Sandstones have been widely used in construction for their abundance, aesthetics, and ease of extraction. To determine sandstones’ quality, it is essential to analyse their petrographic and petrophysical properties and sensitivity (durability and conservation) to environmental agents. This paper evaluates the physical-mechanical changes undergone by Sierra de la Demanda (Burgos, Spain) sandstone under combined and induced water and salt aggression and assesses ESTEL 1100’s effectiveness and suitability as a treatment. This sandstone is porous, permeable, dense and quartz-rich with high hardness and strength. The treatment improved its petrophysical properties by modifying its pore geometry and connectivity, reducing absorbency, permeability and anisotropy, and further increasing its hardness and resistance. Salts did not substantially modify its properties as its porosity type absorbed the crystallisation pressure. Ultimately, its pore system and predominantly quartz composition make it a high-quality, weather-resistant material compatible with the treatment applied.
Las areniscas han sido uno de los materiales de construcción más utilizados por su abundancia, estética y fácil extracción. Para determinar su calidad es esencial analizar sus propiedades petrográficas y petrofísicas, así como su sensibilidad (durabilidad y conservación) a los agentes medioambientales. Este trabajo evalúa tanto los cambios físico-mecánicos sufridos por una arenisca de la Sierra de la Demanda (Burgos, España), ante la agresión combinada e inducida de agua y sales, como la eficacia e idoneidad del tratamiento ESTEL 1100. Esta arenisca es porosa, permeable, densa, rica en cuarzo, y con alta dureza y resistencia. El tratamiento mejoró sus propiedades petrofísicas, modificando la geometría y conectividad de sus poros, y reduciendo su absorción, permeabilidad y anisotropía. Las sales no modificaron sustancialmente sus propiedades, ya que su porosidad absorbió su presión de cristalización. El sistema poroso y el abundante cuarzo hacen de esta arenisca un material de alta calidad, resistente a la intemperie y compatible con el tratamiento aplicado.
Sandstones are sedimentary, granular, porous rocks widely used in construction worldwide because of their great abundance on the planet’s surface, ease of extraction and processing and high aesthetic value. However, sandstones are soft materials likely to suffer severe weathering when exposed to environmental agents (
The presence of specific mineralogy in sandstones’ intergranular space (clay matrices and/or crystalline cement), as well as the empty spaces that define their pore system, are the main features that determine these materials’ hardness and thus their quality and durability as they constrain sandstones’ intergranular cohesion and resistance to aggression from the two most damaging agents of deterioration: water and salts (
Laboratory simulations, such as accelerated ageing tests, are carried out in order to assess the type and extent of damage these stones suffer under real environmental conditions. These tests are usually destructive in the short term, which makes it possible to compare the induced damage with the real damage and thus estimate the degree of durability (
This paper characterises both the petrographic and petrophysical properties of a porous sandstone marketed as ‘Demanda Gold’ (Sierra de la Demanda, Burgos, Spain) and the changes in these properties after treating it with ESTEL 1100 and/or subjecting it to accelerated ageing by salt crystallisation in order to establish the aforementioned sandstone’s quality and durability as a building material. Likewise, it evaluates the treatment’s compatibility with this stone and its effectiveness against the action of water and salts. Analysis will place special emphasis on changes to the pore system.
Demanda Gold sandstone is principally quarried in Palacios de la Sierra, Burgos, Spain (41° 58’ 26.37” N, 3° 6’ 27.46” W), although it is also extracted in Sala de los Infantes, Vilviestre del Pinar, Canicosa de la Sierra and Quintanar de la Sierra (Burgos). It is known commercially as ‘Piedra de la Demanda’, ‘Dorada Urbión’ or ‘Piedra de Salas’. This sandstone has historically been used in the towns and villages close to the extraction sites. It is currently marketed nationally and internationally and is found in buildings in Madrid, Burgos, Soria, Salamanca, Cantabria and the Basque Country. It is used in both modern and traditional architecture, mainly as cladding for façades and exterior walls, although it is also used in masonry and ashlars. Geologically, this sedimentary rock is found in the southern domain of the Iberian Mountain Range in the Cameros Basin. It belongs to the Aptian (Lower Cretaceous) Weald facies, which originated during the second rifting phase (
The Demanda Gold sandstone was supplied by
A: cubic, B: prismatic, C: plates.
