Influence of Silica Fume and Polycarboxylate Ether Dispersant on Hydration Mechanisms of Cement

Abstract

Partial replacement of ordinary portland cement by silica fume (SF) accelerates its rate of hydration reactions. This acceleration is attributed to the enhanced heterogeneous nucleation of the main hydration product, i.e., calcium−silicate−hydrate (C−S−H), on the extra surfaces provided by SF. However, such enhancement of C−S−H nucleation is suppressed in the presence of polycarboxylate ether (PCE) dispersant, which is added to regulate the fluidity and rheological properties of fresh paste. A generalized phase boundary nucleation and growth model with time-dependent growth of C−S−H is used to fit the hydration rates of plain and binary (10% to 30% SF) cement pastes prepared with and without PCE. The results show that while SF accelerates cement hydration, increments in hydration rates are significantly smaller in relation to the extra surface area provided by SF. This is because of the agglomeration of SF particles which renders up to 96% of their surface area unavailable for C−S−H nucleation. Furthermore, it is shown that the hydration of cement, in both plain and binary pastes, is suppressed in relation to the PCE dosage. This is because of (a) adsorption of PCE molecules onto cement and SF surfaces resulting in inhibition of sites for product nucleation and (b) interaction of PCE with C− S−H, which suppresses growth of C−S−H throughout the hydration process. It is shown that the effects of nucleation site inhibition by PCE are more pronounced in SF as compared to cement. The outcomes of this study improve our understanding of the mechanisms that drive the hydration of cement in the presence of SF and PCEs.

Influence of size‐classified and slightly soluble mineral additives on hydration of tricalcium silicate

“…In addition to physical experiments, a modified phase boundary nucleation and growth (pBNG) model was employed to describe the influence of fillers on early‐age hydration kinetics of C 3 S in plain (pure C 3 S) and binary (ie, [C 3 S + filler]) pastes. The pBNG model implemented in this study—including the foundational assumptions, simulation principles, and algorithms—are similar to those implemented in prior studies . Therefore, to avoid replication of information, description of the model in this section is kept succinct; for further details, readers are requested to consult references …”

“…The degree of hydration of C 3 S as a function of time [ α(t) ; Equation ] is directly related to: a unitless constant B , which envelops characteristics of the paste as well as the chemical shrinkage that occurs during hydration; and an X(t) function (unitless), which represents the volume fraction of reactant transformed into a singular product …”

“…In Equation , the parameter SSA solid (m 2 kg C3S −1 ) represents the effective (ie, reactive ) solid surface area of the paste at the time of product nucleation; the parameter, essentially, serves as a measure of substrate area that is available for heterogeneous nucleation of the product . SSA solid , as shown in Equation , incorporates: “as measured” SSAs (m 2 .…”

“…SSA solid , as shown in Equation , incorporates: “as measured” SSAs (m 2 . kg ‐1 ) of the constituent powders (ie, C 3 S and filler); the replacement level of C 3 S with filler ( z , % mass ); and a free simulation variable ( a filler , unitless) that represents the faction of surface area of the filler that is reactive, that is, not affected by agglomeration or other factors that inhibit product nucleation sites for C‐S‐H …”

“…During hydration of C 3 S—a dissolution‐precipitation process—portlandite (CH) and calcium silicate hydrate (C‐S‐H) form homogeneously in the pore‐solution and heterogeneously on the anhydrous particulates’ surfaces, respectively . C‐S‐H formation, which commences within minutes of wetting of C 3 S particulates and continues over several days, principally dictates the hydration kinetics of C 3 S …”