Benefits of Sealed-Curing on Compressive Strength of Fly Ash-Based Geopolymers.

Sujeong Lee,Arie Van Riessen,Chul-Min Chon

doi:10.3390/ma9070598

Abstract

There is no standardized procedure for producing geopolymers; therefore, many researchers develop their own procedures for mixing and curing to achieve good workability and strength development. The curing scheme adopted is important in achieving maximum performance of resultant geopolymers. In this study, we evaluated the impact of sealed and unsealed curing on mechanical strength of geopolymers. Fly ash-based geopolymers cured in sealed and unsealed moulds clearly revealed that retention of water during curing resulted in superior strength development. The average compressive strength of sealed-cured geopolymers measured after 1 day of curing was a modest 50 MPa, while after 7 day curing the average compressive strength increased to 120~135 MPa. In the unsealed specimens the average compressive strength of geopolymers was lower; ranging from 60 to 90 MPa with a slight increase as the curing period increased. Microcracking caused by dehydration is postulated to cause the strength decrease in the unsealed cured samples. These results show that water is a crucial component for the evolution of high strength three-dimensional cross-linked networks in geopolymers.

Highlights

The procedures and formulations that are employed to manufacture geopolymers are not standardized
The alkali hydroxides and sodium silicate are most frequently used as alkaline activators to generate a high pH and activate source materials
It has been demonstrated that formulating the mixture based on amorphous composition of fly ash improves the mechanical performance of fly ash-based geopolymers [3,4,5]

Summary

Introduction

The procedures and formulations that are employed to manufacture geopolymers are not standardized. Low-calcium fly ash, blast-furnace slag and metakaolin are the most widely used silicon and aluminium source material for producing geopolymers. Natural pozzolanic materials are used to produce geopolymers [1,2]. Many studies experimentally derive their own formulations and procedures for mixing and curing to achieve adequate workability and subsequent strength development of geopolymers. Many researchers consider bulk SiO2 /Al2 O3 and Na2 O/Al2 O3 or Na2 O/SiO2 molar ratios to produce optimum mix proportions. Aside from variation in geopolymer formulation there are numerous curing schemes incorporating changes in temperature, and time, including recently-reported intermittent curing [6]. Some researchers start with ambient temperature curing and follow this with heat curing.

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