INFLUENCE OF SELECTED MICRO ADDITIVES CONTENT ON THERMAL PROPERTIES OF GYPSUM

Modern countries and global economies understand and see the needs of environmental protection. It aims at lower greenhouse gas emissions, protection of water and soil, noise reduction, waste management, recycling, the search for alternative energy sources and broadly understood energy saving [1, 2].

Currently, a very broad field of science within the framework of modern, ecological pro-environmental material solutions is the search for insulation and building composites with low values of the thermal conductivity coefficient λ.

Gypsum is a construction material with a very wide application. It is perceived as safe, useful and environmentally friendly. In construction, gypsum is used to perform: internal plasters, decorative architectural details, statues, plasterboards, floors and construction blocks, as well as mortars and glues. This wide usefulness is an effect of the universal and positive physical properties of the material. Gypsum components are completely odourless, environmentally friendly and fire resistant. They also provide thermal and acoustic insulation. Moreover, gypsum possesses the natural mechanism to hygrothermally balance an indoor environment [3]. Excellent performance, attractive appearance, easy application, and its healthful contribution to living conditions have made gypsum one of the most popular finishing materials [4–6].

Gypsum can be modified with various additives. The use of various types of chemical additives may cause differences in the properties of gypsum. Differences in material properties and applications are caused by several kinds of chemical additives: accelerators, retarders, starch, sugars, water-resistant, cellulosic and fiberglass fibres, vermiculite, and others [7–9]. The other group of admixtures are water reduction agents, mainly polymers and copolymers, e.g., ligno-sulfonate, naphthalene sulfonate, and acrylic-polyether. These polymers not only affect water share in a slurry, but also change other physical properties. Therefore, any modifications of gypsum properties affect its useful properties and applicability [10]. The development of thermo-insulating materials has also brought about investigations into other additives such as aerogels [11–13] and microspheres [14, 15] which have insulating properties better than foamed polystyrene. In the presented study, micromaterials were used as an additive, which affects gypsum properties during the aging process. The non-stationary method of measurements allowed determination of thermal properties, especially conductivity, specific heat and thermal diffusivity during the first 28 days. The knowledge of the values of thermal conductivity λ of many materials is very important during engineering practice and in experimental research. Together with specific heat C_p, density ρ and thermal diffusivity a, it is one of the most important parameters of physical and chemical substances.

The main purpose of this study was experimental investigation of gypsum thermal behavior and properties. Gypsum was modified by an addition of microspheres, aerogel and polymer (hydroxyethyl methyl cellulose – HEMC) in different contents. Researchers have studied the thermal conductivity λ of modified gypsums. Selected types of micro additives are widely used in the building material industry and there is a lack of research concerning the thermophysical properties of unripe gypsum composite for aerogels and microspheres.

The research results for three micro additives were presented in the work. They are different in terms of physicochemical properties. In literature, there are no references to comparative research of these composites. In this work, the authors proposed a broader, new view of the problem. Additional measurements were proposed of specific heat C_p , thermal diffusivity a and density ρ.

There is a great deal of scientific research on micro additives applied in concrete. There are few papers about application of micro additives and their influence on thermal parameters of gypsum. Microspheres, aerogels and polymers are well-examined materials. Heim et al. [3] has done some extensive research on how the addition of 1% of polymer to gypsum affects the mineral.

The authors of the current research have decided to apply micro additive of aerogel, also in the amount of 1% for the comparison of both components. Kwan and Chen [15] suggested application of microspheres in concrete in various amounts, including minimal addition of 10%. In the case of gypsum, due to its specific nature in comparison to cement, this amount of additive should be treated as a maximum.

The research was carried out using a non-stationary method with the Isomet 2114 experimental set up for gypsum specimens: 100 × 100 × 100 mm. Each specimen was tested eighteen times. The experiments were devoted to determining thermal conductivity, specific heat and thermal diffusivity for gypsum specimens after hydration of hemihydrate calcium sulphate and in aging of the material for 28 days.

The equation for temperature distribution is derived by considering an infinitely small volume within the medium subject to temperature gradients. In Cartesian coordinates the control volume is a cube of dimensions dx, dy and dz as shown in Fig. 1 [16].

Figure 1.

Fluid flow through a parallelepiped

We will also assume that there is a source of heat generation within the medium, such that the local volumetric heat generation rate is S. The medium has a conductivity λ, a density ρ and specific heat C_p .

The principle of conservation of energy is employed to derive the required equation by balancing the rate of heat storage within the control volume against the net heat input rate. Consider first heat flowing in the direction of the x axis. The component of heat flux entering the left-hand face of the cube is q ˙ x and the component of heat flux leaving the right-hand face is q ˙ x + d x where:

If thermal conductivity λ is constant and there is no internal heat generation then:

where

is the thermal diffusivity of the medium.

