In a number of hydrometallurgical processes used for valuable metal recovery from mineral ores, the rheological behaviour of gangue clay minerals (e.g. muscovite) present may be a predominant issue. Under certain conditions where dissolution of clay occurs, deleterious particle interactions and gelation which impact on pulp processability are observed. In the present work, studies of 0.32 volume fraction (57 wt% solid) muscovite suspensions aging for 4 h at different pH values (7 vs. 1) and 25 ºC were undertaken to investigate the viscoelastic properties and microstructure evolution induced by changes in pulp chemistry. Dispersions at both pH 1 and 7 displayed a non-Newtonian behaviour. The rheology at pH 7 was initially weak and thixotropic. Upon aging, it transformed to anti-thixotropic behaviour, whilst the initially low elastic (G') and viscous (G'') moduli of increased dramatically, suggesting a significant microstructure build-up. The elastic modulus was slightly greater than of the viscous modulus at pH 7, with no significant change observed in their ratio (G'/G'') over 4 h aging. In contrast, at pH 1 stronger rheology and thixotropy, with relatively higher elastic and viscous moduli, were observed initially. However the viscoelasticity decreased markedly upon 4 h aging, suggesting microstructure breakdown. The elastic modulus was more dominant with G'/G'' >3 throughout 4 h aging. This contrasting, microstructure temporal behaviours are ascribed to different pulp chemistries evolving at pH 7 and 1. The ramifications of the outcomes are discussed in terms of a possible time-dependent rheology and its control in concentrated, aqueous clay mineral dispersions.