Development of Passive Fire Protection Mortars

  Citation: Caetano, H.; Laím, L.; Santiago, A.; Durães, L.; Shahbazian, A. Development of Passive Fire Protection Mortars. Appl. Sci. 2022, 12, 2093. https://doi.org/10.3390/ app12042093 Academic Editor: Sang-Hyo Kim Received: 23 December 2021 Accepted: 7 February 2022 Published: 17 February 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). applied sciences Article Development of Passive Fire Protection Mortars Hugo Caetano 1,* , Luís Laím1, Aldina Santiago 1, Luísa Durães 2and Ashkan Shahbazian 1 1Department of Civil Engineering, University of Coimbra, ISISE, Rua Luís Reis Santos, 3030-790 Coimbra, Portugal; [email protected] (L.L.); [email protected] (A.S.); [email protected] (A.S.) 2Department of Chemical Engineering, University of Coimbra, CIEPQPF, Rua Sílvio Lima, 3030-790 Coimbra, Portugal; [email protected] *Correspondence: [email protected]; Tel.:+00351-239-797-261 Abstract: During a fire event, the stability of steel structures may be compromised, and structural collapse may occur due to the loss of their mechanical resistance as the temperature increases. One of the solutions to reduce this problem is the protection with a coating using enhanced fire-resistant mortars. This paper reports a detailed experimental work aiming to develop gypsum and cementbased mortars for passive fire protection and evaluate their composition’s effect in the final thermal performance. Two types of specimens were tested: (i) small specimens composed of a mortar coating (10 mm thick) and one steel plate and (ii) square section short tubular steel columns with 20 mm of coating. The evaluation of the thermal protection was carried out by (a) measuring the thermal gradient between the exposed surface of the protected steel plate under high temperatures and the mortar-steel interface and (b) assessing the fire resistance of the short steel columns. It was concluded that the compositions with gypsum binder present better thermal insulation than the cementitious compositions. Additionally, the introduction of nanoand microparticles of silica still slightly improved the thermal insulation of the tested compositions. Keywords: steel columns; nanoand microsilica; gypsum; cement; mortar; passive fire protection; insulation; fire; heat transfer 1. Introduction There is a growing interest in developing alternative and sustainable construction materials with enhanced properties. As metals are infinitely recyclable, this type of construction is part of a future of “green construction”, thus contributing to a sector of the economy with low environmental impact [1–3]. However, steel structures show some weaknesses, especially their structural behaviour when subjected to fire [ 4 – 9 ]. Due to the high thermal conductivity of the steel, the high section factor of the members, and the rapid degradation of the steel mechanical properties, with the increase of steel temperature, rapid change in the stiffness and mechanical resistance may be noticed in the structure. Its resistance and stability may be compromised, leading to the collapse of some elements or even of the entire structure [10–15]. The thermal protection of these structural elements is crucial. The fire protection of structures can be achieved by combining active and passive fire protection systems and management systems (smoke exhaust systems, communication and evacuation procedures, fire detection systems, and compartmentation [ 16 ]). Active fire protection can consist of systems or items such as fire extinguishers, standpipes, sprinkler systems, and fire blankets [ 17 ]. It is required that these systems have a quick response capacity in extinguishing or controlling the development of a fire in its initial phase [ 18 ]. However, implementing these systems has high installation and maintenance costs, and their results can have low functional reliability and unsatisfactory operational results [19]. Appl. Sci. 2022,12, 2093. https://doi.org/10.3390/app12042093 https://www.mdpi.com/journal/applsci Appl. Sci. 2022,12, 2093 2 of 20 In turn, when applying passive protection, it is almost sure that it will be operational during a fire event. Passive systems include the fire resistance of structural or nonstructural members of a building, such as columns, beams, walls, and ceilings. This purpose ensures adequate evacuation times, minimum safety conditions for firefighting and rescue operations and minimises property, economic, and life losses [20]. The most common method to protect a member from exposure to extreme temperatures is to apply insulation materials around the structural members. Insulation materials used as fire protection in structural steel members can be: board systems (gypsum board or calcium silicate), insulating blankets (ceramic, rock, and glass wool), spray systems, or intumescent coatings [ 21 ]. These materials should be cheaper than the steelwork, easy of application, safe (i.e., not be hazardous during the application and in service, or toxic in the event of fire), and should insulate and remain undamaged during the fire attack (with limited detachments and fractures during the required fire resistance) [22]. One solution that meets these protection requirements is the coating with enhanced fire-resistant mortars due to the introduction of nanosilica particles and thermally stable and porous aggregates [ 23 ]. With the emergence of nanomaterials (silica nanoparticles, titanium dioxide nanoparticles, alumina nanoparticles, and carbon nanotubes, etc.), their incorporation in pastes, mortars, and other cement-based materials has appeared as a possibility to improve their mechanical and thermal properties: higher durability, better resistance to corrosion and better fire resistance [ 24 – 28 ], with the silica nanoparticles being the most studied nanomaterial in cement [ 26 ]. However, little attention has been