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In concentrated alkaline sodium silicate solutions a consistent decrease in the amount of silicate monomer Qo, end-groups Q', cyclic trimer Q: yc groups and middle groups QZ, and also branching groups in ring formations Q:yc, is observed with increasing molar ratio of SiO, to Na,O (R,), whereas for branching groups Q3 and cross-linking groups Q' a consistent increase is found. The interest of this work is particularly focused on the Q4 region, which may indicate the formation of colloidal silicate particles. For this region the ''Si background signal from the NMR probe and the glass tube can cause problems, so the subtraction technique was used. Spin-lattice relaxation times, not previously reported for silicate solutions of concentrations and molar ratios as high as those studied here, are given for all structural units, including Q4. It was found that sodium silicate solutions of R , = 2.0 do contain a Q4 region when they are extremely highly concentrated in S O , (38 wt% SO,) or aged. KEY WORDS Sodium silicate solutions High concentrations Silicon-29 NMR Speciation RelaxationDeparture from extreme narrowing

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