3.1. Materials
Four series of samples were produced, one reference mixture, and three mixtures where cement was replaced by 10, 20 and 30% by mass. In order to understand the effects of its addition to the cement matrix, the primary step of this study was to conduct physical and chemical analysis on the SWWTS as produced, and then thoroughly investigate the obtained LWPCP samples at the age of 28 days.
In the production of all tested concrete mixtures, cement CEM I 52.5 R was used. Expanded clay aggregate with a grain size between 1 and 4 mm in diameter was employed as aggregate in all mixtures. Expanded clay aggregates have a closed pore structure, low bulk density and thermal conductivity. According to the declaration of performance, loose bulk density of this aggregate was 450 ± 65 kg/m3 with water absorption after 24 h of 11 ± 4%. Tap water was used in all mixtures, together with air-entraining and accelerating admixtures.
SWWTS was used as a partial replacement of cement in the amounts of 10, 20 and 30% by mass. This material is a light-gray (grayish) powder with hydrophobic properties (detailed characterization is presented in Chapter 4).
The amount of water remained the same in all of the prepared mixtures, so that the water/(cement + SWWTS) ratio remained constant, but the water/cement ratio was increased, from 0.300 in the reference mixture, to 0.428 in the mixture with 30% cement replacement.
Within the presented study, four series of lightweight pervious concrete pavers were prepared. The images of representative samples are presented in
Figure 1, with magnification of 30×. The amounts (in kg/m
3) of cement, SWWTS, expanded clay aggregate and water are presented in
Table 1. In order to distinguish the samples by color, different pigments were added to each mixture, as described in
Table 1.
3.2. Methods
The presented research was divided into two phases, as shown in
Figure 2. Firstly, detailed characterization of SWWTS was performed, in order to evaluate its properties related to application as SCM. Secondly, LWPCP samples were prepared, as explained above, and their physical and mechanical properties were investigated.
Particle size and particle size distribution were measured by the laser light scattering method using Mastersizer 2000 (Malvern Instruments, Malvern, UK). Furthermore, particle size distribution of powdered samples was measured using Mastersizer Scirocco 2000 analyzer (Malvern Instruments, UK). The results obtained are presented through three dependent parameters: surface weighted mean diameter (SD) (µm) or volume weighted mean diameter (VD) (µm), specific surface area (SSA) (m2/g) and span values.
Chemical composition of SWWTS was measured through the energy dispersive X-ray fluorescence (XRF) characterization; using XRF spectrometer produced by Xepos, Spectro with a binary cobalt/palladium alloy thick-target anode X-ray tube (50 W/60 kV) and combined polarized/direct excitation. Air was used for the cooling system. The same equipment was used for determination of heavy metals content in the specimen. Before testing, samples were dried until constant mass at 105 °C. After drying, they were prepared as pressed pellets (40 mm in diameter and 3 mm in height) by mixing coal and tableting aid wax.
Crystalline phases were identified by X-ray powder diffraction (XRD) using Rigaku MiniFlex 600 diffractometer, Rigaku Corporation, Tokyo, Japan (CuKα radiation, λ = 0.15406 nm; 2θ = 10–70°; scan rate = 0.02 s−1).
Through coupled SEM-EDS technology, morphology of samples and chemical composition of selected areas was determined. Testing was performed on Jeol JSM5800 SEM with a SiLi X-Ray detector (Oxford Link Isis series 300, Oxford, UK), using magnifications of 200, 1000, 3000 and 9000.
The Fourier transform infrared spectroscopy (FTIR) was used for qualitative analysis of functional groups of SWWTS samples. For this purpose, ATR FTIR-Fourier transform infrared spectroscopy (FTIR) spectra of the SWWTS sample were recorded in the absorbance mode using a Nicolet™ iS™10 FT-IR Spectrometer (Thermo Fisher SCIENTIFIC, Bremen, Germany) with Smart iTR™ Attenuated Total Reflectance (ATR) sampling accessories, within the range of 400–4000 cm−1, at a resolution of 4 cm−1 and in 20 scan modes.
Determination of Ca(OH)2 and CaCO3 in the analyzed sample was conducted by titration of a known volume of liquid sample (after dissolution in distilled water in an ultrasound bath). The titration was performed with a standard solution of H2SO4 (0.05 mol/dm3) using phenolphthalein and methyl-orange indicators, respectively.
In order to obtain representative samples, the production of LWPCP was organized at the concrete prefabrication plant, using the existing production line, with the necessary equipment for vibration and cutting of samples. Nominal dimensions of the pavers were 200 × 200 × 60 mm. They were cured in laboratory conditions (temperature 20 ± 2 °C, relative humidity 50 ± 10%) up to the age of 28 days, when the tests were performed.
Bulk density was measured as average value of three specimens for each type of concrete. Volume of the specimens was determined through measurements of their dimensions using a digital caliper with 0.01 mm accuracy and scale of 0.1 g accuracy.
Water absorption was determined through measurements of samples dried to constant mass, and then gradually immersed in water until reaching the constant mass. It was measured as an average value recorded on two tested samples.
Mercury intrusion porosimetry (AutoPore IV 9500, Norcross, GA, USA, Micromeritics) was used to analyze the pore size distribution and porosity. Maximal intrusion pressure used was 228 MPa.
Saturated hydraulic conductivity and thermal conductivity were determined only for the reference mixture. A constant head permeability test is the standard method for determination of the PCPs hydraulic conductivity [
33,
34]. Thermal conductivity measurements were performed according to EN 12667, using guarded hot plate apparatus, with a measurement error lower than 1%.
Flexural strength and ultrasonic pulse velocity measurements were performed on the prismatic samples that were cut out of the pavers. The dimensions of the prisms were 200 × 60 × 60 mm. Flexural strength was determined through a three-point-bending test, using a span of 150 mm. Breaking force was measured to 0.1 kN accuracy. For the ultrasonic test, samples were additionally flattened from the sides with a thin layer of cement mortar. Ultrasonic testing equipment, with probes frequency of 24 kHz, was used. Both properties were calculated as average values of three tests.
Compressive strength was measured on the samples cut out from the original pavers. Their nominal dimensions were 60 × 60 × 60 mm. During the test, the force was transmitted on the samples through steel plates 40 mm wide. Compressive strength was calculated as a ratio of the force measured at breakage and the area of the load transfer as an average value of six measurements. Force was measured with an accuracy of 0.1 kN.
Pull-off tester, with a range between 0 and 16 kN, and accuracy of 0.01 kN, was used for pull-off strength measurements, according to EN 1542. Due to the possible damaging effect, steel plates were only glued to the testing surface, with no additional cutting. Adhesion strength was determined as a ratio between braking force and the fractured surface. It was calculated as an average value of three measurements.