GYPSUM PLASTERING: WASTE EVALUATION Pimentel, L. L. (1) and Camarini, G. (2) (1) PhD, Assistant Professor, Faculty of Civil Engineering, PUC-Campinas (2) PhD, Associate Professor, School of Civil Engineering, Architecture and Urban Design, University of Campinas - FEC/UNICAMP ABSTRACT In civil construction, gypsum plaster has been used as decorative ornaments, plasterboards, dry-wall and renderings. The application procedure of plaster for rendering generates large amounts of waste due to its hydration kinetics, temperature and environmental humidity. Waste production also occurs due to the technique of preparation and application of this material. The need to minimize gypsum waste production is to proposed ways for its reusing. The aim of this work was to quantify the amount of gypsum waste produced by the process of using it as renderings. It was observed the influence of workers applying pastes rendering on gypsum waste production. It was also observed its physical and mechanical properties of commercial plaster. The application process of plaster renderings was followed measuring the generated waste and the time for finishing the process was also observed. It was examined: the application process and each step of plaster paste application was measured and recorded. Results pointed out to waste production values in the range between 18% and 35%. The application technique and physical properties (setting times) influences the amount of waste production. Setting times also have a critical influence due to the kind of gypsum plaster used for renderings. Keywords: Gypsum, gypsum waste, renderings, work technique, mechanical properties. 1. INTRODUCTION In Brazil and in many countries, gypsum-based renders and plasters have become the material of choice for indoor finishing. Excellent performance, attractive appearance, easy application, and its healthful contribution to living conditions have made gypsum a most popular finishing material [1]. This building material is abundant in Brazil and 89% of this production comes from Pernambuco State [2]. The majority of gypsum production, 59% is burnt (calcined), 30% consumed by cement industry, and 11% by the agriculture. It is estimated that hemihydrate consumption is by plates industry, rendering, moulds, ceramics and others [2]. The reactivity of gypsum-based binders is produced by gypsum (CaSO 4 2H 2 O) dehydration. Depending on dehydration temperature, vapor pressure inside of furnace, heating control, fineness and density of the raw-material different products are obtained. Gypsum looses water when heated (calcined) at different temperatures: β-hemihydrate 71
( CaSO4 1 2H2O) is formed when gypsum is heated at temperatures between 100ºC and 180ºC. Anhydrite III ( CaSO4 ε H2O) is formed when gypsum is heated at temperatures below 300ºC. Anhydrite II (CaSO 4 ) is formed when gypsum is heated at temperatures higher than 300ºC. Inside the furnace the temperature is not totally uniform and the commercial plaster for building construction can have both products: hemihydrate (HH) and anhydrite III (AH-III) [3]. Both HH and AH-III at normal temperature react with water forming dehydrate (gypsum). This reaction makes a network of gypsum crystals that produces the properties of gypsum based materials. This reaction releases heat that must be controlled to avoid fast setting [4] [5]. There are some factors that affect the kinetics of HH hydration: water binder ratio, impurities, energy of mixing, mixing water temperature and setting controllers. HH setting can be described as a physical phenomena and it can be divided into three periods: 1 - induction, 2 - nucleation, and 3 hardening (Figure 1). In the initial of reaction gypsum crystal nuclei are formed which grow during the induction period. After the induction period gypsum crystals precipitated and paste consistency increases. This is characteristics of initial of setting. As the reaction occurs and the degree of hydration increases, the paste gets more strength until it is totally hardened. This period is characteristics of final setting [6]. Figure 1. Temperature increase during plaster setting [7] The water/plaster (w/p) ratio is an important factor in pastes properties. Increasing w/p ratio decreases mechanical properties. Great amounts of water changes kinetics of hydration, increase the porosity of the paste, and modify its microstructure. The induction period is changed delaying the initial setting and the gypsum crystals formation [8]. The distance between gypsum particles increase with increasing w/p ratio and dihydrate crystals will have more space to grow and it will take longer to reach each other. In this way, a higher degree of hydration will be necessary to harden the paste and the crystal growth relation will be lower after setting. The water/plaster ratio is a parameter of influence on the kinetics of hydration reaction and consequently on the plaster setting time. However, other factors as water temperature, raw material and the procedure used on the plaster production as well as the energy used on the process of the paste mixing can accelerate the hydration reaction by improving the plaster powder dispersion on the water. 72