given to other properties, such as thermal conductivity at elevated temperatures [29]. When subjected to high temperatures, perlite and vermiculite have been incorporated in cement mortars to improve their thermal performance [ 30 ]. Perlite consists of a siliceous volcanic rock with 2% to 6% water combined. However, when small perlite grains are subjected to high temperatures (870 ◦ C), they strongly increase their volume with numerous tiny, sealed air cells, like popcorn giving rise to lightweight expanded perlite. Lightweight expanded perlite has low thermal conductivity (0.04–0.06 W/mK), relatively high melting point (1260–1343 ◦ C) and low density (loose: 50–400 kg/m 3 ) [ 31 ]. These characteristics make perlite a material with excellent insulating properties. Vermiculite consists of a micalike mineral containing a shiny flake (the phyllosilicate group). It is produced at ambient conditions from the weathering/hydrothermal alteration of phlogopite or biotite [ 30 ]. Similar to perlite, when vermiculite particles are subjected to high temperatures (from 650 to 950 ◦ C), they expand, presenting a density of 80 to 120 kg/m3, a melting point between 1240 to 1430 ◦ C and a thermal conductivity between 0.04 to 0.12 W/mK [ 32 ]. Moreover, due to their highly porous structure, these materials absorb moisture in varying degrees (depending on their type), which when combined with the low thermal conductivity, extends their durability during the fire. Perlite may be a better supplement than vermiculite, not only due to its thermal conductivity but also to the high retraction effect of vermiculite [ 33 ]. As they are materials with good thermal properties, they can be used in the development of plaster or cement mortars as a passive fire protection solution in steel structures. In addition to cement, plaster can also be used as a binder and combined with perlite and/or vermiculite to develop mortars for passive fire protection. Compared to cement, gypsum is much cheaper, easier to produce, and provides a more effective thermal barrier because it has a lower thermal conductivity than cement. It also contributes to the energy loss from fire due to its endothermic dehydration process [34,35]. Studies have shown that perlite–Portland cement and perlite–gypsum coatings are the most effective plasters as fire barriers and in retarding the conduction of high temperatures across their thickness among different kinds of coatings, such as traditional-cement plaster, vermiculite cement/gypsum-based mortar, intumescent coating, calcium silicate board, and LECA-cement plaster [36–38]. Therefore, this paper presents the development of different gypsum or cement-based mortars as passive fire protection materials, the evaluation of the influence of aggregate Appl. Sci. 2022,12, 2093 3 of 20 size, and the addition of silica microand nanoparticles on their thermal performance and assessment of fire resistance to the protected short steel columns. 2. Materials and Methods 2.1. Materials and Compositions In a preliminary phase, several mortars based on cement or gypsum were developed, with different dosages of raw materials, according to the state of the art as presented in the introduction of this paper (Table 1). The following materials were used in the compositions of the mortars: commercial passive protection solutions 1 (IGN) and 2 (VER), Portland cement CEM II/B-L 32.5 (PC), Isidac 40 refractory cement (IRC), Topeca M40 refractory cement (TRC), Eletroland refractory cement (ERC), gypsum powder (GP), expanded vermiculite with dimensions between 0.5 and 3 mm (EV), expanded perlite with dimensions between 1 and 5 mm (EP), polypropylene fibers with an average diameter of 31 µ m and an average length of 6 mm (PP), silica sand with dimensions between 0.01 and 2.00 mm (SS), expanded clay with dimensions between 0.01 and 2.00 mm (EC), silica microparticles with an average diameter of 1000 nm (MS), silica nanoparticles with an average diameter of 200 nm (NS), and water (W). Silica microand nanoparticles were synthesised in the laboratory, according to the procedure described by Vaz-Ramos et al. [ 15 ]. Table 1. Mortar compositions developed at LEMEC (a) (amount of materials in volume %). Mortar Designation Binders Type CPPS Aggregates NS and MS W/B PC ERC IRC TRC GP IGN VER SS EV EP EC PP/B C_1 30% - - - - - - 70% - - - - - 0.50 C_2 29% - - - - - - 70% - - - 1% - 0.50 C_3 28% - - - - - - 70% - - - 1% 1% 0.50 C_4 26% - - - - - - 70% - - - 1% 3% 0.50 C_5 23% - - - - - - 70% - - - 1% 6% 0.50 C_6 29% - - - - - - - - 70% - 1% - 0.50 C_7 29% - - - - - - 35% 35% - - 1% - 0.50 C_8 29% - - - - - - - - - 70% 1% - 0.50 C_9 - 30% - - - - - 70% - - - - - 0.50 C_10 - 29% - - - - - 70% - - - 1% - 0.70 C_11 - 29% - - - - - - - 70% - 1% - 0.70 C_12 - 29% - - - - - 35% 35% - - 1% - 0.70 C_13 - - 30% - - - - 70% - - - - - 0.50 C_14 - - 29% - - - - 70% - - - 1% - 0.70 C_15 - - 29% - - - - - - 70% - 1% - 0.70 C_16 - - 29% - - - - 35% 35% - - 1% - 0.70 C_17 - - - - - 100% - - - - - - - 0.50 C_18 - - - - - 100% - - - - - - - 0.60 C_19 - - - - - 99% - - - - - 1% - 0.70 C_20 - - - 100% - - - - - - - - - 0.50 C_21 - - - 100% - - - - - - - - - 0.70 C_22 - - - 99% - - - - - - - 1% - 0.70 C_23 - - - - - 80% - - 20% - - - - 0.70 C_24 - - - - - 99% - - - - - 1% - 0.70 C_25 - - - - - - 100% - - - - - - 0.50 C_26 - - - - - - 100% - - - - - - 0,60 C_27 - - - - - - 100% - - - - - - 0.70 C_28 - - - - - - 99% - - - - 1% - 0.60 C_29 49% - - - - - - - - 50% - 1% - 1.30 C_30 49% - - - - - - - 50% - - 1% - 3.21 C_31 50% - - - - - - - 25% 25% - - - 2.17 C_32 49% - - - - - - - 25% 25% - 1% - 2.17 C_33 - - - - 100% - - - - - - - - 0.50 C_34 - - - - 100% - - - - - - - - 0.50 C_35 - - - - 100% - - - - - - - - 0.60 C_36 - - - - 40% - - - - 60% - - - 1.25 C_37 - - - - 50% - - - - 50% - - - 1.00 C_38 - - - - 60% - - - - 40% - - - 0.83 C_39 - - - - 40% - - - 60% - - - - 2.10 C_40 - - - - 