Development of Pre-dried and Blended Lime Mortars for the Ready-Mix Market - Essay Example

Materials: Development of Pre-Dried and Blended Lime Mortars For the Ready-Mix Market INTRODUCTION: For increasing sustainability in business operations, there are four key areas that the building industry needs to improve, which are: energy, materials, waste and pollution. Hence, a prudent use of natural resources is essential (CIOB, 2007: 3). This paper proposes to present a literature review on the development of pre-dried and blended lime mortars for the ready-mix market. There is an increasing market for the newly introduced ready mix lime mortar. Through research and development this further promises to evolve into the next generation of mortars, which will be produced without the expenditure of energy; using alternative and novel means to dry the sand. DISCUSSION: Lime mortars harden and gain strength by the evaporation of the water and the absorption of carbon dioxide from the atmosphere. This results in the gradual conversion of the lime into calcium carbonate. There are a number of different types of cements, which may be used to produce mortar (Introduction to Mortar: Learning Text). Lime mortars are valuable because they improve the resistance of brickwork to cracking and reduce the risk of efflorescence on the bricks. As a general rule, therefore, it is not advisable to use a mortar which is stronger than necessary as it will not give added strength to the construction (Nash, 1990: 139). Cements are hydraulic materials, that is they depend upon a reaction with water (hydration) rather than air for strength development. (Cementitious Materials: Learning Text). When 0-40% of cement is added to lime-based mortars, their mechanical strength increases slightly. In cement mortars, the mechanical strength diminishes sharply when a small amount of lime is added (25% of lime gives rise to a drop in strength of around 50%). Lime-rich mortars present a plastic zone that does not appear in the other blended mortars. Therefore, lime-rich mortars are able to absorb a high degree of deformation before the breakage, especially beneficial in renovating old buildings (Arandigoyen & Alvarez, 2007: 774). Cementitious Materials: According to Merritt and Ricketts (2000: 4-1), cementitious materials include the many products which are either mixed with water or some other liquid to form a cementing paste that may be moulded while plastic, but will set into a rigid shape. When sand is added to the paste, mortar is formed. In a standardized test conducted by Jacob and Weiss (1989: 62), typical restoration mortars have been measured for water vapour transmission (WVT) and the values have been reported.The data showed a considerable range of water vapour transmission rate as a function of lime content. Several more tests should be conducted, in order to avoid errors in judgment concerning product selection. In a study conducted by Park et al (2006: 728), the efficacy was examined: of a pneumatic, dry premixing process for producing commercially acceptable, ternary-blend mortars, using less cementitious material. The data presented compare flowability, compressive and flexural strengths, drying shrinkage, and pore size distribution in the mortars using dry, premixed material with that prepared conventionally in a small-scale, high-shear, rotary mixer. A second generation, spout-fluid bed mixer was developed for dry premixing of sand and cementitious materials, which is capable of being scaled to industrial size. This advance provides uniform particle distribution, improves the particle packing density leading to more reproducible mixes, and produces mortars equivalent to those produced with high shear, rotary mixers. Dry premixing allowed the production of commercially acceptable ternary-blend mortars using less cementitious material. At a sand-to-binder ratio of 3.2:1, the compressive strength of the dry premixed mortars was about 10% higher than that of the small-scale, high shear, rotary mixed mortars of the same composition. Other properties of the mortar were also positively affected, including a decrease in the shrinkage, and an increase in the workability (Park et al, 2006: 728). 1. The Drying of Sand for Use in Mortars: According to Ciesielczyk (2005: 1613), drying forms one of the stages in most industrial processes, and its impact on the quality of the product can be crucial. Since its energy requirements are high and since the energy efficiency of industrial fluidized bed dryers is low, innovative actions to improve the process are desirable.. Based on an extensive experimental programme on nine different sand types, De Schutter & Poppe (2004: 517) have found that the apparent dry density of sand proves to be a very important parameter for the estimation of the water demand of the sand as well as for the influence of the sand type on the rheological and strength properties of the mortar. This subsequently impacts the mix design of the mortar. Skuratovsky et al (2003: 1645) have advocated the use of pneumatic