Elevated Temperature Resistance of Mortars Including Ground Granulated Blast Furnace Slag, Fly Ash and Silica Fume

Article History: Received: 11.08.2021 Accepted: 27.10.2021 Published online:08.03.2022 In the current study, elevated heat resistance of the mortars including 15% fly ash, ground granulated blast furnace slag and silica fume is investigated. Fly ash, ground granulated blast furnace slag and silica fume are replaced with cement past by weight in the prepared mortar samples. The prepared samples are kept in a furnace for 2 hours at 400 °C and 800 °C to find out effect of elevated temperature on compressive strength of the mortars. Compressive strength before and after heat exposure are measured in terms of evaluating elevated heat resistance. Control sample that does not contain any admixture, mortar including fly ash, mortar including ground granulated blast furnace slag and mortar including silica fume has 30.81 MPa, 33.62 MPa, 43.91 MPa and 51.83 MPa compressive strength before heat exposure, respectively. Compressive strength of the same samples after 400 °C and 800 °C heat exposure are 25.64 MPa and 8.12 MPa, 28.91 MPa and 11.56 MPa, 34.37 MPa and 15.21 MPa and 39.78 MPa and 21.85 MPa, respectively. The alteration of heat resistance of the mortars with fly ash, ground granulated blast furnace slag and silica fume is attributed to puzzoulanic behavior of the used materials. These materials react with Ca(OH)2 in cement and produce extra tobermorite gel (CSH phase) that provides extra durability to the composite.


Introduction
Cement is one of the most commonly used construction materials in all over the world (Çağlar et al., 2020). Cement based materials are non-flammable and offer important safety advantages over plastic and wood that are used as construction and building materials. Cement based materials also protects rebar in the reinforced structures. The cement based material covers the rebar and protect the material from environmental and chemical events (Erşan et al., 2008). However, exposure of high temperature causes damages in the cement-based structures. Protection of the cementitious composite is limited by exposure time, temperature etc.
Cementitious composites might be subjected to elevated temperatures that damage the structural properties of materials (Zhou et al., 2020). Fire, thermal shock and industrial applications are some of the cases that cementitious composites expose to high temperatures. In most cases high temperature causes deteriorations in the material due to physical and chemical changes that occur in cementitious composites.
According to literature, cement based composites subjected to elevated temperature up to 300 °C are not affected significantly since the elevated temperature could improve or fasten hydration reactions of cement (Setayesh et al., 2017). However, temperature between 350-550 °C causes strength loose in the material since that temperature range results decomposition of calcium hydrate (CH) into lime (Husem, 2006) and water and further elevated temperature range (700-900 °C) results decomposition of calcium-silica-hydrate gels (C-S-H) (Liu et al., 2019).
Elevated temperature not only affects chemical stability of the cementitious composite but also affect physical stability. Due to different thermal strain behavior of the materials in the cementitious composites cracks are formed with exposure of elevated temperature. Cracks formed as a result of mismatched thermal strains also causes strength loose in the material.
In the current paper, physical and mechanical properties of mortars containing GGBFS, fly ash (FA) and silica fume (SF) as replacement of ordinary Portland cement (OPC) at elevated temperature are investigated. The objective of this study is to reveal mechanical and physical properties of mortar including the most available and used puzzolanic admixtures by exposing 400-800 °C heat for 2-hour time duration.

Material and Method
Ordinary Portland cement (CEM I 42.5 R) is used to produce all mortar samples. GGBFS (Iskenderun Iron and Steel Inc., Hatay, Turkey), FA (Afsin-Elbistan Thermal Power Plant, Kahramanmaras, Turkey) and SF (Eti Electrometallurgy Inc., Antalya, Turkey) are incorporated at a single amount of 15% wt partially substituting ordinary Portland cement. Since mineral additives are generally used between 10% and 50% of the cement weight (Puzolanik Maddeler-Panora Makine A.Ş., 2021), in this study, the mineral additive ratio is chosen as 15%, which provides this range. Chemical composition of the GGBFS, FA and SF used are given in Table 1. Calcareous based crushed fine aggregate with maximum grain size of 4 mm and 2.72 specific gravity is used to produce mortar samples.
Polycarboxylate ether-based superplasticizer (SP) (Glenium sky 608, BASF) is used (if required) to reach desired workability.  Standard, 2003). Mix design of the prepared samples is given in Table 2. Dry ingredients are mixed in pan type mortar mixer for 2 min then water is added to the mixture of the binder and aggregate. The mixing procedure is continued until observing homogenous mixture. The fresh mortar that produced after mixing the ingredients are poured into prismatic molds having 40 mm X 40 mm X 160 mm. Molded samples are kept in laboratory environment for 24 hours and mortar samples removed from the molds. After that the mortars are kept in a water tank for additional 27 days. (2) Where , P and A are compressive strength, load and area of the samples load is applied (40 mm X 40 mm), respectively.

Elevated temperature resistance test
Samples broken into two portions after applying flexural tensile strength test are used for elevated temperature resistance test. 28-day water cured samples kept in an oven at 50 °C until observing no change in mas of the specimen. Aim of that procedure is to evaporate free water in the specimens before starting the elevated heat resistance test. After evaporating the free water inside the specimens, the mortar samples are exposed to 400 and 800 °C heat for 2 hours in an oven that temperature inside it increase 5°C/min. It is reported in the literature that the critical temperature range for the loss of compressive strength is 400-800 °C (Chan et al., 1999). For this reason, temperatures of 400 and 800 °C are chosen for the current study. Heat inside the oven starts at 25°C and stays constant at the desired temperature. Specimens are kept inside the oven for 2 hours after the oven reaches the desired temperatures. Specimens are cooled down by switching off the oven. Compressive strengths of the cooled samples are determined to find out elevated heat resistance of the samples.    (Cuesta et al., 2021;Jia et al., 2019;Wang et al., 2020). In addition to that SF has the finest particle size compared to the FA and GGBFS. As SF is more reactive then the FA and GGBFS, it has superior effect on compressive strength development of the mortar.

Results and Discussion
149 Figure 2. Compressive strength results of mortar samples with different inclusions Figure 3 shows compressive strength test results of mortar samples rested at room temperature and samples exposed to 400 °C and 800 °C heat for 2 hours in an oven.  (Saad et al., 1996).

Conclusion
In this study, elevated heat resistance of mortar samples including 15% FA, GGBFS and SF is investigated. Mortar samples with FA, GGBFS and SF are kept in a furnace for two hours at 400  C and 800  C to evaluate elevated heat resistance of the mortars. According to the findings it is revealed that replacement of 15% FA, GGBFS and SF by weight of cement in mortar specimens resulted increase in compressive strength of the mortars as expected. The highest elevated heat resistance observed in mortar samples with SF and the lowest elevated heat resistance observed in the control samples that does not include any puzzoulan. The altered heat resistance with presence of FA, GGBFS and SF is attributed to the puzzoulanic behavior of those materials. These materials react with Ca(OH) 2 in cement and produced extra tobermorite gel (CSH phase) that provides extra durability to the composite. This study reveals that among the studied materials SF provides the best heat resistance to the mortar.

Conflict of Interest
The author declares that there is no conflict of interest.

Author's Contribution
The contribution of the author is 100%.