RMC already owns 22% of its smaller rival, whose shares were trading at 170p before the announcement was made.
Peel Hunt analyst Stephen Rawlinson said there was a general feeling in the market that RMC should be selling some its non-core holdings instead of adding Alexander Russell to its stable.
He added: "But that's unrealistic. The market thinks the timing should always be perfect for these sorts of things, but in reality it's not like that. It's a good deal at a good price. The shareholders wanted out now and RMC have always wanted the company so it was inevitable." Rawlinson said RMC's real target for the takeover was Alexander Russell's 35-40% stake of the sand and gravel reserves in central Scotland. He predicted that it would sell other parts of the business.
Alexander Russell chief executive Graeme Nicolson, who led a failed management buyout last month because the bid was too low, would head the new business, which would also include RMC's Scottish operation.