Innovative Retarders for Controlling the Setting Characteristics of Fly Ash-Slag Geopolymers

Geopolymers, as sustainable alternatives to traditional cementitious materials, offer superior mechanical and durability properties: on the other side, they face challenges with rapid setting, particularly in fly ash-slag systems. Retarders play a crucial role in tailoring the setting behavior and workability of geopolymers, especially in applications where extended handling time or placement under challenging conditions is required. Geopolymers, unlike traditional Portland cement, undergo a rapid alkali-activation process involving dissolution, polymerization, and hardening of aluminosilicate materials. This can lead to very short setting times, particularly at elevated temperatures. In this scenario, the present study investigates the effect of different retarders - including cellulose, starch, borax and their different combinations - on the setting time. The effectiveness of a retarder depends on the geopolymer formulation, including the type of precursor, activator, and curing conditions. The initial and final setting time improved by the addition of retarders, whereas most of the retarders may have a negative effect on compressive strength. The optimum retarder combination was starch and borax with a remarkable improvement in setting time and a positive result on the compressive strength, with a reasonable workability. The retarder was equally effective under both ambient and oven-cured conditions, and for different mix proportions of Fly Ash (FA) and slag, indicating that its effectiveness depends only on the type of precursors used. The study reveals the use of borax along with cellulose/sugar-based compounds which will balance the reaction kinetics, resulting in balanced mechanical characteristics.