Cement composition

Abstract

A cement composition comprising a blend of 50 wt% clinker and 50 wt% calcined clay, said blend being co-ground to have a particle size distribution with d90 of 35 to 40 µm, d50 of 10 to 15 µm and d10

Description

FIELD OF THE INVENTION This invention relates to an improved cement composition BACKGROUND OF THE INVENTION Concrete is the most important construction material in the world enabling the building of homes, roads, railways, etc. Concrete is composed of a cementitious binder with stone and aggregate. The cementitious binder, cement, is typically composed of ground Portland cement clinker along with gypsum and limestone. As concrete is mixed the addition of supplementary cementitious materials or clinker replacement materials becomes a possibility. Portland cement clinker is produced through the reaction of limestone with aluminous and ferrous raw materials at 1400-1500°C in a rotary kiln. The chemical decomposition of limestone, which liberates CO2 to the atmosphere, combined with the energy consumption needed to heat the material to this high temperature results in up to 0.9 kg of CO2 being released to the atmosphere per kg of clinker produced. Cement manufacture contributes up to 8% of global CO2 emissions. Global cement production is likely to rise in the coming decades and, owing to the undesirable emissions from cement, research continues to identify ways of reducing that impact. Options include improving the energy efficiency of the cement manufacturing process, replacing high carbon fuels with low carbon fuels and the use of blended cement in which part of the cement is replaced by other materials, known as supplementary cementitious material (SCM). If these SCMs can be produced with a lower environmental impact while maintaining the necessary properties of the cement I concrete, then this is a route for the construction industry to begin to decarbonise. As a result increased efforts are being undertaken to investigate available sources of SCMs, such as clay. Kaolinitic clay has been demonstrated to be the best clay material for producing SCMs, providing good strength but having the ability to be calcined at a lower temperature, hence reducing CO2 demand. However, in its pure form it is only found in a limited number of deposits around the world. Therefore there is an interest in producing suitable SCM from other locally and more readily available natural clays. However, previous attempts to use calcined clay with a relatively low kaolinite content as a SCM has produced poor strength properties in the finished concrete, limiting the amount of such material that can be added to cement. SUMMARY OF THE INVENTION According to the present invention there is provided a cement composition comprising a blend of 50% clinker and 50% calcined clay by weight, said blend being co-ground to have a particle size distribution with d90 of 35 to 40 pm, d50 of 10 to 15 pm and d10 of 1 to 2 pm. In a preferred embodiment the blend has a particle size distribution with d90 of 38.616 pm, d50 of 11.987 pm and d10 of 1.464 pm. Preferably said calcined clay has a kaolinite content of less than 1 % by weight. The calcined clay may comprise shale clay having a SiO2 content of at least 50%. Preferably said clinker comprises Portland cement. The clinker can comprise CEM1 or CEM2. The calcined clay preferably has a calcite content of less than 15% by weight or less than 10% by weight. The calcined clay may comprise shale clay having a AI2O3 content of at least 15%. DESCRIPTION OF AN EMBODIMENT OF THE INVENTION It has been found that the activation process for various types of clay can be carried out by grinding and heating to a temperature high enough to alter the structure of the clay minerals by di hydroxylation but low enough to avoid recrystallization and the formation of chemically inert phases. Controlling the activation process is essential in order to obtain defined pozzolanic properties of the resulting SCM, because the pozzolanic properties in turn determine the substitution rate of Portland 5 clinker in a composite cement and the compressive strength properties of the concrete made from the composite cement. The raw clay material, having a kaolin content of less than 20% by weight, is mechanically extracted. The material moisture content is to preferably be no more 10 than 10% by mass. The clay has the following composition: Compound Ave Content (%) 1 sd SiO2 51.14 4.26 AI2O3 20.59 2.02 Fe2O3 6.66 1.03 CaO 4.50 3.09 MgO 1.75 0.09 SO3 0.44 0.30 K2O 1.84 0.13 Na2O 0.49 0.12 P2O5 0.24 0.59 TIO2 0.82 0.08 Raw Material (%) Calcite 26.1 Dolomite 0.1 Quartz 31 llite 17.9 Kaolinite 13.6 Chlorite 1.9 Albite 5 K-Feldspar 2.9 Pyrite 1.7 The clay is crushed, generating an initial size reduction of material from quarried rock (>300 mm) to less than <200 mm - an optimum size for milling efficiency. The clay is transported by conveyor belt from the quarry to the stone store before being conveyed onwards to a milling system. The milling system is complex, comprising multiple mills and grinding chambers. The milling system is a closed loop system so all material is circulated through this process until it is fine enough to be transported onwards in the process. The ground material is then transported by pneumatic pump to storage silos. The blend is then extracted from the silos via air-slide to a feed screw which then feeds a bucket elevator which in turn feeds granulators. The granulators are semi-vertical, fixed angle disc granulators which run at a fixed speed. In the granulators, the dry material is mixed with small amounts of water to form semi-dry nodules with a moisture content below 20%. The moisture content is optimised to balance nodule performance (strength and abrasive resistance) with thermal energy efficiency. The thermal energy efficiency is achieved by obtaining a water content of the lowest possible level. Water is fed to the system via spray nozzles rather than traditional pre-wetting or spray bars. This produced nodules with a diameter of less than 30 mm to optimize material drying and calcination. The nodules enter a first chamber of a Lepol grate kiln. Facilitated by carefully managed gas flows, temperatures are managed to avoid a phenomenon of ‘popping’ where the moisture loss is too rapid and the nodules disintegrate. This is achieved between 150-300°C. The reduced moisture content entering the Lepol grate helps reduce the drying temperature, thus improving thermal energy efficiency. A further benefit of managing the initial Lepol grate temperature and the presence of water is a reduction in dust present in the process. This is a cleaner process than other calcination methods and the reduction in dust aids the efficiency of the calcination process. Material continues through the Lepol grate. 40% to 75%, of the calcination process occurs in the Lepol grate before the material enter the kiln. This calcination occurs at a temperature range of 500-650°C. Because much of the calcination process occurs in the Lepol grate at a lower temperature the CO2 is greatly reduced. Material leaves the Lepol grate and enter the kiln. Fuels in the form of ground coal, liquid hydrocarbon, and biomass, either liquid or solid, can be used to fire the kiln. Full calcination or ‘dehydroxlation’ of the clay is completed in the kiln at a temperature of 680-700°C. To avoid the problem of colouration of the calcined clay product due to the oxidation of Iron (III) Oxide (Fe2Os) present in the clay, which would otherwise give the resulting cement an undesirable reddish tint, the availability of excess oxygen in the kiln is controlled by carefully controlling the supply of fuel to the kiln. The calcined clay leaves the kiln and is cooled by a grate cooler system with forced air. This serves two purposes; • To cool the material to aid handling • To recover heat from the material for reuse in the kiln system. The heat recovery, air at approximately, 300-600°C, reduces the quantity of fuel required by improving the combustion conditions and boosting kiln temperatures. The calcined clay is then mixed with CEM1 clinker at a ratio of 1 to 1 by weight (to form a 50% calcined clay blend) and the blend is then co-ground in the milling system to have a particle size distribution with d90 of 38.616 pm, d50 of 11.987 pm and d10 of 1.464 pm and the resulting cement product is bagged. This co-grinding step to grind the blend to an unusually fine particle size has surprisingly been found to produce a cement product with a high calcined clay content having a strength comparable to 100% Portland cement. The grinding step results in a much finer clinker grind, boosting reactivity. As such, the early strengths (e.g. day 2 strength) of the resulting cement product are similar to a CEM II product (with 75-80% clinker and 20-25% calcined clay) both water demand and setting time are similar to that of a conventional CEM I with 90-95% clinker. This is a massive step forward that no other available calcined clay appears to have achieved. Based on the early strengths, a 28 day strength of approximately 60MPa is expected, comparable to 100% CEM1 clinker. In standard tests this blend provides a setting time of 143 minutes, an expansion of 2mm, water demand of 31.6% and provides good chemical resistance and a smooth, glassy finish to the set product. In a further embodiment the calcined clay can be mixed with CEM2 clinker and the blend ground or milled to the abovementioned particle size specification. This blend as also been found to have surprisingly good strength properties when used to make concrete structures. The invention is not limited to the embodiment described herein but can be amended or modified without departing from the scope of the present invention.

