“Fly ash and GGBFS based powder-activated geopolymer binders: A viable sustainable alternative of portland cement in concrete industry”

Highlights

• Powder-activated geopolymer binder can be mixed and handled as conventional Portland cement.

• Geopolymer concrete can be set and harden in ambient curing conditions.

• Geopolymer concrete possess higher tensile and flexural strength than OPC concrete of same grade.

• Geopolymer concrete possess similar modulus elasticity to OPC concrete of same grade and can be estimated using Australian Standard 3600 (2009)

• Geopolymer concrete possess similar drying shrinkage to OPC concrete of same grade at ambient temperature curing.

Abstract

Geopolymer is a new binding material, synthesized by alkali activation of aluminosilicate compounds. Previous researches around the world postulated that concrete from geopolymer binder exhibited superior engineering, thermal and durability properties than ordinary Portland cement (OPC) concrete, such as higher mechanical strengths, higher resistivity to sulphate and acid attacks and higher thermal resistivity.

Geopolymers are generally made from activation of aluminosilicate powders by highly concentrated sodium hydroxide and or sodium silicate solutions, known as liquid-activated geopolymer. Recently, some cement and concrete companies around the world have commenced the production of liquid activator based geopolymer binders. However, this type of geopolymer does not appear as a viable replacement of Portland cement in concrete industry despite its substantial environmental benefits because of its limitation in mixing and handling process. Powder-activated geopolymer contains alkali activators in powder form which are blended together with source material. This geopolymer binder is physically similar to OPC and mixing and handling process are also similar to conventional process. Experimental results showed that concrete from this binder can set and harden in ambient temperature with significant early age strength.

Two types of powder-activated geopolymer binders having different proportions of fly ash and slag were used in this study. Engineering properties of geopolymer concrete of four different strength grades (40, 50, 65 and 80 MPa) were investigated in detail at ambient curing condition and compared with OPC concrete of same grade.

Introduction

Production of ordinary Portland cement (OPC) contributes around 5 to 7% of total global greenhouse gas emission (Hendriks et al., 1998, Meyer, 2009) which possesses a potential problem for the global environment. Several researches were done in the past in pursuit of environmental friendly alternative binding material. Geopolymer technology shows a considerable promises as an alternative binder to Portland cement as proposed by Davidovits (1989). Geopolymer is generally formed by reaction of an aluminosilicate compound of geological origins like clay and metakaolin or industrial by-products, such as fly ash and ground granulated blast furnace slag (GGBFS or slag) with an alkaline solution. The reaction of aluminosilicate powder with alkali solution produces a synthetic alkali aluminosilicate compound which is in amorphous to semi-crystalline state (Davidovits 1991) having good binding property like as calcium silicon hydrate (C-S-H) paste of Portland cement concrete. There are two major environmental benefits of using geopolymer binder over OPC; potential reduced greenhouse gases emissions and utilization of industrial by-products. Geopolymer technology can utilize industrial by-products to produce a good construction material. Considering those benefits, geopolymer is considered as a sustainable binding material (Li et al., 2004, Lloyd and Rangan, 2009).

In last two decades, several studies have been carried out globally on geopolymers; however, majority of these studies were focused on material characterisation, physical and chemical properties of the material and the effects of source material (Duxson et al. 2007). Recently, more focus is given on investigation of compressive strength and other mechanical properties of geopolymer concrete (Diaz-Loya et al., 2011, Hardjito and Rangan, 2005, Sofi et al., 2007). However, these data are still not sufficient to identify the relationship between different mechanical properties of geopolymer concrete. The application of geopolymer concrete in structural elements is in very limited numbers so far because of insufficient proven data.

Geopolymers are generally made from activation of aluminosilicate powders by highly concentrated sodium hydroxide and or sodium silicate solutions, thus called liquid-activated geopolymer. Recently, some cement and concrete companies around the world have begun the production of geopolymer binder under different brand names, such as E-CreteTM (Zeobond 2008), Earth Friendly Concrete, (Wagners 2010) and BanahCEM (Banah 2011). However, liquid-activated geopolymer does not appear as a viable replacement of Portland cement in concrete industry despite its substantial environmental benefits and superior engineering properties because of the difficulties in mixing and handling of this binder. (Hajimohammadi, Provis & Van Deventer 2008).

