A sampling process was taken by a team of divers, who provided samples of marine sediments from all three basins and an additional sample at a reference point in Koper Bay (Figure 1). In each basin, four samples were taken at two depths (at 10 cm and at 100 cm of sediment depth), where the deepening of the basins will be carried out. Two samples were taken at a depth near the shore and two samples at a depth near the sea and two samples at a reference point in Koper Bay. A total of 56 samples with a total volume of 300 litres were analysed [23].

Figure 1.

Red dots represent the sampling points in the Port of Koper [23]

To avoid contamination of the samples, and to prevent changes in the specific physical and chemical properties of the samples during transport and until the beginning of the chemical and physical testing, the samples were appropriately packed, marked and stored [23].

A range of analyses were conducted in order to determine if the marine sediment containing larger quantities of clay was suitable for brick production.

In accordance with the practice in the brick industry, this phase included the following tests:

• A chemical analysis for evaluation of Silicon dioxide (SiO₂), Aluminium oxide (Al₂O₃), Ferric oxide (Fe₂O₃), Calcium oxide (CaO), Magnesium oxide (MgO), Sodium oxide (Na₂O), Potassium oxide (K₂O), Titanium dioxide (TiO₂) content, by means of ARL 8480S X ray fluorescence spectrometry (WD XRF);

• A mineralogical analysis was performed using Philips Norelco equipment for X-ray diffraction analysis with CuKα radiation and a nickel filter;

• A particle size analysis was performed by the wet sieving method;

• The Chloride content (Cl⁻) was determined according to EN 196 2:2013 [24].

Furthermore, the following ceramic technological tests of clay were carried out. Test samples were shaped using a laboratory de-airing extruder at a vacuum of 0.82-0.85 kg/cm². The following shapes were obtained: cylinders, prisms and tiles. Test specimens were dried for 7 days at ambient room conditions, followed by 24 hours at 60 °C and 8 hours at 100 °C in a dryer. Dried samples were then fired using heating rates of 100 °C/h up to 950 °C or 1,100 °C with the holding time at the maximum temperature of 2 hours. The samples were then cooled in the furnace. Determination of linear shrinkage, water absorption, compressive and flexural strength of samples fired at selected temperatures was performed. The test of resistance to freezing of the obtained bricks (samples) was performed according to EN 539-2:2013 [25].

The chemical analysis of samples (see Table 1) indicates minor variations between individual basins as well as between samples taken near the surface and at depth in the sea. These minor variations are expected for natural raw materials. There were slightly larger variations in composition when comparing the results for Koper Bay with the results for individual basins. In the sample from Koper Bay, lower SiO₂ content and higher carbonates content are noticeable. Furthermore, the results closely match the results of X-ray analysis (Figure 2) that also show a lower content of SiO₂ and a higher content of carbonates for the sample from Koper Bay.

Figure 2.

Typical diffractogram of the sample from “Basin 1” (a); with an additional diffractogram of fraction below 2 2m (b)

The results of X-ray diffraction are given in Figure 2. The mineralogical composition of clays and sediments provides an important piece of information in the process of identifying the raw material composition, because the properties of clay and, in particular, the shaping, drying and firing processes depend more on the minerals present in the raw material than on the chemical composition of the material. The content of clay particles in brick clay may vary from 20% to 60%, but it usually amounts to 30%. The clay examined contains about 55% to 63% clay particles. The minerals in the clay belong to the chlorite illite group.

Another important factor is the amount of quartz. In clays it is present as fine sand and its content in brick clay is usually between 20% and 50%. This fact is important as quartz affects the product properties as well as the drying process (reduces the sensitivity) and the firing process, where during the cooling process it is necessary to consider the phase transition of quartz at 573 °C, which is connected to volume changes and may lead to cracking if the cooling process is not properly controlled. As the X-ray analysis of a sample shows, the clay tested contains about 7% to 20% quartz, comparing all the samples taken, the samples from the Port of Koper contain the lowest amounts of quartz but considerably higher amounts of calcite and dolomite.

