Here, Denes Novak and Manfred Tisch of Wopfinger Baustoffindustrie GmbH present the development and properties of a new 'clinker substitute,' which combines high performance with lowered costs and CO2 emissions.
One of the most efficient ways to reduce CO2 emissions in the cement industry is to reduce the limestone content during clinker production. This is because the processing of limestone is responsible for high amounts of CO2 production, firstly due to the decomposition of CaCO3 to CaO and CO2 and secondly due to the high-energy consumption required to achieve this reaction. In addition to the significant ecological and economic advantages, important primary raw material savings can be realised when reducing the limestone content of clinker.
For over 40 years experiments have been made for the advantages of low CaO-content to bring benefits in practice. Several patents and publications1 based on calcium sulphoaluminate cements acknowledge these efforts. Clinkers of such composition are characterised by a high melt-phase content and a narrow melting temperature range, which makes the burning process more difficult. The burnt clinker shows high reactivity, fast setting and high early strength development. These properties make these sulphoaluminate cements excellent for products with such special requirements, for example for fast setting repair mortars.
Because of the high sensitivity of water metering to temperature fluctuations and especially to the precise tuning of the necessary addition of retarding agents, these cements are very difficult to control and, when processed, problematic for use in mass concrete.
A new clinker substitute
The clinker substitute introduced in this article contains a higher percentage of silicon- and aluminium-containing compounds than OPC clinker. These components often occur in cheap secondary raw materials as well as in waste products, such as slightly reactive fly ashes, demolished concrete, brick chippings, foundry sand, aluminium waste, residual construction material and others. Therefore, significantly higher amounts of these additives can be utilised during the manufacture of the new clinker substitute than during the manufacture of OPC clinker. Case study calculations have shown that 25% of waste materials can be used during 'traditional' clinker production while 46% can be used during production of the new clinker substitute. Due to cheaper procurement costs of secondary raw materials, considerable financial advantages are achieved.
A cost saving arises from the fact that during the production of Portland cement clinker, about 1.65t of raw meal is used to make 1t of clinker. Only about 1.21t of raw meal is needed to make 1t of the new clinker substitute, meaning that about 25 - 27% less raw meal needs to be used.
The clinker substitute developed shows lower reactivity than older types of sulphoaluminate cement. It consists mainly of monocalciumsilicate and monocalciumaluminate phases, which show excellent long term strength development. However, due to the slow hardening the early strength development is lower.
After performing numerous experimental tests, it was possible to develop a product with a satisfactory early strength and which allows the production of cements with extraordinary quality properties for a broad spectrum of applications. Both the method of production and the product itself are patented (A 50580/2012).
Compressive strength development
Compressive strength tests (according to EN 196-1) were carried out with the new clinker substitute in combination with various additions to Portland cement clinker. The results are shown in Table 1.
| OPC (%) | 75 | 60 | 50 | 40 |
| New clinker substitute (%) | 25 | 40 | 50 | 40 |
| Concrete making additives (%) | 0 | 0 | 0 | 20 |
| Compressive strength (N/mm2) | ||||
| 1 day | 15.1 | 12.8 | 8.1 | 5.4 |
| 2 days | 26.9 | 21.7 | 15.8 | 12.8 |
| 7 days | 40 | 31.4 | 26 | 24 |
| 28 days | 48.1 | 44.5 | 37.5 | 40.7 |
Above - Table 1: Compressive strength development of OPC and clinker substitute mixtures.
The favourable impact of the clinker substitute on the early strength development of the cements is obvious. Even at a substitution rate of 50% of the OPC clinker with the new clinker substitute there is satisfactory compressive strength development. This enables the use of such mixtures in a broad range of concrete applications.