50% - - - 50% - - - - 1.60 C_41 - - - - 60% - - - 40% - - - - 1.25 C_42 - - - - 20% - - - 40% 40% - - - 3.75 C_43 - - - - 30% - - - 35% 35% - - - 2.50 C_44 - - - - 40% - - - 30% 30% - - - 2.15 C_45 - - - - 99.5% - - - - - - 0.5% - 0.50 C_46 - - - - 99% - - - - - - 1% - 0.50 C_47 - - - - 98.5% - - - - - - 1.5% - 0.50 C_48 - 20% - - - - - - 40% 40% - - - 2.75 (a) Laboratory of Testing Materials and Structures of University of Coimbra. Appl. Sci. 2022,12, 2093 4 of 20 Testing two commercial passive protection solutions tested, it was possible to identify the commercial solution that provided one of the best thermal insulation results, which came to be considered as the reference mortar (CM). From the preliminary tests phase, four different mortars were selected (DCM, DGMP, DGMV, and DRCM) from forty different mortars developed with greater thermal insulation capacity than that provided by the reference mortar. As can be seen in Table 2, the DCM was made with Portland cement, vermiculite and polypropylene fibers, the DGMP was prepared gypsum and perlite, the DGMV was made of gypsum and vermiculite and the DRCM was prepared with refractory cement, perlite, and vermiculite. Table 2. Constitution of each selected mortar (amount of materials in volume %). Mortar Designation CPPS PC RC GP EV EP PP/B W/B CM 100% - - - - - - 0.60 * DCM - 49% - - 50% - 1% 3.21 DGMP - - - 40% - 60% - 1.21 DGMV - - - 50% 50% - - 2.10 DRCM - - 50% - 25% 25% - 2.75 * Water commercial solution ratio in weight %. To analyze the influence of expanded perlite and expanded vermiculite grain size on the thermal performance of laboratory developed mortars, two different grinding methods were used: Los Angeles (LA) and Industrial Mill (IM). The first method was carried out using the Los Angeles method, which fragmented the aggregate by abrasion and shock using steel balls. The particle size of these raw materials was assessed using a particle size analysis (specification LNEC E 195-1966), and it was observed that their size ranged from 0.075 to 0.85 mm. The second method was carried out using an industrial mill, which by the friction of the aggregate with the drum significantly reduced the size of its particles compared to the LA method. The particle size analysis identified a particle size ranging from 0.025 to 0.40 mm. Finally, different dosages of silica micro and nanoparticles were added and tested in DCM, DGMP, DGMV, and DRCM to assess their influence on the thermal insulation of the respective mortars, as described in the following section of the paper. 2.2. Experimental Program The experimental program included two different types of tests, depending on the type of specimens: steel plate (SP) (Table 3) and square section short steel columns (SSC) (Table 4). The experimental program of tests on SP included five different mortar (CM, DCM, DGMP, DGMV, and DRCM), and 45 specimens were produced. Each set of three specimens used in their thermal tests was used to obtain better reliability of results. Table 3. Experimental program on steel plate specimens. Mortar Designation Without NS and MS With NS and MS Total No. of Specimens LA Method IM Method LA Method IM Method CM 3 (*) (*) (*) 45 DCM 3 3 3 3 DGMP 3 3 3 3 DGMV 3 3 3 3 DRCM 3 3 (**) (**) (*) The commercial solution was not modified, so there was no need to test more than three specimens. (**) Since this mortar has the worst results, it has not been tested with the addition of silica microand nanoparticles. Appl. Sci. 2022,12, 2093 5 of 20 Table 4. Experimental program on short steel columns. Different Types of Tested Columns Specimens Designation Number of Repetitions Total No. of Specimens Columns without passive fire protection SSC1 2 12 SSC2 Columns coated with CM SSC3 2 SSC4 Column coated with DCM Without MS and NS SSC5 2 SSC6 With MS and NS SSC11 2 SSC14 Column coated with DGMP Without MS and NS SSC7 2 SSC8 With MS and NS SSC12 2 SSC13 Table 3presents the experimental program defined to evaluate the influence of the size of the aggregates and the addition of silica microand nanoparticles on the thermal performance of the developed mortars. The percentage of silica microand nanoparticles were the same and equal to 0.5% in weight of binder for each one. All specimens were exposed to high temperatures on one side up to 900 ◦ C. Additionally, the experimental program of tests carried out on 12 SSC under fire conditions included three different fire protection mortars (CM, DCM, and DGMP). DGMV and others new ones will make part of another future experimental campaign, in which different aggregates/additives will be studied. 2.3. Preparation of the Specimens The manufacturing process, shape, and dimensions of the SP and moulds were defined to measure the thermal gradient generated between the inner surface of the steel plate exposed to high temperatures and the exposed surface of the fire protection mortar. Thus, a suitable mould was designed and manufactured for these tests (Figure 1). Appl. Sci. 2021, 11, x FOR PEER REVIEW 6 of 22 2.3. Preparation of the Specimens The manufacturing process, shape, and dimensions of the SP and moulds were defined to measure the thermal gradient generated between the inner surface of the steel plate exposed to high temperatures and the exposed surface of the fire protection mortar. Thus, a suitable mould was designed and manufactured for these tests (Figure 1). (a) (b) Figure 1. Mould used in the manufacture of SP: (a) mould components; (b) mould assembly. This mould assigns the desired geometric shape to the specimen, was easy to transport and clean, was reusable for many experimental tests, and was easy to assemble and disassemble when concreting and removing the specimen. During the fabrication of mortars, a balance accurate to 0.1 gr, a graduated beaker, Hobart N50 mixer with 5 L of capacity and a stainless-steel lab spatula were used. The procedure adopted in the fabrication of mortars was as follows: 1. The raw materials were weighed and placed inside the mixer container. 