Sodium or potassium alkalis are the mostly used as activators in geopolymer binder (Diaz-Loya et al., 2011, Fernandez-Jimenez et al., 2006b, Hardjito and Rangan, 2005). There are two major limitations of liquid activated (two- part) geopolymer binder having sodium or potassium hydroxide and sodium silicate as activator. Both sodium and potassium hydroxide solutions are hazardous materials. They can cause severe burns to the human body on contact (ERCOWorldwide 2012). Sodium silicate solution is also a moderately hazardous material, that can cause chemical burns with contact or ingestion (Hartford 2010). Therefore, transporting and handling of these liquids can cause a potential physical risk of workers. Concentrated sodium hydroxide and potassium hydroxide solutions are corrosive to materials such as tin, aluminium, zinc, copper, lead and their alloys and also attack glass and some types of plastics (Helmenstine, 2013, Keith 2008). Liquid sodium hydroxide absorbs carbon dioxide from air and gradually changes to sodium carbonate (OxyChem 2013). They should be stored very securely in air tight vessels made of non-reactive martial.

Change in concentration of alkali activator in the binder can bring differences in the geopolymerization process and properties of concrete as an end product (Mishra et al., 2008, Somna et al., 2011). Increase in concentration of sodium or potassium hydroxide generally accelerates the geopolymerization reaction and results higher strength of concrete but decrease in workability (Chindaprasirt et al., 2007, Ryu et al., 2013). Therefore a skilled manpower is needed for mixing and handling of this type of geopolymer binder in order to maintain the desired concentration of liquid activator.

The limitations of liquid-activated geopolymer concrete have prevented its common utilization in the concrete industry although concrete from this binder possesses superior engineering properties with added environmental benefits. A powder form of geopolymer binder which can be handled and stored safely and mixed in similar way to conventional OPC concrete is therefore needed for a potential repalcement of OPC (Hajimohammadi et al., 2008).

In powder-activated geopolymer binder, both activators (powder form), and source materials are blended together in a fixed proportions to make one-part binder. This type of geopolymer binder can be safely mixed and handled in the similar way to Portland cement because of having chemical activator in solid (powder) form. The mixing process of this binder is very similar with conventional OPC concrete; addition of potable water for desired level of workability. Class F fly ash is the commonly used source material in combination with ground granulated blast furnace slag (GGBFS).

There were two major objectives of developing this binder. The first objective was to develop a binding material that has lower embodied energy by utilizing industrial by-products such as fly ash and GGBFS. The second objective was to make a binder that would be physically similar to conventional Portland cement, such that mixing and handling process would be safe and easy. In order to be used in concrete industry, geopolymer binder can be mixed in the similar way to Portland cement and capable of setting and hardening at ambient conditions (Kidd 2009).

Class F fly ash or metakaolin based geopolymer concrete takes very long time to set and harden at ambient conditions, and therefore most previous research were carried out by heat curing of geopolymer concrete and mortar (Ahmed et al., 2011, Fernandez-Jimenez et al., 2006b, Hardjito and Rangan, 2005, Rovnaník, 2010). In order to be utilized in construction, concrete should able to set and harden at ambient temperature. In general construction, concrete elements are cured in ambient temperature; elevated temperature curing is not practicable and not cost effective. Till date, publish engineering properties of geopolymer concrete cured at ambient temperature are still limited. Replacement of fly ash by GGBFS can make geopolymer concrete set and harden in normal temperature with significantly reduced setting time and higher early age as well as later strength (Ganapati Naidu et al., 2012, Nath and Sarker, 2012, Parthiban et al., 2013). It is suggested that presence of calcium compound accelerate the geopolymerization process (Catalfamo et al. 1997) and there is the coexistence of geopolymeric gel and calcium-silicate-hydrate (C-S-H) in slag-fly ash based geopolymer paste, where C-S-H gel is responsible for developing early age strength (Oh et al., 2010, Yip et al., 2005)

Fly ash and GGBFS based powder-activated geopolymer binder can offer following advantages over liquid –activated geopolymer.

• Concrete mixing and handling process is easier than using liquid-activated geopolymer binders and is similar to conventional Portland cement.

• Storing and transporting are safer than liquid-activated geopolymer binder.

• Properties of concrete are more consistent due to fixed proportions of ingredients in binder.

• Lower workability loss (up to 2 hours) of concrete than from liquid-activated geopolymers. This is due to slowly release of alkali into the binder system and slower rate of initial reaction than in liquid-activated geopolymer.