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MICROSTRUCTURAL STUDY OF CONCRETES INCORPORATING CERAMIC

SANITARY WARE AGGREGATE EXPOSED TO FREEZE‐THAW CYCLES

C. Medina (1); M. I. Sánchez de Rojas (2), M. Frías (2), A. Juan (1), J.M. Morán (1)

(1) Escuela Superior y Técnica de Ingeniería Agraria, University of Leon, León, SPAIN

(2) Eduardo Torroja Institute for Construction Science (CSIC), Madrid, SPAIN

Abstract

The resistance of concretes to the action of freeze-thaw cycles is of particular importance in latitudes with cold, wet winters, since acceptable behaviour under these physical conditions translates into good durability throughout the service life of the material. In the present study, a microstructural analysis was conducted of concretes made with aggregate containing crushed waste from the sanitary ware industry. The study focused on an assessment of the damage sustained by recycled concrete as a result of exposure to freeze-thaw cycles. The physical, mechanical and thermal characteristics of the recycled ceramic aggregate were determined to verify whether they complied with the requirements for coarse aggregates laid down in the standards presently in effect (Spanish Structural Concrete Code EHE-08 and European standard EN 12620). After this aggregate was found suitable for use in concrete production, the design mix was calculated and several concretes were prepared, progressively increasing the percentage of natural coarse aggregate replaced by recycled ceramic aggregate (0%, 20% and 25%). Mechanical strength was also determined before subjecting the experimental concretes to freeze-thaw cycles. The damage sustained by recycled concrete as a result of exposure to successive freeze-thaw cycles was then analysed. The loss of surface mass was determined and a detailed microstructural study was performed, focusing on an analysis of the (natural or recycled) coarse aggregate/paste interface and micro-fissures in the paste. The findings showed that the recycled concretes were slightly more resistant to changes in temperature, indicating that ceramic waste from the sanitary ware industry can be reused as coarse aggregate in concrete production, with the concomitant economic and environmental benefits that this entails.

Key words: microstructure, recycled ceramic aggregate, concrete, freeze – thaw, durability

Second International Conference on Microstructural-related Durability of Cementitious Composites, 11-13 April 2012, Amsterdam, The Netherlands

1. INTRODUCTION

In porous materials such as concrete, durability is directly related to the capacity to resist attack by aggressive physical, chemical and biological agents in the surrounding environment. This property, which must be taken into consideration in concrete design, is defined Euro code 2 [1] as compliance by a material with its intended purpose throughout its expected life in terms of service, strength and stability, with no substantial loss of utility or excessive unplanned maintenance. One particularly harmful physical attack for concretes in latitudes where the autumn is humid and the winter cold [2] is freezing and thawing. Four main types of damage [3] attributable to this agent can be identified. Internal damage and scaling, cracking and popouts affect concrete mechanical strength and durability by increasing its permeability. The use in recent years of recycled crushed construction and demolition waste (C&DW) as an aggregate in concrete manufacture has led to the need to study the long- and short-term behaviour of these recycled concretes to predict and guarantee their suitability. Construction and demolition waste [4] vary widely in composition, primarily comprising stony materials (75% in weight) such as concrete, aggregate, burnt clay materials and asphalt. Fifty-four per cent by weight of C&DW comprises burnt clay products. The existing standards and legislation limit the percentage of this type of material that can be used in recycled aggregate due to its physical, chemical and mechanical properties. The lack of both research on and standards for the re-use of waste from the sanitary ware industry as a concrete aggregate has prompted international studies on the subject. A number of papers [5-10] on the possibility of using fired clay waste (such as roof tiles) as an active pozzolanic addition in cement manufacture have reported satisfactory results in terms of both mechanical strength and durability. Another series of analyses [11-20], addressing the replacement of natural (fine or coarse) aggregate with varying proportions of recycled fired clay material in the manufacture of different types of concrete (precast, filler for road bases or sub-bases, non-structural and structural), have found performance to be satisfactory with respect to a number of properties. The present study forms part of a broader experimental programme on the feasibility of introducing sanitary ware waste as a partial (20% to 25 %) replacement for coarse natural (gravel) aggregate in the manufacture of structural concrete. The findings on the mechanical performance of these concretes after a series of freeze-thaw cycles are discussed below.

2. EXPERIMENTAL PART

2.1. Materials

2.1.1. Aggregates The natural aggregates employed can be sub-divided into two categories: the coarse fraction (gravel), corresponding to a fraction size of 4/20 mm in size, and the fine fraction (sand), with grains of less than 4 mm in size. The recycled ceramic aggregate employed came from a ceramic sanitary ware factory. This ceramic waste was subjected to a crushing process using a jaw crusher, and was then sieved to extract the fraction of 4/12.5 mm in size. The aggregate obtained presented irregular shapes with marked edges (Figure 1), mainly due to the properties of the original product (generally of reduced thickness) and to the crushing process.


Figure 1: Recycled ceramic aggregate

The chemical composition of this ceramic waste was similar to that of other ceramic materials (tiles, bricks, etc.). The internal part was composed mainly of SiO2, Al2O3 and Fe2O3, which constituted 93.81% of the total, whilst in the external part the proportion of the previous components dropped to 68.24%, with zircon (ZrO2) representing 12.62% and calcium oxide (CaO) comprising 11.80%. Meanwhile, the alkalis (MgO, NaO and K2O) formed minority components in both parts.

Further to the physical, mechanical and thermal properties of the aggregates used and shown in Table 1, these materials complied with the requisites established by European standard EN 12620 [21] and in Chapter III of the Spanish Structural Concrete Code (EHE-08) [22].

