Article pubs.acs.org/est
Recycling of Porcelain Tile Polishing Residue in Portland Cement: Hydration Efficiency Fernando Pelisser,*,† Luiz Renato Steiner,‡ and Adriano Michael Bernardin†,§ †
Materials Science and Engineering Post-Graduation Program and ‡Civil Engineering Specialization Program, Santa Catarina Extreme South University, Cricúma, Santa Catarina, 88806-000, Brazil ABSTRACT: Ceramic tiles are widely used by the construction industry, and the manufacturing process of ceramic tiles generates as a major residue mud derived from the polishing step. This residue is too impure to be reused in the ceramic process and is usually discarded as waste in landfills. But the analysis of the particle size and concentration of silica of this residue shows a potential use in the manufacture of building materials based on portland cement. Tests were conducted on cement pastes and mortars using the addition of 10% and 20% (mass) of the residue. The results of compressive strength in mortars made up to 56 days showed a significant increase in compressive strength greater than 50%. The result of thermogravimetry shows that portlandite is consumed by the cement formed by the silica present in the residue in order to form calcium silicate hydrate and featuring a pozzolanic reaction. This effect improves the performance of cement, contributes to research and application of supplementary cementitious materials, and optimizes the use of portland cement, reducing the environmental impacts of carbon dioxide emissions from its production.
1. INTRODUCTION The use of ceramic tile polishing residue for the production of ceramic tiles and other ceramic products has been studied by a large number of researchers,1−3 but studies on the use of ceramic polishing residue in cement-based building materials are scarce.4,5 The ceramic polishing residue is an industrial waste that shows pozzolanic activity as a result of its physicochemical characteristics. The polishing residue is a byproduct of the polishing step during the fabrication of porcelain tiles (known as “porcellanato” in the tile ceramic industry). The polishing residue can maximize the hydration in portland cement because it presents high concentrations of silica and alumina that promote pozzolanic reaction during hydration, besides its filler effect,6,7 when the residue can act as nucleation centers due its small particle size after polishing. Calcium silicate hydrate (CSH) is the main product formed in the hydration of portland cement, being responsible for the resistance and other properties of the final product. Thus, the pozzolanic reaction is the reaction between the pozzolan and calcium hydroxide to form CSH, as shown
increases the durability of the product in acidic environments; and (iii) studies on pore size distribution in hydrated cements showed that the reaction products are more efficient in filling capillary voids, improving the strength and permeability.8 Nowadays, it is increasingly important that the development of mortar and concrete be economical, durable, and sustainable, because the production of portland cement is a highly energy consuming activity. The research in the building materials area evolves constantly in search of more efficient materials, especially those involving improved process efficiency and increased use of supplementary cementitious materials (SCMs),9 improving the cost−benefit ratio. SCMs are ingredients like fly ash, slag cement, silica fume, metakaolin, or other pozzolanic materials added at batching or as components of blended cement. Moreover, there is a constant concern with respect to the reuse of waste materials in other industry segments in order to reduce the environmental impact of the construction industry. The residue derived from the polishing step in the manufacture of ceramic tiles is a common waste in many companies that use polishing and grinding operations in order to produce ceramic tiles, mainly porcelain tiles. In the polishing operation, silicon carbide and diamond abrasives are used in automated machines refrigerated with water, and usually almost 1 mm of the tile surface is removed. Therefore, the polishing residue is formed by a mixture of both the abrasive tool and the
tricalcium silicate + H2O → calcium silicate hydrate (CSH) + calcium hydroxide [Ca(OH)2 ]
(1)
pozzolan + Ca(OH)2 → CSH
(2)
Received: Revised: Accepted: Published:
During the pozzolanic reaction three main aspects are relevant: (i) it is a slow reaction, thereby minimizing the rate of heat release; (ii) the consumption of calcium hydroxide © 2012 American Chemical Society
