Concrete is the most widely used construction material in the world. It is primarily produced using Portland cement as a binder in the ratio of about 10-15% by mass of the resulting concrete. Portland cement production raises environmental concerns over the emission of carbon dioxide. It has been estimated that cement production contributes to approximately 5-7% of the current carbon dioxide emission globally.
This has led to the adoption of waste materials including grounded granulated blast furnace slag and fly ash as better alternatives to the Portland cement due to their ability to improve the chemical, physical, and mechanical attributes of concretes and to reduce the environmental impact. Recent research works have indicated that it is possible to manufacture geopolymer concretes based only on waste materials employing an alkaline activator without using Portland cement. A major upside of geopolymer concrete is that a reduction of about 26-45% carbon emission and Portland cement replacement are realized without major economic impact.
In the process of geopolymerization, silica and alumina species in fly ash react with the alkaline activator solution producing a 3-D polymeric chain and a ring structure. The final geopolymer product is sodium-aluminosilicate gel. It is the composition of this gel that dictates the attributes of low calcium fly ash geopolymer concrete. In alkali-activated slag concrete, on the other hand, calcium silicate hydrate gel is the principal product of the geopolymerization process. This is identical to the principle binder of the Portland cement as well as that of blended cement concretes.
In order for both alkali-activated slag and fly ash geopolymer concretes to work as proper construction materials, it is necessary for both to maintain their performance over the service life of a structure. Chamila Gunasekara, David Law and Sujeeva Setunge at RMIT University in Australia in collaboration with Arie Wardhono at the State University of Surabaya in Indonesia undertook an experimental research program with the aim of investigating an array of mechanical as well as durability characteristics of alkali-activated sag and fly ash geopolymer concretes up to 540 days. The authors assessed flexural and splitting tensile strength, elastic modulus, compressive strength, water permeability and water absorption. Their research work is published in the journal, Construction and Building Materials.
The authors observed that the compressive strengths of fly ash geopolymer and alkali activated slag concrete was in the range of 22-33 MPa and 39-40MPa from 28 to 540 days, respectively. They also observed 48% and 2% increases in compressive strength in the two concrete, respectively during the assessment period.
In addition flyash geopolymer concrete demonstrated approximately a 53% flexural strength increase between 28 and 540 days, as opposed to an approximate 13% decrease for the case of alkali-activated slag. The fly ash geopolymer concrete also achieved twofold splitting tensile strength evolution while alkali-activated slag concrete remained constant.
Alkali activated slag concrete exhibited high elastic modulus than the fly ash geopolymer concrete in the first 3 months. However, the value of the elastic modulus dropped drastically with time and recorded a 43% drop from 28 to 540 days. On the contrary, fly ash geopolymer concrete showed about a 98% increase in elastic modulus in the same period.
Finally fly ash geopolymer concrete had a high water permeability index compared to the alkali-activated slag concrete in the first 3 months. Furthermore, the index reduced considerably with age, unlike the slag concrete.
References
Arie Wardhono, Chamila Gunasekara, David W. Law, Sujeeva Setunge. Comparison of long-term performance between alkali activated slag and fly ash geopolymer concretes. Construction and Building Materials, volume 143 (2017), pages 272–279.
