material while keeping most of the benefits of the scaling ability and low permeability of the bentonite. Gleason et al. (1997) found that about 5% of sodium bentonite and 10-15% of calcium bentonite had to be added to fine sands to achieve a sand-bentonite mix with a permeability of less than 10-9m/s. Hardened bentonite-cement slurry mixes containing 180 kg/m3 of cement and 60 kg/m3 of bentonite had permeabilities of about 10-7m/s with calcium bentonite and 10-8m/s with sodium bentonite. These mixtures arc discussed further below in relation to various different applications. Small quantities of polymers and other chemical additives may also be used to enhance or modify the properties of bentonite slurries for particular applications. These are also discussed further below. [not below here, though, you gotta go read the book if you want more!]