Cement Chemistry, Oxide Basicity and the Solvay Process

Cement clinker sets (hardens) very rapidly on its own — too rapidly to be workable at a construction site. Adding a small proportion of gypsum (CaSO₄.2H₂O) to the cement clinker during manufacture INCREASES (slows down) the setting time, giving workers enough time to mix, transport and place the wet cement before it hardens. Without gypsum, the tricalcium aluminate component of cement would react with water almost instantly (flash setting), making the cement essentially unusable in practice — gypsum retards this specific fast-reacting component.

Basic character of a metal oxide tracks how ionic (versus covalent) the metal-oxygen bonding is, which in turn tracks the metal electropositivity. A transition metal oxide like NiO, with more covalent character, is LESS basic than the alkaline earth oxides. Among alkaline earth oxides, basicity increases going down the group as the cation becomes larger and more electropositive (MgO less basic than SrO). Alkali metal oxides, in turn, are more basic still than alkaline earth oxides of a comparable period (the +1 cations are less polarising than +2 cations of similar size), and basicity again increases going down Group 1 (K₂O less basic than Cs₂O). Putting all of this together gives the full ranking: NiO < MgO < SrO < K₂O < Cs₂O — transition metal oxide least basic, then alkaline earth oxides increasing down their group, then alkali metal oxides, again increasing down their group, most basic of all.

The Solvay process manufactures sodium carbonate starting from brine (NaCl solution), ammonia and CO₂ — NOT from sodium hydroxide. The genuine Solvay-process reactions are: NH₃ + H₂O + CO₂ → NH₄HCO₃ (forming ammonium bicarbonate in solution), NaCl + NH₄HCO₃ → NaHCO3(precipitate) + NH₄Cl (the key step, exploiting NaHCO₃ low solubility to precipitate it out), and 2NaHCO₃ (heat) → Na₂CO₃ + H₂O + CO₂ (calcining the bicarbonate to the final carbonate product, with the released CO₂ recycled back into the process). The reaction 2NaOH + CO₂ → Na₂CO₃ + H₂O, while a perfectly valid way to make sodium carbonate in a general sense, is NOT part of the Solvay process specifically — the Solvay process never uses NaOH as a starting material at all, so this is the reaction to flag as not belonging to the sequence.

Among a set of options that may include a non-alkaline-earth element as a distractor (such as caesium, an alkali metal, listed alongside genuine Group 2 elements), restrict the comparison to the actual alkaline earth metals given. Electropositivity increases down Group 2, so among Be, Mg and Ba, barium is the most electropositive true alkaline earth metal in the set — do not be misled into picking a listed alkali metal just because it happens to be more electropositive than any alkaline earth metal in general; the question restricts the comparison to alkaline earth metals specifically.