2. Then, the mixer was put into operation for 5 min at a slow speed (136 rotations per minute). At the same time, the corresponding amount of water was added, with a constant flow rate to guarantee the homogeneous addition of water in the whole mortar. 3. After this procedure, the mortar was manually kneaded with a spatula to remove parts of the mortar that were on the walls of the container and thus homogenize the mixture, then returning the container to the mixer for another 2 min. 4. The mortar was placed inside the mould (Figure 2). In the production of mortars, the same procedure was followed to ensure that the different properties of the mortars were only dependent on their composition. To minimize possible effects that temperature and humidity might have on the properties of each mortar composition, all mixtures of each composition were manufactured on the same day and placed in a room with controlled environmental conditions. It is well known that the moisture content greatly influences the fire behavior of mortars at elevated temperatures [39,40], in the same way as in the concretes. (a) (b) (c) (d) Figure 1. Mould used in the manufacture of SP: (a) mould components; (b) mould assembly. This mould assigns the desired geometric shape to the specimen, was easy to transport and clean, was reusable for many experimental tests, and was easy to assemble and disassemble when concreting and removing the specimen. During the fabrication of mortars, a balance accurate to 0.1 g, a graduated beaker, Hobart N50 mixer with 5 L Appl. Sci. 2022,12, 2093 6 of 20 of capacity and a stainless-steel lab spatula were used. The procedure adopted in the fabrication of mortars was as follows: 1. The raw materials were weighed and placed inside the mixer container. 2. Then, the mixer was put into operation for 5 min at a slow speed (136 rotations per minute). At the same time, the corresponding amount of water was added, with a constant flow rate to guarantee the homogeneous addition of water in the whole mortar. 3. After this procedure, the mortar was manually kneaded with a spatula to remove parts of the mortar that were on the walls of the container and thus homogenize the mixture, then returning the container to the mixer for another 2 min. 4. The mortar was placed inside the mould (Figure 2). Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 22 (a) (b) (c) (d) Figure 2. Manufacture of specimens: (a) Hobart N50 mixer; (b) fresh mortar in the mixer container; (c) steel plate with thermocouples applied; (d) specimen in the process of curing during the first 48 h. About 48 h after casting the SP in the moulds described above, they were demoulded and placed in the curing process (Figure 3a) for 28 days in the laboratory environment with controlled temperature (25 °C) and a relative humidity (RH) of 55%. The specimens were tested with 6 months of age. (a) (b) Figure 3. (a) Specimens in the process of curing; (b) specimen schematic representation and respective dimensions. The specimens comprised a S355 steel plate with a square section of 250 mm of edge and a thickness of 5 mm; and 10 mm thick fire protection mortar on one side of the steel plate (Figure 3b). The temperature measurement in the SP was carried out by placing 4 type K thermocouples. The thermocouples were placed at different depths across the specimen (Figure 4). (a) (b) Figure 4. Schematic view of thermocouples. (a) A-A’-A’’ cross-section in the middle of the specimen; (b) arrangement and designation of thermocouples. With this distribution of thermocouples, it was possible to determine the thermal gradient between the surface of the mortar exposed to high temperatures (ESMHT—thermocouple 3) and the unexposed surface (USMHT—thermocouple 2), as well as the temperature on the inner surface of the steel plate (ISSP—thermocouple 1) and its external Figure 2. Manufacture of specimens: ( a ) Hobart N50 mixer; ( b ) fresh mortar in the mixer container; ( c ) steel plate with thermocouples applied; ( d ) specimen in the process of curing during the first 48 h. In the production of mortars, the same procedure was followed to ensure that the different properties of the mortars were only dependent on their composition. To minimize possible effects that temperature and humidity might have on the properties of each mortar composition, all mixtures of each composition were manufactured on the same day and placed in a room with controlled environmental conditions. It is well known that the moisture content greatly influences the fire behavior of mortars at elevated temperatures [39,40], in the same way as in the concretes. About 48 h after casting the SP in the moulds described above, they were demoulded and placed in the curing process (Figure 3a) for 28 days in the laboratory environment with controlled temperature (25 ◦ C) and a relative humidity (RH) of 55%. The specimens were tested with 6 months of age. Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 22 Figure 2. Manufacture of specimens: (a) Hobart N50 mixer; (b) fresh mortar in the mixer container; (c) steel plate with thermocouples applied; (d) specimen in the process of curing during the first 48 h. About 48 h after casting the SP in the moulds described above, they were demoulded and placed in the curing process (Figure 3a) for 28 days in the laboratory environment with controlled temperature (25 °C) and a relative humidity (RH) of 55%. The specimens were tested with 6 months of age. (a) (b) Figure 3. (a) Specimens in the process of curing; (b) specimen schematic representation and respective dimensions. The specimens comprised a S355 steel plate with a square section of 250 mm of edge and a thickness of 5 mm; and 10 mm thick fire protection mortar on one side of the steel plate (Figure 3b). The temperature measurement in the SP was carried out by placing 4 type K thermocouples. The thermocouples were placed at different depths across the specimen (Figure 4). (a) (b) Figure 4. Schematic view of thermocouples. (a) A-A’-A’’ cross-section in the middle of the specimen; (b) arrangement and designation of thermocouples. With this distribution of thermocouples, it was possible to determine the thermal gradient between the surface of the mortar exposed to high temperatures (ESMHT—thermocouple 3) and the unexposed surface (USMHT—thermocouple 2), as well as the temperature on the inner surface of the steel plate (ISSP—thermocouple 1) and its external surface (ESSP—thermocouple 4). In Figure 5, it is possible to identify the thermocouples on the specimen following Figure 4. Figure 3. ( a ) Specimens in the process of curing; ( b ) specimen schematic representation and respective dimensions. The specimens comprised a S355 steel plate with a square section of 250 mm of edge and a thickness of 5 mm; and 10 mm thick fire protection mortar on one side of the steel plate (Figure 3b). The temperature measurement in the SP was carried out by placing Appl. Sci. 2022,12, 2093 7 of 20 4 type K thermocouples. The thermocouples were placed at different depths across the specimen (Figure 4). Appl. Sci. 2021, 11, x FOR PEER REVIEW 7 of 22 Figure 2. Manufacture of specimens: (a) Hobart N50 mixer; (b) fresh mortar in the mixer container; (c) steel plate with thermocouples applied; (d) specimen in the process of curing during the first 48 h. About 48 h after casting the SP in the moulds described above, they were demoulded and placed in the curing process (Figure 3a) for 28 days in the laboratory environment with controlled temperature (25 °C) and a relative humidity (RH) of 55%. The specimens were tested with 6 months of age. (a) (b) Figure 3. (a) Specimens in the process of curing; (b) specimen schematic representation and respective dimensions. The specimens comprised a S355 steel plate with a square section of 250 mm of edge and a thickness of 5 mm; and 10 mm thick fire protection mortar on one side of the steel plate (Figure 3b). The temperature measurement in the SP was carried out by placing 4 type K thermocouples. The thermocouples were placed at different depths across the specimen (Figure 4). (a) (b) Figure 4. Schematic view of thermocouples. (a) A-A’-A’’ cross-section in the middle of the specimen; (b) arrangement and designation of thermocouples. With this distribution of thermocouples, it was possible to determine the thermal gradient between the surface of the mortar exposed to high temperatures (ESMHT—thermocouple 3) and the unexposed surface (USMHT—thermocouple 2), as well as the temperature on the inner surface of the steel plate (ISSP—thermocouple 1) and its external surface (ESSP—thermocouple 4). In Figure 5, it is possible to identify the thermocouples on the specimen following Figure 4. Figure 4. Schematic view of thermocouples. ( a ) A-A’-A” cross-section in the middle of the specimen; (b) arrangement and designation of thermocouples. With this distribution of thermocouples, it was possible to determine the thermal gradient between the surface of the mortar exposed to high temperatures (ESMHT— thermocouple 3) and the unexposed surface (USMHT—thermocouple 2), as well as the temperature on the inner surface of the steel plate (ISSP—thermocouple 1) and its external surface (ESSP—thermocouple 4). In Figure 5, it is possible to identify the thermocouples on the specimen following Figure 4. Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 22 Figure 5. Identification of thermocouples, 1–4, on the specimen following Figure 4. Concerning the SSCs tests, specimens were defined by a hollow square section 150 × 150 × 8 mm, with a height of 1250 mm, and the steel grade was S355. At the column ends, it was centered and welded a steel plate (section 300 × 300 × 20 mm), as shown in Figure 8. To evaluate the temperature evolution on the external surfaces of the steel columns during the test, 12 type K thermocouples were welded, equidistant from each other on all the specimen’s surfaces, applied in 3 groups of 4 thermocouples at different heights. These termocouples were welded in the middle of the surfaces of the steel tubular columns. To guarantee a constant and uniform mortar thickness of 20 mm along the steel columns, a modular formwork was developed with the ability to assign the desired geometric shapewith easy assembly and disassembly while concreting the specimen. Figure 6 depicts the location of the three groups of thermocouples and the different concreting steps of the steel columns. The specimens were tested after curing for 6 months. Figure 5. Identification of thermocouples, 1–4, on the specimen following Figure 4. Concerning the SSCs tests, specimens were defined by a hollow square section 150 ×150 ×8 mm, with a height of 1250 mm, and the steel grade was S355. At the column ends, it was centered and welded a steel plate (section 300 × 300 × 20 mm), as shown in Figure 8. To evaluate the temperature evolution on the external surfaces of the steel columns during the test, 12 type K thermocouples were welded, equidistant from each other on all the specimen’s surfaces, applied in 3 groups of 4 thermocouples at different heights. These termocouples were welded in the middle of the surfaces of the steel tubular columns. To guarantee a constant and uniform mortar thickness of 20 mm along the steel columns, a modular formwork was developed with the ability to assign the desired geo- Appl. Sci. 2022,12, 2093 8 of 20 metric shapewith easy assembly and disassembly while concreting the specimen. Figure 6 depicts the location of the three groups of thermocouples and the different concreting steps of the steel columns. The specimens were tested after curing for 6 months. Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 22 (a) (b) (c) (d) Figure 6. Distribution of thermocouples (a) and fabrication of specimens, preconcreting (b), concreting on the short steel columns (c), and concrete specimen without formwork (d). 2.4. Experimental Testing System and Procedure The experimental testing system (Figure 7) used in thermal analysis of steel plates consisted of a cylindrical oven with internal dimensions 400 mm in height and 250 mm in diameter, capable of reaching a maximum temperature of 1200 °C (a) and the respective oven controller (b). A Datalogger TDS-530 was used as a data acquisition system (c) to record the temperature readings. Regarding the test procedure, after sealing all the existing unions and holes of the oven with rock wool, a 8 cm thick rock wool blanket with a circular opening of 210 mm in diameter was also placed on the top of the oven (e), which allows the passage of heat from the interior of the oven to the specimen. Subsequently, the specimen (d) was placed on the top of the oven, i.e., on the rock wool blanket and centred with the barycentric axis of the oven. Figure 7. Experimental setup: (a) cylindrical oven(b) oven controller(c) data acquisition system(d) specimen(e) rock wool. In all tests, the specimens were placed in a suitable position to ensure perfect accommodation with the rock wool and thus avoid heat losses between the specimen and the oven. The specimen was heated at a heating rate of 15 °C/minute until reaching the desired temperature level (900 °C). Temperatures inside the specimen and the oven were Figure 6. Distribution of thermocouples ( a ) and fabrication of specimens, preconcreting ( b ), concreting on the short steel columns (c), and concrete specimen without formwork (d). 2.4. Experimental Testing System and Procedure The experimental testing system (Figure 7) used in thermal analysis of steel plates consisted of a cylindrical oven with internal dimensions 400 mm in height and 250 mm in diameter, capable of reaching a maximum temperature of 1200 ◦ C (a) and the respective oven controller (b). A Datalogger TDS-530 was used as a data acquisition system (c) to record the temperature readings. Regarding the test procedure, after sealing all the existing unions and holes of the oven with rock wool, a 8 cm thick rock wool blanket with a circular opening of 210 mm in diameter was also placed on the top of the oven (e), which allows the passage of heat from the interior of the oven to the specimen. Subsequently, the specimen (d) was placed on the top of the oven, i.e., on the rock wool blanket and centred with the barycentric axis of the oven. Appl. Sci. 2021, 11, x FOR PEER REVIEW 9 of 22 (a) (b) (c) (d) Figure 6. Distribution of thermocouples (a) and fabrication of specimens, preconcreting (b), concreting on the short steel columns (c), and concrete specimen without formwork (d). 2.4. Experimental Testing System and Procedure The experimental testing system (Figure 7) used in thermal analysis of steel plates consisted of a cylindrical oven with internal dimensions 400 mm in height and 250 mm in diameter, capable of reaching a maximum temperature of 1200 °C (a) and the respective oven controller (b). A Datalogger TDS-530 was used as a data acquisition system (c) to record the temperature readings. Regarding the test procedure, after sealing all the existing unions and holes of the oven with rock wool, a 8 cm thick rock wool blanket with a circular opening of 210 mm in diameter was also placed on the top of the oven (e), which allows the passage of heat from the interior of the oven to the specimen. Subsequently, the specimen (d) was placed on the top of the oven, i.e., on the rock wool blanket and centred with the barycentric axis of the oven. Figure 7. Experimental setup: (a) cylindrical oven(b) oven controller(c) data acquisition system(d) specimen(e) rock wool. In all tests, the specimens were placed in a suitable position to ensure perfect accommodation with the rock wool and thus avoid heat losses between the specimen and the oven. The specimen was heated at a heating rate of 15 °C/minute until reaching the desired temperature level (900 °C). Temperatures inside the specimen and the oven were Figure 7. Experimental setup: ( a ) cylindrical oven ( b ) oven controller ( c ) data acquisition system (d) specimen (e) rock wool. Appl. Sci. 2022,12, 2093 9 of 20 In all tests, the specimens were placed in a suitable position to ensure perfect accommodation with the rock wool and thus avoid heat losses between the specimen and the oven. The specimen was heated at a heating rate of 15 ◦ C/minute until reaching the desired temperature level (900 ◦ C). Temperatures inside the specimen and the oven were measured every 5 s. When the target temperature in the specimen was reached, it was maintained uniform during 3 h and after, the test was given as concluded. The experimental layout for the short steel columns (SSC) under fire conditions (Figure 8) consisted essentially of a reaction steel frame (A) to apply the serviceability load on the specimen, a support steel frame (B), a hydraulic jack (C), and an electric furnace (H). Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 22 measured every 5 s. When the target temperature in the specimen was reached, it was maintained uniform during 3 h and after, the test was given as concluded. The experimental layout for the short steel columns (SSC) under fire conditions (Figure 8) consisted essentially of a reaction steel frame (A) to apply the serviceability load on the specimen, a support steel frame (B), a hydraulic jack (C), and an electric furnace (H). Figure 8. Experimental system used in the laboratory to test SSCS. The letters in this figure are defined in the following text. This steel frame was defined by HEB 500 columns and a HEB 600 beam (A), with a stiffness capable of minimizing possible displacements of this steel structure during the tests. Additionally, a support 3D steel frame consisting of two frames (B) with HEB 300 columns and HEB 400 beams accommodate the testing specimen to similar actual boundary conditions. Regarding the test equipment, a 3 MN hydraulic jack (C) and its controller (J), a 3 MN load cell (D), and a 1 MN load cell (E) were used for measuring the compression forces. Ten linear variable displacement transducers were used for displacements measurements (F), a Datalogger (G) for data acquisition, and an electric furnace (H) to heat up the steel columns (I). A hydraulic jack controlled by a servo-controlled central was used, and a preload of 50% of the design value of the loadbearing capacity of the columns at ambient temperature (ULS) was applied (727.8 kN) to simulate a service load on the specimen. After stabilising this loading in the specimen, the furnace was switched on and the specimen heated according to the temperature evolution established by the ISO 834 standard fire curve [41]. Figure 8. Experimental system used in the laboratory to test SSCS. The letters in this figure are defined in the following text. This steel frame was defined by HEB 500 columns and a HEB 600 beam (A), with a stiffness capable of minimizing possible displacements of this steel structure during the tests. Additionally, a support 3D steel frame consisting of two frames (B) with HEB 300 columns and HEB 400 beams accommodate the testing specimen to similar actual boundary conditions. Regarding the test equipment, a 3 MN hydraulic jack (C) and its controller (J), a 3 MN load cell (D), and a 1 MN load cell (E) were used for measuring the compression forces. Ten linear variable displacement transducers were used for displacements measurements (F), a Datalogger (G) for data acquisition, and an electric furnace (H) to heat up the steel columns (I). Appl. Sci. 2022,12, 2093 16 of 20 Table 8. Average temperature value obtained on the specimens of SSC. Different Types of Tested Columns Specimens Designation Average Temperature of Specimens (◦C) Critical Temperature (◦C) Average Failure Time Fire Resistance Rating 15 30 60 90 (Minutes) (Failure Time) (Minutes) Columns without passive fire protection SSC1 525 - - - 560 ◦C (17 min) 17 R15 SSC2 572 - - - 617 ◦C (17 min) Columns coated with CM SSC3 78 111 351 - 560 ◦C (85 min) 81 R60 SSC4 77 108 360 - 531 ◦C (77 min) Column coated with DCM Without MS and NS SSC5 96 220 465 - 566 ◦C (82 min) 83 R60 SSC6 103 242 481 - 585 ◦C (84 min) With MS and NS SSC11 96 193 447 - 582 ◦C (88 min) 90 R90 SSC14 92 186 425 557 559 ◦C (91 min) Column coated with DGMP Without MS and NS SSC7 79 115 341 535 576 ◦C (98 min) 97 R90 SSC8 75 119 342 545 572 ◦C (95 min) With MS and NS SSC12 76 116 328 516 566 ◦C (102 min) 100 R90 SSC13 76 114 328 525 564 ◦C (98 min) Appl. Sci. 2021, 11, x FOR PEER REVIEW 17 of 22 Figure 13. Evolution of temperature in the specimens of SSC as a function of time. Table 8 shows the average temperature values acquired in the specimens after 15, 30, 60, and 90 min and the average temperature values for the failure instants of each specimen. The critical design temperature calculated according to EN 1993-1-2: 2005 [41] for this short steel column was 586.7°C. In Figure 12, the tested SSC2 failed at 17 min, SSC4 at 77 min, SSC6 at 84 min, SSC8 at 95 min, SSC12 at 102 min, and SSC14 at 91 min, corresponding to the temperature of 617, 531, 585, 572, 566, and 559°C, respectively (see also Table 8). In Figure 13 and Table 8, it can be seen that the commercial mortar has a more effective thermal protection for lower temperatures (the highest delay in the temperature rise at the beginning of the test). However, for temperatures higher than 400°C, its thermal capacity tends to decrease due to the degradation of the mortar (the highest temperature rise rate at the ending of the test). Figure 14 depicts the specimen before and after being tested. Regarding the instability modes of the steel columns, local instability was observed despite the column being a class 1 cross-section under fire conditions. Figure 14. Photos of the specimens of SSC before and after tested, as an example. 0 200 400 600 800 1000 1200 010 20 30 40 50 60 70 80 90 100 110 Temperature (°C) Time (minutes) ISO 834 Furnace Temperature TSSC1,SSC2 TSSC3,SSC4 TSSC5,SSC6 TSSC7,SSC8 TSSC12,SSC13 TSSC11,SSC14 Figure 14. Photos of the specimens of SSC before and after tested, as an example. The results from Table 8clearly show that the application of mortars works as a good thermal barrier to fire, since columns without passive fire protection failed after 17 min of testing, whereas the protected columns failed on average, beyond 81 min. Furthermore, the results also show that the thermal protection of mortars developed in the laboratory was more efficient than that provided by the commercial mortars. Finally, it appears that the introduction of the NS and MS slightly improved the thermal performance of the mortars developed in the laboratory. Table 8shows that the specimen without passive fire protection presented a fire resistance rating (FRR) of 15 min (R15), and the protected specimens with the CM allowed a FRR of R60. Concerning the thermal performance of the mortars developed in the laboratory, it was found that the specimens protected with the DCM had a FRR of R60. When NS and MS were added to its composition, a FRR increased to R90. The specimens protected with the DGMP, with and without NS and MS, presented a FRR of R90. The findings results obtained from the short steel columns protected with the different mortars types were in agreement with the ones obtained in the tests carried out on the protected steel plates, which makes these exploratory tests useful for preliminary selection of promising fire protection materials. Appl. Sci. 2022,12, 2093 17 of 20 4. Conclusions The objective of this work was to develop gypsum or cement-based mortars for