Cement additives based on PCE

Additives have been used in cement grinding since the 1930s. Initially, the main purpose of their use was to increase the production rate, whereas substances which additionally raise the strength of the cement have increasingly come into use from the 1970s onwards [1].

A trend towards reducing the clinker factor of cements, and thus enlarging the amounts of clinker substitutes used – such as GBFS, fly ash and limestone – is observable around the world [2]. The quality of such “composite” cements depends to a very great extent on the type and quantity of the clinker substitutes used. These materials may influence the kinetics of hydration and the compatibility of the cement with conventional concrete additives. This phenomenon can be counteracted by adding specifically selected and adjusted cement additives during cement grinding. It is no longer adequate to improve only the grindability and strength development of the cement for this purpose [3]. Greater clinker substitute contents necessitate new and innovative cement additives which also enhance the workability of cements containing such alternative ingredients.

The list of requirements made on modern cement additives can thus be subdivided into three main categories (Figure 1):

– Grindability: Effective grinding is the basic prerequisite for production

– Strength: The strength development must conform with the applicable standards and satisfy customers‘ requirements

– Workability: The cement must be easily workable (water demand, superplasticizer demand, stiffening)

A modern cement additive can be adjusted to meet the specific requirements and influence each of the above-listed items individually.

Surface charges which cause interactions between the particles occur at the fracture surfaces during the comminution of mineral substances. Surfactant substances, which reduce the occurring forces of interaction, are effective grinding aids. This is apparent, in particular, in the form of a reduction in the formation of agglomerates, improved dispersion during air-classifying, and the avoidance of coating in the mill [4] (Figure 2).

Polycarboxylate ethers are used around the globe as admixtures for concretes and mortars. They are highly effective plasticizers and thus assure long-enduring fresh concrete workability, with a significantly reduced water demand.

It is known that the comb polymers adsorb on the cement particles via functional groups of the polymer backbone and that the side chains project into the free space. Steric repulsion of the side chains of various polymers generates a spatial distance between the cement particles, thus increasing the system‘s fluidity [5].

The development of binders with a lower water demand should be possible using polycarboxylate ethers as cement additives. Since PCEs are surfactant substances, it can well be supposed that they might also act as grinding aids. This hypothesis is based on the assumption that the free, negative-charged carboxylate groups bond by adsorption on to the particle surface and thus saturate the surface charges. The side chains are also assumed to cause spatial repulsion between the particles. The basic precondition for this is the use of specially stabilized PCEs which are capable of continuing to act even after cement grinding and storage.

Cement grinding operations for these studies were performed in a Type TTS 100 laboratory ball mill manufactured by Sieb­technik.

Clinker and sulfate sources were crushed to a particle size of < 4 mm for the grinding tests. The clinker was preheated for several hours in a drying cabinet at 105 °C. Indirect heating of the grinding chamber maintained a constant cement temperature of between 100 °C and 105 °C during grinding. Characterization of the ground cement was performed on the basis of particle-size distribution (PSD) by means of laser ­diffraction, sieve residue on an air-jet sieve (32 µm), specific surface area acc. Blaine, water demand and setting time in accord­ance with EN 196, Part 3, and strength in accordance with EN 196, Part 1.

Triethanolamine (TEA) manufactured by Merck was used in the laboratory tests reviewed here. The tested polycarboxylate ethers (PCE-1 and PCE-2) are used in commercially available Sika superplasticizers. They differ primarily in terms of their charge density, with PCE-2 possessing a higher charge density than PCE-1. These additives were used in the form of aqueous solutions.

Grinding tests to determine the influence of the additives on grinding performance:

The “control grind” (grinding with no addition of additives) was ground to a fineness of approx. 2000 cm²/g to 2500 cm²/g (acc. Blaine). This fineness is sufficient to permit conclusions on the efficacy of the grinding aids. The effectiveness of the grinding additives was determined on the basis of the fineness achieved after a constant grinding time.

Grinding tests to determine the effects of the additives on mortar properties:

The fineness of a cement has effects on cement and mortar properties. In order to investigate the influence of the additives on these properties the cement grinding of a test series was continued until a specified fineness had been achieved. The properties of the ground cement samples were analyzed in accordance with EN 196.

