Variation of Wave Velocity and Porosity of Sandstone after High Temperature Heating

This paper reports the variations of mass, porosity, and wave velocity of sandstone after high temperature heating. The range of temperature to which the sandstone specimens have been exposed is 25–850°C, in a heating furnace. It has been shown that below 300°C, porosity and wave velocity change very little. Above 300°C, there is a rapid increase in porosity, but the wave velocity decreases significantly. The results of thermo gravimetric analysis (TGA), differential scanning calorimetry (DSC) and mercury intrusion porosimetry (MIP) suggest that a series of changes occurred between 400 and 600°C in sandstone could be responsible for the different patterns of variation in porosity and wave velocity.


INTRODUCTION
The properties of rocks at high temperature or after heating at high temperature are parameters of growing importance in engineering projects and studies of geological disasters and geological structure formation (Just and Kontny 2012), such as deep geological repositories for heat-generating radioactive wastes (Laloui et al. 2008, Zhao et al. 2009), exploration of geo-thermal energy (Zhang et al. 2001, Zhang et al. 2008, Ranjith et al. 2012, deep petroleum boring (Wu et al. 2003, Shafiei andDusseault 2013), underground gasification of coals (Wu et al. 2005, Tian et al. 2012) and the protection of buildings against fire or building restoration after exposure to fire (Hajpál 2002, Wu et al. 2013. The knowledge on permeability and velocity of wave propagation in rocks under different geological conditions is required in the engineering exploitation of geothermal resource and oil or gas (Somerton et al. 1965, Gong andXie 1989). It is generally accepted that the wave velocity decreases with temperature both under and after high temperature heating treatment (Somerton 1992). However, the porosity increases with temperature (Hajpál 2002) with the increase of porosity, the permeability is also increased gradually (You and Kang 2009).
In most rocks subjected to geological hazards and rock engineering, cracks play an important role, and key information on the structure of cracks in rock mass could be reflected in wave velocity. Variations in the velocity at different temperatures have been considered as a possible precursory phenomenon has been used to evaluate the deformation and fracture feature of rocks (e.g., Lebedev and Zharikov 2000, Qin et al. 2009, Wu et al. 2013, Brantut et al. 2013.
In the past few decades, considerable experimental effort has been taken to quantify the relation between wave velocity and cracks (or stress) of rock. Experiments on a variety of rock types involved measurements of typical wave velocities close to rock damage (e.g., Rao and Murthy 2001, Nasseri et al. 2007, Chaki et al. 2008, Lokajíþek et al. 2012, and the effect of fracture on wave velocity is sufficiently understood. However, the mechanism of wave velocity and porosity variation at high temperature is not clear, probably because the physical and chemical thermal process of rock is not precisely known and the effect of mineral composition is not well understood.
In order to identify the damage level of thermally-cracked sandstone based on the variation of its physical parameters, the variations of some physical properties of sandstone, including mass loss level, porosity, wave velocity, and surface features, were examined after heating treatment in this paper. The heating temperature was varied in a large range, from 25 to 850qC, using a furnace in the laboratory.

EXPERIMENTAL TESTS
Sandstone samples were obtained from Linyi, Shandong province. X-ray fluorescence analyzer showed that the oxides incorporated in sandstone include SiO 2 , Al 2 O 3 , K 2 O, Na 2 O, MgO, CaO, Fe 2 O 3 , CO 2 , etc. (Table 1). X-ray diffraction (XRD) analysis showed that the main components are quartz, feldspar, dolomite/ankerite, and hematite/magnetite structures ( Fig. 1), accompanied by a small amount of biotite and kaolinite structure. Therefore, the cement of the sandstone is of ferrigenous, dolomitic, and/or clayey type. These samples with average bulk density of 2.41 g/cm 3 at room temperature were cut into ĭ 50 × 100 mm cylinders. The mass, volume, porosity, and P-wave velocity of these specimens were tested before and after heating. The porosity was measured by a microporosity structure analyzer apparatus (type 9310) produced by Micromeritics Instrument Corp., and the P-wave velocity was simultaneously collected by a TICO test machine.
The heating apparatus consists of a high temperature furnace (type MTS652.02). The specimens used in all the following tests were placed in the furnace and heated to the designated temperature (25, 75, 100, 150, 200, 250, 300, 330, 340, 350, 400, 450, 500, 550, 570, 585, 600, 700, 800, and 5 10 15  850qC, respectively) at the rate of 30°C/min. Each specimen was kept at its designated temperature for about 30 min before the power was cut off, and the specimen was allowed to cool down naturally with the decline of temperature in the furnace.
The mass, volume, porosity, and P-wave velocity of the specimens were tested before and after being heated. The thermo gravimetric analysis (TGA) and differential scanning calorimetry (DSC) of the sandstone specimens were also made with a synchronous comprehensive thermal analyzer (type STA409C) produced by NETZSCH Co., Ltd. In these tests, the heating rate was 5 k/min, and the gas flow rate was 100 mL/min.

