Gulf Construction Online - Admixtures ease flow of cement mixes

GRC

12-mm-thick GRC panels used to refurbish a small extension.

Admixtures ease flow of cement mixes

The use of chemical admixtures for Portland cement-based mixes has grown steadily over the past three decades with a number of new-generation types developed during this period. GRAHAM GILBERT of Cement Composites Technology* provides an insight into the various types of admixtures used in GRC production.

01 March 2009

THE most common admixtures are those that reduce the water demand and/or improve fluidity.

These admixtures aid the dispersion of cement and are generically referred to as plasticisers with the early forms being based on lignosulfonates. They were improved with the introduction of super-plasticisers based on sulfonated melamine or naphthalene formaldehyde.
Recently, mainly for the self-compacting concrete market, a new range of products has been launched based on polycarboxylate ether chemistry. However, all three types of plasticisers are still freely available.
Increasingly, plasticisers are used with VMAs (viscosity-modifying admixtures), which improve cohesion. They are added mainly to prevent segregation and bleeding when a highly fluid mix is required.
In addition to the above, there are a number of other admixtures that are frequently used for concrete such as set retarders for hot weather, accelerators for cold weather and water repellent/anti-efflorescence chemicals. Admixture types and their applications are well documented, with The Concrete Society’s Technical Report 18* being a good reference source.

Plasticisers
For GRC to flow more easily for a given water:cement ratio, it is necessary to reduce the yield point (force needed to start the mix moving) of the mix. Plasticisers do this by adsorbing onto the surface of the cement particles, reducing flocculation, thus aiding dispersion and reducing drag, which increases fluidity. They have become quite sophisticated; some will give a high degree of fluidity quickly but are short-lived, others give less dramatic fluidity but last over a longer period of time. Sometimes, rapid filling and early demoulding is the key issue but in other cases it is better to accept a lower fluidity so as to have a longer time to fill moulds.
The original and still the cheapest plasticisers are based on lignosulfonate. They have a limited use with fibrous mixes such as GRC because the fibres severely inhibit flow, rendering them less effective.
Nonetheless, they can be cost-effective in some cases and can only be used at low dose levels since at high levels (more than 0.5 per cent weight of cement) they may cause retardation and entrainment of air.
Plasticisers based on sulfonated melamine as well as naphthalene formaldehyde are much more effective at increasing flow or reducing the water content (to increase strength) for a given flow. They were originally given the name super-plasticisers, became widely available in the 1970s and are still extensively used today by GRC manufacturers.
They are custom manufactured so as to give better early strengths, by reducing the water for a given fluidity or longer fluidity retention for rapid mould filling. Dosage is usually in the range 0.7 to 1.5 per cent by weight of cement, allowing for water reduction from 15 to 25 per cent, with a resulting increase in strength. In addition, they are less likely to retard the set of the mix if used within this range. They can also be combined with other admixtures, for example, accelerators or retarders, for secondary effects.
The latest range of plasticisers is based on polycarboxylate ethers (PCEs) and they have dramatically improved the ease of producing GRC, particularly premix GRC. They are the most expensive plasticisers but are also very efficient, so lower dose rates are possible, being typically used in the range 0.1 to 2 per cent. The latter will give a very fluid mix and would probably need to be used in conjunction with a VMA to avoid segregation. At the higher levels, higher fibre content premix can be achieved. They also reduce mixing problems with excellent results being obtained with even the simplest of mixers. PCEs are readily modified by the manufacturers to produce specific properties such as workability retention, cohesion or early strengths. Lowering the water:cement ratio will increase the strength of the GRC as well as improve its resistance to water and chloride ingression. Surface finishes are also usually improved with a reduced number of blemishes.

VMAs
Most VMAs are based on high molecular weight polymers with a high affinity for water and build up a sort of three-dimensional structure in the liquid phase. In this way, VMAs modify the rheological properties of the cement paste in the GRC mix. Typically, they are used at the rate of 0.1 to 1.5 per cent by weight of cement.
Basically, the flow (rheology) of a fresh cement-based matrix depends on two properties: the yield point, which is the force needed to start it moving, and the plastic viscosity, which is the resistance of the mix to flow due to the internal friction of its ingredients.
Slump tests mainly measure the yield point and the V-funnel test measures the plastic viscosity (although this is not part of any current recommended GRC testing programme).
Plasticisers decrease the yield point and therefore increase flow. VMAs, on the other hand, increase the plastic viscosity without affecting the yield point. This helps to hold the constituents together, thus reducing any tendency to segregate, especially in highly fluid mixes. They also help to reduce friction during pumping and reduce bleeding. In this way, VMAs complement plasticisers in GRC mixes and are especially useful for pumped mixes and/or highly fluid mixes.

