Palm oil fuel ash as partial substitute to cement in concrete: performance at elevated temperatures

In this paper, the palm oil fuel ash (POFA) concrete is produced using POFA as partial replacement to ordinary Portland cement (OPC 53 grade) (0%-P0, 20%-P20, 40%-P40 and 60%-P60) by weight. The POFA concrete specimens are subjected to elevated temperatures of 200°C, 400°C and 800°C for duration of 2 hours. The compressive strength, ultrasonic pulse velocity (UPV) and mass loss (%) of POFA concrete are evaluated. The results indicate that the compressive strength is enhanced by the use of POFA in concrete. At elevated temperatures, POFA concrete showed higher resistance than P0 concrete (control concrete) and P20 concrete showed best performance in POFA concrete, when subjected to elevated temperatures. From the experimental results, it can be observed that 20% POFA can be used as partial replacement to cement in producing sustainable concrete.


Introduction
Structural elements when effected by elevated temperatures or fire accidents is a severe problem. Concrete is considered as an incombustible material, but when subjected to higher temperatures or fire accidents, its constituents will be affected, resulting in the worsening of its properties and performance [1]. At elevated temperatures, the performance of concrete is inclined by the mix proportion, size and shape of members, thermal compatibility of constituents, and type of aggregates and exposure duration of target temperature. The degradation of concrete at fire accidents and or elevated temperatures includes causing of spalls, crack formations, reduction of cohesiveness between concrete constituents and large pores [2]. The thermal properties such as conductivity, diffusivity, coefficient of thermal expansion andspecific heatare comparable to both nominal and high strength concrete. The spalling nature of concrete is influenced by the free water and moisture content. Furthermore, at elevated temperatures, the dense microstructure concrete or high-performance concrete with zero moisture will spall. The changes that occur in concrete when exposed elevated temperatures are: (1) at 100 o C, free water in concrete evaporates. (2) at above 180 o C, the calcium silicate dehydrates. (3) between 400 o C and 900 o C, the dissociation of calcium hydroxide takes place. (4) at above 900 o C, complete deterioration and spalling of concrete occurs [3][4]. A huge amount of waste materials and by-products in the size of fines are used as mineral filler or supplementary cementitious materials (SCMs) in enhancing the properties of concrete. At present, the global annual production of solid waste material is about 1.3 billion tonnes and is expected to reach 2.2 billion tonnes by 2025 [5]. Sustainability in concrete production is the major reason for the enhanced use of filler materials or SCMs as alternative binder materials. Partial replacement of cement in concrete by agricultural by-products or by industrial by-products which may act as fine fillers as well as pozzolanic materials can reduce the cement consumption in concrete production which may lead to the reduction of greenhouse gases emission by the cement industries and will result in both economic and environmental benefits. SCMs may be classified as natural pozzolans, artificial pozzolans and non-pozzolanic fillers. Many researchers used the SCMs as partial substitute of cement and found enhancement in properties of concrete such as: Reducing thermal shrinkage and heat of hydration (b) Reducing alkali-aggregate reaction (c) Increasing water-tightness (d) Enhancement in resistance to acid attack (e) Lowering vulnerability to leaching and dissolution (f) Decreasing cost of concrete production Numerous studies have been carried out by researchers on the use of various fillers as SCMs. The addition of fillers will have a chemical impact on cement hydration. The use of fillers expands the cement matrix volume and compensates the irregularity of coarse aggregate filling in concrete. Concrete produced with non-pozzolanic fillers has no negative effect on the strength properties when compared with pozzolanic fillers modified concrete. Moreover, concrete with non-pozzolanic fillers exhibited more resistance to segregation and bleeding than pozzolanic fillers. SCMs usage in concrete improves the microstructure of concrete and reduces the heat of hydration, leading to the enhancement of mechanical and durability properties. Still there is a need to search for alternative materials and an extensive research is to be done to address the nature of alternative materials and to use them in producing sustainable and cost-effective concrete [6][7][8]. The present paper aims to produce a green concrete by replacing cement with POFA and also to study the effect of targeted temperatures on compressive strength, UPVand mass loss (%) of POFA concrete. The elevated temperatures considered are 200 o C, 400 o C and 800 o C for duration of 2 hours.

Materials and Methods
OPC 53 grade cementis used for producing concrete mix.Krishna river sand and coarse aggregate of maximum size 20 mm are used for concrete mixing. The M30 grade concrete (control concrete: P0) mix arrived at was 1:1.59:3.27 (Cement: Fine aggregate: Coarse aggregate), in accordance to IS 10262: 2009 [9]. OPC was partially substituted with POFA (20%-P20, 40%-P40 and 60%-P60) by weight, for casting M30 grade concrete. The water to binder ratio considered is 0.4. Table 1 details the concrete mix proportions. The concrete constituents are mixed uniformly by using a pan mixer and mixed thoroughly to achieve uniformity. The fresh concrete is transferred into cubes of size 100 mm in accordance to IS 516: 1959 [10] and then cured for 28 days. Both control concrete (0% POFA) and POFA concrete are left free in the laboratory for 7 days and then heated to the target temperatures by using an electrically controlled furnace (capacity of 1200 o C) for obtaining the compressive strength, mass loss (%) and UPV of specimens exposed to targeted temperatures.
The elevated temperatures considered are 200 o C, 400 o C and 800 o C and the exposure duration is of 2 hours.The heating rate of target temperatures is kept at 10 o C /min [11] for maintaining the thermal stability of innerand outer portion of concrete specimens. The electric supply of the electrically operated furnace is turned off after achieving 2 hours of exposure duration and then the specimens were left undisturbed for 24 hours. Fig.1 shows a view of POFA used for the experimental study.        Table 5 shows the variation of m temperatures. The highest mass loss (%) of 5.60%, 5.64%, 6.07%, 6.41% respectively. The mass loss at eleva and chemically bounded water.  Fig. 3 shows the variation     At 400 o C, a sharp decrease in the UPV values of POFA concrete is observed. This is due to the change in the phase of aggregates and also by the effect of dehydration of CSH gel. At 800 o C, the quality of POFA concrete changed to doubtful and this could be by the complete breakdown of the cement matrix and also due to the dissociation of hardened CSH.

Conclusions
From the results of POFA concrete, the following conclusions are made: 1. The use of POFA with 0%, 20%, 40%and 60% as partial substitute for OPC cement in producing concrete showed a slightreduction in the compressive strength.
3. After exposure totemperatures of 200 o C, 400 o C and 800 o C, for POFA concrete, the lowest decrease in compressive strength is observed with P20 concrete and the highest reductionis observed with P60 concrete. This could be due to highest replacement of cement and dilution effect of higher volume replacement of POFA content in the produced concrete. 4. Furthermore, the loss in strength could be due to the change in the chemical composition of POFA concrete at targeted elevated temperatures. 5. The increase in the temperature enhanced the mass loss (%) of POFA concrete and the maximum mass loss (%) is observed with P60 concrete at 800 o C. 6. At elevated temperatures, from the results of UPV values it is noticed that up to 200 o C, the POFA concrete showed good results and satisfying as per IS 13311 (Part 1): 1992 and P60 showed medium quality concrete.