The way plaster is employed in constructions, it shows a great material loss. The medium values of this loss are more than 45% [6]. The gypsum waste from rendering is a significant source of material lost. This waste is no longer permitted in mixed landfill because the sulphates may react with organics to form hydrogen sulphide. A limited number of cells have been built for sulphate waste which may encourage this waste transfer to special recycling stations. Due to this great material loss that happens during the use of the plaster in building construction, it becomes useful and necessary a study how it happens. The aim of this work was to evaluate the waste obtained from hydrated plaster in constructions. The application process was observed and the waste was measured. 2. EXPERIMENTAL PROGRAM In order to evaluate the amount of waste formation when plaster was applied for renderings it was chosen three different cities from São Paulo State, in Brazil: Campinas city (1.056.644 hab), Jundiaí city (347.738 hab) and Indaiatuba city (180.524 hab) [9]. The rendering work was performed in a commercial building of 17 floors (Campinas city), and in a familiar residence (Indaiatuba) by a small outsourced company and in a church (Jundiaí city) by a medium-sized enterprise. The work of plaster rendering in each city evaluated different characteristics related to the workers practice on application of plaster in renderings. The application process of plaster renderings was followed by measuring the loss generated and the time taken to finish the process, and the application method used by workers in each different city. Each step of plaster rendering service was measured and recorded. The obtained results were analyzed in order to investigate the main responsible factors for the waste production in the plaster rendering process. 2.1 Commercial Plaster Plaster used in this rendering was a commercial one usually found in the market and employed in buildings in the whole country. The plaster employed in all three buildings was the fine plaster for rendering. A sample of commercial plaster used in each construction work was taken to make laboratory tests. The physical and mechanical properties were determined as follows: physical properties in powder (bulk density and fineness modulus) [10]; physical properties of plaster paste in fresh state (consistency and setting times (initial and final) by Vicat needle) [11]; mechanical properties (hardness and compressive strength) [12]. 2.2 Application process The preparation procedure and application of the pastes of plaster observed in buildings involved: the application time measurement and photographic registration, and the equipment used to make gypsum rendering. 73
2.3 Waste generation The first step to evaluate the gypsum waste began with the investigation of the application process in walls and ceilings. It was observed: Material quantification (commercial plaster) that was used by the workers; Calculus of the application areas: walls or ceilings; Quantify the plaster and the water used in the mixture to produce the paste for rendering; Collect the gypsum waste from the ground after finishing the applications process; Keep the gypsum waste in plastic bags and identifying them correctly; After observation of workers practice, gypsum waste was taken to the laboratory to be weighed. 2.4 Worker productivity Worker productivity (min/m²) was calculated by the ration between time (min) to complete the rendering surface and the area of this surface (m²). 3. RESULTS AND DISCUSSION 3.1 Physical and mechanical properties of commercial plaster Physical and mechanical properties of commercial plaster and minimum values for fine commercial plaster for rendering according to NBR 13207 [13] are shown on Table 1. Table 1. Physical and mechanical properties of commercial plaster Properties Cities Campinas Indaiatuba Jundiaí Brazilian Standards minimum values Bulk density (kg/m 3 ) 740,76 701,52 709,89 700 Fineness modulus 0,65 0,94 0,8 < 1,1 w/p ratio for normal consistency 0,6 0,4 0,4 - Setting time - initial (min:s) 08:09 10:25 10:00 > 10:00 Setting time - final (min:s) 27:26 25:55 23:00 > 45:00 Hardness (N/mm²) 44,97 43,94 51,57 > 30 Compressive strength (MPa) 5,51 6,01 6,21 > 8,0 The values obtained for bulk density met the requirements of the standard (> 700 kg/m 3 ) and the values obtained for the modulus of fineness classify the material as fine plaster for 74