The influence of PCE-1 on the water demand and flow table spread of a Type CEM I cement was investigated in a fundamental test. 95 % clinker with 5 % natural gypsum were ground for this purpose. The composition of the cement ­clinker is shown in Table 1.

These grinding tests were performed with various rates of addition of 40 % PCE solution. The samples were ground to a Blaine value of 4300 cm²/g ± 60 cm²/g. For comparison purposes, a 40 % solution of triethanolamine was used as an additive.

The cement‘s water demand decreases, and its flow table spread becomes greater, as rate of PCE-1 solution dosage rises. A noticeable effect occurs only at comparatively high additive levels, however. In the sample to which triethanolamine was added, water demand was increased and flow table spread reduced compared to the control sample (Table 2).

The effectiveness of PCE-1 as a grinding aid was evaluated in a further series of tests. Cement grinding tests with various aqueous grinding-aid contents were performed for this purpose ­(Figure 3). All samples were ground for 60 minutes.

PCE-1 solution noticeably increases grinding efficiency, but not as greatly as the TEA solution.

The properties of triethanolamine as a grinding aid were then combined with the effect of PCE-1 on water demand and flow table spread. For this purpose, various mixtures of PCE-1 and TEA were used as the additive in new series of tests. Total active-ingredient content of the solutions was 40 %, while the amount of the solutions added was 0.10 % of the cement weight. The mixing ratios (parts by mass) can be found in Figure 4. The ground samples had finenesses of 4100 cm²/g ± 50 cm²/g.

As in the initial series of tests, slight improvements in water demand and flow table spread when PCE-1 is added are also apparent here. The positive influence of PCE-1 is gradually neutralized in relation to the increasing TEA content in the mixture. However, when a mixture consisting of one part PCE-1 and two parts TEA is added, water demand is lower and flow table spread greater than in the case of the TEA solution.

The effectiveness of the mixtures mentioned above as grinding aids was determined by means of cement grinding tests at a constant grinding time, with subsequent determination of fineness (Figure 5). The additive dosage was 0.05 % referred to cement weight, with an active-ingredient content of 40 %.

The ground cement following the addition of a PCE solution as a grinding additive was not as fine as after the addition of a TEA solution. Fineness increases as TEA content in the mixtures rises, up to a mixing ratio of 1:1. Fineness remains constant within the fluctuation range if TEA content is further increased. A linear rise in fineness based on the PCE-1:TEA ratio in a series PCE-1, 10:1, ...,1:10, TEA had however been anticipated.

These tests were repeated using other cement clinkers, and the results stated above qualitatively confirmed. 0.05 % of a PCE‑1 solution, of a PCE-1/TEA mixture (mixing ratio: 1:2) and of a TEA solution were used as the additives for this purpose ­(Figure 6). The active-ingredient content of these solutions was 40 %.

The effects of the grinding additives on grinding of clinkers C1, C2 and C3 are comparable. The PCE-1/TEA mixture is just as effective as the TEA solution. Only in the case of C2 is the TEA solution slightly more effective than the mixture. The PCE-1 solution achieves the greatest fineness in the case of clinker C4, however.

Tavares et al. [6] ascertained a general correlation between grindability and the phase composition of a clinker. The possibility of a correlation between clinker composition and an optimum PCE‑1/TEA mixing ratio has not been investigated up to now. Further tests should, in addition, take account of the degree of crystallization of the clinkers.

The laboratory experiments were followed by grinding tests under real conditions at a number of cement plants. For this purpose, various cement types were ground in two-chamber ball mills incorporating dynamic third-generation separator. The additives were added to the clinker belt immediately upstream of the mill inlet; the required rate of addition was selected by means of an adjustable diaphragm pump. Cement samples were taken from the finished-product conveyor for the purpose of further testing. Unlike the laboratory grinding tests, the effectiveness of the grinding additives was assessed on the basis of production rate at constant fineness. Raw materials for the additives were used in technical quality. The production data stated are averages across four hours of stable production with constant parameters (such as separator settings, for instance).

The first example shows the results of grinding of a CEM I 42.5 R (Table 3). The cement was initially ground with no additive, while a 40 % solution of triethanolamine was added as a grinding aid in the second test. In the following phase, a formulation consisting of two parts TEA and one part PCE-1 was used as the grinding additive. The total active-ingredient content of the formulation was 40 % in each case.

