To improve the workability in gypsum plasters, additives are sometimes used, including citric acid, which provides acceptable setting times for low w/g ratios, maximizing the mechanical properties of the material. The influence of citric acid on the fire response of gypsum coatings is not well known, and so our aim was to analyze the effects that citric acid produces on the behavior of gypsum plasters exposed to fire. Temperature measurements were made with sensors and thermal imaging cameras while other instrumental techniques, including SEM, XRD and TG, were used to characterize the microstructure and composition of gypsum materials subjected to the action of fire. The fire had a greater effect on gypsum plasters containing citric acid as revealed by the cracking patterns and heat propagation profiles observed. Likewise, micro-cracks were observed in gypsum specimens, containing and non-containing citric acid, exposed to fire. In all cases, the alterations were consistent with the temperature profiles and chemical composition of the faces whether exposed to fire or not.
Habitualmente, para mejorar la trabajabilidad del yeso se utilizan aditivos, entre ellos el ácido cítrico, que proporciona tiempos de fraguado aceptables para relaciones a/y bajas, potenciando al máximo las propiedades mecánicas de este material. La influencia del ácido cítrico en la respuesta frente al fuego de revestimientos de yeso no se conoce bien. El objetivo de este trabajo consiste en analizar los efectos que produce el ácido cítrico en el comportamiento frente al fuego del yeso. Para ello se usaron medidas de temperatura con sensores, cámara termográfica y otras técnicas instrumentales (SEM, XRD y TG) para caracterizar la microestructura y composición de los materiales de yeso sometidos a la acción del fuego. El fuego tuvo mayor efecto en yesos que contienen ácido cítrico, tal como revelaron los patrones de fisuración y los perfiles de propagación de calor obtenidos. Asimismo, se apreció formación de micro-fisuras en probetas de yeso aditivadas con ácido cítrico y sin aditivar, expuestas a fuego. En todo caso, la manifestación de las alteraciones fue coherente con los perfiles de temperatura y composición química de la cara expuesta y no expuesta al fuego.
Fire regulations require that construction and structural elements reduce to acceptable limits the risk for the users of a building that suffers damage in the face of a fire of accidental origin (
For many centuries, gypsum has been widely used in construction because, among other reasons, it is an inexpensive and abundant material. In addition, it is one of the most environmentally acceptable binders because it requires relatively low firing temperatures to obtain the hydraulically active form (bassanite or calcium sulphate hemihydrate). Gypsum plasters offer important features as easy installation and finishing (workability), good thermal insulation and, above all, suitable behavior against fire due to its ability to absorb a considerable amount of energy. In fire events, part of the energy is used to evaporate a significant amount of crystallization water from CaSO4·2H2O, thereby preventing the energy from contributing to increasing the temperature. (
The first scientific results on the thermal behavior of gypsum did not appear until the middle of the 20th century (
Time-temperature curves (normalized fires) are used to study the fire behavior of many materials, measuring the temperature of the air in the proximity of the surfaces of an element as a function of time. The Technical Building Code (
In this standard the following equation represents the theoretical fire (
where θg is the gas temperature on the surface exposed to fire (ºC), and t is time elapsing since the fire started (min).
One of the main features of gypsum is that it sets very quickly, which considerably limits its workability: the lower the w/g ratio, the higher the mechanical properties, but the lower the workability and open time. To solve this problem several types of retarders, such as polycarboxylic acids and more specifically, citric acid are used in gypsum plasters. Numerous studies have been published on the influence of citric acid as an additive in gypsum, mainly focusing on its influence on the microstructure (
However, the influence of citric acid on the fire behavior of gypsum is not well known, and, although several studies report on practical properties as workability, setting and open time of plasters, the influence of citric acid on the response of gypsum to fire has received less attention (
The commercial gypsum used in the study (CaSO4·½H2O) was supplied by a local manufacturer, and is a construction plaster classified as B1 according standard EN 13279-1 which establishes the characteristics that construction plasters must meet (
Citric acid was used as a setting retarding additive. As regards the time at which it should begin to set, the norm indicates that for manual application it must be greater than 20 minutes and greater than 50 minutes for mechanical application.
Gypsum specimens without additives were made up with w/g ratios of 0.4 and 0.7. W/g ratios in the order of 0.4 are common in commercial gypsum products. This ratio provides gypsums with high mechanical resistance, but excessively short workability times. For this reason, citric acid concentrations were varied until a setting start time of around 50 min was achieved, while the different doses were tested by studying the workability of each blend. The citric acid dose that led to an open time of around 50 min was 600 ppm (0.6 g of additive per kg of gypsum).
The standard EN 13279-1 covers the general definitions and requirements made for gypsum plasters (
Gypsum specimens were made with the following dimensions; 200 mm x 200 mm x 20 mm with w/g ratios of 0.4 and 0.7. In addition, 0.4 w/g specimens were made to which 0.6 g of citric acid was added per kg of gypsum (0.4 w/g + 0.6 g/kg C. A.) (
The specimens were cured for 90 days in a laboratory environment (23±3 ºC y 55±10% HR). Before carrying out the tests, the specimens were kept in an oven at a temperature of 40 ºC until the mass was constant.
