A combined understanding of particle size evolution during grinding and chemical interactions during cement hydration can be used to develop high performance cement additives. This customisation approach ensures that cement plants maximise efficiency while delivering superior performing products to their customers.

Portland Pozzolan Cements (PPCs), in which part of the Portland cement clinker has been substituted by fly ash, are well established in India with clinker replacement levels often being greater than 30 per cent. This high level of substitution poses a challenge for the cement plant during grinding as well as in ensuring that the final performance of the PPC meets the requirements of customers. Consequently, many cement plants are turning towards customised cement additives to ensure efficiency in the grinding process while enhancing the cement performance.

Particle Size Distribution
PPCs are typically produced by the co-grinding of the clinker, fly ash and gypsum in a ball mill (with or without roller press) or vertical roller mill. This range of grinding technologies combined with the significant differences in the hardness of the clinker and fly ash can result in large variations in the particle size distribution of the produced cement, which consequently impact the performance.

A significant amount of research has been published which demonstrate the impact that the different particle size fractions have on cement strength performance in both ordinary Portland cement as well as blended cements [1, 2, and 3]. Consequently, it is sensible to question whether it is possible to combine different cement additive raw materials in such a way as to modify the particle size distribution of the finished cement and thereby optimise its performance. It is well known that the particles distributed in the 5 to 30 ?m size fraction contribute the most to the overall strength development. Therefore, if we are able to enrich the amount of particles in this size range, we should be able to increase the compressive strength of the cement.

In our laboratory we prepared a series of PPCs (67 per cent OPC clinker, 29 per cent fly ash and 4 per cent gypsum) while using various cement additive raw materials to improve the grinding efficiency. After 35 minutes of grinding, samples were taken and Blaine surface area, sieve residue and particle size distribution (laser diffraction) measurements were conducted. Some of the key results are presented in the following tables and figures.

After grinding, a Blaine specific surface area of around 350 m2/kg was achieved and the particle size distributions of three cements are given in figure 1. The reference cement ground without an additive and the cement produced with cement raw material, RM 2, are almost identical. This is not surprising as RM 2 was chosen for its chemical effect on the hydration reaction and not for its physical effect on grinding.

However, a significant effect on the particle size distribution can be seen when the cement is ground using cement additive RM 1. The cement additive RM 1 was chosen as it is known to have a positive effect on the particle size distribution during grinding. In the particle size range 5 to 30?m; there is around a 10 per cent increase in the volume of particles for the cement produced with RM 1 compared with the reference cement produced without any additives.

What is more interesting is the change in the volume of particles in the range 5 to 15 ?m as these are known to contribute the most to the early strength development. The difference between the reference cement and the cement produced using additive RM 1 was around 50 per cent. The values for both ranges are given in table 1.

The increase in the 5 to 15 ?m range is particularly important in the case of PPC. In a cement containing fly ash, the early strength is one of the more problematic areas when trying to increase the clinker substitution rate as the fly ash itself contributes almost nothing to the one-day strength. This is because for the pozzolanic reaction to be initiated, it requires the presence of calcium hydroxide, which is released from the hydration of the main clinker minerals. Consequently, if the amount of clinker particles in the 5 to 15 ?m range increases, this will lead to a faster rate of reaction of the main clinker minerals, thereby producing more calcium hydroxide to help boost the reaction with the fly ash.

Cement Hydration and Strength Development
The hydration of cement is a complex process, but for a clinker or cement with the same mineralogical and chemical composition, grinding it finer, i.e., producing a larger number of smaller particles will increase the rate of reaction. Consequently, this will increase the compressive strength of that cement and is essentially the physical effect that we presented in the previous section.

However, many of the cement additive raw materials also impact the hydration process directly by reacting with the various clinker minerals. Depending on the additive used, this interaction will vary and could, for example, accelerate the conversion of ettringite, produced from the hydration of the aluminate phase (C3A) with sulfate, into mono-sulfate or accelerate the rate of hydration of alite (C3S), thereby changing the rate of formation of the calcium silicate hydrates (CSH).

The impact on the hydration process can be evaluated by monitoring the rate of heat evolution for a hydrating PPC and in our case we used a TA Instruments TAM-Air isothermal calorimeter for this purpose. The heat of hydration curves for the two selected cement additive raw materials, RM 1 and RM 2 along with the reference cement, are shown in figure 2.

The curves shown mainly refer to the hydration of alite and there are clear differences between the three cements. Although the initiation of the main hydration reaction as well as the overall shape of the curves are similar for the reference cement and the cement containing RM 1, the area under the curve is larger for the cement with RM 1. The peak height is also greater and together this would tend to suggest that the reaction had progressed to a greater extent over the same time period when the additive RM 1 was used.

