Hygral Behavior of Superabsorbent Polymers with Various Particle Sizes and Cross-Linking Densities

Abstract

This study focuses on investigating the effects of particle size and cross-linking density on the hygral behavior of superabsorbent polymers (SAPs), which are increasingly used as an internal curing material for high-performance concrete. Four SAPs with different mean particle diameters and cross-linking densities were tested under controlled wetting and drying conditions to measure free absorption and desorption kinetics. Absorption capacities of SAPs under actual mixing conditions were additionally measured and verified by means of mortar flow and semi-adiabatic hydration heat measurements. In addition, the effects of SAP type and dosage (i.e., 0.2, 0.4, and 0.6% by mass of cement) on the mechanical properties of hardened mortar were assessed. The results indicated that: (1) the absorption capacity increased with decreased cross-linking density and increased particle size under both load-free and mixing conditions; and (2) the greater the cross-linking density and the lower the particle size, the shorter the desorption time. It was also confirmed that while the early-age mechanical properties were more related with the gel strength of swollen SAP, the later-age mechanical properties were more affected by the water retention capacity and spatial distribution of SAP in the matrix.

Internal Relative Humidity, Autogenous Shrinkage, and Strength of Cement Mortar Modified with Superabsorbent Polymers

“…First, it can be seen that SAP C, which has the greatest cross-linking density and largest particle size, maintained the highest IRH profile over the measurement period. The most probable reason is that the more numerous the cross-links between polymer chains, the higher the elastic retraction forces are applied to the network structure, rapidly squeezing out the water absorbed in SAP to the matrix [37]. On the contrary, SAP D, having a medium grain size and cross-linking density, exhibited the greatest IRH drop during the first 50 days.…”

“…The mixture proportions for plain and SAP-modified mortars used in this study are shown in Table 4. Three different levels of SAPs (i.e., 0.2%, 0.4%, and 0.6% of cement by weight) were used for SAP D. The total water content of each mixture was adjusted per the absorption capacities of SAPs measured under mixing condition so as to keep the workability comparable with each other [37]. All the mixtures were fabricated at an effective water-cement ratio ( w / c ) of 0.30 and a fine aggregate-to-cement ratio of 2.75.…”

“…SAP is a cross-linked polymeric substance that can momentarily absorb a substantial amount of water or aqueous solutions (up to 500 g within a couple of minutes). In the concrete industry, since Jensen and Hansen initiated a pioneering work on “water-entrained concrete” using SAP [15,16], numerous follow-up studies have been done in the last few decades to seek potential possible uses of SAPs as an internal curing agent for cement-based materials, for instance, autogenous shrinkage mitigation [16,17,18,19], later-age mechanical properties improvement [20,21,22,23], rheology modification [24,25,26], self-sealing and healing capability [27,28,29,30,31], shrinkage crack control [20,32], coefficient of thermal expansion (CTE) control [33], frost damage mitigation [34,35], and absorption and desorption kinetics under alkaline environment [17,36,37]. The efficiency of IC is closely concerned with the absorption and desorption kinetics of SAP.…”

Mechanical and Thermal Performance of Cement Mortar Incorporating Super Absorbent Polymer (SAP)

“…Moreover, differential moisture loss produces humidity gradient since moisture loss is higher at the surface of the plaster compare to that of in the core of mortar plaster. The osmotic pressure increases with the decrease in humidity inside the mortar and the water absorbed by SAP is progressively released once the relative humidity of the mortar declines below 90% [11,12]. As a result, SAP contributes to maximize the cement hydration in cement mortar [13][14][15].…”

“…Moreover, the hydrogel, produced as a result of water absorbed by the SAP, has very low stiffness compare to the hydrated cement matrix [2] and thereby decreases the overall compressive strength of cement mortar [5,38]. Therefore, major work in this field suggested that most of the SAP mixtures have an adverse effect on the compressive strength of mortar compared to the reference mixture [12,39,40], particularly at the early phase of mortar age [41]. The decrease in compressive strength also attributed to the overestimation of the absorption capacity of SAP [36].…”