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September 6, 2011 January 26, 2012 January 26, 2012 February 8, 2012 dx.doi.org/10.1021/es203118w | Environ. Sci. Technol. 2012, 46, 2368−2374
Environmental Science & Technology
Article
addition, by mass, of 0% (standard), 10%, and 20% of the PPR. The consistency index for all mixtures was maintained constant at 27 ± 2 cm (Flow-Table Test, EN 1015-3, 2007).15 Brazilian type CPII-Z32 portland cement (equivalent to ASTM MH-II standard) and standard sand (NBR 7215, 1996)16 were used, mixing the sand in four particle sizes (main diameters of 0.15− 0.3, 0.3−0.6, 0.6−1.2, and 1.2−2.4 mm). The PPR was collected during 2 days (approximately 5 kg each day) when the porcelain tile was being polished (during production, the sludge from the porcelain tile polishing operation is separated from the glazed and unglazed tile polishing operation and from the grinding operation). In sequence, the PPR was dried in an oven at 60 °C, fractioned in an agate mortar, and dried again in an oven. The PPR was characterized by chemical analysis (X-ray fluorescence, PW-2400/Philips; fused sample), FT-IR, XRD, chlorides and sulfates analysis (Standard Methods Examination of Water and Wastewater), and particle size distribution analysis (laser diffraction, 1064/Cillas; 0.04−500 μm). The sample for the infrared spectroscopy analysis was prepared by pressing the material into a pellet shape, using 95 wt % potassium bromide (KBr) and 5 wt % PPR. The pressing load for the 13 mm diameter pellet was 9 tons. A Perkin-Elmer 16PC Fourier transform-infrared spectrometer (FT-IR) was used in direct transmission mode in the range 4000−400 cm−1 (4 cm−1 resolution). For the X-ray diffraction analysis (XRD), a Shimadzu XRD 6000 diffractometer was used operating with Cu Kα radiation (λ = 1.5418 Å) at 40 kV and 30 mA output. Scanning was performed from 10° to 80° (2θ) in 2°/min reading time. The mortar mixtures were characterized by compressive strength (ASTM C 1231/1993),17 isothermal calorimetry (cement paste), and thermogravimetry (samples from the fractured mortar). The compressive strength was carried out in a universal testing machine (EMIC/PC 200I, 0.5 MPa/s) on three specimens with 5 × 10 cm (diameter × length) dimensions, and the mortar compressive strength results were analyzed by analysis of variance (ANOVA) and by the Duncan test, used to determine the significant differences between group means in the analysis of variance. The Duncan’s multiple range test is a comparison of all treatment means so that any difference between any other treatment will be reflected in this analysis. The calorimetry tests were carried out at 22 °C in a calorimeter (TA Instruments TAM) under air atmosphere. The cement pastes (water/cement ratio w/c = 0.40) were mixed before testing (11 ± 0.1 g sample mass). The thermogravimetry (TG) test was performed in a thermal analyzer (TA Instruments SDT Q600). The samples, cured during a 56 day period, were maintained at 50 °C during 10 min for drying (to prevent moisture changes in the samples), and the temperature was raised to 800 °C with a heating rate of 10 °C/min and 100 mL/min N2 flow using a platinum crucible. Previously, the samples were ground in a micromill with an agate mortar and screened to 75 μm mesh, being maintained in a vacuum oven (50 °C) before analysis. Through the thermal analysis results, the amount of calcium hydroxide (CH) was calculated from the mass loss that occurs during the decomposition of the water released in the thermal decomposition of PMCH (calcium hydroxide mass loss) phase by employing the equation18
ceramic tile that is processed in effluent treatment plants. In the ceramic tile industry there is a significant source of this type of waste with relatively similar characteristics. As an example, the ceramic tile companies in the South of Santa Catarina State, Brazil, produce an estimate of 1000 tons of the waste each week. This volume of waste can be used on an industrial scale in a cement factory − added at 5 wt % on clinker, for example, or in specific products with higher added value, such as adhesive mortars. Votorantim Cement Co. is among the 10 largest global players in the sector of basic construction materials (cement, concrete, aggregates, and complementary products) with several plants in Brazil and the Americas (producing 42 million tons per year of cement products). The