# Enhancing Cement Paste Properties with Biochar: Mechanical and Rheological Insights
## 2 Materials and Methods
### Methods  
The water retention capacity was measured following the method suggested by [PERSON] et al. [17]: two biochar samples, each of 10 g, were placed in a beaker, then put in a ventilated oven at 90 \({}^{\circ}\)C for 24 h in order to remove the moisture that was adsorbed. Then, each of the samples was filled with 100 g of distilled water and left to stand for 48 h in each beaker. A vacuum filtration test was prepared, in which a fritted glass funnel (G4) was used to remove the solid phase from the liquid one. The fluid retention capacity was calculated by taking the wet biochar weight and subtracting the dry one. The thermal stability was assessed using a thermogravimetric analyzer (TGA, Mettler Toledo 1600) in air condition. The air was supplied at a constant flow rate (50 mL min\({}^{-1}\)). A roughly 33 mg sample was placed in an alumina crucible and the sample was heated from 25 \({}^{\circ}\)C to 1000 \({}^{\circ}\)C with a constant heating ramp of 10 \({}^{\circ}\)C min\({}^{-1}\). Regarding mechanical tests, 40 prismatic specimens of 20 mm \(\times\) 20 mm \(\times\) 80 mm were prepared. For each experimental set, eight specimens were made, with four tested after 7 days of curing and the remaining four tested after 28 days (Table 2). The flexural tests were conducted according to the ASTM C 348, while for compressive strength the procedure of ASTM C 109 was followed. Regarding the rheological test activity, a total of 12 mixed specimens were made, 2 for each experimental set (Table 2). The specimens were prepared following the mix design shown in Table 3. Ground biochar was dispersed in a solution of water and superplasticizer. The solution was transferred into a mixing bowl and hand-mixed for 1 min until yielding a homogeneous solution. The cement was gradually poured into the solution in the first two minutes while operating the mixer at 480 rpm for three minutes. Then, the mixer speed was increased to 840 rpm for three more minutes, reaching a total mixing time of six minutes. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline
**Oxide** & **SiO\({}_{2}\)** & **Al\({}_{2}\)O\({}_{3}\)** & **Fe\({}_{2}\)O\({}_{3}\)** & **CaO** & **MgO** & **SO\({}_{3}\)** & **Na\({}_{2}\)O** & **K\({}_{2}\)O** & **Loss on Ignition** \\ \hline Amount (\%) & 20.02 & 4.12 & 1.87 & 63.23 & 4.2 & 3.43 & 0.003 & 0.0015 & 0.8 \\ \hline Phase & **C\({}_{3}\)S** & \multicolumn{2}{c}{**C\({}_{2}\)S**} & \multicolumn{2}{c}{**C\({}_{3}\)A**} & \multicolumn{2}{c}{**C\({}_{4}\)AF**} \\ \hline Composition & 49.10\% & \multicolumn{2}{c}{19.70\%} & \multicolumn{2}{c}{7.91\%} & \multicolumn{2}{c}{5.20\%} \\ \hline \end{tabular}
\end{table} 
Table 1: Composition of the used cement as reported by the supplier. \begin{table}
\begin{tabular}{c c c c} \hline  & **Mechanical Test Activity** & **Rheological Test Activity** \\ \hline
**ID** & **N\({}^{\circ}\) Specimens 7 Days** & **N\({}^{\circ}\) Specimens 28 Days** & **N\({}^{\circ}\) Specimens** \\ \hline Ref. OPC & 4 & 4 & 5 \\ B 1\% & 4 & 4 & 5 \\ B 2\% & 4 & 4 & 5 \\ B 3\% & 4 & 4 & 5 \\ B 5\% & 4 & 4 & 5 \\ B 7\% & - & - & 5 \\ \hline \end{tabular}
\end{table} 
Table 2: Set of experimental samples. The samples subjected to tests for rheological activity were immediately loaded into a co-axial cylinder rotary viscometer for testing (Malvern Kinexus Pro+, Malvern, UK). Two different measurements were taken at 23 \({}^{\circ}\)C--steady shear and dynamic oscillatory shear, with an angular frequency \(\omega\) of 6.28 rad/s (\(\omega\) = 2\(\pi\)f with f, frequency = 1 Hz). Five minutes of temperature stabilization was allowed after the pastes were introduced into the cup, while the gap between the base of the cup and the bob was set to 5 mm. Successively, the samples used for mechanical tests were transferred to the prismatic stainless-steel molds and we left them to cure for 24 h at room temperature, covered to maintain the level of humidity at 90%.