Cubic specimens | Prismatic specimens | Plate specimens | ||
---|---|---|---|---|
ADD | Original | A7-A12 ( |
B4-B6 ( |
C4-C6 ( |
ADDs | Salts | A7-A9 ( |
B4-B6 ( |
C4-C6 ( |
ADDT | Treatment | A1-A6 ( |
B1-B3 ( |
C1-C3 ( |
ADDTs | Salts + treatment | A1-A3 ( |
B1-B3 ( |
C1-C3 ( |
The ESTEL 1100 chemical conservation product, provided by
The following abbreviations are used in this paper to refer to each type of sample according to whether they have been treated and/or degraded with salts: Demanda Gold sandstone (ADD), Demanda Gold sandstone with salts (ADDs), Demanda Gold sandstone with treatment (ADDT) and Demanda Gold sandstone with treatment and salts (ADDTs) (
The techniques and tests used in this paper followed the UNE-EN 16515:2016 standard (
Compositional and textural characterisation of the samples were carried out by macroscopic and microscopic description as per the UNE-EN 12407:2020 standard (
Surface properties: The following tests were carried out on the cubic specimens: 1) colour measurement of surfaces (UNE-EN 15886:2011 (
Dynamic properties: On the cubic specimens, the ultrasonic propagation velocity of the P-waves (Vp) was determined (UNE-EN 14579:2005 (
Structural and hydric properties: Mercury intrusion porosimetry (MIP) was used to determine pore structure (percentage, size and shape of the pores, and their distribution) (ASTM D4404:2010 (
Mechanical properties: The Leeb surface hardness test (HLD) was performed (UNE-EN ISO 16859-1:2016 (
In order to determine the durability of Demanda Gold sandstone and to assess the effectiveness of the conservation treatment applied, this lithological variety was subjected to a highly aggressive accelerated ageing test combining the action of both water and salts (Na2SO4·10H2O at 14%) (UNE-EN 12370:2020 (
After the salt resistance test, the specimens (ADDs and ADDTs) were again characterised both petrographically and petrophysically in order to quantify the changes undergone and to estimate their quality and durability, as well as the suitability and effectiveness of the treatment applied (
This lithological variety is a detrital, homogeneous, massive, light beige-colored and strongly cohesive sedimentary rock (ADD) (
A: Powdery appearance with salts on the surface, B-C: Appearance after removal of the salts from the test specimens.
Microscopically (PLM) and mineralogically (XRD), this sandstone (ADD) is mainly formed of single-crystal subangular quartz grains (> 75%) and some polycrystalline quartz, feldspars (5- 10%), and muscovites, biotites and tourmalines (< 5%) (
In the ADDT, the treatment appears as a cracked and irregular surface coating (hydrophobic layer of oligomeric polysiloxanes) up to 80 µm thick, and as a micrometric film (< 40 µm) with high optical relief surrounding the grains on the inside (silicic acid consolidating film), but without filling the intergranular porosity (
The presence of abundant syntaxial cement between the quartz grains is mainly responsible for both the high intergranular cohesion of this sandstone and its hardness. In addition, the existence of large pores seems to favour its resistance to salt crystallisation, as only the smaller pores are able to generate fissures (
Although the FTIR technique is very effective for the detection of treatments, in this study it has been hampered because the ESTEL 1100 product has a significant siliceous component (Si-O-Si and Si-OH groups), so its vibration bands (1170, 1090, 799-780 and 450 cm-1) are the same as those of quartz (Si-O). Nevertheless, the treatment could be identified by the presence of the C- H alkane groups of the water repellent (CH2-CH3: 2990 single peak and 1450-1380 cm-1 double peak).
