Parameter Study of Superabsorbent Polymers (SAPs) for Use in Durable Concrete Structures

Abstract

Superabsorbent polymers (SAPs) can be added to a concrete mixture to provide internal curing and reduce the risk for early-age shrinkage cracking. Hence, they can help to increase the overall durability of concrete structures. The type, swelling characteristics, kinetics of water release, amount and particle size of the SAPs will dictate their effectiveness for this purpose. In this paper, SAPs with different cross-linking degrees, particle sizes and amount of solubles are investigated. By varying these parameters, insight can be gained on the influence of each of these parameters on SAP properties such as the swelling capacity. In a next step, the SAPs can be implemented in mortar to assess their influence on mortar properties like workability, compressive strength or hydration kinetics. Based on these results, the ‘ideal’ SAP with tunable properties for a specific concrete application can be selected. For this purpose, an anionic SAP was synthesized with varying amounts of cross-linker and ground to particle sizes with d50 varying between 10 and 100 µm. The swelling capacity in demineralised water of 40 µm SAP particles increased with a decreasing degree of cross-linker from 66 g/g SAP with 1 mol% cross-linker to 270 g/g SAP in case of 0.15 mol% cross-linker, and was about three to four times larger than the swelling capacity in the prepared cement filtrate. The SAPs were tested for their effect on mortar workability, cement hydration kinetics and mechanical properties of the hardened mortar. With proper compensation for the absorbed water by the SAPs, the mortar workability was not negatively affected and the reduction in flow over the first two hours remained limited. The SAPs with the lowest swelling capacity, resulting in the smallest total amount of macro pores formed, showed the smallest negative effect on mortar compressive strength (a reduction of 23% compared to the reference after 28 days for an addition of 0.5 m% SAP) and a negligible effect on cement hydration. The difference in strength with the reference decreased as a function of mortar age. When using SAPs with particle sizes in the range of 10–100 µm, no significant differences between the studied particle sizes were found concerning the mortar properties. With the ease of upscaling in mind, the need to purify the SAPs and to remove the non-cross-linked soluble fraction was further investigated. It was shown that the solubles had no effect on the mortar properties, except for increasing the setting time with almost 100%.

The Use of Superabsorbent Polymers in High Performance Concrete to Mitigate Autogenous Shrinkage in a Large-Scale Demonstrator

“…The final setting time measured by penetrometer was 5.5 h for the REF mixture and 9 h for the SAP mixture. In the case of SAP addition, there is a delay in setting time, which is in accordance with earlier findings of the authors [31], and with findings reported in the literature [12,47,48].…”

“…In addition, Ca 2+ (or any divalent cation) can act as cross-linker which offers an additional reduction of the swelling properties; the higher the cross-linking degree, the lower the swelling capacity [28][29][30]. The evolution in time of the swelling capacity of SAPs in demineralized water and in cement filtrate for different types of SAPs and concentrations can be found in [31]. Therefore, the decrease in the swelling capacity of the SAPs used in this research at 24 h, could be linked to the additional change in the ion concentration with time of the cement filtrate solution that leads SAPs to be highly cross-linked.…”

Influence of Environmental Factors on the Swelling Capacities of Superabsorbent Polymers Used in Concrete

“…At the same time, the hydration process is based on exothermic reactions causing the temperature to increase [ 22 ]. Both effects have a strong impact on the durability [ 23 ] and performance of the hardened concrete. The utilisation of SAPs can solve these problems by acting as an internal on-demand water reservoir [ 24 ].…”

Experimental Study on the Permeability of SAP Modified Concrete

“…SAP can be used as an internal curing material for low water cement ratio concrete. After absorbing water, the SAP particles become distributed uniformly in concrete, thus effectively providing the hydration water in the course of concrete hydration as well as effectively reducing the autogenous shrinkage of concrete and preventing concrete cracking [ 5 , 6 , 7 ]. SAP can also be used as an air-entraining agent, thereby improving the freeze-thaw resistance of concrete [ 8 , 9 , 10 , 11 ].…”