The use of natural calcined clays as a main constituent in cement

The use of pozzolanic and/or latent hydraulic cement constituents instead of Portland cement clinker represents a key leverage factor for the environmentally important reduction of CO₂ emissions within the cement industry. In view of the predicted significant increase in demand for cement and the necessity of further reducing the clinker factor, calcined clays are becoming more important for the cement industry as a pozzolanic main constituent in cement.

The use of calcined clays as a main constituent in cement in accordance with EN 197-1 is possible if the content of the reactive silicon dioxide is at least 25 mass %. Although suitable raw material resources exist in Europe and some cement manufacturers have access to these, calcined clays have rarely been used in cement manufacture up to now, mainly due to a lack of research work in this field. In particular, the influence of the chemical and mineralogical composition of the clays on their suitability as a main constituent in cement has not yet been sufficiently investigated.

Calcination of clays

Fifteen different mined clays from clay deposits, which could be assigned to the four groups “kaolinitic clays”, “bentonitic clays”, “kaolinitic-illitic clays” and “chloritic clays” were investigated concerning their suitability as a cement main constituent.

The determination of the optimum burning conditions is an important step to generate a sufficient pozzolanic reactivity of the clays. Therefore, between 2 and 9 calcination steps were carried out per clay, varying in burning temperature (500 – 1300 °C) and duration (5 or 30 minutes). After fixing suitable burning conditions for each clay, larger amounts of calcined clays were produced accordingly. Figure 1 shows exemplarily the X-ray diffractograms of the calcined samples of kaolinitic-illitic clay in comparison to the starting sample. It is recognisable that after calcination at 1000 °C all clay minerals (illite/muscovite, kaolinite and montmorillonite) were converted to amorphous reactive phases. New, less reactive high temperature phases were sillimanite and/or mullite, and hematite from as low as 900 °C.

Figure 1. X-Ray diffraction pattern of a kaolinitic-illitic clay, burning parameters seen on the right.

Figure 2. Reactive silicon dioxide acc. EN 197-1 of the calcined clays depending on their insoluble residue in HCl/KOH acc. EN 196-2.

Figure 2 shows the link between the content of reactive silicon dioxide according to EN 197-1 and the insoluble residue in KOH/HCl according to EN 196-2 for all calcined clay samples. Regardless of the relevant burning conditions, the EN 197-1 requirement of having at least 25 mass % of reactive silicon dioxide was met whenever the clay had insoluble residue of under 50 mass %. The insoluble residue in KOH/HCl is therefore a significant parameter for estimating the quality of clay calcination. Due to selecting suitable burning conditions, the requirement of the standard regarding the content of reactive silicon dioxide could be met in each of the 15 clays investigated.

Investigation of clay-containing cements

CEM II/A-Q with 20 mass % and CEM IV/B (Q) with 40 mass % of the respective clay components were produced by intensively mixing the calcined clays with a CEM I 42.5 R. Additionally, cements with an industrial metakaolin as reference were manufactured the same way.

The water demand closely following EN 196-3 of the CEM II/A-Q was between 28.0 and 34.5 mass %; the water demand of the CEM IV/B (Q) was between 28.5 and 38.5 mass %. As expected, the water demand increased as the proportion of clay in the cement increased.

The compressive strength tests on the CEM II/A-Q were carried out with a water/cement ratio of 0.5, and on the CEM IV/B (Q) with a water/cement ratio of 0.6 for process-related reasons. The strengths of the CEM II/A-Q were between approximately 21 and approximately 29 MPa at the age of 2 days and between approximately 48 and approximately 66 MPa at the age of 28 days. The strengths of the CEM IV/B (Q) were between approximately 9 and approximately 13 MPa at the age of 2 days and between approximately 24 and approximately 49 MPa at the age of 28 days. The highest levels of compressive strength were reached with the kaolinitic clays.

Figure 3 shows the compressive strengths of the clay-containing cements that reached the highest levels of compressive strength at an age of 28 days in each of the four different clay groups. It is evident that the cements with the kaolinitic clay each displayed the highest levels of compressive strength. The levels of strength at the age of 2 days were affected by the dilution of the clinker by the calcined clay in both types of cement, and were therefore relatively close together. Overall, the cements with the chloritic clay showed the lowest compressive strengths. Of the tested CEM II/A-Q, only the latter cement did not achieve the level of compressive strength of the meta-kaolin-containing reference cement after 28 days of hydration, and also remained clearly below the level of strength of the CEM I 42.5 R reference. However, it was possible to produce CEM II/A-Q in strength class 42.5 R with all clays investigated, without either granulometric optimisation or additional sulfate adjustment. The influence of clay mineralogy on the contribution to the strength of the cement provided by the calcined clays was confirmed, with the ranking kaolinite, montmorillonite, muscovite/illite.

Aspects regarding the optimisation of strength development and the durability of cements with calcined clays are currently being investigated in several research institutes.

Figure 3. Compressive strength of CEM II/A-Q (left, w/c = 0.5) and CEM IV/B (Q) (right, w/c = 0.6) compared to the respective cements with metakaolin and to the CEM I as references.

This article was originally published in Newsletter 3/2014 of the European Cement Research Academy and is reproduced by kind permission of ECRA.

Read the article online at: https://www.worldcement.com/special-reports/17092014/the-use-of-natural-calcined-clays-as-a-main-constituent-in-cement-496-/