Surface properties: Aesthetically, ADD is a light yellowish-grey luminous sandstone with a high lightness value (L* = 71.5) and yellowness index (YI = ~25) and a low chroma value (C* < 14), albeit with yellow tonality (b* = 13.2) (
ADD | ADDs | ADDT ( |
ADDT ( |
ADDTs ( |
|
---|---|---|---|---|---|
L* | 71.5 ± 0.1 | 74.2 ± 1.3 | 62.2 ± 0.4 | 67.7 ± 0.4 | 69.2 ± 0.1 |
a* | 3.5 ± 0.1 | 3.1 ± 0.3 | 5.1 ± 0.3 | 4.0 ± 0.2 | 3.7 ± 0.0 |
b* | 13.2 ± 0.3 | 11.1 ± 0.8 | 17.1 ± 0.4 | 14.3 ± 0.5 | 13.7 ± 0.5 |
C* | 13.7 ± 0.3 | 11.5 ± 0.9 | 17.8 ± 0.4 | 14.9 ± 0.5 | 14.2 ± 0.5 |
WI | 0.7 ± 0.9 | 9.3 ± 3.6 | -11.6 ± 0.8 | -3.9 ± 1.3 | -1.6 ± 1.4 |
YI | 24.7 ± 0.5 | 20.5 ± 1.7 | 34.3 ± 0.8 | 27.6 ± 0.9 | 26.1 ± 0.9 |
ΔE* | |
---|---|
ADD vs ADDT ( |
10.3 |
ADD vs ADDT ( |
4.1 |
ADD vs ADDs | 3.4 |
ADDT ( |
1.7 |
ADD vs ADDTs ( |
2.4 |
The static contact angle between a water droplet and the surface of this sandstone establishes its strong hydrophilic character, producing similar results both before (θADD = 29° ± 5.2°) and after (θADDs = 32.7° ± 7.2°) ageing tests. The treatment makes it hydrophobic (θADDT > 106° ± 6.8°) until after the attack with salts, which returns it to its initial hydrophilic state, albeit with a greater contact angle (θADDTs > 56° ± 8.3°) due to the cracking and partial dispersal of the hydrophobic agent on the surface (
Dynamic properties: These porous sandstones’ high intergranular cohesion, due to the natural syntaxial cement around the dominant quartz grains, results in high P-wave propagation velocities (VpADD ~2667 m/s). These values are similar to the averages of quartz-rich sandstones (quartz arenites-subarkoses) with crystalline cement (
Vp (m/s) | dM (%) | dm (%) | dMm (%) | |
---|---|---|---|---|
ADD | 2667 ± 53 | 11.79 ± 2.3 | 7.24 ± 2.1 | 19.03 ± 4.1 |
ADDs | 2811 ± 66 | 8.83 ± 1.1 | 3.88 ± 1.4 | 12.70 ± 2.4 |
ADDT | 3832 ± 53 | 7.03 ± 0.9 | 4.43 ± 1.1 | 11.47 ± 1.1 |
ADDTs | 3487 ± 85 | 7.50 ± 1.9 | 6.32 ± 1.7 | 13.81 ± 0.6 |
WI | 0.7 ± 0.9 | 9.3 ± 3.6 | -11.6 ± 0.8 | -3.9 ± 1.3 |
YI | 24.7 ± 0.5 | 20.5 ± 1.7 | 34.3 ± 0.8 | 27.6 ± 0.9 |
Structural and hydric properties: Untreated sandstone (ADD) has a high real density (~2640 kg/m3) due to its high quartz composition (
ADD | ADDs | ADDT | ADDTs | ||
---|---|---|---|---|---|
|
2641 ± 0.7 | 2637 ± 0.9 | 2588 ± 3.5 | 2604 ± 1.0 | |
|
2148 ± 8.0 | 2148 ± 9.2 | 2184 ± 5.2 | 2175 ± 5.8 | |
|
18.83 ± 0.3 | 18.7 ± 0.4 | 15.75 ± 0.2 | 16.62 ± 0.2 | |
|
|
0.26 | 0.22 | 0.29 | 0.17 |
|
1.2 | 1.14 | 0.6 | 1.14 | |
|
10.4 | 16.9 | 14.5 | 16.1 | |
|
Micro: 29.0 Macro: 71.0 | Micro: 34.2 Macro: 65.8 | Micro: 28.9 Macro: 71.1 | Micro: 30.7 Macro: 69.3 | |
|
10.1 | 6.59 | 18.2 | 2.37 | |
|
0.81 | 0.81 | 0.84 | 0.83 | |
|
8.69 ± 0.2 | 8.63 ± 0.2 | 7.13 ± 0.1 | 7.57 ± 0.1 | |
|
5.52 ± 0.1 | 5.33 ± 0.1 | 1.11 ± 0.1 | 1.76 ± 0.1 | |
|
0.03308 ± 2.23·10-3 | 0.03208 ± 1.61·10-3 | 0.002598 ± 7.53·10-6 | 0.002875 ± 2,1·10-4 | |
|
1.63·10-9 ± 3.7·10-11 | 2.97·10-9 ± 1.1·10-10 | 1.14·10-9 ± 1.5·10-10 | 2.73·10-9 ± 6.9·10-10 | |
|
772.53 ± 326.2 | 484.15 ± 50.4 | 315.98 ± 40.6 | 380.75 ± 74.7 |
Application of ESTEL 1100 reduces porosity in this sandstone to 15.7% without causing any changes in its modal distribution or its micro/macroporosity ratio (
Salt aggression does not cause significant changes to the structural and hydric properties of the original sandstone (ADD and ADDT;
Mechanical properties: The Demanda Gold sandstone shows high surface hardness and strength (~427 HLD, ~48 RL and > 55 MPa) (
ADD | ADDs | ADDT | ADDTs | |
---|---|---|---|---|
Leeb hardness (HLD) | 426.7 ± 25.3 | 382.3 ± 5.8 | 422.2 ± 55.8 | 363.8 ± 32.6 |
Schmidt hardness (RL) | 47.92 ± 2.8 | 47.67 ± 2.8 | 52.00 ± 1.9 | 50.33 ± 1.5 |
UCS (MPa) | 55.58 ± 11.8 | 54.67 ± 10.8 | 72.58 ± 9.9 | 64.67 ± 6.5 |
Demanda Gold sandstone is a Cretaceous subarkose with good compositional and textural maturity that determine its petrographic and petrophysical properties. These intrinsic properties make this sandstone an outstanding building material. Moreover, its response to the weathering and salt crystallisation tests would seem to confirm its durability. The dominant monomineralic composition (quartz grains), the high-strength cement and the high monomodal porosity (~19%) determine its physical and mechanical properties. Its mineralogy makes it a dense material (> 2600 kg/m3) with high ultrasonic propagation velocities (2660 m/s) and high hardness (~48 RL, > 55 MPa). Because of its dominant macroporosity (71% measuring 10-30 µm) it has high water absorption (embedded and capillary) and air and water vapour permeability coefficients.