drying by a two-fluid, two-dimensional model. They have found that the large surface area for heat and mass transfer with increased drying capacity, and the high convective heat and mass transfer coefficients are leading to high drying rates. Drying by Porous Material in Fluidized Bed: The drying characteristics of porous material in fluidized bed were examined theoretically and experimentally. When the pore diameter of sample was relatively large, the fluidizing particles were adhered on the sample surface. From the results, the following facts were clarified: (1) The temperature decrement is caused by the increment of drying rate due to decrement of adhered particle mass. (2) The effect of fluidizing particle diameter on the drying time is very small. (3) The water content in sample governs the mass of adhered particle. (4) The mass of adhered particle becomes smaller as the fluidizing particle diameter is larger. (5) The excess increment in drying gas temperature contributes very little to shortening the drying time (Tatemoto et al, 2001: 1316). Batch-drying Kinetics in a Two-Zone Bubbling Fluidized Bed: Batch drying of particulate solids in a two-zone bubbling, fluidized bed is one of the most important modern drying methods. This method is relatively easy, convenient, and, moreover, it helps in attaining high product quality (Ciesielczyk, 2005: 1613). On the basis of the model, its simplified version has been developed, called the equilibrium model. In order to verify the theoretical model, the concept of the generalized drying curve has also been used, forming a link between the analytical and semi-empirical methods of analysis of batch drying in fluidized bed systems (p.1635). 2. The Drying Action of Lime on Sand: Limes produce hardening which is due to the conversion of hydroxides to carbonates, and were formerly widely used as the sole cementitious material, but their slow setting and hardening are not compatible with modern requirements. Hence, their principal function today is to plasticize the otherwise harsh cements and add resilience to mortars and stuccoes. Use of limes is beneficial in that their slow setting promotes healing, the recementing of hairline cracks. The desorptivity is defined as a parameter characterising the water retaining properties of wet mortar mixes. A test is conducted for measuring the desorptivity of wet mixes. Experimental results are reported for a range of wet cement mortars including mixes containing lime and air-entraining and water-retaining admixtures. These show that 1:3 cement:sand and equivalent mixes containing lime all have very similar water-retaining characteristics, but are all much less water-retaining than a 1:3 lime:sand mix. (Green et al, 1999: 1743). Sebaibi et al (2003: 689) conducted a study to evaluate the influence and drying action of lime on sand. The characteristics of lime have a certain impact on the water-retention capacity of a lime-cement-sand mortar. Experimental results have highlighted the combined influence of both lime characteristics and lime content, especially in terms of pore structure and chemical composition. It is widely accepted that though lime does not increase the strength of mortar, it contributes considerably to water-retention capacity (Sebaibi et al, 2003: 689). In the case of low lime content, which corresponds to optimal mechanical strength, hydrous exchanges are related to both the specific surface area and total lime porosity. Hybrid mortars which are composed of a mix of lime and cement, have been the subject of a number of scientific works. Among those the research conducted by Sriboonlue and Wallo (1990: 206) are significant. They were interested in the influence of mix proportions of a lime-cement mortar on both the mechanical strength and elastic modulus of the mortar. Philippi et al. (1994: 219) have studied the lime mortar microstructure. The purpose of this work was to develop methods for analysing the microstructure of porous materials that present a wide range of pores. This work was followed up by Philippi and Souza (1995: 667), who proposed a three-dimensional model to represent the heterogeneous pore structure of this mortar and went on to study the influence of this model on both water retention and humidity transfer. In 1996, Colantuono et al. (1996: 861) studied the capillarity phenomena occurring in porous materials bound by lime-cement mortars. This work has allowed establishing the influence of oxide or hydroxide magnesium present in the lime on the mechanical strength of a lime mortar. In a study conducted by Plawsky et al (2003: 255) a spout-fluid bed device was developed for dry premixing sand and cement to produce mortar. The goal of the work was to explore the efficacy of a new method for dispersing cement in sand to produce a mortar with better mechanical and physical properties. This strategy was found to work best at high sand/cement ratios, indicating that the dry premixing is more effective as the cement content is reduced; and that it may be possible to produce commercially acceptable mortars with a lower cement content, with added benefits of decrease in the shrinkage and an increase in the workability. 