## 1 Introduction  
Owing to the increasing number of high fire risk infrastructures built around the world, including concrete tunnels, petrochemical plants, nuclear reactors, and oil refineries, concrete structures with a relatively high strength capacity and superior fire resistance are in high demand. Reactive powder concretes (RPC) were first developed in the early 1990s by a French Corporation [1]. They are currently used as an ultra-high performance concrete, where compressive strengths in the range of 150-800 MPa, tensile strengths between 6-13 MPa, flexural strengths in the range of 30-60 MPa and fracture energy in the range of 1200-40,000 J/m\({}^{2}\) can be achieved [1; 2; 3; 4]. Related studies also reported that further enhancements in mechanical properties can be achieved through the reduction in the water to cement (w/c) ratio, exclusion of large aggregate particles, decrements in the calcium oxide to silicon dioxide ratio and through the addition of steel fibres as the reinforcement agent [5; 6]. Typical dry ingredients of RPC include cement, as the binding material, together with silica fumes, an ultra-fine spherical shaped material, having an average diameter of about 0.15 \(\upmu\)m and aggregate fillers, such as silica flour and fine sand/quartz particles, typically less than 600 \(\upmu\)m in size [3; 6]. The w/c ratio of RPC is reported to be between 0.1-0.25, whereas conventional concretes consist of a w/c ratio between 0.35-0.7 [6]. Superplasticizers (SP), which reduces the use of water whilst obtaining the required workability conditions, is considered as another key ingredient. A study on the influence of naphthalene, melamine and acrylic polymer superplasticizers on the strength and w/c ratio of RPC found that the incorporation of acrylic polymer superplasticizer generally requireda low w/c ratio [7]. Furthermore, specimens having the acrylic polymer superplasticizer recorded higher compressive strengths compared to the specimens that incorporated either of the other two ingredients. In spite of its higher performance in terms of strength, RPCs behave very poorly in fire scenarios, where it often undergoes spalling, which is associated with the breaking away of concrete layers, thus causing degradation and loss of mechanical properties of the base matrix [8; 9; 10; 11; 12]. The compressive strength, tensile strength and fracture energy of RPCs when exposed to temperatures of up to 600 \({}^{\circ}\)C was investigated, where significant levels of spalling were also found to occur [8]. In addition, after being exposed to temperatures of 400 \({}^{\circ}\)C, the compressive strength was found to decrease significantly. It was also reported that the temperature range of 400-600 \({}^{\circ}\)C is the critical temperature range for spalling. When some similar properties were tested [11], the residual strength was found to vary, where it was found to decrease up to 100 \({}^{\circ}\)C, then increased over a temperature range of 200-500 \({}^{\circ}\)C and finally was observed to decrease above 600 \({}^{\circ}\)C. In addition, explosive spalling conditions were also found to occur during a temperature range of 260-520 \({}^{\circ}\)C.  
On the other hand, geopolymer (GP) concretes, which is a mixture of a material(s) rich in silica and alumina, an alkaline liquid (most commonly a combination of sodium hydroxide, or potassium hydroxide and sodium silicate, or potassium silicate) and aggregate particles [13], have been reported to possess similar, if not better, performance as compared to conventional ordinary Portland cement (OPC)-based concretes, especially when exposed to heat/fire [14]. Reports also showed that GPs possess a relatively higher fire resistance owing to its ceramic-like properties and that GP composites are ideally suited for construction, transportation and infrastructure, where fire endurance is part of the requirements [15; 16].