Accelerating admixtures
Accelerating admixtures are generally not required for GRC. GRC is cement rich, so it has a natural exothermic (heat-developing) reaction and is made under factory conditions. Therefore, setting in 12 to 16 hours is not normally a problem. However, in factories that are not heated overnight in very cold climates, they can help to ensure that the product sets hard enough to demould the next morning. If GRC is to be left under these cold, overnight conditions, it would be better to consult with the suppliers of plasticisers so as to find one which has an accelerator pre-blended in.
The most common accelerators are based on calcium chloride but care must be taken when using this product if there is any steel (for example. fixings parts) embedded in the GRC. A number of chloride-free accelerators are available. If sprayed GRC is being produced, one use of these accelerators could be of interest since it is added at the nozzle head of wet sprayed concrete in tunnels to increase the strength development. Perhaps this is a way to increase mould turn around?

Retarding admixtures
In hot weather, manufacturing GRC can become problematic because of its high cement content and resulting exothermic heat development. In hot weather, it is also best to keep all ingredients cool and use iced water, but when this is not possible chemical retarders can be used.
The majority of retarders are sugar-based and are most effective if they are added a few minutes after mixing is complete. They should be used in addition to other admixtures, such as plasticisers, so as to have a fluid mix to start with. Retarders do not normally affect long-term strengths but they can affect 24-hour properties.

Air-entraining admixtures
Air-entraining admixtures are not usually used with GRC since it is already relatively lightweight and such admixtures generally reduce the mechanical strength of the finished product. However, if the weight of a product on-site is a problem and, even for GRC this is increasingly the case, then there are several proprietary admixtures available that can easily reduce the density of GRC. If air entrainers (or low-density aggregates) are used, then the mechanical properties of the GRC should be verified.

Polymers
For the last 30 years, polymers have been used with GRC and their use in all forms of concrete and cement-based blends is growing. In GRC, they are used for the following reasons:
• To eliminate wet curing while still achieving good seven- and 28-day strengths;
• To greatly improve workability and application even without the use of other admixtures;
• To improve the long-term mechanical properties of GRC, particularly the aged flexural strain to failure; and
• To achieve harder, denser surface with no ‘dusting’ and less crazing.
The polymers used are white latex, usually acrylic, emulsions with approximately 50 per cent solids content. The recommended dosage is five to six per cent solids (10 to 12 per cent latex) by weight of cement with lower levels also being used (accepting lower benefits). The polymer must be resistant to alkali and be UV (ultraviolet) stable. They function by forming a network of flexible polymer bridges between the brittle mineral ingredients, helping to bind them together. The polymer film formed traps in the water preventing premature evaporation and sealing the surface. This results in a harder surface, which also reduces moisture ingress, and especially the rate of water penetration.

Anti-efflorescence
GRC has a high cement content and so produces more lime during setting. This makes it prone to efflorescence since lime is the primary cause of efflorescence. Replacing some of the cement with a high reactivity pozzolanic material will drastically reduce the amount of lime generated and almost eliminate efflorescence. However, pozzolans are added at 15 to 20 per cent dose levels and, therefore, fall into the category of additives, so are not discussed here.
By reducing the water content and curing the GRC correctly, efflorescence is kept to a minimum.
Also, the use of polymers is beneficial since it ensures good cure and helps to seal the surface of the product preventing or reducing the evaporation of water from the surface, which leads to efflorescence.
There are also a number of compounds that can be added into the mix to increase the water-repellent nature of the surface after setting, including the more recent integral crystalline compounds. Anything that helps reduce evaporation of water from the surface will automatically reduce efflorescence.

* A guide to the selection of admixtures for concrete. Technical Report 18, The Concrete Society, Camberley, 2002.
** This article is reproduced from The Concrete Society’s Concrete magazine October 2008.