Enhancing anti-washout behavior of cement paste by polyacrylamide gelation: from floc properties to mechanism

Polyacrylamide (PAM) is one of the commonly used anti-washout admixtures (AWAs) in cement-based materials. However, PAM chains tend to be contracted under attacks from alkaline and metal ions in cement paste, resulting in low efficiency of washout resistance enhancement. Herein, the gelation of PAM is implemented to surmount this obstacle, in which the influence of gelation on the floc properties is evaluated by the light scattering method and the enhancing mechanism is explored as well. Unlike the contraction of PAM chains in cement paste, the resulting PAM-based hydrogel can unfold a three-dimensional network that enlarges the contact surface with cement particles and promotes the bridging interaction. As a result, the gelation of PAM essentially improves the floc strength and helps maintain its originally compact floc structure with an unchanged fractal dimension of approximately 2.50 under the elevated shear rates. Furthermore, fresh cement paste with the addition of 0.006% PAM-based hydrogel achieves a decrease of 10% in washout mass loss compared to that with the same dosage of PAM. The result of breakage-regrowth experiment shows the PAM-based hydrogel facilities a rapid reaggregation with a recovery factor of 76%, while the flocs in PAM modified cement paste are irreversible after breakage as its recovery factor is only 37%. This understanding is of great importance to chemical admixtures where gelation is a promising way to promote their interaction with cement particles. Moreover, floc properties provide a new perspective on understanding the early-stage behavior of fresh cement paste.