An investigation into the use of magnesia based cement substitutes.

Abstract

Hydrated Magnesia has very poor cementitious properties in comparison with hydrated OPC, thus its use as a hydraulic binder compound as an OPC replacement material is not possible. Hydrated Magnesia possesses no or very low axial compressive properties, revealing its poor cementitious features. Magnesia in OPC is highly deleterious, as volumetric expansion takes place whilst Brucite transforms into Nesquehonite, and due to the facts that Magnesia possesses very little or no cementitious properties. Brucite can be easily synthesised with confidence in wet cure. Although it possesses very little binding properties, the observed compressive resistance at 3MPa is below the required 7MPa as a minimum to be considered as a binder compound. Whilst Brucite carbonates in the presence of moisture and CO2, cracks and fissures are developed in a self-induced expansion, which contributes to deterioration of the solid. Magnesium Silicate Hydrates have very little binding properties, thus they cannot be utilised as cementitious materials. Whilst MSH pastes develop a very low density porous structure, they significantly shrink, which also prohibits their use in applications where OPC excels. Reliance on Nesquehonite as a binder compound is not possible, due to the low levels of carbonation taking place. Magnesia on its own or in the presence of silicates in a hydrated or carbonated form is not a substance that can replace OPC on any levels in the absence of additives. However, it has been established that hydrated Magnesia in the presence of sulphates form a considerably strong cementitious media, which possesses similar characteristics to OPC in many aspects, with lower porosity, lower density and aesthetic features that may well be utilised in the construction sector as an OPC replacement product. The material blend for this can be produced at roughly 25% of heat and energy input required in comparison to OPC, considering raw material extraction and treatment using the dry method in a modern rotary kiln. This also means that the CO2 emissions produced in the manufacturing process is also reduced in line with the stated reduction. Both the hydration and carbonation processes of Magnesia and its by-products produce high heat levels, in chemical processes that drive off moisture contents in dry cure leaving behind a very porous structure. It has been observed that the rate of hydration of MgO is relatively fast, and that carbonation takes place to a limited extent only. Carbonation is limited in the wet phase, whilst prolonged carbonation in the solidified stage leads to micro-structural volumetric expansion taking place, which in turn leads to the deterioration of the solidified paste in time. Since the expected rate of carbonation is very low, it is safe to say that Nesquehonite as a binding compound is unreliably ii
synthesised, thus its use a binder material is restricted. It has been observed that MSH pastes develop an inconsistent structure, with large pores around the forming individual crystals. With the introduction of sulphates, the alkaline solution changes the role of water from reagent to median agent, since it appears that is no longer water causing the cure, but sulphates in some sort of alkali-sulphate reaction, where water contents are also locked away by the forming gel, making this a semi-hydraulic chemical reaction. From the DMA pictures it is clearly visible that MgO in hydration develops a very porous macro-structure. The SEM pictures highlight the inconsistency of the developing solid on a micro-structural level. Both the DMA and SEM pictures underpin the discussed physical properties, and provides suggestions on the reasons for the observed performance levels.
It is interesting to find that many claims unveiled in the literature review have no credibility at all, in particular to those claiming Magnesia can be used as an OPC replacement product in a manner similar to OPC is used. Magnesia cannot be used as a hydraulic binder compound on its own nor with silica additives, however, it may be a very promising alternative to OPC if used with sulphates as additives. The properties of the produced concrete and paste as a result is comparable and similar in many aspects to the OPC equivalent, hence it’s potential on the mass markets as OPC replacement.
Magnesia based cement alternatives can provide a sustainable alternative to OPC.