Composite application of naphthalene and melamine-based superplasticizers in alkali activated fly ash (AAFA)

Abstract

Workability could be a serious drawback for fresh alkali activated fly ash (AAFA) due to its high viscosity caused by alkaline activators. However, the reported optimum superplasticizers (SPs) only produced a slight improvement of fluidity while brought ambivalent effect on the strength in final NaOH-activated AAFA products. To obtain AAFAs with high fluidity and strength, 8M NaOH-AAFAs with low w/b at 0.24 were synthesized, and the effect of composite SPs consisted of naphthalene and melamine on fluidity and strength was investigated. Compared with naphthalene and melamine used separately, the composite SPs gave a maximum reduction in plastic viscosity, corresponding to the largest mini-slump and the lowest fluidity collapse. The workability improvement caused by composite SP was provided from the synergism of melamine and naphthalene, as the significant improvement of melamine on initial fluidity and the excellent fluidity retention supplied by naphthalene. Moreover, the application of composite SPs at lower dosage (1%) could give a significant improvement on the compressive strength of AAFAs. Further, the stability and water reduction mechanism of composite SP were also assessed, and results revealed that the decomposition of sulfonate in naphthalene and triazine ring in melamine was delayed; indicating naphthalene and melamine could maintain their structural stability within 60 minutes when adopted together. In addition, the steric resistance effect cannot be ignored in the dispersion mechanism of composite SPs, while the electrostatic repulsion played a dominant role when naphthalene and melamine used separately. The outcome of this study will be supportive to the ongoing research related to the construction performance of AAFAs in the aim of promoting their in-situ applications