Variation of surface features
The variations of colour and surface features of the heated sandstone specimens are shown by Fig. 2. From 25 to 400°C, the colour of sandstone faded gradually. Above 400qC, sandstone turns into brick red. Figures 3 and 4 also show that when the heating temperature is higher than 400°C, the colour of sandstone is changed significantly (Su et al. 2014). This change resulted from the loss of water of different kind. Free water would escape at around 100qC and the bound water escapes between 100-300qC. Above 300qC, the loss of crystal water and structural water would lead to reduced hydroxyl (-OH), damage of mineral crystal lattice skeleton, increased sandstone defects, and colour change. Additionally, dolomite/ankerite and magnetite, carbonates and organic materials in sandstone are disintegrated into oxides, carbon dioxide, and water when the temperature is above 300qC (especially between 300 to 600qC).

Variation of mass
The variation of mass loss level versus temperature is shown in Fig. 5. Combining the results of TGA and DSC analyses (as shown in Fig. 6), three phases can be identified in the curve shown in Figs. 5 and 7.
(1) Room temperature to 300qC. When the treatment temperature is lower than 300°C, mass loss is generally caused by the evaporation of water. In the process of heating, the state of the water existing inside sandstone would change. The absorbed water, bounded water, and mineral water (i.e., crystal water, structural water, and zeolite water) would escape from sandstone at different temperatures. Free water would escape at around 100°C, and bounded water escapes between 100-300°C.
(2) 300 to 700°C. Above 300°C, the loss of crystal water and structural water would lead to the loss of hydroxyl (-OH), damage of mineral crystal lattice skeleton and increase in the number of defects of sandstone. Additionally, carbonates and organic materials in sandstone are disintegrated into oxides, carbon dioxide, and water when the temperature is above 300qC (especially between 300 to 600°C). At roughly 573°C, quartz experiences a phase transformation from the Į phase to ȕ phase (Glover et al. 1995); in the   range of 470-520°C and above 540°C, dehydration reaction occurs in kaolinte (as shown by Eqs. 1 and 2). Between 400 and 600°C, especially from 500 to 600°C, chemical changes would occur to the minerals such as dolomite/ankerite, magnetite, illite, and kaolinite.
(3) Over 700°C. When the treatment temperature is higher than 700°C, there is a significant reduction in mass (shown in Fig. 5 and the TG curve in Fig. 6). At roughly 736.9°C, there is an endothermic peak in the DSC curve, corresponding to the change of potassium feldspar into potassium microcline.

