Effect of Graphite Addition on Thermal Behaviour of Metakaolin-Geopolymer Coating for Fire Protection of Steel

Geopolymers are inorganic binders that resulted from the alkali activation of aluminosilicate powder. Usually, geopolymers possess better high-temperature resistance than Portland cement-based materials. This makes them a promising candidate for high-temperature applications as well as for fire protection coating. The geopolymers pastes have been the focus as an intumescent coating. This study emphasizes the geopolymer pastes from metakaolin and silica fume precursors with the additive of graphite powder and intumescent compounds. The coupons test was made of carbon steel plate 50mm x 50mm x 4mm. Since steel loses a major of its mechanical properties at 500°C, then the time-temperature curve for the coated-steel plates was created from fire testing, compared to the naked steel. Moreover, the visual appearance and the SEM structure were recorded. It is found that the addition of graphite to geopolymer improves the fire resistance of geopolymer but does not affect the intumescent behaviour. is found that the geopolymer paste graphite addition did not result in better fire resistance.


Introduction
Numerous buildings use steel as construction materials since it has compromised mechanical properties and economical concerns.Even though steel is not prone to fire, it has a considerable drawback as it loses approximately 50% of mechanical strength when exposed to temperatures around 500 o C.This brings to the risk of collapsing structure.An appropriate passive fire system provides adequate time to evacuate humans and properties [1].The usual technique of passive fire protection is the use of thinlayer fire-retardant coatings of intumescent type.In fire cases, the intumescent coating is able to produce a foam-oxide shielding layer many times thicker than the original coating [2].Whereas organic binders are usually liable to thermal attack and toxic release upon heating, then inorganic binders become a potential alternative material for an intumescent coating system.
Studies on a number of intumescent geopolymer coatings have been conducted by researchers [3]- [16].Saadi [11] found that graphite addition to geopolymer decreases the intumescent temperature, hence improving the protection of the substrate.A study by Watolla [16] concluded that the addition of sodium tetraborate pentahydrate to geopolymer prolongs the time to reach 500 o C when the coated steel is exposed to the fire test.Sarazin [3] reported that foam-coating metakaolin-silica fume geopolymer performed better thermal barrier than bulk geopolymer.Commonly intumescent coating is composed of active components bound together by an organic binder such as epoxy resin.The active components of intumescent are, carbon source, an acid source, and gas source.Carbon sources can be carbon-rich polyhydric compounds such as glucose, pentaerythritol, and starch.This compound is then dehydrated by an acid source promoting the development of carbonaceous char.Ammonium phosphate, diammonium phosphate, and other phosphates are the general compounds of acid sources.A gas source has the function to expand the coating during heating.It can be a nitrogen or halogen compound.The most usual is melamine, however, urea, or chlorinated paraffin are acceptable [17].
This study focused on evaluating the thermal properties of metakaolin-silica fume geopolymer with the additive of graphite powder and intumescent compounds.The compounds were mixtures of diammonium phosphate, starch, and urea, as there are easily found commercially in Indonesia.While these organic intumescent compounds were generally applied for organic binders, this research was concerned with the inorganic binder.

Methodology 2.1 Materials
Geopolymer precursors used in this study were metakaolin and silica fume.Metakaolin (MK) was prepared by calcinating kaolin at 750oC for 5 hours.These two materials are majorly composed of amorphous phases as identified from the broad peaks of the X-ray diffraction (XRD) pattern (Fig. 1, 2).

Figure 1. XRD pattern of metakaolin
Metakaolin used in this study were composed of silica 52.6% (SiO2) and alumina 43% (Al2O3).Silica fume was supported by PT BASF Indonesia with SiO2 content of 97.4%.The activator was a mixture of 23% NaOH solution and water glass (WG).Water glass had a composition of 15% Na2O and 30% SiO2; the rest is water.To increase fire resistance, this study used additional materials, namely graphite (G) and a mixture of diammonium phosphate -starch-urea (DSU).