3 The Benefits of Dehydrating NHL, PC and GGBS a). Natural Hydraulic Lime (NHL): Hydraulic limes are made by calcining a limestone containing silica and alumina to a temperature short of incipient fusion so as to form sufficient free lime to permit hydration, and at the same time leave unhydrated sufficient calcium silicates to give the dry powder its hydraulic properties. Because of the low silicate and high lime contents, hydraulic limes are relatively weak. They are mainly used in masonry mortars. A hydraulic lime with more than 10% silica will set under water (Merritt & Ricketts, 2000: 4-7). Specimens with more binder content, show the highest compressive and flexural strengths (Lanas et al, 2004: 2200). When dehydrated hydraulic lime is used in mortar, it helps in increasing the strength of the mortar. b. Ground Granulated Blast Furnace Slag (GGBS): The use of ground granulated blastfurnace slag (ggbs) and pulverized fuel ash (pfa): a pozzolana, in mortar has increased in recent years. These materials not only impart technical benefits to both the fresh and hardened properties of mortar they are also environmentally friendly. Both materials are products resulting from industrial processes and their use, therefore, reduces the quantity of primary raw materials that have to be extracted from the ground. Ground granulated blastfurnace slag is classified as a latent hydraulic material. This means that it has inherent cementitious properties, but these have to be activated. The normal means of achieving this is to combine the material with Portland cement. Reaction mechanism of GGBS: The hydration mechanism of a combination of ggbs and Portland cement involves the activation of the GGBS by alkalis and sulfates to form its own hydration products. Some of these combine with the Portland cement products to form further hydrates which have a pore blocking effect. The result is a hardened cement paste with more of the very small gel pores and fewer of the much larger capillary pores for the same total pore volume. The rate of strength development is slower than for a Portland cement mortar (MIA Data Sheet 16). GGBS Utilisation: GGBS has been used in mortars for many years, generally in ready-to-use retarded mortars. Increasingly, the dry silo system is coming into use and GGBS is also being used in this method of producing mortar. GGBS has been used at between 25 and 50% replacement of the Portland cement with/ without the addition of lime, according to the Mortar Industry Association (Data Sheet 16). c. Portland Cement (PC): Varas et al (2005: 2055), state that early cements can be divided into two groups, natural and artificial (Portland) cements. Portland Cement is still the most important and predominant today. The main differences between natural and artificial cements arise during the manufacturing process. The final properties of the cements are greatly influenced by differences in the raw materials and burning temperatures employed. Portland cement (PC) concrete is the most popular and widely used building materials, due to its availability of the raw materials over the world, its ease for preparing and fabricating in all sorts of conceivable shapes, state Li et al (2003: 55). Due to the restriction of the manufacturing process and the raw materials, some inherent disadvantages of portland cement are still difficult to overcome. There are two major drawbacks with respect to sustainability: About 1.5 tons of raw materials is needed in the production of every ton of PC, at the same time, about one ton of carbon dioxide (CO2) is released into the environment during the production. Therefore, the production of PC is an extremely resource and energy intensive process (Li et al, 2003: 56). 4. The Different Types of Natural Hydraulic Limes (NHL) Vicat’s (1820) classification of hydraulic limes into “feebly”, “moderately” and “eminently hydraulic”classes was widely adopted from 1820 until the demise of most of the traditional lime industry in the UK, as it was both practical and appropriate to the end uses of the materials. This classification has now been superseded by the natural hydraulic lime classifications of NHL 2, 3.5 and 5 enshrined in the Euronorm EN 459-1: 2001. However, these tripartite classifications are not equivalent. The weakest modern class of NHL 2 is more closely equivalent to Vicat’s “moderately hydraulic” class, whilst NHL 3.5 is nearer to “eminently hydraulic”. NHL 5 limes can easily reach strengths equivalent to natural cements (“Context”, 2006). Hydraulic lime sets quickly without exposure to air and will even set under water. The Cultural Heritage Team, U.K. states that the following classification of hydraulic limes is based upon the speed at which they set under water: Feebly hydraulic lime - NHL 2 Moderately hydraulic lime - NHL 3.5 Eminently or very hydraulic lime - NHL 5 The more hydraulic a lime, the greater is its strength but