Lignosulfonates Water Reducers for Concrete

What are water reducers?

Water reducers (such as Lignosulfonates) are a type of admixture that is added to concrete during the mixing process. Water reducers can reduce the water content by 12-30% without compromising the workability of concrete or the mechanical strength of concrete (which we usually express in terms of compressive strength). There are other terms for Water reducers, which are Superplasticizers, plasticizers or high-range water reducers (HRWR).

Types of water-reducing Admixtures

There are multiple types of water-reducing admixtures. Manufacturing companies give different names and classifications to these admixtures such as water-proofers, densifiers, workability aids, etc.

Generally, we can categorize water-reducers into three types according to their chemical composition (as in Table 1):

lignosulfonates, hydroxycarboxylic acid, and hydroxylated polymers.

We will split the types of water-reducers into two articles; one for lignosulfonates and one for the other two, to avoid having very long articles.

The first post will deal with lignosulfonate water-reducers.

The second post (click here) will be on the hydroxycarboxylic acid and hydroxylated polymers admixtures,

Where does Lignin come from?

Lignin is a complex material which represent roughly 20% of the composition of wood. During the process for the production of paper-making pulp from wood, a waste liquor is formed as a by-product containing a complex mixture of substances, including decomposition products of lignin and cellulose, sulfonation products of lignin, various carbohydrates (sugars) and free sulfurous acid or sulfates.

Subsequent neutralization, precipitation and fermentation processes produce a range of lignosulfonates of varying purity and composition depending on a number of factors, such as the neutralizing alkali, the pulping process used, the degree of fermentation and even the type and age of the wood used as pulp feedstock.

Composition of Lignosulfonates Water Reducers

Commercial lignosulfonates used in the manufacture of admixtures are based on calcium or sodium with sugar contents of 1–30%.

The next Table 2 provides an example for the analysis of two commercially available lignosulfonate water-reducing admixtures.

A substituted phenyl propane unit with hydroxyl, carboxyl, methoxy, and sulfonic acid groups makes up the lignosulfonate molecule. A polymer that contains the repeating unit depicted in Figure 1 serves as one possible representation.

The molecular weight of the polymer might range from a few hundred to 100 000, with an average of 20 to 30 000. The way and circumstances of refining determine the range of molecular weight that is present in the lignosulfonate. These procedures include fermentation, heat treatment at a certain pH, and ultra-filtration.

Figure 2 is a typical molecular weight distribution curve for a lignosulfonate.

The lignosulfonate polymer generates spherical microgels of the kind seen in Figure 3 rather from being a simple linear flexible coiled “thread” like many high molecular weight polymers are. As a result, the spheroid’s carboxyl and sulfonate groups within are not ionized, and the charges are primarily on the exterior of the spheroid. Only 20–30% of lignosulfonates are ionized, according to conductivity tests.

Sugar in Lignosulfonates

Lignosulfonates include a variety of sugars, both in kind and concentration, depending on their source, type, and level of refinement. Pentoses make up the majority of the remaining sugars in the refined lignosulfonates because throughout the fermentation process, the microorganisms used preferentially consume hexoses rather than pentoses.

Figure 4 depicts the different kinds of sugars that were discovered, and Table 3 provides a breakdown of the sugars in two examples of commercial water-reducing admixtures and untreated sulfite lye.

Salts in Lignosulfonates

Although several different salts of lignosulfonates are commercially available, the calcium and sodium derivatives are the most widely used in admixture formulation. The sodium salt tends to maintain its solubility at low temperatures, thus avoiding sedimentation in winter conditions. In addition, the sodium salt has a higher degree of ionization in solution than the calcium salt. This is reflected in the observation that solutions of higher concentrations of the calcium salt are required to obtain the same reduction in water–cement ratio obtained by using the same dosage of a sodium-salt-based water-reducing admixture. However, calcium lignosulfonate raw materials are invariably cheaper than sodium lignosulfonates so that the higher concentrations can be offered on an approximately equal cost-effectiveness basis.

Lignosulfonates as Water-reducers in Concrete

1. Lignosulfonate superplasticizer dose is typically 0.25 percent, which can result in water reductions of up to 9 to 12 percent in cement content (0.20-0.30%). As used in the proper dosage, concrete strength improved by 15-20% when compared to the reference concrete. Strength grew by 20 to 30 percent after 3 days, by 15-20 percent after 7 days, and by the same amount after 28 days.
2. Without altering the water, concrete may flow more freely, making it easier to work with (i.e. increasing workability).
3. By using one tonne of lignosulfonate superplasticizer powder instead of cement, you may save 30–40 tonnes of cement while maintaining the same concrete slump, intensity, and reference concrete.
4. In the standard state, concrete mixed with this agent may delay the peak heat of hydration by more than five hours, the final setting time of concrete by more than three hours, and the setting time of concrete more than three hours compared to reference concrete. This is advantageous for summer construction, commodity concrete transport, and mass concrete.
5. Lignosulfonate superplasticizer with micro-entraining can enhance the concrete’s performance in terms of freeze-thaw impermeability.

Forms of water-reducers

Reducers come in either powder or liquid format.

Users should conduct an experiment to find the ideal dose of lignosulfonates, which are employed as a water-reducing agent in concrete and have a suggested dosage range of 0.2 percent to 0.6 percent (cement weight metre). Liquid lignosulfonate water reduction agent is available for usage. Users can conduct experiments to choose the content and ratio.

Conclusion Points

In the composition of water reducing admixtures from lignosulfonate (Table 1), the following points are relevant:

1. A significant amount of lignosulfonates, especially the less pure varieties and those made from hardwood lignins, entrain a tiny amount of air into the concrete. This is frequently an unintended side effect but may be advantageous if an air-entraining substance is needed to improve durability or cohesiveness. This means that a little amount of an air-detraining agent can be applied during the manufacturing of typical water-reducing admixtures. Though dibutyl phthalate, alcohols that cannot be dissolved in water, borate esters, and silicone derivatives can also be used, tributylphosphate, which makes up less than 1% of the lignosulfonate, is the most common ingredient.
2. The cement’s hydration is delayed by the lignosulfonate molecule as well as the sugars that are naturally present in lignosulfonate materials. In the case of the materials with a greater sugar content, this is used to create water-retarding admixtures that allow for longer transit or placement times. Triethanolamine additions are occasionally applied at a level of around 15% of the lignosulfonate content of the admixture since this effect is undesired for typical water-reducing admixtures. Triethanolamine functions as an accelerator at this degree of addition, offsetting the lignosulfonate’s and its impurities’ retarding effects. It has been demonstrated that this has certain negative consequences on some of the qualities of the resulting concrete.
3. The water-reducing admixtures that accelerate the setting time of concrete are straightforward mixes of lignosulfonate or hydroxycarboxylic acid salts with calcium chloride, nitrate, thiocyanate, or formate. In some circumstances, it would not be feasible to generate a solution that is entirely sediment-free, and agitation of storage tanks could be required. In practice, a solution containing calcium lignosulfonate 4% and calcium chloride 33% by weight in water would be employed.
4. As mentioned previously, impure lignosulfonate raw materials can serve as the basis for air-entraining water-reducing admixtures that contain lignosulfonates when just 2-3 percent more air is needed. Surfactants are added since this air could not be the exact quantity, kind, or stability needed. There are several varieties that can be employed, but most of them are based on fatty-acid soaps or alkyl-aryl sulfonates, such as sodium dodecyl benzene sulfonate (e.g. the sodium salt of tall-oil fatty acids). These sorts of additions will make it possible to incorporate enough stable air with the right bubble size to fulfil durability requirements during freeze-thaw situations.