Claims

1. A cement composition comprising a blend of 50% clinker and 50% calcined clay by weight, said blend being co-ground to have a particle size distribution with d90 of 35 to 40 pm, d50 of 10 to 15 pm and d10 of 1 to 2 pm.

2. A cement composition as claimed in claim 1 wherein said blend has a particle size distribution with d90 of 38.616 pm, d50 of 11.987 pm and d10of 1.464 pm.

3. A cement composition as claimed in any preceding claim, wherein said calcined clay has a kaolinite content of less than 1 % by weight.

4. A cement composition as claimed in any preceding clay, wherein said calcined clay comprises shale clay having a SiO? content of at least 50%.

5. A cement composition as claimed in any preceding claim, wherein said clinker comprises Portland cement.

6. A cement composition as claimed in claim 5, wherein said clinker comprises CEM1 or CEM2.

7. A cement composition as claimed in any preceding claim, wherein the calcined clay has a calcite content of less than 15% by weight.

8. A cement composition as claimed in claim 7, wherein the calcined clay has a calcite content of less than 10% by weight.

9. A cement composition as claimed in any preceding claim, wherein said calcined clay comprises shale clay having an AI2O3 content of at least 15%.05 11 24AMENDMENTS TO THE CLAIMS HAVE BEEN FILEDAS FOLLOWSCLAIMS1. A cement composition comprising a blend of 50% clinker and 50% calcined clay by weight, said blend being co-ground to have a particle size distribution with d90 of5 35 to 40 pm, d50 of 10 to 15 pm and d 10 of 1 to 2 pm and wherein the calcined clay has a calcite content of less than 15% by weight.

2. A cement composition as claimed in claim 1 wherein said blend has a particle size distribution with d90 of 38.616 pm, d50 of 11.987 pm and d 10 of 1.464 pm.

103. A cement composition as claimed in any preceding claim, wherein said calcined clay has a kaolinite content of less than 1 % by weight.

4. A cement composition as claimed in any preceding claim, wherein said calcined 15 clay comprises shale clay having a SiO2 content of at least 50%.

5. A cement composition as claimed in any preceding claim, wherein said clinker comprises Portland cement.20 6. A cement composition as claimed in claim 5, wherein said clinker comprises CEM1 orCEM2.

7. A cement composition as claimed in claim 1, wherein the calcined clay has a calcite content of less than 10% by weight.

259. A cement composition as claimed in any preceding claim, wherein said calcined clay comprises shale clay having an AI2O3 content of at least 15%.30