rendering (MF <1.1). The values obtained for the w/p ratio to reach the paste of normal consistency, and initial and final setting were evaluated according to NBR 13207 [13]. The results obtained for hardness test are in accordance with the limits established by the Brazilian Standard NBR 13207 [13] for all commercial plaster; however, the results for compressive strength were not in accordance with Brazilian Standard NBR 13207 [13] (R > 8.0 MPa). 3.2 Equipment used in rendering process The rendering process is executed using four main tools (Figure 2): 1 - kneading through; 2 - PVC hand float, to spread and to remove the excess of paste; 3 - aluminum ruler to finish up the corner of the walls, windows and door frames; and 4 steel hand float to finish the windows and doors frames and to give the final touch. In all visited buildings the workers from different enterprises used the same tools. 3.3 Application Process For the construction visited in each city the application process are described below. It is detailed: the kind of surface application, the mixture preparation, and the applying process. 4 1 2 3 Figure 2. Equipments used for plaster paste application 3.3.1 Surface a) Campinas city. Two surfaces: ceramic block and concrete beam. Three measurements of the work process were made at ceramic surface and one measurement was made on a concrete beam surface. Both ceramic and concrete beam surfaces had a smooth texture. b) Indaiatuba city. Ceramic block. Two measurements of the work process were made. c) Jundiai city. Ceramic block. Four measurements of the work process were made. 3.3.2 Mixture preparation a) Campinas city. The mixing process was manual. The worker mixed 20 kg of plaster powder with water on two measurements of work process (ceramic surface) and the worker mixed 40 kg of plaster powder with water on the other two measurements (ceramic and concrete surfaces). The water/plaster ratio was 0.75, value higher than w/p ratio for normal consistency. The paste preparation followed the steps 1 to 4. 75
Step 1 The water was placed inside a kneading through. Step 2 Plaster powder was spread on water inside the kneading through. Step 3 After spreading plaster powder on water, the worker waited a time between 4 and 11 minutes to begin the mixture. Step 4 Only half of the material inside the kneading through is mixed. After using this first half the second half is mixed and used. b) Indaiatuba city. The mixing process is manual. The worker prepared 40 kg of plaster in the all measurement. The water/plaster ratio was 0.6, value higher than w/p ratio for normal consistency. The paste preparation followed the steps 1 to 4 described on item 3.3.2. The only difference was on step 3: the waiting time before mixing was of 30 minutes. c) Jundiai city. The mixing process is manual. The worker prepared 35 kg of plaster. The water/plaster ratio was 0.7, value higher than w/p ratio for normal consistency. The paste preparation followed the steps 1 to 4 described on item 3.3.2. The only difference was on step 3: the waiting time before mixing was of 30 minutes. 3.3.3 Application process a) Campinas city. The workers used the tools described on section 3.2. PVC hand float is used to spread the paste on walls and beams surfaces. The worker fill the PVC hand float with fresh plaster paste and spread it on surfaces with rapid movements over the surface to be covered. He used the steel hand float to make the same procedure. The rendering application was not organized. The fresh plaster paste was applied in a disorderly manner. So, the application process took long time of implementation. Aluminum ruler is filled with fresh plaster paste and is placed at wall corners and is fixed until the end of setting, when the ruler is withdraw. The final touch occurred when plaster is reaching the final setting time. The worker used the steel hand float in vertical and horizontal directions in order to eliminate any imperfection on surface rendering. b) Indaiatuba city. The workers used the tools described on section 3.2. The same procedure described for Campinas city was used to make the gypsum rendering. Rendering application was carried out in consecutive bands, obtaining a precise leveling on the thickness of the layer. The application was made with PVC hand float and the final touch with steel hand float. c) Jundiaí city. The workers used the tools described on section 3.2. The same procedure described for Campinas city was used to make the gypsum rendering. But The application process was more organized. The worker followed a regular sequence of bands which covered the entire area uniformly. In this construction the steel and PVC hand float were used for application and steel hand float final touch. The surface final touch initiated when the plaster paste consistency increased and arrived near the final setting. The worker makes horizontal, and upward and downward movements in order to eliminate any imperfection on rendering. Figure 3 shows the plaster rendering application on the three buildings. 76
