Synergy of retarders and superplasticizers for thickening time enhancement of hematite based fly ash geopolymers

Geopolymers are an evolving technology to replace ordinary Portland cement (OPC) in oil and gas industry. Geopolymers can be formed using aluminosilicate materials such as some waste and rock-based materials. Heavy weight geopolymers suffer from low thickening times especially at high temperatures. Thickening time is affected by downhole pressures and temperatures, the latter has the most significant effect. As downhole temperature increases, the cement slurry sets faster. Thickening time is a critical parameter for cementing operations to avoid risks of an early or a delayed setting. This study focuses on increasing thickening time of hematite-based class F fly ash (FFA) geopolymers at downhole temperatures. Firstly, the effect of hematite as a weighting agent on FFA geopolymer thickening time was investigated. Next, different combinations of retarders, dispersants and superplasticizers were tested to prolong the thickening time of hematite based FFA geopolymers. After that, the effects of the proposed mixture on the rheological and mechanical properties were assessed. The thickening time of the hematite based FFA geopolymer decreased from 430 to 80 min with increasing hematite from 25 to 75% by weight of binder (BWOB). For the 75% hematite geopolymer, a proposed mixture of a poly naphthalene sulfonate (PNS) superplasticizer and a modified lignosulfonate retarder increased the thickening time from 80 to 580 min at 195 °F. The proposed additives mixture developed a formulation with the highest unconfined compressive strength (UCS) and Poisson's ratio (PR) as compared to other formulations that use each additive separately. In addition, this mixture decreased the plastic viscosity of the high density geopolymer slurries by 29% which facilitates mixing and pumping. This study introduced a solution to thickening time and pumpability challenges associated with heavy weight fly ash geopolymers at high temperatures.