Blended Cements with Limestone
Portland Limestone cement has distinct advantages in terms of performance, controlling strength, reducing cost, and enhancing sustainability. There are international standards that regulate the use of Portland Limestone cement. For example, the European Standard EN 197-1 permits a number of cement types as well as setting criteria for the limestone itself. Typically the limestone needs to have CaCO3 purity >75%, have a limited clay content (methylene blue test not exceeding 1.20g/100g), and total organic carbon not exceeding 0.20% by mass for "LL" and 0.50% for "L".
The percentage of limestone used has a significant impact on the cement grinding process and ultimately on the final cement performance characteristics. It affects the grinding efficiency of the clinker, mill retention, mill internals coating, moisture input, grinding temperature, and cement flowability. Limestone also generally produces a wider particle size distribution (PSD) for the cement, with both a higher Blaine value and higher residues. Each additional 1% of limestone typically increases the Blaine SSA by 5-8 m2/kg (constant mill kWh/t).
Higher Blaine SSA and wider particle size distribution (PSD) reduces dry flowability, and this tends to be more difficult for limestone cements. It increases material filling in the mill, which is detrimental to the grinding efficiency when it is above the optimum value of void filling. The high fineness and increased moisture input increase agglomeration and coating, also leading to a negative impact on grinding efficiency.
The performance of the cement is strongly impacted by the quantity of the limestone. A wider PSD reduces paste water demand and higher fineness reduces the tendency for bleed water. However, the most noticeable effect concerns strength development, resulting from both the substitution of clinker and the effects from the PSD. The amount that the limestone reduces strength depends on the limestone percentage, the effects on PSD, cement fineness, and any cement additive present.
Cement additives can make an important contribution to grinding efficiency to counter the negative impact on flowability, void filling, coating, and PSD. Given that strength is substantially reduced in these situations, selecting the appropriate quality-improving additive is critical. Typically, there is a strong requirement to improve 28-day strength; additives based on higher alkanolamines are often the most appropriate.
Some improvements in 28-day strength are possible through finer grinding, accomplished by reducing mill output. However, the strength reduction due to higher limestone levels cannot be fully offset by a reduction in mill output alone.
Choosing the right cement additive has a marked impact on the percentage of limestone possible to achieve the target performance. For a typical 5MPa gain from the additive, it would be possible to increase the limestone by approximately 7% for the same mill output. Identifying the correct cement additive has more of an impact on limestone cement production than the effect of the mill output. Importantly, it is also usually more economically and logistically advantageous.
In planning the production of Portland Limestone cement:
- Select the desired cement performance characteristics, such as strength targets.
- Decide on the production mill output that is acceptable to suit logistics and volumes.
- Evaluate the benefits of an appropriate cement additive.
- Identify the limestone content that can satisfy the cement performance requirements.
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