Indaiatuba city Campinas city Jundiaí city Figure 3. Application process 3.4 Discussion of rendering process Figure 4 shows a sequence of the entire application process. It is possible to observe: (a) gypsum bags in a wagon, (b) plaster spreading on water, (c) waiting period, (d) manual mixing, (e) application with aluminum ruler, (f) application with PVC hand float, (g) final touch with steel hand float, (h) cleaning of equipment. a) b) c) d) 77
e) f) g) h) Figure 4. Steps of plaster application process As summary of the whole application process, the plaster workers follow the six steps described bellow. [1] Commercial plaster powder was spread slowly on water. The amount of plaster was in according with the w/p ratio used; [2] Period of waiting after spreading plaster powder. This time is necessary to ensure the uniform dissolution of plaster, which will provide better quality of the mixture, avoiding the formation of lumps [3] Mixing procedure immediately before the application. Mixing is made in just half of the mixture of step 1. The remaining half of the mixture is mixed after finishing the use of the first half; [4] Application of paste on the surface during the nucleation period [6]; [5] Surface finishing: starts when the hydration reaction of the paste arrives near the final setting time; [6] Cleaning the equipment: the worker washes the equipments and tools, eliminating the wastes remained from the application process. This material joined the waste generated along whole the process. The process of applying plaster renderings on the three constructions were largely made with the use of PVC hand float. The fresh plaster paste was placed on the tool and then it is spread on the wall or ceiling surface. The rendering can be applied to various substrates, but it must be to ensure adhesion and thickness ideal for both technical and economic factors. The thickness of plaster in general 78
depends on the substrate, but technically is recommended a thickness of 5 mm ± 2mm [14]. Brazilian Standards NBR 13867 [15] does not specify the thickness of rendering, but prescribes that "[ ] the layer of coating of should have a thickness as uniform as possible and the paste carefully spread [ ]". In all buildings the thickness was higher than the maximum value recommended. 3.5 Waste production and worker productivity Calculus of gypsum waste was made in three different buildings. In each building the workers were from different enterprises. For each work it were performed at least two measurements. Table 3 shows the amount of waste produced and worker productivity. From these results one can observe that there is no criteria for the amount of commercial plaster used related to the surface to be covered. At Campinas city when the area increases about 30% the amount of commercial plaster increases 100%. The amount of waste increased about 57%. At Jundiaí city the area increases 100% and the amount of commercial plaster used is the same. This indicates that the thickness of gypsum rendering is not uniform and the technique employed to make the mixture and paste design must be better understood. The waiting time was different for Campinas city building. This can be a reason for the higher amount of waste produced. This waiting time is important to improve plaster workability. This can be explained by the lack of practice and also a methodology to make this kind of service. The technical properties of the plaster should be better understood by the worker in order to reduce the waste production. Security equipments to make the work are important. Only one worker used them partially (Campinas city). This worker did not use gloves. In other buildings the workers did not use security equipments. An interesting result was found at Jundiaí city building. The performance on ceiling was better than the performance on walls. This indicates that the same procedure should be used on both surface, but this is not the reality on work result. These different work performance leads to different amount of plaster waste produced. This is shown on their productivity: it is very different showing that they do not have a standard of working with this kind of product. 79