The specimens were exposed to direct fire using a blow torch fed by propane gas, anchored to a mobile support with a graduated path, which was gradually moved from position 0 to position 5, and remained in each position for 2, 3, 4, 5, 6 and 10 minutes respectively. Through this procedure it was possible to vary the temperature reached in the exposed face (EF) from room temperature to approximately 800 ºC.
At the same time, images of the specimen surface not exposed to fire were obtained by thermographic camera in order to analyze both the temperature and the heat distribution. For this, the specimens were placed on a stainless steel support and the fire protocol described above was applied, performing 30 minutes tests in which a thermal image was recorded every minute using a FLIR T400 camera attached to a tripod in fixed position (
In addition, the temperature of the fire was measured on the exposed face by means of a chromel-alumel K type thermocouple located in the action center of the flame. In this case, thermographic measurement could not be used because the heat from the flame saturates the camera detector and is not capable of measuring the temperatures reached on the fire-exposed face. An Omron ZR-RX 25 Data Logger was used to record the temperature measurements made by the thermocouple.
SEM images were obtained from the untested gypsum samples and the samples that had been exposed to fire, in which case both the exposed face (EF) and non-exposed face (NEF) were recorded, using a Hitachi S-3500N scanning electron microscope. The SEM observation was made at a voltage of 15 kV and 2000 magnification using Backscattered Electrons (BSE).
The gypsum powder samples, the non-fire tested gypsum and the fire tested samples corresponding to the exposed face (EF) and the non-exposed face (NEF) were subjected to XRD analysis, which provided important chemical information about the mineral phases present since they are sensitive to temperature. The samples were gently ground in a mortar and the mineral phases were identified using the Cu K-alpha line using a Bruker D8 Advance powder diffractometer for powder analysis. A scan angle range (2-theta) of 10° to 70° with a resolution of 0.05° was used.
The powdered gypsum samples of the non-tested gypsum and the two samples corresponding to the exposed face (EF) and the non-exposed face (NEF) were studied by thermogravimetric TG analysis, which measures the variation in mass of a specimen as a function of temperature or time. For this, a Mettler-Toledo TGA / DSC HT thermogravimetric analyzer was used, whose horizontal oven has a temperature range running from room temperature to 1600 ºC with an accuracy of ± 0.5 ºC used with a heating rate of 20 ºC / min. N2 at a flow rate of 20 ml / min and O2 at 50 ml / min were used in the oven atmosphere.
A metal blade was used to take the samples, with which the most superficial layer of the samples was eroded. The amount of sample per test used was approximately 10 mg.
A previous XRD study carried out on the powdered gypsum confirmed the presence of bassanite (CaSO4·½H2O) as the main mineral phase with a much lower presence of anhydrite or anhydrous calcium sulfate (CaSO4), calcite (CaCO3) and dolomite (CaMg(CO3)2) (
Gypsum powder was also studied by TG. A first mass loss of 4.88% was obtained corresponding to the transformation of calcium sulfate hemihydrate (CaSO4·½H2O) into anhydrite (CaSO4) at around 130 ºC. Subsequently, at a temperature of approximately 670 ºC, a second mass loss of 2.49% was observed, again due to the decarbonation of calcite (CaCO3) accompanied by a loss of CO2 (CaO + CO2) (
Finally, a study of the material was carried out using XRF, from which the percentage concentration of the elements that make up the gypsum expressed in the form of oxides was obtained (
Concentration (%) | |
---|---|
Oxide | Binder B1 |
H2O* | 4.8772 |
CO2* | 2.4943 |
MgO | 0.80 |
Al2O3 | 0.38 |
SiO2 | 1.19 |
SO3 | 49.10 |
K2O | 0.16 |
CaO | 40.47 |
TiO2 | 0.02 |
Fe2O3 | 0.19 |
SrO | 0.23 |
Other | 0.09 |
*: H2O and CO2 were determined by TG.
Once set and cured, the heat transfer of the tested plasters increases with their density (
If the results obtained for the ratios 0.4 w/g and 0.4 w/g + 0.6 g/kg C. A. are compared, it can be seen that upto about minute 7 both the maximum and mean temperatures increased at a similar rate. From that moment onwards, the slope flattens out substantially in boths, corresponding to the evaporation of part of the crystallization water of the gypsum. This plateau lasted about 2 minutes less in the samples containing citric acid, indicating that the generation of CaSO4·2H2O by hydration of CaSO4·½H2O was hampered in the presence of citric acid, as demonstrated in recent studies (
However, the average temperatures obtained throughout the test were very similar. Three tests per dosage were performed, confirming the trends described and the consistency of the measurements (
The lowest degree of heat transmission almost throughout the test was that of the 0.7 w/g gypsum board, although during the final few minutes, the temperatures were slightly higher than for the 0.4 w/g ratio probably because its greater porosity led to a greater alteration due to exposure to fire. Little difference was observed between the gypsum with and without added citric acid for the same w/g ratio. For the 0.4 w/g ratio, the temperature peak was lower than for the 0.4 w/g ratio + 0.6 g/kg C. A., but the heat was distributed over a wider area of the specimen, which meant that the average surface temperatures of the specimens were practically similar.
