The Effect of Particle Fineness on Properties of Portland Pozzolan Cement Mortars
The change in particle fineness effects firstly setting times and the other properties like compressive strength and volumetric changes.In this study, the effects of the change in cement particle fineness on the properties of portland pozzolan cement were studied. Experiments were conducted with samples of portland pozzolan cement in five different fineness values.
It was found that as particle fineness increase, the first seven days compressive strengths of these types of cements increase and their setting times decrease. In addition, it is seen also that when particle fineness increases, volume expansion values increase a little. These results were discussed detailed by the help of tables and graphics.
Keywords: Cement, Fineness, Cement Additives, Compressive Strength, Volume Change.
1. Introduction
Natural pozzolans, also known as trass, which contributes cement by high durability level [1-4] and energy saving [5-6], is added to cement after kiln process, in grinding stage. Thus, an increase in cement particle fineness is seen because of their ability of grinding finely. This increase in fineness improves many properties of cement positively.
Turkey is rich in natural pozzolan sources [7-8]. In there, almost about one third of the total production in recent years was “trass cement” which is a portland pozzolan cement [9]. In most Mediterranean countries, situation is also the same [8]. Therefore, recognizing properties of portland pozzolanic cements is very important.
Former studies showed that cement particle fineness has an effect on concrete’s mechanic strength. It is understood that finely grinded cements gain strength more quickly, however after a long time; this strength does not show an evident increase in respect of coarse grinded cements [10-12]. Actually, one other study show strength does not always increase in direct proportion with cement’s specific surface [13]. In addition, some researches show the increase in cements’ fineness, increase cements’ workability, and contribute formation of a denser matrix besides increasing their compressive strength [14-16].
Researchers state also that in mortars and concretes, where the mixture of portland cement and natural pozzolan are applied together as a mixture, initial set and final set show
development depending on the replacement ratio, fineness and reactivity of pozzolan cement in respect to portland cement [16-18].
Previous studies also state that the harmful materials in cements such as free CaO, MgO and SO3 can damage the volume stability. The existence of these materials in cement mixtures should be kept under the specific limits. The ratio of free CaO should be less than 2% (by mass) and the ratio of MgO should be less than 5% (by mass) [5]. In spite of this, it is known that natural pozzolan addition decreases the existence of these compounds by getting into a reaction with them according to its fineness [19].
The objective of this study is to examine the effect of the change in portland pozzolan cement’s fineness on cement’s compressive strength, setting times, and volume expansion properties. In this respect, cements in five different fineness were produced by fixing natural pozzolan type and ratio (25%) constant and required tests are conducted. In addition, it is also examined that whether the natural pozzolan sample taken from Esiroglu (Trabzon / North – East Turkey) district can be used in cement industry or not.
2. Experimental Methods
2.1. Natural Pozzolan
Mineralogical and petrographical properties of the natural pozzolan samples were identified under the polarizing microscope by using their thin sections (Figure 1) and X-ray powder diffraction (XRD) analysis of the powdered bulk samples were carried out (Figure 2). Major element analysis of the samples comprising SiO2, A12O3, Fe2O3, MgO, CaO, Na2O and K2O were carried out by means of wet chemical analysis (Table 1). The pozzolanic activity (Table 2), the behavior of pozzolan in mortar, was determined according to the Turkish standards; TS 25 [20] (similar as ASTM C 311).
In a previous study [7] it is stated that high SiO2 content of natural pozzolan implies high pozzolanic activity (strength). In Table 1, high SiO2 content can be seen. These especially come from quartz (Figures 1 and 2). Besides, there are clay group minerals (Figures 1 and 2) and some zeolitization (Figure 2) in the natural pozzolan sample. Zeolitization also have a positive effect on pozzolanic activity.
2.2. Cement Samples
The experiments designed in the scope of this study were conducted according to the suggested principles in EN 196. Five different cement mixtures with known fineness values (Table 3) were prepared and exposed to cure. Then, compressive strength experiments for 1st, 2nd, 7th and 28th days were conducted (Figure 3) according to Rilem – Cembureau Method in EN 196. In addition, their setting times (Table 4) and volume expansion (according to Le Chatelier soundness test) values (Table 4) were determined. For setting times, Vicat Apparatus in EN 196 was used.
3. Determining Material Properties
3.1. Natural Pozzolan and Clinker
The natural pozzolan used in the experiments was taken from Esiroglu District (Trabzon/Turkey), and its chemical (Table 1), mechanical (Table 2) and mineralogical properties (Figures 1 and 2) were presented. Besides, chemical analysis of clinker used in cement samples was also given in Table 2.
3.2. Substitution Proportions of Natural Pozzolan
If masses of the materials were taken into consideration, cement mixtures were constituted from 25% natural pozzolan, 72% clinker, and 3% plaster. These cements are in Portland Pozzolan Cement (CEM II/BP) class with 25% natural pozzolan ratio according to EN 197-1.
3.3. Particle Fineness of Cements
Five different cements (Table 3) were produced in different fineness by grinding the mixture in different periods. In addition, these were exposed to different experiments expressed above. Moreover, a portland cement sample (PC) was added to these experiments for comparison. Specific surface values were realized according to Blaine method as suggested by related standards.
Since portland pozzolan cements were produced by mass grinding in Turkey, the cement samples in this study was prepared by
grinding natural pozzolan, clinker and plaster together.
Although sieve analyses were carried out, when the effects of fineness were examined during the study, specific surface (cm2/g, Blaine surface) values were taken into consideration. Comparisons and evaluations were made according to these values.
