To accelerate the progress of pozzolanic reaction of the mineral admixtures, such as fly ash (FA) and metakaolin (MK), dispersed nano-sized hydrated calcium silicate-polycarboxylate superplasticizer (C-S-H-PCE) were employed to regulate their early-age pozzolanic reactivity in saturated Ca(OH)₂ solution. The ion dissolution process of the mineral admixtures was investigated through conductivity determination, inductively coupled plasma optical emission spectroscopy (ICP), and pH value measurement. The pozzolanic reaction products of the mineral admixtures were studied by chemically bound water content analysis, Fourier transform infrared spectroscopy (FTIR), and Brunner-Emmett-Teller (BET) based on nitrogen adsorption. The enhancement effect of nano-sized C-S-H-PCE on the cement-mineral mortar was analyzed by compressive strength, and the regulatory mechanism underlying the pozzolanic reactivity was discussed.

As a kind of commercial product, nano-sized C-S-H-PCE was provided by Jiangsu Sobute New Materials Co., Ltd., China. The synthetic C-S-H was dispersed via the adsorption of the comb-like PCE on its surface, and then the final C-S-H-PCE with the particle sizes of 50-100 nm was obtained via ultrasonic treatment. The mass concentration of nano-sized C-S-H-PCE was 12%. The conductivity, inductively coupled plasma optical emission spectroscopy (ICP), pH value measurement, chemical bound water analysis, Fourier transform infrared spectroscopy (FTIR) and N₂ adsorption were applied to characterize the samples in this investigation.

As the dosage of nano-sized C-S-H-PCE increasing, the conductivity of MK and FA systems decreased, indicating that the concentration of free ions in the system gradually decreased, and the conductivity of MK system decreased more significantly in relative to the FA system with the addition of nano-sized C-S-H-PCE. The pH values of both MK and FA systems rapidly decrease with time, and the corresponding OH^- consumption increases with the addition of nano-sized C-S-H-PCE after 24 h reaction. The results of conductivity and pH value indicated that nano-sized C-S-H-PCE promoted ion dissolution from mineral admixtures and thus enhancing the pozzolanic activity.

As the dosage of nano C-S-H-PCE increases, the chemically bound water content corresponding to both MK and FA systems gradually increases, and the chemically bound water content corresponding to MK system was significantly higher than that of corresponding to FA system. The FTIR spectra indicated that the polymerization degree of silicon oxygen tetrahedra in mineral admixtures MK and FA in saturated Ca (OH)₂ solution continuously increased with the addition of nano-sized C-S-H-PCE, and the corresponding Si—O bond stretching vibration peak number and integral area of MK system were higher. The results of nitrogen adsorption showed that the nitrogen adsorption capacity and specific surface area of MK and FA samples increase with the increase of nano-sized C-S-H-PCE content, promoting the progress of pozzolanic reaction.

The mechanism of nano C-S-H-PCE regulating the pozzolanic reactivity of mineral admixtures was that nano-sized C-S-H-PCE reduced the free energy ∆G required for the nucleation barrier and nucleation of C-S-H gel, that is, the ions dissolved from mineral admixtures can nucleate and crystallize on the surface of nano-sized C-S-H-PCE at a lower saturation concentration, thus quickly forming hydration products. The formation of hydration products on the surface of nano-sized C-S-H-PCE reduced the ion concentration in the solution, leading to an increase in ion unsaturation and further promoting the ion dissolution from mineral admixtures, thereby accelerating the pozzolanic reactivity of mineral admixtures.

The results showed that the conductivity and pH values of the MK and FA systems both could decrease as the amount of nano-sized C-S-H-PCE increased, accompanying with an accelerated breakage of Si—O and Al—O bonds. After 24 h reaction, the chemically bound water content gradually increased, while the polymerization degree of silicate tetrahedrons continuously increased. In addition, the nitrogen adsorption capacity and specific surface area of the samples gradually expanded. The aforementioned results indicated that nano-sized C-S-H-PCE accelerated the ion dissolution from mineral admixtures and the formation of pozzolanic reaction products. Nano-sized C-S-H-PCE reduced the nucleation energy barrier of C-S-H gel, facilitating the nucleation and crystallization of ions dissolved from mineral admixtures on the surface of nano-sized C-S-H-PCE, thereby accelerating the formation of hydration products. This process could promote the dissolution of ions from mineral admixtures, ultimately enhancing their pozzolanic reactivity.