Influence of superplasticizers on the rheology and stability of limestone and cement pastes

The influence of superplasticizers on the rheological properties and dynamic stability of cement and reference limestone pastes were examined at room temperature. The pastes were initially formulated to exhibit nearly identical rheological parameters and bleeding–segregation characteristics, with w/c = 0.50 for the limestone and 0.55 for the cement. The former was examined at equilibrium pH ~ 10 and at pH 12.5 following addition of Ca(OH)2 to allow distinction of effects related to high pH and Ca+2 from those related to cement hydration reactions. Both polynaphthalene- (PNS) and polyacrylate-type (PC) superplasticizers were investigated, adjusting the dosages to cover the same range of paste fluidity. Superplasticizer–particle interactions were monitored through binding isotherms and zeta potential measurements. The rheology of the pastes was evaluated through the mini-slump test and dynamic viscosity measurements which yielded key rheological parameters: yield stress, elastic and loss moduli (G′ and G″) and zero-shear viscosity (η0). The paste stability was monitored as function of time, i.e. migration of solids and liquid phase measured in-situ and in “real time”, through surface bleeding measurements and from a multipoint conductivity method. The results provide new insight on the relative modes of action of PNS- and PC-type superplasticizers as dispersants. Also, the combined rheology and stability data allow an improved description of the processes responsible for bleeding and segregation in cementitious and reference systems.