Variation of porosity
The porosity of sandstone after being heated at different temperatures was obtained by Mercury intrusion porosimetry (MIP). The test range of pore diameter is between 0.001 to 200 ȝm. According to the pore size (pore diameter represented by d), the pores can be divided into four levels: (i) macropore d > 100 ȝm) -pore liquid can flow freely; (ii) medium-pore (100 t d > 10 ȝm) -in the natural state, and when certain head pressure is imposed, pore liquid can flow; (iii) micro-pore (10 t d t 1.0 ȝm) -under high pressure, the liquid can flow, but the permeability is very low; (iv) smaller micro-pore (d < 1.0 Pm) -the liquid usually cannot flow.
The variation of total porosity versus temperature is shown in Fig. 7. Below 400°C, the porosity changes very little. After that, it increases with temperature. At 600°C, the porosity is nearly 1.23 times the initial value (under 25°C). Figure 7 also shows a plot of porosity under the atmospheric conditions as a function (shown in Eq. 4) of temperature. The increment of porosity with temperature below 400°C is small, but becomes significant from that temperature onwards. The variation of porosity with temperature may be either caused by thermal expansion and altered micro-crack network, or driven by the structural damage of rocks (Hajpál and Török 2004). The cementing materials and initial porosity of sandstone play very important roles in this process. where n is the porosity, and T is the temperature [qC]. Scanning electron microscope analyses were used to record different types of cement materials in the studies of surface grains (Fig. 8). Crystal morphology was used in the identification of mineral phases. The sharpedged ( Fig. 8a and b), opened cracks at minerals contacts ( Fig. 8c and d) or even inside of the minerals (Fig. 8e and f) were formed owing to thermal expansion. Figure 8 shows that with the increasing temperature, the number of microcracks increases significantly, especially at temperatures higher than 400°C, consistent with the MIP result. Additionally, when the temperature is lower, cracks only develop at the edges of grains. When the temperature reaches a threshold, the cemented mineral crystals in sandstone rupture. For example, in Fig. 8f we can find that a large number of fractures have developed on the idiomorphic dolomite cement on quartz grain in the carbonatecemented sandstone. Porosity increases with the heating temperature; bulk density and wave velocity decrease with the heating temperature.

Variation of longitudinal wave velocity
The velocity of longitudinal or P-wave propagation in the sandstone specimens was examined before and after heating treatment. As wave velocity is closely related to other physical and mechanical properties (density, moisture content, structural characteristics, porosity, etc.) of sandstone, the variation of wave velocity due to high temperature may reflect the variation of these properties of sandstone samples. Therefore, determining the change in wave velocity after heating may provide a means of evaluating the effect of heating. Figure 9 shows that the wave velocity decreases with increasing temperature during heating treatment. The gradient of the curve indicates that there is little variation in wave velocity at temperatures below 300°C, but between 300 and 850°C (especially between 340 and 600°C), there is a significant reduction in wave velocity. Figure 10 shows that the change of wave velocity is correlated to thermal expansion and the variation of micro-crack network, or driven by the structural damage (mass loss level). From the same figure, we may infer that at temperatures higher than 300°C, the temperature effect on wave velocity should be mainly caused by changed mineral content.
In the range of 300 and 600°C, the mineralogical composition and texture of sandstones significantly influence the wave velocity and porosity of sandstone against high temperature. Quartz is the main rock constituent, which shows reversible change at 573°C (Į-quartz becomes to ȕ-modifica- Fig. 9. Variation of wave velocity after different treated temperature. tion), which causes crack increase. This results in the increase of porosity and decrease of wave velocity. The cemented minerals of dolomite/ankerite and clay or magnetite for sandstone show important changes in mineral composition and in physical properties even at lower temperatures. The most important change is that the structure of clay minerals and carbonate disintegrates when the heating temperature is higher than 450°C. Additionally, above 300°C, the loss of crystal water and structural water would lead to the loss of hydroxyl (-OH), damage of mineral crystal lattice skeleton, and increase the number of defects of sandstone. After high temperature heating, sandstone can be seriously damaged. With increasing damage, the wave velocity decreases quickly.
According to the analysis presented above, the thermal damage of sandstone gets larger as the temperature increases. The relation between wave velocity and damage factor can be described by the equation (Zhao et al. 2009). , where D is the damage index, V 0 is the initial wave velocity (before heating), and V p is the wave velocity after heating.  The measured temperature effect on thermal damage, obtained in this study, is shown in Fig. 11, which clearly displays that above 340°C, the thermal damage of sandstone increases rapidly with temperature.

CONCLUSION
To assess the effect of high temperature on sandstone, the characteristics of mass, pores, wave velocity, and surface features of sandstone specimens before and after heating at different temperatures have been examined in this paper. The major findings in this study are listed below: Being heated at temperatures from 25 to 300°C, the sandstone color faded gradually. Above 400°C, sandstone turns into brick red. The increase in porosity depends on the mineralogy of particles and cement. The minor cracks, quartz, feldspars and dolomite/ankerite, agnetite and clay minerals show significant changes. Sandstone mineral cements often show changes in mineral composition or they are even destroyed at high temperatures. The most important change is that the structures of minerals disintegrate between 400 and 600°C. Between 300 and 850°C (especially between 340 and 600°C), there is a significant reduction in wave velocity. The change of wave velocity is correlated to structural damage and the variation of mineral content.