The collaboration served as a cementing of the long-term partnership between the two companies.

cementing 是动词 cement 的现在分词形式。cement 本义是"水泥"，作为动词时比喻为"巩固、加强"，就像水泥使建筑结构更牢固一样。在这里 cementing 表示"起到巩固作用"的动作。