Building chemistry laboratory exercises

Transcription

1 POLITECHNIKA WARSZAWSKA WYDZIAŁ INśYNIERII LĄDOWEJ KATEDRA INśYNIERII MATERIAŁÓW BUDOWLANYCH WARSAW UNIVERSITY OF TECHNOLOGY FACULTY OF CIVIL ENGINEERING DIVISION OF BUILDING MATERIALS ENGINEERING al. Armii Ludowej 16, p. 551, Warszawa, POLAND; tel.: (+48 22) , fax: (+48 22) , Lech Czarnecki Paweł Łukowski Andrzej Garbacz Bogumiła Chmielewska Building chemistry laboratory exercises collaborative work under the chairmanship of Lech Czarnecki 8. LIME BINDERS Theoretical background Practical task 1. Determination of active calcium oxide content in quicklime Practical task 2. Determination of non-decomposed calcium carbonate content in quicklime

2 8. LIME BINDERS THEORETICAL BACKGROUND Burnt lime (quicklime) an air mineral binder Quicklime is classified as an air mineral binder, i.e. it hardens when mixed with water in contact with air. Lime binders are made from limestone, which apart from calcium carbonate CaCO 3 (58% 99%) contains magnesium carbonate, clayish minerals, quartz, ferric oxides. Clay content worsens the properties of binder and makes the slaking process more difficult. However, with more than 6% of clay in the raw material (marl lime), the product acquires hydraulic properties (hydraulic lime). Burnt lime production Lime burning process is a reaction of thermal decomposition of calcium carbonate CaCO 3 = CaO + CO 2, H > 0 where H reaction enthalpy. It is a reversible endothermic reaction, i.e. it is necessary to provide energy to the system in order to start the reaction and achieve the state of balance between products and substrates. According to Le Chatelier-Braun s principle, an increase in temperature causes a shift of balance of endothermic reaction towards products. In this case, the rising temperature increases the pressure of released CO 2. At 898ºC, the pressure reaches the value of atmospheric pressure (Fig. 8.1). In thus defined balance, in an open system (exchanging mass and energy with the environment) it is possible to remove one product (CO 2 ) from the reaction environment. System tendency to restore the state of balance makes the reaction irreversible and carbonate decomposition complete. Fig. 8.1 Pressure of CO 2 released in CaCO 3 decomposition process In industrial practice, higher temperatures up to ºC are used to increase the speed of reaction. The choice of temperature depends on the quality of raw material, its structure and chemical constitution. 1

3 In the burning process, apart from the above described reaction, there are also reactions whose substrates are other components of the raw material. At ºC, magnesium carbonate is decomposed. At ºC, calcium oxide reacts with aluminum, silicon and iron oxides producing calcium aluminates, silicates and ferrates. Lime burning at over 1300ºC increases the contents and degree of liming in these compounds, causing at the same time product sintering, which makes hydration difficult (so called dead-burnt lime). High temperatures are not beneficial, either, when the content of MgCO 3 is high. Heating of MgO (formed at about 550ºC) contributes to the loss of its binding properties. Temperature that is too low may be the reason for a high content of non-decomposed CaCO 3. Lime slaking Lime slaking is an exothermic hydration reaction (with heat release) run according to the formula CaO + H 2 O = Ca(OH) 2, H < 0 The temperature goes up and a substantial amount of water evaporates. The amount of water used in slaking is a differentiating factor of the final product: hydrated lime, obtained by slaking of quicklime with possibly the smallest amount of water; beside the amount of water resulting from reaction equation (stoichiometric), some extra amount is needed in this process due to evaporating and adsorption of water, lime putty plastic mix of calcium hydroxide and saturated solution of calcium hydroxide in water, produced as a result of lime slaking with excess water; high degree of hydroxide comminution and surrounding its particles with water diminishes the force of internal friction and makes the mix plastic, whitewash colloidal suspension of hydroxide in water formed as a result of lime slaking with huge excess of water or by lime putty dilution. Standard requirements for building lime Standard requirements for building lime are given in PN-B-30020:1999 norm. It distinguishes 3 kinds of lime depending on raw material used for production, i.e. limestone: - pure; CL (calcium lime), - containing dolomite; DL (dolomitic lime), - clayish; HL (hydraulic lime). Depending on a total content of calcium oxide and magnesium oxide, we have 3 varieties of CL lime and two varieties of DL lime (Table 8.1). Selected requirements according to PN-B-30020:1999 Table 8.1 Properties Calcium lime Dolomitic lime CL 90 CL 80 CL 70 DL 85 DL 80 CaO+MgO content, not less than, % Content of free water, % - quicklime and hydrated lime 2 - whitewash Reactivity of ground quicklime: - slaking temperature, ºC > 60 - slaking time, minutes