company recently opened a new factory in the city of Imbituba (in the southern region of the Santa Catarina state, Brazil) with a capacity of 1.1 million tons of cement per year, justifying the use of the PPR residue. The global production of portland cement is 2.8 billion tons per year,10 reaching an overall average consumption of 0.8 tons of clinker per ton of portland cement produced.8,11 Therefore, the annual CO2 emissions associated with cement production can reach almost 2.2 billion tons, amounting to over 6% of total CO2 global emissions.12 Results of the 13th International Conference on Chemistry of Cement (2011) show that global consumption of cement is increasing and in 2025 the cement industry will emit CO2 at a rate of 3.5 billion tons/year,13 roughly equal to the total emissions in Europe today,14 and for 2050, the estimated CO2 emissions from the cement industry will be 17% of the global emissions.11 Given the guidelines questionable or not to reduce CO2, that is a high percentage for a single industry, even exposing all the social and economic development. Nevertheless, concrete and mortar materials can be considered “green” materials because they use large amounts of waste from other industries, sequester CO2 from the atmosphere, and may be used for disposal of radioactive waste and be recycled indefinitely. An alternative way to reduce the environmental impact of cement production and to produce more efficient concrete and mortar, also reducing the amount of portland cement in the composition of these materials, is the use of SCMs. Currently the main sources of SCM are plants that use coal as fuel and metallurgical furnaces producing cast iron, silicon metal and alloys, and other materials such as natural pozzolan, silica, rice husk, microsilica and metakaolin. The efficiency of an SCM depends on its chemical composition, fineness, and the amount of amorphous phases present in it. In this context, the use of the residue from the polishing process of porcelain tiles (PPR), produced in large amounts by the tile ceramic industries in Brazil, mainly in the Santa Catarina State, is suitable for the production of cementitious materials such as mortars, paving concrete blocks, and masonry. Therefore, the objective of this study was the use of ceramic tile polishing residue as a pozzolanic agent in portland cement mortars in order to improve the mortar characteristics. The residue was added in a mortar formulation maintaining its processing characteristics, and the performance of the mortar hydration was measured in order to estimate the effect of the residue addition in the compressive strength of the mortar samples.
2. MATERIALS AND METHODS Three mixtures of cement, sand, and water (1:3:0.60 for the cement/sand/water ratio, by mass) were performed with the
CH = PMCH(%) × mmCa(OH)2 /mmH2O 2369
(3)
dx.doi.org/10.1021/es203118w | Environ. Sci. Technol. 2012, 46, 2368−2374
Environmental Science & Technology
Article
Table 1. Chemical Analyses (mass %) of the Polishing Residue and Cement (XRF) polishing residue cement
SiO2
Al2O3
CaO
Fe2O3
Na2O
K2O
MgO
TiO2
ZnO
P2O5
LOI
66.7 22.0
20.4 6.5
2.4 53.6
1.7 3.3
3.1 0.1
2.4 1.0
1.1 5.9
0.7
0.2
0.1
0.5 5.4
where mmCa(OH)2 is the molar mass of calcium hydroxide (=74) and mmH2O is the molar mass of water (=18).
3. RESULTS AND DISCUSSION The chemical analysis of the polishing residue (PPR) (Table 1) shows that the waste contains silica (SiO2) and alumina (Al2O3) as main components: the amount of SiO2, Al2O3, and Fe2O3 is greater than 70% in accordance with ASTM C-618 (standard for pozzolan use).19 However, the high alkali content (Na2O and K2O) is not recommended for concretes subjected to deleterious alkali−silica reactions in the presence of amorphous silica. The cement chemical analysis shows a typical portland cement composition, with silica and calcium oxide as the main components (Table 1). The PPR shows 0.07 mass % of total chlorides and 0.06 mass % of total sulfates, therefore not affecting the cement composition. Regarding the particle size distribution (Table 2), the polishing residue (PPR) presents 90% of the particles under
Figure 1. Compressive strength of mortars with PPR addition (0.5 MPa/s, 3 samples per run, ASTM C 1231/1993). The graph shows an increase in the compressive resistance with PPR addition at the ages of 28 and 56 days.
Table 2. Particle Size Distribution of the Polishing Residue and Cement
Table 3. Compressive Strength Results
particle diameters (μm) polishing residue cement