Coefficient a shows the speed with which the temperature from one plane to another changes, that is, receptivity of p=terial to compensation of temperature while heating up or tooling in specific places. Specific heat of solids and liquids is a feature depending only on the chemical structure of these bodies and not depending on their shape and size. Specific heat of most of the substtnces changes insignificantly with changes of temperature even within the one state. Thermal conductivity is one of the irreversible phenomena. It is a symptom of reaction of the thermodynamic system to the disturb ance of the equilibrium state. This reaction aims at liquidation of the occurred disturbance.

In the experimental work [3] the scope of literature knowledge related to thermal research is described very precisely, in particular with determining the thermal conductivity of building materials.

The material, mechanical and thermal properties of pure gypsum and its components are very well known and described in the literature [17–19]. As a traditional, unmodified building product, it has the thermal conductivity λ, varying in the range of 0.23 to 1.00 W/(m⋅K), and density ρ = 1000 kg/m³ for gypsum boards and blocks. The averaged specific heat is approximately 840 J/(kg⋅K) [3, 19]. The influence of various additives like: aerogels, microspheres and polymers on the thermal conductivity of gypsum is currently not well recognized.

General and basic information of porous materials can be found in Carson et al. work [20]. Some investigations were done for vermiculite [21], rocks [22] and cement [23–25]. In the case of concrete [23], the effects of inorganic polymer on pore size and thermal conductivity are substantial. Hydrate additives in cement mortar were investigated by Choi and Noguchi [26]. They showed that these hydrates then fill up the pores in the hardened cement.

A more homogenous and denser cementitious matrix can be obtained using superabsorbent polymers (SAPs) [27]. The effect of strengthening the structure of cement was changed by three different types of polymers (PVAA, MC i HEC) and described by Knapen and Van Gemert [28].

Hydroxyethyl methyl cellulose is a nonionic polymer characterized by high viscosity and nontoxic and water-soluble properties. HEMC is one of the cellulose ethers and is widely applied in the building construction sector. It can be used to modify building materials that are made based on any mortars, such as cement or gypsum [23, 25]. The particle size of polymer added to gypsum was in the range 150–250 μm. Nowadays aerogels are one of the best thermal insulation materials. For this work, silica aerogel in the form of particles of fraction 0.7–4.0 mm was used [29]. The granules feature hydrophobic properties and their ipecific density ranges between 120 and 150 kg/m³. They are the only fire resistant materials that offer theimal conductivity values as low as 0.012–0.018 W/(m K) without the need for vacuum or gas sealed systems [30]. This is achieved by forming the structure in a supercritical drying process. Supercritical drying is performed to replace the liquid in a material with a gas to isolate the solid component from the material without destroying the material’s nanostructured pore network of diameter of approximately 20 nm.

Microspheres are light, thin-walled hollow spheres which are by-products of the combustion of pulverized coal at thermal power plants. Due to their properties they are a potentially interesting filler and may be used for cement-based composites production [31]. The particle size of microspheres from fly ash, added to gypsum was in the range 50–150 μm. The main chemical constituents in used microspheres: Al₂O₃ (34–38%), Fe₂O₃ (1–3%), SiO₂ (50–60%), CaO (1–4%), MgO (0.2–2%) and TiO₂ (0.5–3%).

Micro additives are new and little-known materials used in construction. Even when thermal properties of pure gypsum have been precisely investigated, there are no experimental studies and analyses, especially comparisons of novel gypsum composites modified with aerogel, microspheres and polymers, which have been considered in the presented paper.

Figure 2.

Experimental setup to measure conductivity λ, specific heat capacity C_p and thermal diffusivity a: 1 – tested specimen, 2 – surface probe, 3 – microprocessor-controlled Isomet instrument, 4 – PC computer, 5 – AC/DC power supply

Gypsum is a traditional construction material with a relatively low density under normal production methods. The main constituent of commercial gypsum plaster is calcium sulphate hemihydrates (CaSO₄⋅0.5H₂O). Hemihydrate gypsum can be obtained in two main phases, namely and phases. -hemihydrates gypsum is frequently used in the construction industry. phase achieves a certain level of fluidity with much less water. Due to its better workability and higher strength, -hemihydrates gypsum has been applied in moulding, special binder systems, and dental materials, as well as the construction industry [32, 33].

As a starting material, the natural gypsum powder CaSO₄⋅0.5H₂O (hemihydrate) widely available on the market and meeting the standard requirements was selected.

