Kinetics of SAPs During Hardening, Drying and Healing in Cementitious Materials Studied by NMR

Abstract

At early ages, plastic shrinkage can arise when a cement paste is subjected to harsh drying conditions during hardening. Furthermore, when using a low water-to-binder ratio, the cementitious material may show autogenous shrinkage during setting. Super Absorbent Polymers (SAPs) are a promising admixture to mitigate shrinkage in cement pastes. By introducing internal curing by means of the stored mixing water in the SAPs, the plastic shrinkage can be partially mitigated, next to the mitigation of autogenous shrinkage during setting of the cement paste. The kinetics of water release by the SAPs towards the cementitious matrix are an important factor. To effectively and non-destructively monitor the effects induced by the SAPs during the plastic and hardening period as a function of time, Nuclear Magnetic Resonance (NMR) was applied. Using NMR, a clear distinction could be made in terms of the free water signal and the entrained water signal for SAP particles. The SAPs are able to protect the cement paste internally from the harsh ambient drying conditions by sustaining the internal relative humidity. The plastic settlement was reduced and there was less plastic shrinkage measured. By mitigating shrinkage, shrinkage cracking can be partially prevented. However, upon acting mechanical stresses, the cementitious material may crack nevertheless. SAPs are also interesting to first seal a crack due to their swelling capacity and to heal cracks afterwards by promoting autogenous healing. This healing was also monitored by NMR as a function of healing cycles and the amount of healing products formed were estimated based on the water signals obtained by NMR. Part of the water going into the crack was used to trigger further hydration of unhydrated cement particles. Healing in wet/dry cycles was stimulated by means of SAPs and healing at high relative humidity conditions only occurred in samples containing SAPs.