## 3 Results  
To prepare specimens from porous concrete, it was necessary to determine the optimal values of the cement paste content. An excess of cement paste leads to an increase in the density and stratification of the mixture, and a lack of paste leads to a decrease in the quality and strength of contacts between the aggregate grains. The optimum content (volume) of cement paste \(V_{c,p}\) in porous concrete can be obtained from the following equation:  
\[V_{c,p}=\delta\cdot S \tag{1}\] \begin{table}
\begin{tabular}{c c c c c c c} \hline
**Type of Aggregate** & **Grading** & **Density** & **Water** & **Bulk Density** & **Void Content** & **Content of Sludge** \\  & **(mm)** & **(kg/m\({}^{3}\))** & **Absorption** & **in Loose State** & **(kg/m\({}^{3}\))** & **(\%)** & **Impurities** \\ \hline Granite screenings & 2.5–5 & 2700 & 0.82 & 1550 & 42.6 & 5.0 \\ Granite crushed stone & 5–10 & 2700 & 0.75 & 1530 & 43.3 & 4.0 \\ \hline \end{tabular}
\end{table} 
Table 1: Physical properties of aggregates. Figure 1: **(a,b)** Test specimens of porous concrete with granite screenings. where \(\delta\) is the thickness of the cement paste layer that covers and glues the aggregate grains, and \(S\) is the total surface area of the aggregate grains. Problems in the technology of porous concrete are mainly related to the variability of the \(S\)-value caused by the heterogeneity of aggregates in terms of grain composition and grain shape. The heterogeneity of aggregates is not only meant in cases of low quality. Even when all the rules and standards are fully complied with, i.e., when aggregates are fractionated and meet the requirements of the standards, their total surface area varies significantly (within 20% or more) [7]. Aggregate heterogeneity leads to concrete heterogeneity, and for porous concrete, this factor is much more significant than for conventional concrete. In the presented experiment, the surface area of the aggregate grains was determined using two equations. 1. Equation (2) is recommended by [35] to calculate the specific surface area of concrete aggregates: \[S_{a}=\frac{0.0036}{\rho_{b.d}}(0.55b_{1}+b_{2}+2b_{3}+3.9b_{4})\] (2) where \(b_{1}\), \(b_{2}\), \(b_{3}\), and \(b_{4}\) are the percentage amounts of the 40-60 mm, 20-40 mm, 10-20 mm, and 5-10 mm fractions, while \(\rho_{b.d}\) is the bulk density of crushed stone. Assuming that the aggregate is mono-fractional and all grains remain on the 5 mm sieve, Equation (2) can be given as follows (3):  
\[S_{a}=\frac{0.0036}{\rho_{b.d}}(3.9\times 100)=\frac{1.404}{\rho_{b.d}} \tag{3}\]  
Considering that the bulk density of crushed stone, determined experimentally, was 1530 kg/m\({}^{3}\), the specific surface area of the grains will be \(S_{a}=1.404/1.530=0.918\) m\({}^{2}\)/kg. 2. For granite screenings of the 2.5-5 mm fraction (grain size typical for construction sand), the specific surface area can be calculated by the formula of [PERSON]. [PERSON] [35]:  
\[S_{a}=\frac{6.35k}{1000}(0.5a+b+2c+4d+8e+16f+36g) \tag{4}\]  
where \(k\) is a coefficient depending on the type of sand (for stone screenings and their mixtures with quartz sand \(k=2\)); \(a\)-\(f\) are the partial residues on sieves with holes measuring from 5 to 0.16 mm, expressed in %; and \(g\) is the passage through a sieve with 0.16 mm holes, expressed in %. Assuming that a pure mono-fractional screening of the 2.5-5 mm fraction was used, it can be considered that there is only a 2.5 mm residue on the sieve. Therefore, Equation (4) is written as follows:  
\[S_{a}=\frac{6.35k}{1000}(100)=\frac{6.35\times 2}{10}=1.27\,\mathrm{m}^{2}/ \mathrm{kg} \tag{5}\]  
Cube specimens with a size of 100 mm \(\times\) 100 mm \(\times\) 100 mm were prepared from porous concrete at the two specified fractions. The water/cement ratio was varied (0.35, 0.4, 0.45, 0.5) and the cement paste layer thickness (CPLT) was set at different levels. The cement consumption was calculated from Equation (1).