Table 1: Physicals, mechanicals and thermals properties of coarse aggregates Characteristic Gravel Ceramic EN 12620 / EHE-08

Fine content (wt%) 0.22 0.16 <1.5 Dry sample real density (kg/dm3) (EN 1097-6) 2.63 2.39 - Water absorption (wt%) (EN 1097-6) 0.23 0.55 < 5 Flakiness Index (wt%) (EN 933-3) 3 23 < 35 “Los Ángeles” coefficient (% weight) (EN 1097-2) 33 20 < 40 Total porosity (% volume) (MIP) 0.23 0.32 - Loss of mass with freeze-thaw cycles (wt%) (EN 1367-1) 0.33 0.05 - Magnesium sulfate value (wt%) (EN 1367-2) 4 2 - 2.1.2. Cement The cement used was pure Portland cement (type CEM I 52.5 R), which fulfils the specifications given in the European Standard EN 197-1 [23].

2.1.3. Concretes mixtures Three types of concrete were mixed; a reference concrete (RC) and two concretes containing recycled aggregates in the proportions 20% and 25%. These were denominated CC-20 and CC-25, respectively. The design and calculation of these mixes was carried out using the de la Peña method [24], in which a characteristic compressive strength of 30 MPa and a constant water content is established according to the desired consistency (soft) and maximum size of the aggregate (20 mm). Mix proportions of the various components are given in Table.

Table 2: Mix proportions of concretes

Concrete mix Materials (kg/m3)

Sand Gravel Ceramic Cement Water Referente concrete (RC) 716.51 1115.82 0.00 398.52 205.00 Concrete containing 20% recycled aggregate (CC-20) 725.81 892.66 216.43 387.64 205.00 Concrete containing 25% recycled aggregate (CC-25) 728.14 836.87 270.53 384.91 205.00

2.2. Experimental procedure


The properties studied in the hardened concretes analyzed include compressive strength (20 samples/concrete) and freeze-thaw resistance (4 samples/concrete) European standard CEN/TS 12390-9 EX) [25]. The samples tested (Figure 2) were prepared as described in the aforementioned standard and subsequently exposed to 56 freeze-thaw cycles, in which the mean freezing temperature of the medium (water) ranged within the interval shown in Figure 3.

Figure 2: Samples exposed to freeze-thaw cycles

Figure 3: Mean freezing temperature

The microestructural damages were determined using backscattered electron microscopy (BSE). A scanning electron microscope (PHILIPS model XL 30) with tungsten source was used, which enables spot chemical analyses to be carried out using energy-dispersive X-rays, together with a silicon/lithium detector and an EDX analyser model DX4i. In this study specimens for the backscattering analysis were prepared by epoxy impregnation, followed by precision sawing and careful polishing of a flat surface for examination.

3. RESULTS AND DISCUSSION

3.1. Mechanical strength The results obtained [26] for compressive strength assays at 7, 28 and 90 days all presented the same trend, whereby mechanical behaviour was observed to improve as the replacement ratio increased. Figure 4 gives the values obtained for the 28-day compressive and splitting tensile strength for the different concretes. It can be seen, as commented earlier, that the incorporation of recycled ceramic aggregates improved the mechanical behaviour as regards both compressive and tensile strength in recycled concretes, compared with the reference concrete. This effect intensified with rising replacement ratios. Thus, the concrete mix


containing 25 % natural coarse aggregate was (CC-25) exhibited 12 and 25 % higher compressive and splitting tensile strength, respectively, than the reference concrete (RC).

Figure 4: Compressive and splitting tensile strength at 28 days

3.2. Freeze-thaw strength The findings on loss of surface mass after exposing the specimens to a variable number of freeze-thaw cycles are shown in Figure 5. After 56 cycles, the recycled concrete lost no more mass than the reference concrete, which exhibited poorer performance than the new materials. All the concretes tested were highly resistant to popout effects, as measured according to the criteria laid down in Swedish standard SS 137244 [27].

Figure 5: Surface detachment from concrete after freeze-thaw cycles These results can be attributed primarily to the higher resistance of fired clay to changes in temperature (see Table 1). Moreover, as this new aggregate was crushed, its surface was rougher and had a larger contact area, favoring better bonding to the paste than the smoother and rounded natural aggregate [28, 29]. This difference is illustrated in Figure 6.


Figure 6: Natural and recycled coarse aggregate/paste ITZ after 56 freeze-thaw cycles

(BSE X 200) Figure 7 shows the fissures in the reference concrete (RC) and concrete with 25 % ceramic aggregate (CC-25). Note that the former cracked more deeply than the recycled concrete, which has shallow surface cracks only.

Figure 7: Fissures after 56 freeze-thaw cycles (BSE 350X): a) RC; b) CC-25

4. CONCLUSIONS

The following conclusions may be drawn from the present findings. 1. The physical-mechanical characteristics of recycled ceramic waste make it apt for use

as an aggregate in structural concrete manufacture. 2. The new aggregate exhibits higher resistance to changes in temperature than natural

coarse aggregate. 3. Recycled concrete exhibits higher mechanical performance than the control, and

compressive and tensile strength rise with increasing replacement ratios. 4. The new concretes show higher resistance to changes in temperature, with less scaling

and surface microcracking than conventional concrete. 5. According to the studies conducted to date under the present project, these recycled

concretes may be apt for use in structural concrete, although more extensive research on durability is required.


5. ACKNOWLEDGEMENTS

The present study was funded by the Spanish Ministry of Science and Innovation under coordinated research Project (BIA2010-21194-C03-01).

6. REFERENCES

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