4 Setting and hardening of lime mortar Slaked lime mixed with sand and water makes lime mortar. Mortar sets within a few hours due to water evaporation and crystallizing of calcium hydroxide. Then a slow, even a few years long, hardening process, which is a carbonatization reaction, starts: H 2 O Ca(OH) 2 + CO 2 = CaCO 3 + H 2 O Sand in the mortar facilitates CO 2 penetration and accelerates carbonatization. Mortar hardness and strength depends on a degree of mortar carbonatization and a size of crystals formed. Reaction acceleration by an increase in CO 2 concentration in the air and/or raising temperature reduces mortar strength; obtained crystals of calcium carbonate are smaller. Hydraulic lime Beside calcium oxide (CaO), hydraulic lime contains dicalcium silicate (2CaO SiO 2 ), tricalcium aluminate (3CaO Al 2 O 3 ) and dicalcium ferrate (2CaO Fe 2 O 3 ), giving hydraulic properties to the binder. After burning, hydraulic lime is slaked with a little amount of water to make calcium oxide change into calcium hydroxide and not to let silicates, aluminates and ferrates undergo hydration. The process of hydraulic lime setting and hardening consists in crystallization and carbonatization of calcium hydroxide as well as in hydration of calcium aluminates, silicates and ferrates. PRACTICAL TASK 1. DETERMINATION OF ACTIVE CALCIUM OXIDE CONTENT IN QUICKLIME The equipment necessary to perform the task scale, 250 cm 3 conical flasks (4) funnels, 25 cm 3 pipette, 50 cm 3 measuring cylinders with stoppers evaporating dish, filter, watch glass, 250 cm 3 measuring flask. Reagents used: quicklime, distilled water, methyl orange, 0.25 m HCl, 10% water solution of phenol. Task completion The phenol method may be used to determine the content of active calcium oxide in quicklime. The method consists in adding a phenol water solution (ATTENTION! STRONG POISON) to a sample of slaked lime. As a result of calcium hydroxide (base) reacting with phenol (acid) we obtain a salt calcium phenate (a salt of a weak acid and a strong base). CaO + H 2 O = Ca(OH) 2 2C 6 H 5 OH + Ca(OH) 2 = (C 6 H 5 O) 2 Ca + 2 H 2 O After filtering the solution, the amount of salt obtained is determined by titration of a sample with hydrochloric acid using methyl orange as an indicator (C 6 H 5 O) 2 Ca + 2HCl = 2C 6 H 5 OH + CaCl 2 The number of CaO moles = the number of Ca(OH) 2 moles = the number of calcium phenate moles = ½ c HCl V HCl, where: c HCl molar concentration of HCl, mole/dm 3 V HCl volume of acid used in titration, dm 3 3

5 So 1 cm 3 of HCl of 0.25 mol/dm 3 concentration used to titrate a sample containing calcium phenate matches moles of CaO, i.e g of CaO. The testing procedure is as follows: place a 12.5 g sample of quicklime in the evaporating dish, cover with a watch glass and then put out slowly using distilled water, flush the content of the evaporating dish (using a funnel) to a 250 cm 3 measuring flask; refill the flask up to the calibration mark with distilled water, pour 25 cm 3 of turbid liquid form the measuring flask to each of the two 50 cm 3 measuring cylinders, trying to shake the flask rhythmically so that the sediment will not settle, fill both cylinders up to 50 cm 3 with a 10% phenol solution, close them and shake for one minute; leave for about 40 minutes, shaking frequently, after this time, pass the content of both cylinders through one filter to the conical flask, using a 25 cm 3 pipette, take three samples from the filtrate in the flask and pour them into three conical flasks, add to each three drops of methyl orange and titrate with 0.25 m HCl until it changes colour, calculate the percentage of CaO in the sample tested, assuming that 1 cm 3 of 0.25 m HCl is equivalent to g CaO; the determined amount of calcium oxide is related to 1/20 part of the solution tested on account of twice conducted dilution of sample solution. Hydrating capacity of lime binder and a kind of quicklime should be estimated on the basis of CaO content in quicklime. PRACTICAL TASK 2. DETERMINATION OF NON-DECOMPOSED CALCIUM CARBONATE CONTENT IN QUICKLIME A set of equipment necessary to perform the exercise: gasometric apparatus (Fig.8.2), thermometer 25cm 3 pipette, porcelain crucible, scales, barometer. Reagents applied: quicklime, 1 m HCl. Task completion The aim of the exercise is to determine the underburnt particles in quicklime by determination of decomposed calcium carbonate in the binder. It may be done by the gasometric method, which consists in the measurement of CO 2 released during a reaction of hydrochloric acid with carbonates (mainly CaCO 3 ) in lime CaCO 3 + HCl CaCl 2 + CO 2 + H 2 O Determination is conducted in the apparatus for gasometric analysis (Fig.8.2), which consists of a calibrated burette (B), an equalizing container (Z) and a reaction vessel (N). The vessel (N) is closed with a stopper which holds a tube connected with the burette by means of a rubber hose. The rubber hose also connects the burette (B) with the container (Z). The burette (B) and the container (Z) are filled with a saturated solution of NaCl. Changing a container (Z) position, adjust the burette meniscus level at 0 mark and in this position fix the container (Z) in the stand. 4

6 Measurement taking Fig The gasometric analysis apparatus weigh 0.5 quicklime in a porcelain crucible on a laboratory scales, using a pipette, drop in 25 cm 3 of 1 m HCl solution into the vessel so that the crucible with lime placed on the bottom has the acid around it, cap the vessel (N) with a stopper and connect with the burette using a glass tube and a rubber hose, tilt the vessel to make the crucible knock over and to help the acid get into the tested sample; calcium carbonate reacts with HCl; the releasing carbon dioxide increases the volume of gas in the burette, read the position of NaCl meniscus in the burette immediately after the reaction, wait for 15 minutes and repeat the reading, read the ambient temperature (t), pressure (p), and in Table 8.2 vapour pressure over the saturated NaCl solution (f), reduce the volume (V) of released CO 2 in t temperature at p pressure and f vapour pressure over the saturated NaCl solution to the volume in standard conditions V 0 (273 K, 0.1 MPa) using a formula V 273 ( p f ) V0 = ( 273+ t ) calculate the percentage of CO 2 in quicklime using a formula d V0 %CO2 = 100 m where m mass of tested sample of burnt lime in grams, d g/cm 3 calculate the percentage of CaCO 3. Relation between vapour pressure (f) and temperature (t) Table 8.2 t, ºC f, hpa