Synergistic effect of shrinkage mitigating materials on rheological properties of flowable and thixotropic cement paste

This study investigates the effect of individual and combined additions of CaO-based expansive agent (EA), shrinkage reducing admixture (SRA), and super absorbent polymer (SAP) on key characteristics of flowable cement paste mixtures proportioned with a water-to-cement ratio of 0.40. Of special interest is the effectiveness of these admixtures to mitigate shrinkage of cementitious materials for 3D printing. Static and dynamic yield stress, plastic and apparent viscosities, and thixotropy were evaluated at 20-min intervals up to 90 min. Compressive strength development and autogenous shrinkage were also determined. A factorial design approach was developed to evaluate the synergetic effects of the shrinkage mitigating materials (SMM) on performance. Test results showed that the use of any given SMM increases the static and dynamic yield stress, plastic viscosity, and thixotropy. The individual use of SRA and the combination of EA-SRA presented the highest effect on reducing 20-min plastic viscosity and dynamic yield stress, respectively. The combinations of EA-SAP almost tripled the 90-min static yield stress. The individual or combined use of the SMM, especially EA and SAP, significantly reduced autogenous shrinkage. Such systems, however, reduced the 28-d compressive strength by 14%-65%. The lowest value was obtained with the paste made with EA-SRA. A multi-objective optimization based on the measured properties was performed. The results revealed that the use of 10% EA either with 0.125% SAP or 1% SRA can result in producing flowable and thixotropic cement paste with high pumpability and buildability for 3D printing applications, no autogenous shrinkage, and high strength.