Long-Term Strength Evolution in Ambient-Cured Solid-Activator Geopolymer Compositions

Vincent Wong,Aditya Rawal,Benjamin Fishburn,Charles Christopher Sorrell,William Jervis,William Joe,Pramod Koshy,Takafumi Numata

doi:10.3390/min11020143

Abstract

The major downsides of cement manufacturing are the high CO2 emissions and high energy usage. Geopolymers, which are fabricated by activation of blends of fly ash (FA) and ground granulated blast furnace slag (GGBFS) using an alkaline activator, offer a promising solution to this issue. However, to enhance the replacement of cement in construction applications, geopolymer compositions have to be designed such that they can be activated on site by just adding water, similar to how cements are used. Therefore, the present work uses solid sodium metasilicate (MS, Na2SiO3) as the alkaline activator in order to design an add-water-style FA/GGBFS-based geopolymer composition. These compositions were designed by optimising the binder (FA/GGBFS) ratio, Na2SiO3/binder ratio, and water/binder ratio individually to assess the effects of these parameters on the setting times and mechanical (flexural and compressive) strengths over extended curing times (three months). The major factors affecting the strength development and setting times (initial and final) were the amounts of GGBFS and Na2SiO3, with the former demonstrating the more dominant effect. The consistent strength development with curing time was attributed to calcium aluminium silicate hydrate (CASH) gel formation in the early curing times which was affected by the slag addition levels, and sodium aluminium silicate hydrate (NASH) gel formation at later curing times which was influenced by the metasilicate addition levels. The metasilicate amounts were observed to impact on CASH gel formation in early stage curing. Geopolymer compositions with FA/GGBFS ratio of 35/65 and MS/water ratios of 0.2 showed high compressive strengths of ~70 MPa at 28 days, which are superior to values seen in conventional ordinary Portland cement (OPC) mixes for the same curing times.

Highlights

Concrete is the most widely used building material in the world, owing to its low cost and ease of availability of raw materials
* N/A = not available or provided clearly in paper; ** dash (–); = not tested. These results show that geopolymers can be successfully produced using only solid activators under ambient conditions resulting in good compressive strengths
(60/40/0.25), the strength to increase to the delayed formation gel and its contribution to later-age strength development

Summary

Introduction

Concrete is the most widely used building material in the world, owing to its low cost and ease of availability of raw materials. Geopolymers commonly represent the crystalline products of the reactions of aluminosilicate pozzolanic raw materials with alkaline hydroxide and/or alkaline silicate solutions. The resultant blends are activated by an alkali solution resulting in the formation of a binder gel. Both fly ash and slag are by-products of industrial processes; fly ash is obtained from the electrostatic precipitators in coal combustion power plants while slag is the by-product of iron production in blast furnaces [4]. Geopolymers provide a functional use for these waste materials that would otherwise occupy the limited storage available at industrial sites or in landfill

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Results

Conclusion