The TEA solution made it possible to increase the production rate by 18 %. This figure rose to 22 % compared to the control test when the formulation consisting of PCE-1/TEA was added. Such a further increase in production due to the use of the PCE‑1/TEA mixture, which was not anticipated on the basis of the laboratory tests (Figure 6: Clinker 1), can be explained by the dispersing action of the PCE in the separator. The enhanced effectiveness of the PCE‑1/TEA mixture on grinding performance was also confirmed by the results obtained at other cement plants [7].

The production data for a CEM II/A‑S 52.5 R are shown in Table 4. This in-plant test compared an aqueous solution of diethylene glycol (DEG) against an aqueous solution of PCE‑1 and DEG (mixing ratio: 1:2). The active-ingredient content of the solutions was in each case 50 %. It is apparent that PCE‑1 also generates an additional boost to production when used in mixtures containing DEG.

The mechanical strength which a cement is ca­pable of developing in concrete or mortar is influenced by a number of factors. For strengths measured in accordance with EN 196, these factors are, essentially, clinker composition, cement composition and fineness of cement [8].

An increase of 100 cm²/g in the specific surface area of a CEM I cement results in an increase of around 1.0 MPa (2 d) and around 1.5 MPa (28 d) in strength. These data should be regarded as averages, and may vary from cement to cement. In the case of grinding in commercial cement plants, an increase in cement fineness will result in a fall in production output rate. This loss can be compensated by the use of effective PCE-based grinding aids.

The reactivity of the cement can be modified by the addition of chemical substances. The PCEs make no contribution to chemical activation of the cement in this context. As has already been demonstrated elsewhere (Table 5) [9], product formulations containing other substances as well as PCE can permit systematic adjustment of early and ultimate strength, however.

The essential criteria for the workability of fresh concrete are flow table spread and flow retention (maintenance of consistency). Due to the specified strength values, cements with a high clinker substitute content must be of correspondingly high fineness. Water demand and plasticizer demand also increase as fineness rises, however. The various clinker substitutes exhibit great differences in this respect. Natural and synthetic clinker substitutes may contain both mineral and organic impurities which reduce the workability of cementitious systems.

Cements containing large amounts of pozzolans or ground limestone as clinker substitutes show a particularly low flow table spread in the concrete, and have a stronger stiffening effect. PCEs which permit a high flow table spread and long flow retention are therefore particularly suitable for use as superplasticizers in the production of concrete from these cements. The amount of these superplasticizers added to concrete is generally between 0.5 % and 1.5 %, referred to the weight of cement.

If specially stabilized PCEs are added to the cement during grinding, even low dosage can achieve a significant increase in flow table spread and flow retention. This is illustrated by the following example of grinding of a CEM II/B‑M (V‑LL) 42.5 N cement. Pure PCE‑2 (active-ingredient: 100 %) which only improves workability without affecting strength development was used at two different dosages.

Table 6 shows that production of the cement was slightly increased when the PCE‑2 additive was used, while maintaining cement fineness and strength development. Water demand and air content were also not affected by the additive.

The essential result of these tests consists of the improvement of flow table spread and of enhanced flow retention when the PCE-2 is used (Figure 7). It is apparent that the beneficial effects increase as dosage rate rises.

Laboratory and in-plant tests have verified that PCE-based additives fulfill the requirements made on modern cement additives.

PCEs make a contribution to increasing grinding efficiency; it is apparent that their benefits as grinding aids are particularly pronounced in product formulations containing conventional grinding aids, such as triethanolamine or diethylene glycol, for instance. This effect is particularly noticeable in the production of cement by means of mill-separator systems. It was demonstrated here that mixed products consisting of PCE and “traditional” grinding additives increase production more significantly than additives containing no PCE.

PCE-based cement additives also make it possible to increase strengths. Adjustment of separator settings permits the production of a finer cement. The resultant loss in production output can be balanced out by the use of effective grinding additives. In addition, products consisting of PCE and chemically active substances which directly influence cement hydration can also be formulated.

The workability of cement can be systematically improved by means of addition of a PCE additive at the grinding stage. Addition at customary “grinding aids rates” (250 g/t – 500 g/t) has usually no, or only a slight, influence on workability. It is apparent  that flow table spread and flow retention can be controlled by increasing the amount of additive used. Further studies into the influence of cement additives of various structures on workability will be reported at a later date.