The Effect of Gradation and Grain-Size Properties of Fine Aggregate on the Building Mortars

Abstract This work is about the effect of fine aggregate properties on the physicomechanical characteristics of hardened mortars. The results indicated that the increase in grain-size of fine aggregate increases the bulk density of hardened mortars. The strength of mortars including limestone fine aggregate is higher than that of the silica-sand. Regardless of the aggregate origin, the strength of the mortars with well-graded fine aggregate for all grain-size is greater than of with uniform fine aggregate. This indicates that grading of fine aggregate increases the strength, while uniformity decreases it. The strength of mortars with well-graded fine aggregate increases as the grain-size increases. Regardless of the aggregate origin, the strength of mortars with uniform fine aggregate increases with increasing grain-size until the grain-size range of 425-1000 µm, but after this range it decreases with increasing grain-size. The thermal conductivity increases with the increase in the grain-size. Notedly, the relationship between thermal conductivity and maximum grain-size of well-graded fine aggregate has a very strong positive correlation. Further, the thermal conductivity value for mortars formed with uniform fine limestone aggregate is minimum at the grain-size range of 425-1000 µm, while it has greatest values close together from this grain-size range


Introduction
Fine aggregate as filling material with cement makes up approximately 90-99% of building mortars. Filling materials can be defined as fine aggregate grade materials with or without binder. Filling materials brought to desired dimensions using crushers and sieves; in other words, it is inevitable to find stone dust (micronized aggregate) passing through 75µm sieve in crushed sand (Ramyar et al., 1995). This size of material is also called stone dust.
There are not many studies on the effects of micronized aggregate or stone dust on building materials. It has been determined that fine aggregate up to a certain amount has a positive effect on the strength and insulation of concrete (Ramyar et al., 1995;Özgan, 2008). Şahin 2011, indicated that the micronized pumice had a positive effect on the strength of a very special size composite plaster.
In this study, the effects of type and grain size distribution, which are the main components of building mortars, on the long-short term physicomechanical properties are revealed. For this purpose, the effects of grain-size distribution variation on changes in physicomechanical characteristics of hardened mortars were investigated by using limestone and silica-sand fine aggregates as filling materials. Few studies are available to determine the effects of the grain size and distribution of fine aggregate on the physicomechanical properties of building mortar. In this research, an experimental research was carried out in line with the objectives mentioned above.

Experimental material
Karaisalı limestone taken from the region of Adana city (south Turkey) and silica-sand taken from the Silis Maden company located in Tarsus (south Turkey) were used as the material in the study (Fig 1 and 2). The limestone aggregate and silica-sand micronized by grinding in the crusher are well-graded (Fig. 3).

Method
In this study, the effects of changes in the grain size distribution of fine aggregates on the physicomechanical behavior of building mortars were investigated. First of all, the grain size distributions of the two different aggregates were determined by sieve analysis following the EN 1015-1 standard. Subsequently, well-graded and uniform fine aggregate (<4000 µm) were prepared for both materials (limestone and silica sand aggregate) and mortars were prepared in accordance with EN 998-1 standard and their physicomechanical properties were determined. In order to compare the test results, each mixture was prepared in accordance with the standards, adhering to a fixed formula seen in Table 1, and the only variable was fine aggregate grain size and distribution characteristics. For this purpose, five well-graded fine aggregates with different maximum grain-size and uniform fine aggregates with various range of grain-size were prepared for each two type of aggregate. Thus, fine aggregates with 10 different particle sizes and range were composed for each material type. Special sieve apertures were used in the preparation of aggregates. Details of the particle size properties of the prepared fine aggregates are given in Table 2.
Six 160x40x40 mm prism-shaped hardened mortars for each aggregate mixture were prepared in accordance with EN 1015-11 standard. The tests of density, flexural and compressive strength, and thermal conductivity were carried out on the hardened mortar samples cured 28 days at 65C 0 (Fig. 4) in accordance with EN 1015-10, EN 1015-11 and EN 1745 standards, respectively. Thus, the effects of the particle size and grading change on the physicomechanical properties of the hardened mortar were analyzed.  [C: Well-graded limestone fine aggregates with various maximum grain size from 250 µm (C1) to 4000 µm (C5), Cr: Uniform limestone fine aggregates with various grain size formed from the material remaining between two special sieves from 125-250 µm (Cr1) to 2000-4000 µm (Cr5), S: Well-graded silica sand limestone fine aggregates with various grain size formed from the material remaining between two special sieves from 125-250 µm (Sr1) to 2000-4000 µm (Sr5), Sr: Uniform silica fine aggregates with various grain size formed from the material remaining between two special sieves from 125-250 µm (Sr1) to 2000-4000 µm (Sr5)]