passive fire protection and to evaluate the influence of aggregate size and the addition of silica microand nanoparticles on its thermal performance. The thermal performance of these mortars in short steel columns under a compression service load and subjected to high temperatures was assessed. The following conclusions can be drawn: • The furnace, the dimensions of the specimens, and the test procedure adopted in the tests at high temperatures allowed an adequate thermal exposure of the specimens. It allowed the evaluation of the thermal performance of the various mortars tested. • Some mortars developed in the laboratory (DCM, DGMP, and DGMV) have better thermal performance when compared with the best commercial solution tested (CM). Furthermore, in this set of mortars without nanoand microparticles of silica, the mortars with vermiculite in their constitution were those with the best thermal performance. • Most mortars with raw material milled through the Los Angeles method had better thermal performance than mortars developed with raw material processed through the Industrial Mill method. However, if nanoand microparticles of silica are added in their composition, the mortars developed with raw material obtained through the Industrial Mill method may have a slightly superior thermal performance. • The addition of nanoand microparticles of silica improves the insulating capacity of the mortars. • Overall, the results demonstrate that the reduction in grain size of the raw materials used (perlite and vermiculite) did not benefit the thermal performance of the tested compositions. • Regarding the cracking of the mortars, it was concluded that perlite (DGMP) contributes to its low value. It was also concluded that, in general terms, the addition of NS and MS tends to increase the cracking degree of the developed compositions. • The compositions that use gypsum as a binder (DGMP and DGMV) had the best thermal insulation capacity. Under the tested conditions, it was found that 10 mm of mortar coating was sufficient to form an efficient thermal barrier, reducing the ISSP temperature by approximately 70% of the temperature recorded inside the oven (900 ◦C). • The thermal protection level of columns with the mortar developed in the laboratory with the best overall thermal performance (DGMP with nanoand microparticles of silica) was 19% more efficient than the commercial solution and increases by 5.9 times the fire resistance of an unprotected short steel column. These results demonstrated the actual impact that the application of such mortars can have as passive fire protection of steels structures. Bearing in mind the experimental findings obtained in this research study, further experimental tests on short steel columns with the developed mortars and new ones by using different additives as well as with different loading conditions will be carried out in the near future. Author Contributions: Conceptualization, H.C. and A.S. (Aldina Santiago); methodology, H.C.; software, A.S. (Aldina Santiago); validation, L.L., L.D. and A.S. (Ashkan Shahbazian); formal analysis, H.C.; investigation, H.C.; resources, A.S. (Aldina Santiago); data curation, A.S. (Aldina Santiago); writing—original draft preparation, H.C.; writing—review and editing, L.L.; visualization, H.C.; supervision, H.C.; project administration, A.S. (Aldina Santiago); funding acquisition, A.S. (Aldina Santiago). All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Portuguese Foundation for Science and Technology (FCT), grant number PTDC/ECI-EGC/31850/2017. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Appl. Sci. 2022,12, 2093 18 of 20 Acknowledgments: The authors gratefully acknowledge the Portuguese Foundation for Science and Technology (FCT) for its support under the framework of research project PTDC/ECI-EGC/31850/2017 (NANOFIRE—Thermal and Mechanical behaviour of Nano Cements and their application in steel construction as fire protection) and also to the University of Coimbra (UC) for their support under the Scientific Employment Stimulus Programme given to the first author, as well as to the European Regional Development Fund, the European Social Fund, and European Structural and Investment Funds. This work was also financed by FEDER funds through the Competitivity Factors Operational Programme—COMPETE and by national funds through FCT within the scope of the project POCI01-0145-FEDER-007633 and through the Regional Operational Programme CENTRO2020 within the scope of the project CENTRO-01-0145-FEDER-000006. Conflicts of Interest: The authors declare no conflict of interest. Abbreviations CM Commercial passive protection solution used as a reference mortar CPPS Commercial passive protection solution DCM Developed cementitious mortar DGMP Developed gypsum mortar with perlite DGMV Developed gypsum mortar with vermiculite DRCM Developed refractory cementitious mortar EC Expanded clay EP Expanded perlite ERC Eletroland refractory cement ESMHT Exposed surface of the mortar to high temperatures ESSP External surface of the steel plate of the test specimen EV Expanded vermiculite GP Gypsum powder IGN Commercial passive protection solution 1 IM Industrial mill method IRC Isidac 40 refractory cement ISSP Inner surface of the steel plate of the test specimen LA Los Angeles method MS Microparticles of silica NS Nanoparticles of silica PC Portland cement CEM II/B-L 32.5 PP Polypropylene fibers PP/B Polypropylene fibers cement ratio in weight % RC Refractory cement RH Relative humidity SD Standard deviation SP Steel plate SS Silica sand SSC Short steel columns T Temperature t Time TH Thermocouple TRC Topeca M40 refractory cement USMHT Unexposed surface of the mortar to high temperatures VER Commercial passive protection solution 2 W Water W/B Water binder (cement or gypsum) ratio in weight % References 1. 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