The Effect of Different Curing Temperatures on the Properties of Geopolymer Reinforced with Micro Steel Fibers

In this study, geopolymer mortar was designed in various experimental combinations employing 1% micro steel fibers and was subjected to different temperatures, according to the prior works of other researchers. The geopolymer mortar was developed using a variety of sustainable material proportions (fly ash and slag) to examine the influence of fibers on its strength. The fly ash weight percentage was 50%, 60%, and 70% by slag weight to study its effect on the geopolymer mortar's properties. The optimal ratio produced the most significant results when mixed at a 50:50 ratio of fly ash and slag with 1% micro steel fibers at curing temperature 240 o C for 4 hours through two days. The compressive strength of the geopolymer mortar increased by 11%, 11.5%, and 14% after 3, 7, and 28 days when utilizing fibers. The result shows that fly ash with a ratio of 50% by weight of slag improved the compressive strength of the mixture. It was discovered that a combination with 50% of the weight of fly ash with micro steel fibers, when treated at 240 o C for curing age of 3, 7, and 28 days, had a flexural resistance rate of 28%, 30%, 33% higher than a mixture without fibers. Keywords-sustainable material; geopolymer mortar; fly ash; ground granulated blast furnace; slag


INTRODUCTION
The primary binder used in concrete production is Ordinary Portland Cement (OPC). The environmental concerns surrounding the manufacturing of OPC are well-known. Furthermore, the amount of energy needed to manufacture only steel and aluminum is equal to that necessary to build OPC [1]. Cement production varies widely due to differences in the availability of raw materials [2]. Sustainable materials are being developed to limit the emission of CO 2 from the cement industry and help recycle the industrial waste by incorporating environmentally friendly materials into civil engineering project [3]. Many cement composites may be found in addition to regular concrete. Various byproducts may be employed to produce blended cement classified as supplementary cementitious materials, such as GGBFS, Fly Ash (FA), and Silica Fume (SF) [4]. FA and slag waste from thermal power plants pose a severe environmental hazard if not treated and repurposed. Since the FA from thermal power plants is buried, it does long-term damage to the environment by polluting the groundwater and harming the farmlands [5]. The byproduct of combustion is a fine powder. A significant amount of this garbage is produced every year, creating a substantial environmental threat. Pozzolanic (siliceous and aluminous) materials may be utilized to minimize waste to some degree [6].
In 1978, the term "geopolymer" was developed to designate a group of mineral binders having a similar chemical makeup to zeolites. On the other hand, geopolymers use polycondensation of silica and alumina precursors instead of traditional Portland/pozzolanic cement to form the matrix. Geopolymers are mainly composed of source materials and alkaline liquids. Alumino-silicate-based raw materials with high silicon (Si) and aluminum (Al) content should be used. Byproducts like FA and SF may be added to the mix. As opposed to other alumino-silicate compounds, geopolymers are a unique creation (e.g. alumino-silicate gels, glasses, and zeolites). Geopolymerization produces higher concentration of solids than alumino-silicate gels or zeolite synthesis [7]. Geopolymer concrete, made from FA and slag, is an excellent alternative to conventional concrete because it is more environmentally friendly and has diverse ingredients and qualities [8]. Therefore, in this paper, the effect of different proportions of FA by weight of slag with deferent curing temperatures is studied.

A. Fly Ash
The acquired FA is a fine, glassy powder produced by coal combustion throughout the generation of electrical energy at the ISKEN-MENT-Turkey power station. The chemical composition of the FA used in this research is shown in Table  I.

B. Slag
The slag utilized in this study complied with ASTM C618 [8] requirements, as shown in Table II.

C. Sodium Hydroxide
Commercially available NaOH flakes have a 98% purity. NaOH is used to make geopolymer mortar solutions. NaOH is made by melting caustic soda flakes with water. Based on the ratio of the soda flakes added to the water, different molar concentrations may be obtained.

D. Sodium Silicate
The concentration of Na 2 SiO 3 is determined by the ratio of Na 2 O to SiO 2 and H 2 O. The Na 2 SiO 3 used in this formulation was produced in the United Arab Emirates.

E. Water
Distilled water was used to dissolve the caustic soda flakes to make the NaOH solution. Tab water was added to the geopolymer mix to improve workability. It conforms to IQS 1703 [10].