In the case of the cement produced with additive RM 2, there is a clear reduction in the alite induction period demonstrated by the fact that the hydration heat starts to rise rapidly around 30 minutes earlier than the reference cement. This decrease in the induction period should also have a positive effect on the strength development. Compressive strength measurements were conducted using the three laboratory cements and the results are provided in table 2. The cement produced with RM 1 shows the best overall performance in terms of strength improvement and this can be attributed to the combination of particle size effects and chemical interactions. Additionally, the strength gain at later ages is quite significant and most likely occurs due to the enhancement of the fly ash reaction.

The use of cement additives affects both the physical nature of a cement, i.e., its particle size distribution and also the chemical reactions that occur during the hydration reaction. These effects are often overlapping and understanding these interactions can lead to the formulation of more effective customised cement additives.

Practical Application
The previous two sections have highlighted some of the more theoretical aspects when using cement additives in a controlled laboratory environment. However, the critical aspect is how well can this information be transferred to a real life application to achieve the desired performance. The following example demonstrates how we have applied this knowledge to optimise the performance of a PPC for one of our customers in India. The cement plant is producing a PPC with around 25 per cent fly ash addition in a ball mill with a capacity of around 210 T/hr.

The key objective for the customer was to increase the mill output and boost the one-day strength performance. Over the trial period, several products and dosages were evaluated in order to determine the optimum product. An overview of some key results from the trial is given in table 3.

All three products that were evaluated in plant scale trials demonstrated a marked improvement in both the mill output and one-day compressive strength when compared to the reference level. This allowed the customer to select the product that was best aligned with their expectations in terms of cost and performance.

Therefore, by developing an understanding of both the physical effects occurring during grinding and the chemical effects during cement hydration, there is no need for a cement plant to compromise on performance, quality or cost. Moving away from standard products towards a customised approach can lead to significant benefits for a cement plant and is an ideal solution, when compromising on cost and performance is not an option.

This article has been authored by Sridhar Gowda and Nilesh Malave, Fosroc Chemicals (India) Pvt Ltd, Fatzunnahar Ngopil, Fosroc International Limited (Malaysia) and Martyn Whitehead, Fosroc Asia Sdn Bhd (Malaysia).

A combined understanding of particle size evolution during grinding and chemical interactions during cement hydration can be used to develop high performance cement additives. This customisation approach ensures that cement plants maximise efficiency while delivering superior performing products to their customers.

Portland Pozzolan Cements (PPCs), in which part of the Portland cement clinker has been substituted by fly ash, are well established in India with clinker replacement levels often being greater than 30 per cent. This high level of substitution poses a challenge for the cement plant during grinding as well as in ensuring that the final performance of the PPC meets the requirements of customers. Consequently, many cement plants are turning towards customised cement additives to ensure efficiency in the grinding process while enhancing the cement performance.

Particle Size Distribution
PPCs are typically produced by the co-grinding of the clinker, fly ash and gypsum in a ball mill (with or without roller press) or vertical roller mill. This range of grinding technologies combined with the significant differences in the hardness of the clinker and fly ash can result in large variations in the particle size distribution of the produced cement, which consequently impact the performance.

A significant amount of research has been published which demonstrate the impact that the different particle size fractions have on cement strength performance in both ordinary Portland cement as well as blended cements [1, 2, and 3]. Consequently, it is sensible to question whether it is possible to combine different cement additive raw materials in such a way as to modify the particle size distribution of the finished cement and thereby optimise its performance. It is well known that the particles distributed in the 5 to 30 ?m size fraction contribute the most to the overall strength development. Therefore, if we are able to enrich the amount of particles in this size range, we should be able to increase the compressive strength of the cement.

In our laboratory we prepared a series of PPCs (67 per cent OPC clinker, 29 per cent fly ash and 4 per cent gypsum) while using various cement additive raw materials to improve the grinding efficiency. After 35 minutes of grinding, samples were taken and Blaine surface area, sieve residue and particle size distribution (laser diffraction) measurements were conducted. Some of the key results are presented in the following tables and figures.

After grinding, a Blaine specific surface area of around 350 m2/kg was achieved and the particle size distributions of three cements are given in figure 1. The reference cement ground without an additive and the cement produced with cement raw material, RM 2, are almost identical. This is not surprising as RM 2 was chosen for its chemical effect on the hydration reaction and not for its physical effect on grinding.