If carbonates are finely dispersed, their content may reach up to 20-25%, but a problem may arise with lime inclusions larger than 1 mm. There is no sign of lime inclusions exceeding 1 mm in the clay examined, the carbonate content ranges from 15% to 19% in Basin 1 to about 29% in the Koper Bay. If lime inclusions (as free lime ‒ CaO) are bigger than 1 mm in the clay body and it they are near the surface of the brick unit, white burst or pop outs can be formed as the free lime hydrates and becomes Calcium hydroxide [Ca(OH)₂]. This reaction results in a volume increase. Exposure to air and consequently to Carbon dioxide (CO₂) in the air, further forms Calcium carbonate (CaCO₃). If the quantity is not very high, this does not impair the product quality but may be disturbing from the aesthetic point of view.

The particle size distribution in the raw material also has a significant effect on the properties of clay during the shaping, drying and firing processes as well as on the properties of dry and fired products. The results of the particle size analysis are given in Figure 3.

Figure 3.

Particle size distribution for the sample “Basin 1”

All the tested parameters show great similarity to the typical clay used for brick production and they are also similar to the properties of some other dredged sediments [15], only the chloride content is high, which might cause problems during firing due to the corrosion of equipment and the presence of chlorine in flue gases. The chloride content was 1.6% in a non-fired/wet state. Water soluble chlorides are still present (0.6%) when firing at a lower temperature (950 °C), the chloride content drops after firing at temperatures above 1,050 °C, their level is only 0.036% (for reference, the limit for chlorides in cements is 0.1% according to SIST EN 196 1:2005).

The samples were prepared using a vacuum (de-airing) extruder for making tiles with dimensions of 50 × 20 × 8 mm.

The analysis of the firing process in a gradient furnace provides information about linear shrinkage and water absorption as a function of the firing temperature (shown in Table 2 and Figure 4) and shows how likely it is for the clay to deform within a specific range of firing temperatures. Results of firing in a gradient furnace suggested that the examined clay is a highly sensitive type of raw material because it “closes” very quickly at temperatures exceeding 1,070 °C. This means that as the firing temperature rises, the water absorption quickly drops while the shrinkage increases. When firing at about 1,150 °C, clay is vitrified because the water absorption falls below 1%. The clinker point (water absorption 6%) is at 1,112 °C while the sintering point (water absorption 2%) is at 1,143 °C.

Figure 4.

Shrinkage and water absorption in relation to firing temperature

Further on, material from Basins 1 and 3 was dried and homogeneously mixed. The sample from the Basin 2 was excluded because it was polluted (there is an impact of the Rižana River that can be polluted by external influences, such as factory and septic tank emissions in the river, agriculture, etc.)

Results on the conditions of shaping and the characteristics of clay after firing and drying are given in Table 3.

The final properties of dried and fired products deviate from the expected values for typical brick clay. Drying shrinkage especially is very high, about 12% (depending on the orientation during shaping). The products fired at 950 °C and 1,050 °C also have high water absorption (23.1 and 22.6 respectively), when fired at 1,100 °C the water absorption drops to 7%, but these samples tend to bend and inflate. The values of compressive and flexural strength are satisfactory and the compressive strength amounts to 25.7 MPa and 36.4 MPa, and the bending strength 3.2 and 8.7 when fired at 950 °C and 1,050 °C, respectively. Both strengths significantly increased when fired at 1,100 °C, the bending strength was 19.3 MPa and the compressive strength was 96.8 MPa.

Frost resistance is important when products are to be exposed to severe climatic conditions. For clay products this is usually the case for outward facing bricks or clay roofing tiles, these products have to guarantee resistance to freezing [26]. Testing the frost resistance of laboratory samples has shown that the material is resistant to frost when fired at 1,100 °C but non-resistant to frost when fired at 950 °C and 1,050 °C. When fired at lower temperatures water absorption is still high and this greatly contributes to the frost non-resistance.

To improve the brick production process and to achieve optimal properties in the technological process of shaping ceramic products, further studies have to be carried out using possible additions that reduce plasticity and the drying shrinkage. Suitable additions could include waste material containing quartz such as silica sludge, or slag [18], [27]. The sediments from the Port of Koper contain large amounts of water (more than 70%) and therefore have to be dried beforehand. For this reason it is best to find a dry additive, for example paper sludge [16] or fly ash because it facilitates the regulation of the water content needed in the shaping process [17].