Pore structure of affected zone around saturated and large superabsorbent polymers in cement paste

Superabsorbent polymer (SAP) has been widely used as internal curing agent for cement-based materials, duo to its high internal curing efficiency. Pore structure of affected zone around SAP is important for understanding the internal curing mechanism. This paper presents a study of the pore structure evolution of affected zone around SAP in cement paste with water/cement ratio 0.24 and 0.30 from 3 up to 28 days. Pore structure of affected zone and matrix was tested by mercury intrusion porosimetry and nitrogen adsorption and desorption. Surface fractal dimensions were determined using two different fractal models. Results indicated that SAP had no effect on the pore shape of affected zone, excluding the pore volume and pore diameter at early and late ages. Higher critical pore diameter but lower pore volume were found in affected zone at late ages. All the samples showed a multi-fractal property at micro- and macro-fractal scale, and the surface fractal dimension of affected zone was always larger than the matrix, showing a higher pore complexity. Some new findings on internal curing were achieved from an affected zone perspective. The calculated mechanical strength and permeability of affected zone from the pore structure parameters were found higher than the matrix. These new findings may give insight into the effect of internal curing with SAP from the perspective of affected zone.