it is less permeable and flexible. Therefore it is important that the right balance is obtained for each specific job (Cultural Heritage Team, U.K.). CONCLUSION: This paper has highlighted the background for the development of pre-dried and blended lime mortars for the ready-mix market. Drying technology and science are developing innovative methods for incorporating more efficient methods of drying sand for use in mortars. The aim is to develop cost-effective, sustainable equipment and procedures for the drying process. Research is focused on developing mortars which have the required strength for various uses, and with water absorption and evaporation mechanisms, for durability and sustainability. The characteristics of lime and mortar, the properties of Natural Hydraulic Lime, Portland Cement, and Ground Granulated Blast Furnace Slag have been identified, for use in mortars. ---------------------------------------- References Arandigoyan, M. & Alvarez, J. I. (2007). “Pore Structure and Mechanical Properties of Cement- Lime Mortars”. Cement and Concrete Research, 37, 767-775. Cementitious Materials: Learning Text. Web site: www.cement.org.uk/downloads/lt-intro_to_cement.pdf Ciesielczyk, W. (2005). “Batch Drying Kinetics in a Two Zone Bubbling Fluidized Bed”. Drying Technology, 23(8), 1613-1640. CIOB (The Chartered Institute of Building). (2007). “Sustainability and Construction”. Web sites: www.businesseye.org.uk/519733.pdf www.ciob.org.uk Colantuono, A., Dal Vecchio, S., Marino, O., Mascolo, G., Vitale, A. (1996). “Cement-Lime Mortars Joining Porous Stones of Masonries Able to Stop the Capillary Rise of Water. Cement and Concrete Research, 26(6), 861–8. “Context”. (2006). “The Lime Spectrum”. The Journal of the Institute of Historic Building Conservation. Web site: www.ihbc.org.uk Cultural Heritage Team. “Repointing Your Building”. Peak District National Park Authority. Web site: www.peakdistrict.org/repointing.pdf De Schutter, G. & Poppe, A. M. (2004). “Quantification of the Water Demand of Sand in Mortar”. Construction and Building Materials, 18, 517-521. Green, K. M., Carter, M. A., Hoff, W. D., Wilson, M. A. (1999). “The Effects of Lime and Admixtures on the Water-Retaining Properties of Cement Mortars”. Cement and Concrete Research, 29, 1743-1747. Jacob, J., Weiss, N. R. (1989). “Laboratory Measurement of Water Vapour Transmission Rates Of Masonry Mortars and Paints”. APT Bulletin (Association for Preservation Technology International), 62-70. Introduction to Mortar: Learning Text. Web site: www.mortar.org.uk/downloads/lt-intro_to_mortar.pdf Lanas, J., Perez, J. L., Bello, M. A., Alvarez Galindo, J. I. (2004). “Mechanical Properties of Natural Hydraulic Lime-Based Mortars”.Cement and Concrete Research, 34, 2191-2201. Li, Z., Ding, Z., Zhang, Y. (2003). “Development of Sustainable Cementitious Materials”. International Workshop on Sustainable Development and Concrete Technology, 2003. Web site: www.cptechcenter.org/publications/sustainable/lisustainable.pdf Merritt, F. S., Ricketts, J. T. (2000). Building Design and Construction Handbook. New York: McGraw-Hill Professional. Mortar Industry Association. “The Use of Ground Granulated Blast Furnace Slag and Pulverized Fuel Ash in Mortar”. Data Sheet 16. Web site: www.mortar.org.uk Nash, W. G. (1990). Brickwork. United Kingdom: Nelson Thornes. Park, K. B., Plawsky, J. L., Littman, H., Paccione, J. D. (2006). “Mortar Properties Obtained by Dry Pre-Mixing of Cementitious Materials and Sand in a Spout-Fluid Bed Mixer”. Cement and Concrete Research, 36, 728-734. Philippi, P. C., Rosendo, Yunes. P., Fernandes, C. P., Magnani, F. S. (1994). “The Microstructure of Porous Building Materials: Study of a Cement and Lime Mortar”. Transport Porous Media, 14, 219-45. Philippi, P. C., Souza, H. A. (1995). “Modelling Moisture Distribution and Isothermal Transfer in a Heterogenous Porous Material”. International Journal of Multiphase Flow, 21(4), 667–91. Plawsky, J. L., Jovanovic, S., Littman, H., Hover, K. C., Gerolimatos, S., Douglas, K. (2003). “Exploring the Effect of Dry Pre-Mixing of Sand and Cement on the Mechanical Properties of Mortar”. Cement and Concrete Research, 33, 255-264. Sebaibi, Y., Dheilly, R. M., Queneudec, M. (2003). “Study of the Water Retention Capacity of a Lime Sand Mortar: Influence of the Physicochemical Characteristics of the Lime”. Cement and Concrete Research, 33, 689-696. Sriboonlue W., Wallo E. M. (1990). “The effect of Constituent Proportions on Stress-Strain Characteristics of Portland Cement-Lime Mortar and Grout”. In: Matthys J. H., (Ed.), Masonry: Components to Assemblages, Special Technical Publication, Philadelphia, 206- 14. Tatemoto, Y., Bando, Y., Yasuda, K., Senda, Y., Nakamura, M. (2001). “Effect of Fluidizing Particle on Drying Characteristics of Porous Material in Fluidized Bed”. Drying Technology, 19(7), 1305-1318. Varas, M. J., Alvarez de Buergo, M., Fort, R. (2005). “Natural Cement as the Precursor of Portland Cement: Methodology for its Identification”. Cement and Concrete Research, 35, 2055-2065. Read More