Methods
The geopolymer paste composition is present in Table 1.The first step in comprising geopolymer paste was preparing a homogenous dry mixture of metakaolin, silica fume (and graphite for MS_G)).In a separate container, the activator was prepared by mixing the NaOH solution and water glass and then allowed to cool.The activator was then poured into a container containing the dry precursor and stirred until uniform with a mechanical mixer for about 4 minutes, yielding a neat geopolymer paste.The paste was applied on a surface-treated steel of 50mm x 50mm with a layer thickness of about 3 mm.The samples were marked as MS and MS_G.For the MS_GD sample, additive diammonium phosphate-urea-starch (DSU) was added to the neat geopolymer paste.This final geopolymer was applied to steel in the same manner as the former specimens.The coated specimens were left in the open air for one week before the fire testing.
The fire test was conducted by exposing the geopolymer-coated side of the steel coupon to flame with a temperature of around 1000 o C. The uncoated side of the coupon was connected to a thermocouple and the temperature was recorded every two minutes.The observation was conducted for up to 30 minutes or until the substrate temperature reached 500°C.Referring ISO 834-1:1999 standard, a temperature-time curve was made based on the recorded data.The scheme of the fire test is presented in Fig. 3.

Results and Discussion
Geopolymer coating application for MS and MS_G samples was quite easy due to its high workability.On the other hand, the MS_GD application is rather difficult because less workable.However, after 7 days cracks were found on MS (Fig. 4a) and MS_G coatings (Fig. 4b), while for the MS_GD (Fig. 4c) coating no crack was identified.The cracks of the MS sample (without graphite) appear to be deeper than those in the MS_G mixture (with graphite).The crack was probably due to water evaporation during the curing.This result is relevant to [18] state that higher water content tends to higher shrinkage which leads to coating delamination.
After the fire test, the MS and MS_G specimens were peeling off.Visually MS specimen has more severe cracks than the MS_G (Fig. 5 a and b).It can be said that the addition of graphite to the geopolymer slightly increases the fire resistance of the geopolymer For both sample MS and MS_G, the thickness difference before and after the fire test was not identified.It means that graphite does not affect the intumescent behavior of geopolymer, However, further research is still needed with more holistic data to study this feature.Otherwise, the MS_GD specimen remained well attached until the fire test was completed (Fig. 5c).This coating has a good adhesion even though undergoes shrinkage and swelling during curing dan fire test, respectively.The MS_GD layer experienced a remarkable expansion due to heating (Fig. 4 d).The thickness of the layer increased by 2.5-fold compared to the initial thickness.Although the expansion was not as significant as commercial intumescent coatings, the MS_GD coating provides adequate fire protection for steel substrates.This is shown in Fig. 6, the time-temperature curve for MS_GD compared to the naked steel.
Fig. 6 indicates that the MS_GD layer is capable to provide steel protection against exposure to fire at a temperature of around 1000 o C. Up to 30 minutes, the steel temperature is still around 350 o C. In contrast, the naked steel experienced a temperature increase to 500 o C in just 14 minutes.Observations with SEM (scanning electron microscope) were conducted to study the effects of adding graphite in terms of microstructure.Fig. 8 shows that the MS_G specimen has a more compact structure than the MS specimen (Fig. 8).In other words, the addition of graphite increases the geopolymerization reaction.However, the mechanism is still not well understood.8 shows that the paste structure is not homogeneous.It is suspected that the addition of diammonium phosphateurea-starch disrupts the geopolymerization.However, heating the MS_GD sample was not damage the structure, and even result in a more compact structure.Again, a comprehensive study needs to be conducted to identify the cause and mechanism.

Figure 3 .
Figure 3.The scheme of fire test

Figure 6 .
Figure 6.The time-temperature curve of geopolymer coated steel (MS_GD) and the naked steel.

Figure 7 .
Figure 7. SEM image of (a) metakaolin-silica fume geopolymer (MS) and (b) metakaolin-silica fume-graphite geopolymer (MS_G)Adding diammonium phosphate-urea-starch mixture to the geopolymer caused changes in the microstructure observed by SEM.It is shown by comparing Figure7(b) with Figure8(a).Figure8shows that the paste structure is not homogeneous.It is suspected that the addition of diammonium phosphateurea-starch disrupts the geopolymerization.However, heating the MS_GD sample was not damage the structure, and even result in a more compact structure.Again, a comprehensive study needs to be conducted to identify the cause and mechanism. Figure