How Superabsorbent Polymers Work

SAPs are chemical polymers made up of cross-linked polyelectrolytes which swell when they come in to contact with water or any aqueous solution (Friedrich, 2012). Figure 2.45 shows the difference between a dry SAP particle (left) and a swollen SAP particle (right), which is formed into a hydrogel after being in contact with water.

From the figure, it can be seen that the swollen SAP particle increases drastically in size and volume when saturated. The amount of water that a single dry SAP particle can absorb is dependent on a number of factors including particle size, chemical make-up and the surrounding conditions.

Craeye et al. (2011) reported that SAP can absorb water up to 500 times its own weight, while Tange et al. (2012) reports that SAP can absorb water up to 1000 times its own weight. It should be noted that not all types of SAP behave the same and are therefore not always suitable for use in concrete and cement-based materials. These extreme absorption capacities are often not reached when the polymers are added to concrete.

Figure 2.45: Dry and swollen SAP particle (Friedrich, 2012)

SAPs are most commonly used in the hygiene industry, specifically baby diapers and explain how television commercials could cut through a soaked diaper without any liquid dripping from it. Only recently has SAPs been used in more technical applications such as concrete technology, cable isolation and soil mechanics.

There has been a number of research studies conducted on the use of SAPs as internal curing agents in HPC over the last few years. The results from these studies are both conclusive; in that SAPs are proven to successfully mitigate autogenous shrinkage in HPC, but also inconclusive in that the results from these studies are specific to the materials used in that particular study itself and cannot always be predicted. The sensitivity of results obtained from studies involving SAPs in HPC is owing to the type of SAP used, the amount of additional internal curing water added and other factors which relate to the composition of the surrounding concrete or cement-based material (Mechtcherine, 2012). 2.3.1.1 Absorption

The SAPs used in concrete are covalently cross-linked polyacrylates and copolymerized polyacrylamides or polyacrylics which have an ionic nature and ionic interconnected structure (Jensen & Hansen, 2001b). This ionic nature allows water to be absorbed into these polymers. The cross-linking of the polymers in its dry and swollen state are schematically represented in Figure 2.46.

The kinetics of the water absorption of SAPs can simply be explained by the process of osmosis. From the dry SAP particle in Figure 2.46, it can be seen that ions in the polymer network are closely packed, resulting in the polymer having a high ionic concentration, or high osmotic pressure inside (Friedrich, 2012). When these polymers are suspended in water, which has a much lower ionic concentration, the water migrates into the polymer to decrease the osmotic pressure inside the initially dry particle. As the polymer swells while it absorbs water from outside, the osmotic pressure inside the particle is reduced. When the SAP is fully saturated, they act as stable, water-filled reservoirs which are able to release this free water back into the capillary pore network when needed (Lura et al., 2012).

Figure 2.46: Illustration of cross-linking of a dry and swollen SAP particle (Friedrich, 2012)

The rate of water migration from the concrete into the SAP particle is also explained by osmosis. Initially, the absorption of water into a dry SAP particle is rapid due to the high osmotic pressure gradient between the dry particle and the surrounding fluid. However, as the ionic concentration inside the SAP particle decreases with the ingress of water, so does the rate at which the water enters the polymer because of the progressively decreasing osmotic pressure gradient (Jensen & Hansen, 2001b).

2.2.1.2 Desorption

Although more information on the absorption kinetics of SAP is available, the desorption kinetics is perhaps more valuable. Albeit, not much is known about the physical desorption of SAP, as it is hard to observe. Desorption is mostly governed by diffusion and the rate of desorption is dependent on the internal relative humidity in the capillary pores (Mönnig, 2005).

Mönnig observed that for an internal relative humidty of 80%, the desorption rate is 0.032 mℓ/min and when the internal relative humidty dropped to 40%, the desorption rate increased to 0.10 mℓ/min. This is because internal curing water is released from SAPs when the internal relative humidity decreases in the capillary pore system due to hydration (Filho et al., 2012).

In addition to this, is the reverse direction of the osmotic pressure gradient as the pore solution surrounding the swollen SAP particles may have a higher ionic concentration at setting time (Lura et al., 2012). Mönnig (2009) described the absorption and desorption kinetics as a competition for water to relieve the ionic pressure between the inside of the SAP particle and the surrounding cement paste. Schröfl et al. (2012) found that SAPs that absorbed water quickly also desorbed it quickly. Fast desorption is not particularly desirable. Schröfl et al. (2012) further concluded that autogenous shrinkage was continuously reducing in mixes containing SAP that released internal curing water more slowly. The rapid absorbing and desorbing SAP only reduced autogenous shrinkage over an initial period and then ceased to be effective after a few hours.