Table 2 shows comparison of the new clinker substitute to CEM II / A-L, CEM II / A-S and CEM II / B-M with two mixtures containing the new clinker substitute. Despite using higher amounts of the new clinker substitute, significantly higher early strength was achieved than with cements containing usual additives.
| CEM II / A-L | CEM II / A-S | Clinker substitute A | CEM II / B-M | Clinker substitute B | |
| OPC (%) | 80 | 80 | 75 | 65 | 60 |
| New clinker substitute (%) | 0 | 0 | 25 | 0 | 40 |
| Limestone (%) | 0 | <20 | 0 | 0 | 0 |
| Concrete making additives (%) | <20 | 0 | 0 | <35 | 0 |
| Compressive strength (N/mm2) | |||||
| 1 day | 12.6 | 13.7 | 15.1 | 7.8 | 12.8 |
| 2 days | 25.9 | 26.4 | 26.9 | 16.8 | 21.7 |
| 7 days | 45.2 | 47.4 | 40 | 30 | 31.4 |
| 28 days | 55.8 | 60.5 | 48.1 | 41 | 44.5 |
Above - Table 2: Comparison of CEMII/A-L, CEMII/A-L, CEMII/B-M and cements with 25% and 40% clinker substitute.
Physical advantages
The good early strength development reduces the danger of the concrete surface drying, which favourably influences the amount of curing components.
The lifetime and durability of concrete buildings is increased significantly by the use of the innovative clinker substitute. A strong long term strength development and a dense concrete structure are achieved by the monocalcium silicates and monocalcium aluminates which originate from the low CaO content.
Less heat develops in the formation of the monocalcium silicates and monocalcium aluminates phases. In this way, the heat of hydration is lower and the risk of crack formation is significantly reduced.
As a result of the decreased tricalcium aluminate content, the sulphate-resistance is increased. Research has also confirmed that concrete made of cement containing the new clinker substitute is resistant to the application of de-icing salts. Due to these properties, a broad range of applications can be covered with cements containing the new clinker substitute.
Economic and environmental benefits
The production of the new clinker substitute generates significantly less CO2 and considerably less energy is used during the burning process. Due to the reduced limestone content, 50 - 60% less CO2 is generated than with OPC-clinker. The CO2 emissions are reduced due to the decomposition of less CaCO3 and a reduction in the amount of fuel needed to perform this reaction.
Furthermore, the firing temperature is lower than usual. Therefore the stress on the combustion chamber is substantially reduced. Usually, OPC clinker is made at 1380°C, but the innovative clinker substitute can be produced at about 1200°C. It is also important to note that the new clinker substitute can be produced in existing cement plants with minimal investment prior to production.
To prove these advantages, burning tests with OPC clinker and with the new clinker substitute were carried out. 10t of the clinker substitute and 8.4t of OPC clinker were produced. Table 3 shows the results of these tests. Despite the 9% higher performance, it was possible to reduce the consumption of natural gas by 50%. This amounts to total energy savings of more than 50%. The reduction of CO2 emissions was 57%.
| OPC | New clinker substitute | Saving | |
| Raw power feed (kg/hr) | 593 | 674 | |
| Natural gas rate (Bm3/hr) | 90 | 45 | 0.5 |
| Temperature before filter (°C) | 117 | 87 | |
| Furnace intake temperature (°C) | 814 | 468 | |
| Sintering zone temperature (°C) | 1394 | 1202 | |
| O2-content (%) | 1.6 | 2.6 | |
| Free lime (%) | 0.63 | 0.34 |
Above - Table 3: Burning tests with the new clinker substitute.
The performance of the rotary kilns is in the first place defined by the highest possible introduction of heat to the raw materials. The greatly-reduced energy demand of clinker substitute therefore allows for a significantly increased performance of the rotary kiln compared to OPC burning. If the parameters, for example the capacity of the raw meal and clinker transport or the rotational speed, have to be matched to the higher furnace output, this can be done with appropriate investment.
The firing proceeded without disturbance due to the lower stress on the combustion chamber and without increased coating-formation. The obtained clinker substitute has a consistent quality and a significantly better grindability than OPC.
Conclusion
Using the new clinker substitute over traditional cement mixtures enables conservation of primary raw material reserves and significant reduction in CO2-emissions. It offers significantly lower raw material costs, greatly reduced energy demand, considerably better furnace performance and higher grindability.
The beneficial properties of the cements that contain the new clinker substitute allow for a broad range of applications with significant amelioration of sustainability.
Consistent product quality by controlling the raw material setup and burning process is straightforward and there are almost unlimited production options. It offers significant cost and energy saving potential to countries that have little or no clinker additives and those where clinker additives are expensive.
1. Sacharow, L.A. 'Tonerde-Belitzement,'Silikattechnik 11/1971.