F. Fine Aggregates
This research used natural sand from the Al-Ekhadir (Karbala city) area as fine aggregates. It belongs to zone 2 and complies with the physical and chemical characteristics of the Iraqi standard requirements IQS (No.45/1984) [11] as shown in Table III. Table IV shows the properties of the micro steel fibers utilized in this research.

H. Superplasticizer
A high range water reducer (superplasticizer) based on modified sulfonated naphthalene formaldehyde condensate was used to improve the geopolymer mortar's workability. It confirmed to ASTM C494 [17].

A. Alkaline Solution Preparation
The sodium hydroxide molar concentration was set to 12 molars when making the geopolymer mortar in this experiment. The sodium silicate-to-sodium hydroxide ratio was set at 2:1, and the solution-to-cementitious-material ratio was set at 0.45. Table V shows the sodium hydroxide flakes weight.

B. Mixing
The day before it was utilized, the alkaline liquid was produced and then mixed with the superplasticizer before the mixing procedure. The dry components (GGBS, FA, fibers, and sand) were first incorporated by hand for about 2 minutes before the alkaline liquid and superplasticizer were combined at a 75% concentration. After 5 minutes of mixing, the mixture was left for roughly 15 seconds before the remaining 25% of the mixed alkaline liquid was added and then it was recombined for another 5 minutes. Approximately 10 to 15 minutes of mixing time were required to achieve homogeneity as shown in Table VI.

IV. RESULTS AND DISCUSSION
Mortar testing was carried out in accordance with ASTM: C109-2 [13]. Since compressive strength is an essential feature of concrete mixtures or mortar, regular patterns in standard specifications are highly reliant on compressive strength at 3,7, and 28 days of curing age, with varying curing temperature. The test was performed on 50×50×50 mm cubic test samples as shown in Table VII  The increase of the proportion of the replacement of FA with GGBFS giving optimal compressive strength was at 50% FA and 50% slag (G1) with an additional 1% of fibers (G2) at 3,7, and 28 days. This conclusion is in accordance with the findings in [14]. The compressive strength of geopolymer mortars increases by increasing curing temperatures from 80 o C to 160 o C and 240 o C. That conclusion is in accordance with the findings in [15]. The results of G1, G3, and G5 show increases in compressive strength with the increasing percentage of GGBFS, compared with G1 at 240 o C and 28 days of curing as shown in Figure 3. Compressive strength of geopolymer mortar at 28 days with curing temperatures.

Fig. 2.
Geopolymer mortar cubes. Flexural strength testing is a method of determining the way materials respond to basic beam loading. The test was carried out according to ASTM C 293M-16 [16]. Cubic 50×50×50 mm specimens were used in this test with different curing ages (3, 7, 28 days). The result is shown in Table VIII. The optimal proportion mix was G1 (50% FA : 50% Slag) and G2 (50% FA : 50% Slag : 1%Fiber).   Figure 4 indicate that increased compressive strength leads to increased flexural strength, which is clearly shown by the experimental results of these two properties. This study found that at 3, 7, and 28 days of curing age, the flexural strength of geopolymer mortar supplemented with micro steel fibers was increased by 28%, 30%, and 33% compared with the mixture without fiber at the same temperature of 240 o C, which is the ideal dry heat curing temperature for fly ash-slag blends with a 50:50 weight ratio. However, the introduction of fibers in concrete increased the capacity to regulate fracture propagation, transforming the brittle cementitious material into a ductile composite. These results are in accordance with the findings in [18]. V. CONCLUSION The main conclusions according to the results of the current study on geopolymer mortar are: • Geopolymer is an ecologically acceptable substitute for OPC in structural applications.
• FA and slag-based geopolymer mortar are preferable to conventional concrete because of their sustainable ingredients and improved characteristics.
• Best geopolymer mortar qualities may be achieved by adding 1% by volume micro steel fibers to the mixture.
• The additional micro steel fibers in geopolymer mortar increase strength by 10%, improving bond strength.
• The optimum strength of geopolymer mortar was acquired with proportions of 50% of FA: 50% of GGBFS with 1% of micro steel fibers (Mix G2).
• The addition of GGBFS to geopolymer concrete as an alternative to FA improved compressive strength in mixtures G2, G4, and G6 with ratios of 11%, 8.5%, and 5% respectively, with 240 o C curing temperature at 28 days.
• Geopolymer mortar can develop high early strength by using high curing temperatures.
• By increasing temperatures of curing age from 80 o C to 240 o C, the compressive strength increased by about 25% without fibers and 28.5% with micro steel fibers (at 28 days).