Investigations on the properties of geopolymer mortar and concrete with mineral admixtures: A review

Highlights

• A comprehensive review of geopolymer mortar and concrete with mineral admixtures.

• A total of 126 research papers spanning a period of 22 years are reviewed.

• Mineral admixtures effects on fresh, mechanical and durability properties.

• Use of OPC, GGBFS, Nano-silica, Metakaolin, and Alccofine in Geopolymer products.

• Microstructure analysis of geopolymer mortars and concrete is reviewed.

• Research gap has been identified and elaborated.

Abstract

The climatic changes associated with global warming due to greenhouse gases emission from cement production attracted researchers towards geopolymer based cementitious materials. Nowadays researchers are in a continuous process to improve the mechanical properties of geopolymer mortar and concrete at ambient temperature curing condition so as to broaden its scope of utilization in building construction. The review paper summarizes the effect of various mineral additives on mechanical, durability and microstructure characteristics of geopolymer mortar and concrete. Research findings revealed that geopolymer products blended with these materials showed a significant improvement in mechanical and durability properties at normal temperature conditions. The results advocate that geopolymer mixtures with desired properties can be designed for ambient temperature curing condition with minerals additives which may further promote them as an environmentally friendly construction material.

Introduction

Globally, concrete is still a leading construction material owing to its wide-ranging mechanical properties, better long-term performance, easy application, and cost-effectiveness. The concrete production which is estimated to be approximately one cubic meter per capita requires the manufacturing of cement on a massive scale [1]. The global cement demand is estimated at over 4.216 billion metric tons in 2018 as per International Cement Review Research Report which further requires nearly 9.476 × 10⁷ Joules/ton of energy consumption in its production process [1]. It is estimated that carbon dioxide released during cement manufacturing process contributes approximately 5 to 7% of the total carbon dioxide released in the environment which is considered as a prominent reason to accelerate global warming.

Growing industrialization is also responsible for the release of waste by-products such as fly ash, rice husk ash, ground granulated blast furnace slags, which are pozzolanic in nature. Environmentally compatible dumping of these waste materials requires suitable techniques. The world earth summits also warned cement industry to switch over from Portland cement to a greener alternative binder with desirable structural as well as durability properties so that the increased emission of greenhouse gases to the atmosphere can be controlled.

Researchers globally explored the ancient construction materials and found that geopolymer materials were used in Egyptians pyramids during 2630 BCE–2611 BCE [2]. In this quest, Joseph Davidovits, a French material scientist proposed a term ‘geopolymer’ to represent a broad range of materials characterized by chains or networks of inorganic molecules and also pointed out the possibility of its use as a binder material in concrete. The special features of the geopolymer, such as the development of high early strength and better resistance to chemical attacks fascinated the scientific community. Geopolymer materials require raw materials which are rich in silicon (Si) and aluminum (Al) and alkaline activator solutions. The binding properties of these materials are obtained by the process of polymerization which primarily differentiates them from the conventional cement based binder materials. Geopolymeric materials don’t require cement as a binder which emits greenhouse gases, so it makes it an environmentally friendly material.

Geopolymers may address the issue of disposal of industrial waste materials by utilizing industrial by-products such as rice husk ash, ground granulated blast furnace slag (GGBFS), fly ash, silica fume, and red mud as a source material. Sodium hydroxide (NaOH) or potassium hydroxide (KOH) and sodium silicate (Na₂SiO₃) solution can be used to activate the source materials. Another important aspect of geopolymers is that water plays no significant role in the chemical reaction but helps in producing a workable mixture which reduces the water requirement [3]. Geopolymeric gel obtained from the polymerization process possess cementitious properties and act as a binder in geopolymer concrete [4], [5].

The curing process adopted for geopolymer concrete (GPC) differentiates it from conventional cement concrete. Instead of water curing, heat curing i.e. steam curing and air curing is generally used in geopolymer concrete. The heat curing process is generally used to activate the polymerization reaction. Geopolymer concrete cured at ambient temperature could not achieve desirable compressive strength [6], [7], [8]. Therefore most of the research on geopolymer concrete was conducted by using electric hot air oven technique for elevated temperature curing [9], [10]. The fly ash based geopolymer concrete when exposed to heat curing system could achieve desired mechanical properties [11], [12], [13], [14] but lacked in case of normal room temperature curing conditions [8], [15]. A significant share of research used heat cured conditions for curing of geopolymer concrete and the curing temperature was generally between 60 °C and 100 °C [10], [16], [17], [18], [19]

The heat curing mechanism requires special arrangement which may not be cost-effective and limit the GPC uses in precast construction practices. Therefore to broaden the scope of geopolymer based concrete in general construction use, researchers focussed their attention towards developing GPC at ambient temperatures which may be cost-effective as well as easy to practice.

Several Researchers observed that the mechanical strength and durability properties of geopolymer concrete can be significantly enhanced with the blending of ordinary Portland cement [20], ground granulated blast furnace slag [21], [22], nano-silica [23], [24], [25], [26] and Alccofine [6], [27] at ambient temperature curing.

This paper presents a comprehensive review of the enhancement of properties of geopolymer mortar and concrete with the inclusion of the various type of mineral admixtures at ambient curing conditions.