City Campinas Indaiatuba Jundiaí Measurements *ceiling Table 3. Measurement of the work in buildings Area Commercial plaster Plaster loss Application time Worker productivity (m²) (kg) (%) (minutes) (min/m²) 1 3,28 20 30 43 12,9 2 3,28 20 33,7 50 15,2 3 4,32 40 27 37 8,5 4 4,33 40 47,2 24 5,5 1 2.00 40 23,78 39,2 24,6 2 2.86 40 21,16 46,84 16,4 1* 6,24 35 14,9 52 8,21 2 4,81 35 20,4 51 10,65 3 4,81 35 15,8 54 11,37 4 3,04 35 19,9 66 21,72 4. CONCLUSIONS From the results obtained on this work enable us to make the following conclusions. The application process of the plaster for rendering includes the stages of preparation of the paste and application. This procedure does not follow a standard for all workers. There are differences in the amount of commercial plaster used, in the amount of water used, and in the performance to cover the surface with plaster paste. These differences are important because it made variable the waste production, the productivity is changeable, and the final product does not reach the desirable quality. The waste can be related to the material lost in plaster rendering and also to the excess of material used for rendering by increasing the thickness due to lack of correct practice. The properties of plaster can be important for this application process. This must be discussed with worker to follow a standard to obtain a good material with the desirable performance. ACKNOWLEDGMENTS The authors would like to thank the undergraduate student Natália Haluska de Sá for the data collection and PUCCAMP for the undergraduate scholarship. 80
REFERENCES [7] Arikan, M.; Sobolev, K. The optimization of a gypsum-based composite material, Cement and Concrete Research, 32 (2002) 1725 1728 2002. [8] DNPM. National Departament of Mineral Production. Gipsita. 2007 Available on http://www.dnpm-pe.gov.br/detalhes/gipsita.htm. [9] De Milito, J.A. Avaliação do comportamento de pastas de gesso com cimento portland e sílica ativa para revestimento (Evaluation of pastes performance with plaster, Portland cement and silica fume for renderings), MSc Thesis, School of Civil Engineering and Urban Design, University of Campinas, Campinas, São Paulo, Brasil, 2001 (in Portuguese). [10] Maksoud, M. A.; Ashour, A. Heat of Hydration as a Method for Determining the Composition of Calcined Gypsum. Thermochimica Acta. v. 46, 303 308. 1981. [11] Hincapié, A. M.; Oliveira, C. T. A.; Cincotto, M. A.; Selmo, S. M. Revestimento de Gesso II (Gypsum rendering II), Revista Téchne nº 22, Maio/Junho, 1996b (in Portuguese). [12] Antunes, R. P. N. Estudo da influência da cal em pastas de gesso (Study of influence of lime on plaster pastes), MSc Thesis, University of São Paulo, São Paulo, 1999, 145p. (in Portuguese). [13] Gmouh, A. et al. Changes in plaster microstructure by pre-stressing or by adding gypsum grains: microstructural and mechanical investigations, Materials Science and Engineering A, 352 (2003) 325-332. [14] Nolhier, M. Contruire em plâte. Paris, L Harmattan, 1986. [15] IBGE, 2008 - http://www.nossosaopaulo.com.br/ acessed 16/08/2009. [16] Associação Brasileira de Normas Técnicas (ABNT). NBR 12127: Gesso para construção Determinação das propriedades físicas do pó. (Plaster for renderings Powder Physical Properties), Rio de Janeiro, 1991 (in Portuguese). [17] Associação Brasileira de Normas Técnicas (ABNT). NBR 12128. Gesso paraconstrução Determinação das propriedades físicas da pasta (Plaster for renderings Paste Physical Properties). Rio de Janeiro, 1991 (in Portuguese). [18] Associação Brasileira de Normas Técnicas (ABNT). NBR 12129. Gesso paraconstrução Determinação das propriedades mecânicas (Plaster for renderings Mechanical Properties). Rio de Janeiro, 1991 (in Portuguese). [19] Associação Brasileira de Normas Técnicas (ABNT). NBR 13207. Gesso para construção civil (Plaster for civil constructin). Rio de Janeiro, 1994 (in Portuguese). [20] De Milito, J. A. (2007), Avaliação do desempenho de Aglomerante à Base de Gesso com Cimento Portland de alto forno e sílica ativa (Evaluation of Gypsum binders with Blastfurnace Slag Cement and Sílica Fume), PhD Thesis, University of Campinas, São Paulo, Brasil (in Portuguese). [21] Associação Brasileira de Normas Técnicas (ABNT). NBR 13867. Revestimento interno de paredes e tetos com pasta de gesso Materiais, preparo, aplicação e acabamento (Plaster for civil construction). Rio de Janeiro, 1997 (in Portuguese). 81
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