Influence of Silica Modulus on the Activation of Amorphous Wollastonitic Hydraulic Binders with Different Alumina Content: Study of Hydration Reaction and Paste Performance.

This study investigates how different sodium silicate SiO2/Na2O MS ratios (0.75, 0.9, and 1.2) affect the hydration behavior of amorphous wollastonitic hydraulic (AWH) binders containing various amounts of Al2O3 content (4, 7, 10, and 12%wt). The effects of and interaction between the MS ratio of the activator and the Al2O3 content of the sample on the hydration reaction and paste performance were investigated. The reaction was followed by calorimetry, and the pastes' compressive strength performances were tested at different curing times (2, 7, and 28 days). The hydrated pastes were characterized by FTIR, thermogravimetry analysis, and X-ray diffraction. The calorimetric results show that a higher Al2O3 cContent and a higher MS ratio result in a longer induction period. In terms of paste performance, an increase of the Al2O3 coupled with an activation with a 1.2 MS ratio results in a lower compressive strength after 28 days of hydration; the results range from 76 to 52 MPa. A decrease of the MS ratio to 0.9 allowed the obtention of a narrower range of results, from 76 to 69 MPa. Even though a decrease of the MS ratio to 0.75 led to higher hydration kinetics and high compressive strength results at early ages, at 28 days of curing, a decrease in compressive strength was observed. This may be a consequence of the fast kinetic of the mixture, since the rapid growth of hydration products may inhibit the dissolution at later ages and increase the porosity of the paste. Moreover, the high Al intake in the hydration product, facilitated by the high sodium content of the activator, promotes the formation of a higher number of calcium aluminate silicate hydrate structures (C-A-S-H) to the detriment of calcium silicate hydrate structures (C-S-H), decreasing the compressive strength of the samples. The TGA results indicate that the samples hydrated with the MS075 solution resulted in a higher number of hydrated products at early ages, while the samples hydrated with the MS09 and MS1.2 solutions exhibit a steady increase with curing time. Hence, an equilibrium in the hydration kinetic promoted by Si saturation-undersaturation appears to be fundamental in this system, which is influenced by both the MS ratio and the Al(OH)4- content in solution. The results of this study suggest that for this type of binder, optimal performance can be achieved by decreasing the MS ratio to 0.9. This composition allows for a controlled kinetic and overall higher compressive strength results in pastes produced with this AWH precursor.

The Use of Solid Sodium Silicate as Activator for an Amorphous Wollastonitic Hydraulic Binder.

To ensure the acceptable mechanical strength of amorphous wollastonitic hydraulic binders (AWHs), activation with a sodium silicate solution is necessary. However, the use of this type of activator increases the final cost and the complexity of the product's overall use. In this work, we focus on enhancing the manufacturing of the alkaline activator by producing three Na2SiO3 powders using cost-effective raw materials. The procedure consisted of heating a mixture of NaOH pebbles with either sand, glass, or diatomite to a temperature of 330 °C for 2 h. After synthesis, the powders were characterized by Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Diffraction (XRD) techniques. Finally, mortars made with AWHs were activated using the synthesized powders that were added either as a solid or dissolved in an aqueous solution. The compressive strength results in these mortars show that the lab-made activators are competitive with the traditional sodium silicate activators. Furthermore, the synthetized activators can be added in either solid form or pre-dissolved in a solution. This innovative approach represents a more economical, sustainable and easy-to-use approach to enhancing the competitiveness of AWHs.

The Apparent Activation Energy of a Novel Low-Calcium Silicate Hydraulic Binder.

In this work, the apparent activation energy (Ea) of a novel low-calcium binder was, for the first time, experimentally determined, using a calorimetric approach. Additionally, a correlation between the Ea, measured at the acceleration period with the C/S ratio of the hydration product is proposed. The Ea of the prepared pastes was determined through isothermal calorimetry tests by calculating the specific rate of reaction at different temperatures, using two different approaches. When comparing the Ea, at the acceleration period of this novel binder with the one published for alite and belite, we observed that its value is higher, which may be a result of a different hydration product formed with a distinct C/S ratio. Finally, to study the temperature effect on the compressive strength at early ages, a set of experiments with mortars was performed. The results showed that the longer the curing time at 35 °C, the higher the compressive strength after 2 days of hydration, which suggests a higher initial development of hydration products. This study also indicated that the novel binder has a higher sensitivity to temperature when compared with ordinary Portland cement (OPC).

Alternative Clinker Technologies for Reducing Carbon Emissions in Cement Industry: A Critical Review.

Currently, the production of one ton of ordinary Portland cement (OPC) releases considerable amounts of CO2 into the atmosphere. As the need and demand for this material grows exponentially, it has become a challenge to increase its production at a time when climate-related problems represent a major global concern. The two main CO2 contributors in this process are fossil fuel combustion to heat the rotary kiln and the chemical reaction associated with the calcination process, in the production of the clinker, the main component of OPC. The current paper presents a critical review of the existent alternative clinker technologies (ACTs) that are under an investigation trial phase or under restricted use for niche applications and that lead to reduced emissions of CO2. Also, the possibility of transition of clinker production from traditional rotary kilns based on fuel combustion processes to electrification is discussed, since this may lead to the partial or even complete elimination of the CO2 combustion-related emissions, arising from the heating of the clinker kiln.

Influence of Pseudowollastonite on the Performance of Low Calcium Amorphous Hydraulic Binders.

A systematic investigation of the influence of pseudowollastonite on the performance of a new family of low calcium hydraulic binders is described. Samples of the new low calcium binder were produced by an innovative process consisting of heating and homogenizing the mix of raw materials (limestone, sand, and fuel cracking catalyst) at a constant temperature followed by the rapid cooling of the mixture itself. Different maximum temperatures, close to the melting temperature of the mix, were tested, and materials with CaO/SiO2 (C/S) ratios of 0.9, 1.1, and 1.25 were produced into the form of the amorphous phase with small percentages of pseudowollastonite. Compressive strength results were determined at 7, 28, and 90 days of hydration, and the hydrated phases were analyzed using isothermal calorimetry, X-ray diffraction (XRD) analysis, thermogravimetry analysis (TGA), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC). The present work is focused on the influence of the percentage of the pseudowollastonite phase on the binder compressive strength performance.