Distilled water was used for mixing gypsum with the polymer, aerogel or microspheres addition. The properties of a gypsum block were characterized by density ρ = 1.026 kg/m³ and conductivity λ = 0.3113 W/(m⋅K) after 28 days of ß hemihydrate hydration (in a dry mass state). Other specimens were produced using pure gypsum by mixing with a water solution of the polymer, aerogel or microspheres additive. Polymer additives were used as a cellulose ether methyl 2-hydroxyethylcellulose produced by Sigma-Aldrich (Warsaw, Poland). Aerogel was translucent material produced by Cabot Corporation (Leuven, Belgium) and microspheres were produced by Eko Export Inc (Bielsko-Biala, Poland).

The mixture was prepared from 2 kg of gypsum powder mixed with micro additives (HEMC or aerogel or microspheres) dissolved in 1.5 L of distilled water. Components were stirred using a slow rotary agitator for 1 minute at a temperature of 20°C. The water-to-gypsum ratio was assumed to be constant at the level of w/g = 0.75. The gypsum slurry was modified by addition of HEMC in the amount of 1%, aerogel 1% and microspheres 10% of gypsum. After the mixing process, the mixture was poured into a cube-shaped form.

Three measurements were carried out using a measuring set up (Fig. 2): thermal conductivity, volumetric heat capacity and thermal diffusivity. The specific heat was obtained by dividing the measured volumetric heat capacity by the bulk density of the material. This method is used by many researchers [29, 34–36] and based on non-stationary measurement. Its measurement is based on the analysis of the temperature response of the analysed material to heat flow impulses.

This is a microprocessor-controlled commercial instrument for direct measurement of the thermal properties of materials by means of exchangeable probes. The signal from the probe was sent to a computer by serial port RS232C and recorded.

This is a transient method for determining thermal conductivity. During the course of the measurement, a known amount of heat produced by the line source results in a heat wave propagating radially into the specimen. The temperature rise of the line source varies linearly with the logarithm of time. This relationship can be used directly to calculate the thermal conductivity of the material [34, 36].

In this experiment the given measurements were carried out by a surface probe with a built in memory and calibration constants stored in the memory. In principle, the time dependence of thermal response on pulse transmitted from the heat flow into the material being measured is analysed. The heat flow is generated by dissipated electrical energy by means of the probe that is in direct contact with the material being measured. Temperature, depending on resistance, is sensed by a semiconductor sensor and a time change in the temperature is sampled in discrete points - regression polynomials that pass through the specimens are constructed using the least squares method and coefficients of relevant regression polynomials enable the analytical calculation of required parameters.

The thermal conductivity of gypsum as fresh and aging specimens was established using the non-stationary method. The results were obtained for each of four specimens after 1, 3, 7, 14, 21, 28, and 35 days of experiment. Specimens were conditioned in temperatures of 20–22°C and RH = 52 ± 2%. After 28 days, the specimens were dried at a temperature of 65°C for 7 days. Each specimen was tested eighteen times and, finally, the results were averaged. The effect of contents of polymer, aerogel and microspheres after gypsum aging on thermal conductivity, specific heat and thermal diffusivity were also analysed. The history of conductivity up to 35 days is presented in Figs. 3a–3c. The measurements done for individual days show the same effect. The values for gypsum modified by polymers, aerogel and microspheres always have lower conductivity than the pure gypsum specimens (Fig. 4). After 28 days, the gypsum specimens reached the air-dry state.

After 35 days the gypsum with polymer content resulted in more than 15% lower thermal conductivity in comparison to the specimen without HEMC. The gypsum with aerogel and microspheres content resulted in more than 8% and 7% respectively lower values in comparison to the pure gypsum without micro additives.

Figure 3.

Thermal conductivity changes of gypsum specimens during aging process for pure gypsum with different additives: a) polymer, b) aerogel, c) microspheres

Figure 4.

Thermal conductivity changes of modified gypsum specimens during aging process between 14 and 35 days of measurement

In the case of specific heat and thermal diffusivity, results of both parameters were higher for gypsum with micro additives in comparison to the specimen of pure gypsum. After 35 days an increase of specific heat and thermal diffusivity with added micro additives was observed. Specific heat increased in the range of 5–7% in comparison to specimens with pure gypsum and thermal diffusivity that increased almost 8%. The history of specific heat and thermal diffusivity up to 35 days is presented in Figs. 5a–5c and Figs. 6a–6c.

The results of specimens with thermal properties after 35 days, standard deviations and uncertainty of variation u are presented in Table 1.