4. Results and Discussions
4.1. Availability of Natural Pozzolan Sample in Cement Industry
After mineralogical and chemical analysis, it is understood that this rock sample contains quartz, amphibole and plagioclase minerals. Its SiO2 content is %70.88. Therefore, it can be classified as dasidic tuff.
It is seen also that total basic compounds is greater than TS25 requirement (%82.91>%70). As mechanically, the pozzolan sample has better properties. Its compressive strength is 11 N/mm2 and it achieves conditions of 4 N/mm2 and its flexural strength is 4.45 N/mm2 and this also provides condition of 1 N/mm2.
Thus, both chemical experiment results and mechanical strengths prove that the material was taken from Esiroglu District (Trabzon / North – East Turkey) appropriate according to TS 25 standard requirements, (Table 2).
4.2. Effects of Particle Fineness on Compressive Strength of Portland Pozzolan Cement
As it can be seen from Figure 1, when specific surface of portland pozzolan cement and so its particle fineness increases its compressive strength increases, too. This situation is valid for each cement sample. For example, the finest sample (C5) shows 9.2 N/mm2 of compressive strength at initially. However, after 28 days, it shows 49.1 N/mm2. On the other hand, relatively less fine sample (C1) shows 6.7 N/mm2 for 1th day, and 35.5 N/mm2 for 28th day. In addition, the same figure also shows that effect of fineness increase on 7th and 28th days compressive strength are greater than the effect on 1st and 2nd days’. The difference between the slopes of 1st and 2nd days compressive strength lines and 7th and 28th days compressive strength lines shows this rise.
However, when 7th and 28th days compressive strengths are compared with each other, it is seen that lines’ slopes are nearly constant, and this shows that there is not a serious increase in compressive strength after the seventh day. As it can also be seen from 28th day compressive strength curve, an increase around 1000cm2/g (approximately 38%) in cement fineness causes approximately 40% compressive strength increase. Natural pozzolan additive’s contribution to strength in general can be based on material’s particle fineness first. Because finer natural pozzolan particles strengthen the framework by filling the micro pores in cement matrix.
4.3. Effects of Particle Fineness on Setting Times of Portland Pozzolan Cement
As it can be understood from the results (Table 4) when portland pozzolan cement samples’ specific surfaces were increased in obtained levels, initial set times did not change significantly. On the other hand, although portland cement’s specific surface is lower than natural pozzolan added cement samples’ (2847<3714, cm2/g), initial set time was very short with respect to these cement samples (150<175, min.). It can be also concluded that relative increase in particle fineness of portland pozzolan cement samples did not affect initial set time significantly. However, it is considered that portland pozzolan cement samples have a slowing effect on setting times as compared to portland cement samples in general. This should be also linked with the finely grinding ability of natural pozzolan. Because SiO2 (Quartz) which is available high ratio in pozzolan can be grinded finely. By this reason it can attach hydration water by absorbing it from the environment in large amounts and lowers the existence of CaO that is the main reason of the reaction. Although, portland pozzolan cement sample has nearly the same particle fineness as portland cement, it causes a significant delay (C2, 35 minutes) in initial set time, and this proves the claim stated above. In addition, portland cement, which has nearly the same fineness values as C1 and C2 samples, gives close final set time values. It is interesting to see that portland cement and portland pozzolan cement samples, which have the same fineness values, have the same final set times. Nevertheless, researchers say that cements that include 20-25% natural pozzolan in mass,
show close setting time values to comparison samples that include portland cement. However, it is known that addition of greater natural pozzolan amounts (30–70% in mass) delay setting times in considerable amounts [17, 21].
4.4. Effects of Particle Fineness on Volume Expansion of Portland Pozzolan Cement
According to the obtained results, portland pozzolan cement samples’ volume expansion amounts increases as their specific surface increases, and this can be seen in Table 4. In addition, portland cement’s expansion, which is the reference sample, is found as 7.0 mm with the same methods. C1 and C2 samples which nearly have the same fineness values with portland cement show 3.0 mm and 4.0 mm volume expansion respectively.
This situation can be explained that 25% in mass
natural pozzolan addition reduces total CaO
amount and free CaO ratio. As a result of this,
reaction temperature decreases and so, reaction
rate decreases. Thus, the material crystallizes
perfectly. However, it is understood that as the
fineness increases, similar volume expansion value of portland pozzolan cement to portland cement’s is caused by grinding of CaO in clinker more. In here, it was investigated once more that CaO ratio in natural pozzolan has to be low.
5. Conclusions
Based on the study, it is understood that the increase in particle fineness of cement including 25% natural pozzolan has influences on following properties of it:
· The increase in cement mixtures’ strength is especially influenced up to seven days because of the increase in particle fineness. Increase in particle fineness does not have a significant contribution to strength results obtained after seven days.
· The increase in particle fineness does not have an influence on portland pozzolan cements’ initial set significantly. However, it
considerably decreases final set time of portland pozzolan cements.
· Volume expansion values of portland pozzolan cements are lower than portland cement’s volume expansion, which has the same fineness. When particle fineness increases, volume expansion values increase a little.
· It is clear that natural pozzolan sample, used in the experiments, which is taken from Esiroglu (Trabzon / Turkey) district is available for cement industry.
Acknowledgments
This study is supported by Karadeniz Technical University (Project No.118.001.1). Authors thank to Trabzon Cement Factory for laboratory possibilities.
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