Fig. 2. Distribution of aggregates in concrete specimens: from top (left) and from the side  (right).

Table 4

Mixture proportions.

Code Cement type Cement (kg/m3) Fly ash (kg/m3) Fly ash (%) Water (kg/m3) Waste marble (kg/m3) Marble Dust (%) Marble Dust (kg/m3) W/C

A1 CEM I 350 0 0 190 1829 6 21 0.54

A2 CEM I 315 35 10 165 1889 6 21 0.52

A3 CEM I 280 70 20 140 1948 6 21 0.50

A4 CEM I 245 105 30 120 1995 6 21 0.49

A5 CEM IV 350 0 0 190 1808 6 21 0.54

428 I. Tekin et al. / Construction and Building Materials 133 (2017)   425–432

on the fresh concrete to determine the workability. Air content of fresh concrete was measured according to ASTM C 231-04 [31]. Three specimens were used to determine the properties of hard- ened concrete for each   test.

From each concrete mixture, six specimens were cast in molds of 350 × 700 × 150 mm. Some representative specimens are shown in Fig. 2. After 24 h, specimens were demolded and all were cured by wetting twice a day in the laboratory. Unit weight accord- ing to ASTM C 138 [32], ultrasound pulse velocity according to ASTM C 597 [33], Schmidt hardness cording to ASTM C805 [34] and compressive strength tests were performed. The carbonation depth was measured according to the CPC-18 RILEM procedure [35].

3. Results and discussion

3.1. Workability

Slump values are presented in Fig. 3; they range between 55 and 90 mm. As seen in Fig. 3, workability increases with increasing fly ash concentration in the mixtures while W/C ratios are reduced. Compared to A1, slump increased 36.4% for A2, 27.3% for A3 and 63.6% for A4. We achieve here an important objective, substituting fly ash for a portion of cement increases the slump value. Appar- ently nearly spherical shape of fly ash particles provides a ball bearing effect that reduces internal friction in fresh concrete and thus increases the flowability and compaction of concrete. As also seen in Table 4, the water demand decreases from A1 to A4. More- over, because of its lower density, FA increases the paste volume leading to improved workability. It is thus that the use of costly plasticizers or so-called superplasticizers can be avoided. Addition- ally, increasing the amount of FA in the mixture increases the fine- ness of concrete, resulting in improved cohesion. All these positive effects of FA impart improved workability of concrete—leading to easy handling without segregation.

A5 has a slump value 18.2% higher than that of A1. This out- come results from the waste marble in A5 that  increases  the cement paste volume and the workability of concrete. On the other hand, marble dust increases water demand, what might have mit- igated the slump values to within the range 60–90 mm. All mix- tures are workable. We note that an increase in the waste marble content increases weight – as some of us have discussed earlier [16].

According to several reports, waste marble dust aggregates decrease the slump loss thanks to their  smooth  surface textures and lower water absorption [23,36,37]. The same authors have shown that fresh marble coarse aggregate concrete improves workability and is more cohesive than ordinary limestone aggre- gate concrete [36]. All our  mixtures contain waste  marble     aggre- gates; thus, the slump changes are related to the cement type, pozzolan type, and water to cement ratio. We conclude that fly ash significantly improves  workability.

The volume of waste marble aggregate increases along with the increase in volume of FA. As known, FA has lower specific gravity than  Portland  cement.  The  specific  gravities  of  FA  and  A1  are

2.55 and 3.13, respectively. To prepare specimens of the same vol- ume, therefore, we had to decrease A1 amounts more than the amounts of FA introduced as a replacement. In order to obtain 1 m3 concrete mixtures, we also had to increase the volume of waste marble aggregates and modify the water content because of the absence of superplasticizers. The improved workability of A2, A3 and A4 series, with respect to A1 and A5 series, can be attributed to the increases in the volumes and amounts of both FA and waste marble aggregate. Both FA and waste marble aggregate contribute to  more  smooth surfaces.