Preparation of Lignin-Based Superplasticizer by Graft Sulfonation and Investigation of the Dispersive Performance and Mechanism in a Cementitious System

A practical
graft sulfonation process was developed to synthesize
a lignin-based superplasticizer using acid-precipitated lignin from
wheat straw black liquor as raw material. The graft-sulfonated lignin,
prepared in optimal reaction conditions, was named GSL. The properties
of GSL in a cementitious system were investigated. The adsorption
isotherms of GSL fractions separated by ultrafiltration and the thickness
of their absorbed films on cement particles were measured to reveal
the dispersion mechanism. Also, it was found that the molecular weight
of graft-sulfonated lignin increased with the dosage of acetone and
formaldehyde, but having too high a molecular weight reduced its dispersive
performance. High sulfonic group content in graft-sulfonated lignin
severely inhibited the increase of molecular weight, resulting in
a decrease of dispersive performance. GSL has stronger compressive
strength enhancement in concrete and lower hydration heat temperature
than the commercial naphthalene-sulfonated formaldehyde superplasticizer.
Moreover, the strong dispersion of GSL with high molecular weight
is mainly attributed to strong steric hindrance among cement particles.