The uncertainty of the absolute measurement of the thermal conductivity of gypsum can be described as u(λ) = Δλ. Similarly for the specific heat u(C_p ) = ΔC_p and thermal diffusivity u(a) = Δa. Because the real value of thermal conductivity is unknown, it can be assumed with a reasonable probability that it falls within the range of λ-Δλ≤λ≤λ+Δλ. Similarly for the other parameters C_p-ΔC_p ≤λ≤ C_p + ΔC_p and a - Δa ≤ a ≤ a + Δa at 95% confidence. Both analyses of average values and standard deviation, as well as conducted statistical tests show that differences between the values of thermal properties are statistically significant in almost all cases. The only case with no statistically significant difference between the results is the value of thermal diffusivity for pure gypsum and gypsum with microspheres.

Additional physical properties of gypsum specimens with and without micro additives addition after an aging period were obtained using the standard test method. Bulk density was determined as a ratio of mass and volume of the gypsum specimens. Total porosity was calculated based on bulk density with reference to the density of the structure. The results of specimens’ bulk density and total porosity are presented in Table 2. The specific density of gypsum was ρ = 2.350 kg/m³ for all specimens. The bulk density of a specimen decreases and the porosity increases with different micro additives in the gypsum product. The total porosity was calculated from the formula:

where: ρ _b is bulk density of specimens and ρ is the specific density of pure gypsum.

Figure 5.

Specific heat changes of gypsum specimens during aging process for pure gypsum with different additives: a) polymer, b) aerogel, c) microspheres

Figure 6.

Thermal diffusivity changes of gypsum specimens during aging process for pure gypsum with different additives: a) polymer, b) aerogel, c) microspheres

During the first few days of hydration, the specimens contained the water that was not used in chemical processes and evaporated during aging. The higher density corresponded to the higher water content. The specimens with micro additives content are characterized by lower bulk density and lower thermal conductivity, higher specific heat and lower thermal diffusivity in comparison to pure gypsum specimen. The relations between the bulk density and thermal properties during the aging process are presented in Figs. 7a–7c, 8a–8c and 9a–9c.

Figure 7.

Thermal conductivity coefficient versus bulk density of gypsum specimens with different moisture contents and different micro additives: a) polymer, b) aerogel, c) microspheres

Figure 8.

Specific heat versus bulk density of gypsum specimens with different moisture contents and different micro additives: a) polymer, b) aerogel, c) microspheres

The correlations between thermal conductivity, specific heat, thermal diffusivity and density are presented in Figures 7–9. These relations do not explain behaviour of materials and their mechanisms. However, obtained correlations seem to be interesting and necessary for gypsums with micro additives presented in the publication. The thermal properties mentioned above can be calculated on the basis of simple measurement of density. Coefficients of determination R2 in table 3, show high conformity of obtained measuring points with proposed mathematical correlations.

Figure 9.

Thermal diffusivity versus bulk density of gypsum specimens with different moisture contents and different micro additives: a) polymer, b) aerogel, c) microspheres

For all the studied gypsum specimens generalized dependencies have been proposed (5–7):

where constants of equations A, B, C, D, E and F are presented in Table 3.

The correlations for different micro additives may change. More variables such as stability, viscosity, segregation degree and interfacial characteristics should be taken into account in the future to get information about behaviour of the materials. The examination of these parameters will be interesting and lead to better knowledge of the properties of modified gypsum.

For gypsum with a content of polymer, the bulk density is 4% lower than for pure gypsum specimens and for gypsum with a content of aerogel and microspheres, the bulk density is 8% lower respectively. The bulk density of gypsum specimens also changed during gypsum hydration as an effect of noncrystallizable moisture released during deceleration [37]. The greatest changes were observed during the first 7 days from the moment of specimen preparation. After that period, the noncrystallizable moisture diffused and the gypsum specimens achieved the air-dry state. Specimens with higher bulk density and λ > 0.5 also signify lower porosity, where the porous are filled with water. In Fig. 10, the common effect of conductivity changes versus bulk density and type of micro additives are presented. The main differences are visible for low bulk density, which is characteristic of the specimens after 21 days of hydration. All micro additives caused a decrease in thermal conductivity. However, the lowest thermal conductivity values were obtained for the polymer.

Figure 10.

Thermal conductivity coefficient versus bulk density for different type of micro additives content after 35 days of aging

The current study, described in this paper, targeted experimental investigations of thermal properties (thermal conductivity, specific heat, thermal diffusivity) of micro additives modified gypsums in the setting and aging processes. To achieve this aim a non-stationary method with the Isomet 2114 experimental setup was used.

An additive in the form of micro additives changes the structure of the new composite gypsum, which is reflected in the density and thermal properties of the final product.

On the basis of results analysis of applied research on the thermal properties changes of modified gypsum in its setting and aging process, the following conclusions can be presented.

Modification of pores by micro additives leads to an observed decrease of thermal conductivity and increase of specific heat and thermal diffusivity. The analyzed new gypsum composites are thus environmentally friendly materials with improved insulating performance.