Idea & Principles
Celitement releases less carbon dioxide during its manufacture than classic Portland cement klinkers.
Around 770-870 kg of carbon dioxide is generated by the manufacture of a tonne of ordinary Portland cement clinker. Around 75% of this is caused by the deacidification (or calcination) of the main raw material, calcium carbonate (CaCO3). Less limestone is needed for the manufacture of Celitement (see video clip on material balance), so less CO2 is released. In addition to cement clinkers, though, modern cements also contain additives such as gypsum, limestone meal, granulated slag, fly ash or natural pozzolans. These additives can also be combined with Celitement and enable the CO2 intensity to be even further reduced. When comparing the GWP (Greenhouse Warming Potential) of new binding agents with traditional cements, the basis for comparison selected must be taken into account. Is the comparison based on pure Portland cement clinkers, Portland cement (CEM I), a composite cement (CEM II) or even what is known as CEM VI cement? Our benchmark is Portland Cement Clinker.
Classic Portland cement clinkers:
In the adjacent diagram, the assumption is made that Portland cement clinkers are only made of the main mineral tricalcium silicate, mineral name alite (C3S or Ca3SiO5) by way of a simplification. When the raw materials are fired for manufacture of clinker, three units of carbon dioxide are released from the calcium carbonate. The intermediate product calcium oxide (CaO) binds with silicon dioxide (SiO2) to form the alite mentioned (C3S) during the firing process. Alite reacts with three parts of water to form two parts calcium hydroxide portlandite Ca(OH)2 and one part calcium silicate hydrate C-S-H. It is primarily the C-S-H which is responsible for the strength of mortars and concretes. It is effectively the “glue” in concrete or mortar.
A fundamental reduction in the emission of carbon dioxide caused by raw materials is almost impossible in the conventional manufacture of Portland cement. Both the high firing temperature of 1.450°C and a minimum calcium content in the recipes are essential for the desired qualities of Portland cement, otherwise the products will be unreactive.
Celitement or hCHS:
Celitements have a low lime content or calcium to silica ratio. In current recipes it is between 0.5 and 2.0. The material balance is shown in a very simplified manner with a molar ratio of calcium to silicon of one in the adjacent diagram. Some carbon dioxide is emitted when the calcium oxide e.g. burnt lime (CaO) is produced. Calcium oxide, silicon dioxide and water are converted to Celitement (hCHS) in two reaction steps. These already contain a bit of water in a special binding form. When additional water is added, it is exclusively calcium silicate hydrate C-S-H, which is known from the hydration of alite (C3S) or Portland cement clinker, that is produced.