The slightly different behavior of gypsum with added citric acid from the gypsum without additive could be due to the effect of citric acid on the microstructure of gypsum, which would affect the transmission of heat through the plate.
Once the 30-minute fire test had finished, photographs of the face that had been exposed to fire (EF) were taken.
As can be seen, citric acid has a strong effect on the gypsum microstructure. The specimens that do not contain citric acid had acicular crystals with twins and crystals behaved differently had a or had crystalline morphologies (
After the test, the specimens exposed to fire from both the non-exposed face (NEF) and the exposed face (EF) were observed by scanning electron microscope. As can be seen in
From the analysis of the results obtained, it is clear that, once cured and before being tested by fire, the gypsum is rich in calcium sulfate dihydrate, with small amounts of bassanite and anhydrite, as expected in this type of material (
Additional XRD tests were carried out on both faces of the specimens tested by fire, to verify the effect of temperature on the mineral phases described above. On the non-exposed face (NEF), the greater presence of the most hydrated mineral in the series (gypsum) was clearly observed for the sample without citric acid (
However, the results obtained on the exposed face (EF) indicate that after that time of exposure to fire practically all of the gypsum had become anhydrite (
A thermogravimetric study allows the degree of hydration of gypsum to be based on its resistance and degree of exposure to fire (
In specimens not subjected to fire (
The second and third mass loss in all the thermograms corresponded to the thermal decomposition of carbonates (calcite) into calcium oxide and carbon dioxide at around 650-700 ºC and the decomposition of anhydrite sulfate (CaSO4) in calcium oxide and carbon sulfur dioxide at 1300-1350 ºC.
In the thermograms made on specimens from the face not exposed to fire (NEF), after 30 minutes of testing, the three above described steps are also evident (
This result corroborates the analyses obtained for the NEF in the XRD, where more intense diffraction signals of the gypsum mineral were observed for the 0.4 w/g dosage with respect to the same sample with citric acid. There was also a small difference in mass loss between the 0.4 and 0.7 w/g coating which was slightly greater in the latter.
Again, there a second mass loss occurred between 650 and 700 ºC due to the thermal decomposition of carbonates (calcite) and a third loss between 1300 and 1350 ºC due to the decomposition of anhydrous calcium sulfate (anhydrite).
These results are very different in EF (
Small differences were found in the CO2 mass loss associated with carbonates. The specimen least affected by exposure to direct fire was 0.4 w/g since the loss of CO2 was 1.46% which was higher than that detected in the 0.7 w/g and 0.4 w/g samples with added citric acid (0.64% and 0.34%, respectively). It is important to emphasize that on the face exposed to the fire, the sensors recorded temperatures at around 800 ºC. Therefore, the duration of the experiment was prolonged by exposing the coating for 30 minutes to temperatures high enough to partially decarbonate the carbonates and even eliminate them in their entirety. Differences in the compactness and microstructure of the coatings may explain the differences observed in the thermograms of
When citric acid was used, slightly higher temperatures were recorded in the area of direct exposure to fire than when the additive was not used for the same w/g ratio
On the face exposed to fire (EF) there were fewer but thicker cracks in the coatings made with citric acid. Whether or not citric acid was used, the cracks were clearly concentrated in the area exposed to fire. Therefore, such cracks may be related not only to the expansion and contraction processes (thermal stress) that the material may suffer in a fire, but also to substantial modifications of the coating composition.
The XRD tests performed on the face not directly exposed to fire (NEF) confirmed that at a depth of 2 cm from the surface of the coating, the material retained a large part of the minerals originally present. However, the first symptoms of gypsum dehydration due to the formation of bassanite (CaSO4·½H2O) at the expense of gypsum (CaSO4·2H2O) were evident.
From a practical point of view, the experimental results indicate that a 2 cm thick gypsum coatings provides a quite effective protection against fire, taking into account the high temperatures reached on the exposed face (around 800 ºC). The integrity of the material was confirmed not only by temperature measurements but also from the relatively high concentration of hydrated minerals such as gypsum and bassanite.
In general, all the studies carried out by SEM, DRX and TG confirmed that fire has a greater effect on gypsum containing citric acid. In the absence of citric acid, better the behavior against fire is better as it does not inhibit the formation of the mineral CaSO4·2H2O. The evaporation of the crystallization water of this mineral delays the increase in temperature of the sample and, with it, the destruction of other mineral phases of the coating, which may influence the behavior of gypsum coatings for passive protection of other construction and structural elements.