Clay　often　has　severe　detrimental　impacts　on　cement-based　materials.　Therefore,　it　is　necessary　to　investigate　the　mechanism　causing　the　deterioration　to　improve　the　service　life　of　cement-based　materials.　Based　on　accurate　dimensional　analysis,　a　mechanism　that　influences　clay　is　proposed:　the　intercalation　of　the　side　chains　of　superplasticizer　molecules　in　the　interlayer　space　of　the　clay.　To　lessen　this　harmful　effect,　a　new　clay-resistant　admixture　(CRA)　possessing　cationic　groups　of　small　molecular　size　was　synthesized　through　a　novel　dimensional　design.　The　length　and　width　of　the　side　chains　of　this　superplasticizer　molecule　were　9.50-17.50　and　025-0.40　nm,　respectively,　with　a　radius　of　similar　to　3.74　nm　in　solvent,　which　is　larger　than　the　interlayer　spacing　of　montmorillonite　(i.e.　1.09-2.14　nm).　The　longitudinal　and　latitudinal　lengths　of　the　CRA　molecule　were　0.468　and　9.456　nm,　respectively,　ensuring　intercalation　in　the　interlayer　of　montmorillonite.　The　increase　in　interlayer　spacing　of　the　clay　was　0.364　nm　following　addition　of　polycarboxylate　superplasticizer　(PCE)　plus　CRA　and　0.632　nm　following　addition　of　PCE,　which　suggests　that　the　CRA　plays　the　rote　of　a　＇sacrificial　agent＇　that　is　preferentially　intercalated　into　the　interlayer　space　of　clay　to　further　prevent　the　side　chains　of　the　superplasticizer　molecules　from　entering　the　interlayer.　The　aim　of　this　study　was　to　propose　a　suitable　means　of　synthesizing　a　new　CRA　to　address　the　impact　of　clay　through　dimensional　design　and　mechanism　analysis,　which　contributes　to　the　theoretical　study　and　technological　improvement　of　cement-based　materials.