Development of Magnesium Silicate Hydrate Binder System with the Addition of Alkali Carbonates

Portland cement is currently the most common globally used cement, responsible for most of the carbon dioxide emission in cement industries. Magnesium cement may become a potential alternative to the Portland cement, especially for some specific applications, and offers solutions to mitigate the carbon dioxide emission in traditional cement industries by reducing the energy requirements. One of the magnesium cement types is the magnesium silicate hydrate (M-S-H) cement. It obtains strength from M-S-H gel which corresponds to the calcium silicate hydrate (C-S-H) in the Portland cement. However, currently the application of M-S-H cement for construction is limited by its long setting time and relative insufficient strength at early stage. Thus, the development of the M-S-H gel needs to be investigated.

This project aims to investigate the development of the M-S-H gel, in order to obtain a reliable and feasible methodology to develop the M-S-H cement for general applications with acceptable period of hardening time. The effects of additives are studied, including sodium bicarbonate (NaHCO3) and sodium carbonate (Na2CO3), aiming to change the pH of the system as well as sodium and carbonate ion concentration in the cement at batch preparation stage. The change of the pH is expected to influence the solubility of different materials in the batch, and the evolution of the M-S-H gel is investigated under different conditions. The effects of different ions in the additives are also studied on M-S-H evolution.

The development of the M-S-H gel is first tested by the addition of NaHCO3 and Na2CO3 solutions with various concentrations. The obtained results show that the carbonate additives have the ability to aid the reaction, but the reaction rate is affected by the additives concentration to different extent. The maximum acceleration was achieved generally when the concentration equals to the saturation. The addition of NaHCO3 resulted in the formation of intermediate hydromagnesite, which appeared to accelerate the reaction of Mg(OH)2, while this intermediate phase was identified only in a limited amount in the Na2CO3 system. The final products are also affected by the concentration of the solutions, that some magnesium carbonate species appear in the NaHCO3 samples when concentration equals to the saturation but disappeared when the concentration is reduced; talc or dolomite may appear in the Na2CO3 samples depending on the concentration of Na2CO3.

In the next step, alternative raw materials are used to test their feasibility to develop the M-S-H cement. The Mg dross from alloy industries is used as the alternative magnesium source and calcined clay is used as the alternative silicon source in the M-S-H formation. The results indicate that using the Mg dross is able to form M-S-H gel, and the reaction can be accelerated by the presence of NaHCO3, while the formation of M-S-H gel is limited when using calcined clay in the condition used. Additional phases are presented in final products for both systems due to the impurity contents of the raw materials. The compressive strength of the samples is limited, likely due to the too much water contained in the samples. The results suggest that the addition of the carbonate may also reduce the total strength.

To improve the strength of the M-S-H cement, the effect of water reduction and the effect of superplasticisers are tested in the final step. The M-S-H batches with same composition but different water-to-solid (w/s) ratio are tested. The results suggest that the addition of NaHMP as the superplasticisers effectively reduced the water requirements for making the paste but may retard the reaction. The reduction of water in the cement successfully increased the compressive strength of the cement and can also accelerate the reaction. However, the addition of NaHCO3 may hinder the effect of NaHMP.