Application of the ESTEL 1100 treatment effectively fulfils a dual function, cementing (consolidating) up to a depth of ~5 mm and waterproofing (water-repellent) on the surface, substantially improving the sandstone’s physical-mechanical properties. Although the sandstone’s original porosity is only reduced to 16-17%, the treatment does significantly modify the geometry of its pores and capillary connections, making them more irregular and sinuous. Not only does this restrict water, water vapour, air and even heat ingress and mobility inside the stone, it also hinders their egress. This results in reductions of 80-90% in water absorption capacity (embedded and capillary), of 60% in air permeability and of 30-35% in water vapour permeability. Similarly, the treatment does not significantly modify the sandstone’s aesthetic appearance as it does not change its chromatic parameters or its gloss, although it does reduce its original anisotropy. It also increases its ultrasonic propagation velocity by almost 30% (3800 m/s), its surface hardness by ~8% (52 RL) and its UCS by > 23% (> 72 MPa). In conclusion, the treatment improves the quality of the stone by making it harder and less absorbent, although it may leave a certain amount of water retained inside (3%), which can influence its durability.
The accelerated ageing test with Na2SO4·10H2O confirms these sandstones’ low susceptibility to degradation by the effects of crystallisation of this salt inside their pore system. The large pore size (> 10 µm) seems to be able to contain the high crystallisation pressure of this type of salt so that the physical-mechanical variations suffered by this sandstone are minimal and non- significant. It should be noted that the presence of salts on its surface seems to have generated microporosity to the detriment of the macroporosity it fills, thus doubling its permeability to water vapour. The salts affect the treated sandstone by causing significant changes in the geometry and connectivity of its pores, which have been previously modified by the treatment and which, in this case, makes them more regular and straighter. Here, the salts break up and detach part of the surface treatment, increasing the sandstone’s porosity somewhat (5%) and generating fissural microporosity that increases its water absorption capacity (embedded ~37% and capillary ~10%), which was strongly reduced after the treatment. Another consequence of this action was a reduction in its mechanical properties (of 3.2% in surface hardness and ~11% in UCS). Thus, the amount, size, modal distribution and geometry of the pores of this sandstone seem to significantly influence its resistance to weathering, improving its durability.
This work was funded under the Region of Madrid’s TOP-HERITAGE-(Technologies in Heritage Science (S2018/NMT-4372) project. The authors are members of Complutense University of Madrid research team n.921349. The authors wish to acknowledge professional support of the Interdisciplinary Thematic Platform from CSIC Open Heritage: Research and Society (PTI- PAIS).
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Conceptualization: M.J. Varas-Muriel. Data curation: A. Gómez Marfil. Formal analysis: M.J. Varas-Muriel, A. Gómez Marfil. Funding acquisition: M.J. Varas-Muriel. Investigation: A. Gómez Marfil. Methodology: A. Gómez Marfil. Project administration: M.J. Varas-Muriel. Resources: M.J. Varas-Muriel. Software: A. Gómez Marfil. Supervision: M.J. Varas-Muriel. Validation: M.J. Varas-Muriel, A. Gómez Marfil. Visualization: A. Gómez Marfil. Writing, original draft: M.J. Varas-Muriel, A. Gómez Marfil. Writing, review & editing: A. Gómez Marfil.