# Study About the Influence of Diatoms on the Durability of Monumental Limestone
## 2 Materials and Methods
### Methodology
#### 2.2.3 Water Absorption Test by Capillarity  
The water absorption test by capillarity was performed following the method described in the UNE-EN 1925:1999 standard for the determination of the coefficient of water absorption by capillarity in natural stone [33]. The test procedure described in this standard consists in immersing the test surface of the samples in water to a depth of 3 \(\pm\) 1 mm. As time passes, measurements are taken to verify the increase in mass that each sample experiences due to the absorbed water.  
According to the standard, the samples used for the present work were cubic with an edge of 50 mm; three samples contained diatoms and were named D1, D2, and D3, and three samples were used as control samples and named C1, C2, and C3. As shown in Figure 7, the samples were arranged in closed containers to start the capillarity test. Previously, the specimens had first been dried for 5 days in a ventilated oven at 68 \({}^{\circ}\)C. In this case, to isolate as much as possible the upper face of the samples with diatoms and to prevent water entry through the four lateral faces adjacent to the studied face (which are in contact with water), they were waterproofed with fiberglass paint to a height of 1 cm.  
This is necessary because diatom biodeposition occurs most significantly on the side parallel to the pond surface (the upper face). Similarly, four faces adjacent to a random face were partially covered in the control samples.  
Figure 6: Samples arranged to start water absorption test at atmospheric pressure.  
After allowing the paint to air dry for 1 day, the samples were placed in a ventilated oven for an additional four days at 68 \({}^{\circ}\)C to achieve a constant dry mass before starting the test. The dry samples were then positioned in a container or tank on supports that ensure a separation among them and allow the test face to be immersed without being in full contact with the tank surface. In this way, the samples rest partially on their base, minimizing any interference with their absorption capacity. The same plastic grid used in the previous test was employed for this one. Water was added until the test face of each sample was submerged to a depth of 3 \(\pm\) 1 mm, and timing began immediately. After initially short and then progressively longer intervals, the samples were removed from the water, and the slightly submerged base was dried with a damp cloth, carefully removing only surface water drops without removing moisture from the sample. The samples were then weighed immediately. Once the mass values had been recorded, the samples were returned to the container to continue the test.  
According to the standard, a minimum of seven measurements must be taken, and the choice of time intervals depends on the absorption capacity of the material. For samples with high water absorption capacity, the recommended intervals are 1, 3, 5, 10, 15, 30, 60, 480, and 1440 min. For samples with low absorption capacity, the suggested intervals are 30, 60, 180, 480, 1440, 2880, and 4320 min. For the present study, the following time intervals were used: 1, 3 min, 5 min, 10 min, 15 min, 30 min, 1 h, 2 h, 3 h, 6 h, 11 h, 24 h, and thereafter every 24 h until the end of the test. Time intervals must be measured with a precision of \(\pm\)5%, and the test is considered complete when the difference between two consecutive weighings is less than or equal to 1% of the mass of water absorbed by the sample.  
Similarly, to evaluate the effect of the 1 cm lateral sealing with fiberglass paint--designed to isolate exclusively the upper surface under study--the test was also repeated using samples without this lateral sealing. This comparison was performed considering the concentration of diatoms on the lateral faces to be low or practically negligible compared with that on the upper face.