Results and discussion
The grain size distribution curves obtained by sieve analysis show that both micronized fine aggregates are wellgraded, in other words, they contain a balanced amount of each grain size (Fig. 3). As a result of physicomechanical experiments on 28-day prismatic hardened mortar samples prepared with uniform and well-graded special fine aggregate mixtures with different gradation produced from these well-graded aggregates, very remarkable results have emerged.
Average values of bulk density, compressional strength, flexural strength and thermal conductivity experiment results on hardened mortar samples are given in Table 3. The experimental results revealed that analysis on the basis of grading/uniformity, the grain size change and significantly affected the physicomechanical characteristics, as well. Bulk density of the mortar specimens increases meaningfully as the grain size of fine aggregate increases. The relationships between bulk density, and maximum and range of particle size in the fine aggregate (fill material) for the 28day hardened mortar samples are shown in Fig 5-8. The correlation coefficient in the statistics was classified as strong, medium and weak for R≥0.8, 0.8<R<0.2 and R≤0.2, respectively (Smith, 1986). These graphs states that the increment of grain size amount increases the bulk density with strong correlations for both well-graded and uniform fine aggregates. The strength test results show that both the compressional and flexural strength values of the 28-day hardened mortars including limestone fine aggregate are higher than those of the silica sand (Fig 8-11). On the other hand, irrespective of the aggregate type (origin), both the compressive and flexural strength of the mortars formed with well-graded fine aggregate for all grain-size category is particularly greater than of with uniform fine aggregate ( Fig. 8 and 9).
The compressive and flexural strength of hardened mortars prepared using well-graded fine limestone aggregate increase exponentially with a high correlation coefficient as the grainsize increases (Fig. 9). On the other hand, both the compressional and flexural strength of mortars prepared with uniform fine limestone aggregate increases with increasing grain-size until a certain grain-size range, but after that maximum strength point at grain-size range between 425-1000 µm it decreases with increasing grain-size (Fig. 10). This situation indicates a bell curve model behavior. A similar situation mentioned above is valid for the mortars prepared with fine silica sand aggregate (Fig 11 and 12), provided that they have lower strength values than those prepared with fine limestone aggregate. However, it is noteworthy that the correlations between both the compressive and flexural strength values of the mortars prepared with a well-graded fine silica-sand aggregate and the maximum grain-size are much stronger than those prepared with limestone. This means that the increase relationship in strength of the hardened mortars formed with well-graded fine silicasand aggregate due to the maximum grain size increase has a much stronger correlation coefficient than those with limestone aggregate. The thermal conductivity of the hardened mortars generally increases in proportion to the aggregate grain size increase. Especially, the relationship between thermal conductivity and maximum grain-size of well-graded fine aggregate for both limestone and silica-sand has a very strong positive correlation (Fig 13 and 15). The correlation coefficient of that relationship for the hardened mortars formed with uniform fine aggregates is strong, similar to those with wellgraded and unlike medium (Fig. 14 and 16). Interestingly, the thermal conductivity value for the mortars formed with uniform fine limestone aggregate is minimum at the grainsize range of 425-1000 µm, where the strength is maximum, while the thermal conductivity for the mortars with uniform fine silica-sand aggregate has greatest values close together from this grain-size range. All of test results indicate that the grain-size range of 425-1000 µm for uniform fine aggregate is critical for strength and thermal conductivity.

Summary and conclusion
Numerous series of laboratory tests were performed to investigate the effect of a change of grain-size of well-graded and uniform fine aggregate on the physicomechanical characteristics such as bulk density, compressive and flexural strength, and thermal conductivity of 28-day hardened mortar. From the data and observations presented in this paper the following conclusions can be drawn:  The increase in grain-size content of well-graded and uniform fine limestone and silica-sand aggregate increases the bulk density of hardened mortars.  Both the compressional and flexural strength of hardened mortars including limestone fine aggregate is higher than that of the silica sand.  Irrespective of the aggregate type (origin), both the compressive and flexural strength of the mortars formed with well-graded fine aggregate for all grain-size categories is particularly greater than of with uniform fine aggregate. This indicates that grading of fine aggregate make hardened mortars stronger, while uniformity decreases the strength.  The compressive and flexural strength of hardened mortars prepared using well-graded fine aggregate increase exponentially with a high correlation coefficient as the grain-size increases.
 Regardless of the aggregate type, the compressional and flexural strength of mortars formed with uniform fine aggregate increases with increasing grain-size until the grain-size range of 425-1000 µm, but after this maximum strength point at that grain-size range it decreases with increasing grain-size.  The thermal conductivity of the hardened mortars generally increases with the increase in the grain-size of aggregate. Especially, the relationship between thermal conductivity and maximum grain-size of well-graded fine aggregate for both limestone and silica-sand has a very strong positive correlation.  The thermal conductivity value for mortars formed with uniform fine limestone aggregate is minimum at the grain-size range of 425-1000 µm, where the strength is maximum, while the thermal conductivity for the mortars with uniform fine silica-sand aggregate has greatest values close together from this grain-size range. All of test results indicate that the grain-size range of 425-1000 µm for uniform fine aggregate is critical for strength and thermal conductivity.