However, a significant effect on the particle size distribution can be seen when the cement is ground using cement additive RM 1. The cement additive RM 1 was chosen as it is known to have a positive effect on the particle size distribution during grinding. In the particle size range 5 to 30?m; there is around a 10 per cent increase in the volume of particles for the cement produced with RM 1 compared with the reference cement produced without any additives.

What is more interesting is the change in the volume of particles in the range 5 to 15 ?m as these are known to contribute the most to the early strength development. The difference between the reference cement and the cement produced using additive RM 1 was around 50 per cent. The values for both ranges are given in table 1.

The increase in the 5 to 15 ?m range is particularly important in the case of PPC. In a cement containing fly ash, the early strength is one of the more problematic areas when trying to increase the clinker substitution rate as the fly ash itself contributes almost nothing to the one-day strength. This is because for the pozzolanic reaction to be initiated, it requires the presence of calcium hydroxide, which is released from the hydration of the main clinker minerals. Consequently, if the amount of clinker particles in the 5 to 15 ?m range increases, this will lead to a faster rate of reaction of the main clinker minerals, thereby producing more calcium hydroxide to help boost the reaction with the fly ash.

Cement Hydration and Strength Development
The hydration of cement is a complex process, but for a clinker or cement with the same mineralogical and chemical composition, grinding it finer, i.e., producing a larger number of smaller particles will increase the rate of reaction. Consequently, this will increase the compressive strength of that cement and is essentially the physical effect that we presented in the previous section.

However, many of the cement additive raw materials also impact the hydration process directly by reacting with the various clinker minerals. Depending on the additive used, this interaction will vary and could, for example, accelerate the conversion of ettringite, produced from the hydration of the aluminate phase (C3A) with sulfate, into mono-sulfate or accelerate the rate of hydration of alite (C3S), thereby changing the rate of formation of the calcium silicate hydrates (CSH).

The impact on the hydration process can be evaluated by monitoring the rate of heat evolution for a hydrating PPC and in our case we used a TA Instruments TAM-Air isothermal calorimeter for this purpose. The heat of hydration curves for the two selected cement additive raw materials, RM 1 and RM 2 along with the reference cement, are shown in figure 2.

The curves shown mainly refer to the hydration of alite and there are clear differences between the three cements. Although the initiation of the main hydration reaction as well as the overall shape of the curves are similar for the reference cement and the cement containing RM 1, the area under the curve is larger for the cement with RM 1. The peak height is also greater and together this would tend to suggest that the reaction had progressed to a greater extent over the same time period when the additive RM 1 was used.

In the case of the cement produced with additive RM 2, there is a clear reduction in the alite induction period demonstrated by the fact that the hydration heat starts to rise rapidly around 30 minutes earlier than the reference cement. This decrease in the induction period should also have a positive effect on the strength development. Compressive strength measurements were conducted using the three laboratory cements and the results are provided in table 2. The cement produced with RM 1 shows the best overall performance in terms of strength improvement and this can be attributed to the combination of particle size effects and chemical interactions. Additionally, the strength gain at later ages is quite significant and most likely occurs due to the enhancement of the fly ash reaction.

The use of cement additives affects both the physical nature of a cement, i.e., its particle size distribution and also the chemical reactions that occur during the hydration reaction. These effects are often overlapping and understanding these interactions can lead to the formulation of more effective customised cement additives.

Practical Application
The previous two sections have highlighted some of the more theoretical aspects when using cement additives in a controlled laboratory environment. However, the critical aspect is how well can this information be transferred to a real life application to achieve the desired performance. The following example demonstrates how we have applied this knowledge to optimise the performance of a PPC for one of our customers in India. The cement plant is producing a PPC with around 25 per cent fly ash addition in a ball mill with a capacity of around 210 T/hr.

The key objective for the customer was to increase the mill output and boost the one-day strength performance. Over the trial period, several products and dosages were evaluated in order to determine the optimum product. An overview of some key results from the trial is given in table 3.

All three products that were evaluated in plant scale trials demonstrated a marked improvement in both the mill output and one-day compressive strength when compared to the reference level. This allowed the customer to select the product that was best aligned with their expectations in terms of cost and performance.

Therefore, by developing an understanding of both the physical effects occurring during grinding and the chemical effects during cement hydration, there is no need for a cement plant to compromise on performance, quality or cost. Moving away from standard products towards a customised approach can lead to significant benefits for a cement plant and is an ideal solution, when compromising on cost and performance is not an option.

This article has been authored by Sridhar Gowda and Nilesh Malave, Fosroc Chemicals (India) Pvt Ltd, Fatzunnahar Ngopil, Fosroc International Limited (Malaysia) and Martyn Whitehead, Fosroc Asia Sdn Bhd (Malaysia).