DEVELOPMENT OF BLENDED CEMENT FROM ACTIVATED AND SINTERED FLY ASH

Transcription

1 CHAPTER 6 DEVELOPMENT OF BLENDED CEMENT FROM ACTIVATED AND SINTERED FLY ASH 6.1. Activated Sintered Flyash as blending material for cement. Investigations have been made to develop flyash-blended cements using flyash in activated form. Activated flyash has been mixed in the ratio of 20%, 30%, 40% and 50% by replacing Portland cement clinker and cements have been prepared by grinding in ball mill with 3% of gypsum. Different physical properties of the cements thus prepared have been examined. It has been found that up to 40% flyash in a activated form can be used for manufacturing blended cements as per Indian standard Necessity of Activation for Indian Flyash. Using flyash as raw material earlier several investigation have been carried out for cement manufacturing [38,44]. The flyash of Indian thermal power plants contains considerable amount of crystalline compounds like quartz, mullite, magnetite etc. Quartz and mullite particles are stable phases of coal ash and they show very weak reactivity towards lime and cement. The combustible carbon particles present in flyash cause deterioration in the strength of cement and concrete. The above factors stand as barrier for high percentage use of flyash in cement and concrete. In order to materialise the direct bulk use of flyash as blending material in cement and concrete some investigations have been made by mechanical, chemical and thermal means [59,60] to improve the pozzolanic activities of flyash. The thermo chemical activities of coal flyash have been increased to a considerable extent by adopting down draft sintering technique. By this technique, activated sintered flyash aggregates (ASFA) are manufactured by incorporating lime and iron bearing waste materials and sintering in the temperature range 1300 C C. ASFA contains more high temperature 63

2 polymeric phases of quartz such as tridymite and crystoballite, caleiumalumino silicates and dicalcium silicates[12]. Mullite, originally present in flyash gets transformed to calcium and iron bearing mineral compounds[12]. Besides it also contains 50%-60% non crystalline phases. The presence of polymeric forms of lower elementary silica calcium, iron containing silicates and other amorphous phases maintain the lime reactivity value of ASFA to as high as 7Mpa [12]. Generally the lime reactivity values of Indian flyash are in the lower side around 4Mpa, which prevents its direct use in blended cement On the other hands activated flyash, which has high lime reactivity (LR) values of around 7Mpa due to the presence of polymer forms of lower elementary silica calcium, iron containing silicates and other amorphous phases, has been suecessfiilly used in high percentage in blending with cement. This clinker gives better strength of cements than inactivated flyash. In the present context use of ASFA along with Portland cement clinker has beat examined and investigation have been carried out as regards their suitability as per National & International Standards Materials and Methods for the preparation of Blended Cement Materials Portland cements clinker and activated flyash are used as the main ingredient for the preparation of blended cements, a). Activated fly ash A typical flyash from a thermal power plant of Orissa containing 57.82% Si02,24.13%A1203,5.12% Fe203,1.49% Ti02,0.85 % CaO, 0.50% MgO, 0.67% Na20,0.63% K20,0.65% P2O5 and 7.50 % LOI was used to prepare activated fly ash aggregate by the agglomeration technique. The powdery flyash ( 75% ) is mixed with 10% lime sludge waste, 10% semiplastic clay and 5% coke breeze powder and granulated in a disc granulator using water as a binder. Pellets prepared were below 15mm size. The granulated pellets were sintered in a port 64

3 grate furnace [12], which operates on the principle of chain grate sintering system commonly adopted for agglomeration of iron ore fines in steel plants. A laboratory setup (400 x 400) mm cross section and 500mm height sinter pot has been used to produce about 30 kg of activated flyash aggregate by sintering at 1260 C. Solid carbon present in the charge acted as fuel for sintering. b). Portland Cement Clinkers manufactured by rotary kiln process were collected for a mineralogical study. The cement clinker contains 58% tricalcium silicate (C3S), 27% dicalcium silicate and interstitial phases of tricalcium aluminate (C3A), tetracalcium alumino ferrite (C4AF)) and glass of 155. Chemically the cement clinker contains 20.6% SiCh, 5.1% MgO, 0.3% S03, 63.0% CaO, 5.6% A1203j and 0.8% alkalines( mainly K20 and Na20). This cement clinker is used as a control cement, which is prepared by grinding in a ball mill in presence of 3% byproduct gypsum Preparation of blended cement Cement Clinker, activated flyash, and byproduct gypsum were mixed and ground in the laboratory ball mill to prepare blended cements. Four different mix proportions (Table-6.1) were made to prepare 6kg cement keeping cement fineness constant at about 3000 cm2/gm. Gypsum has been kept 3% in all the cement to keep uniform content of SO3. In order to study the effect of flyash, a blended cement of cement clinker and activated sintered fly ash with proportions given in Table-6.1 with by product gypsum 3% was prepared of similar fineness. 6.4 Tests for activated sintered fly ash and blended cement The sintered materials were subjected to sieve analysis to determine percentage of different size fractions. Representative sintered sample from each type was ground to powder for chemical analysis. Different mineral phases present in sintered Aggregate were examined in a Phillips Make XRD using Cu 65

4 ka radiation. The physical properties of the blended cements such as normal consistency, setting time, Le-Chatelier expansion, autoclave expansion and compressive strength has been determined following BIS specifications. Refractoriness of different sintered products containing alumina were determined using Lica Make heating microscope. Table-6.1 Physical Characteristics of Blended Cements with Activated Sintered Fly Ash SL Characteristics Mix.1 Mix2 Mix3 Mix.4 No. 1 Percentage ofopc Clinker Percentage of Activated Sintered Fly Ash 3 Percentage of Gypsum Fineness Sieve Analysis ( Residue % ) Blains Surface Analysis ( Surface area in sq cm/gm) >3500 >3600 >3600 > Normal Consistency (%) 6 Setting Time (hr.min) Initial Final Soundness LeChateliers Expansion Negligible Autoclave Expansion Negligible 66

5 Table Chemical Characteristics of Blended Cements with Activated Sintered Fly Ash [Asper BIS (Part.l ) 1991 ] SL Characteristics and required Characteristic Values ( % by mass) No. values Mix.l Mix.2 MixJ Mix.4 1 Loss on ignition: % by mass (Max. 5%) 2 Magnesia (MgO ) % by mass (Max. 6%) 3 Sulphuric anhydride ( SO3) %by mass (Max.3%) 4 Insoluble materials [ Max. % by mass = x (100-x)/100 where x is the % of activated sintered fly ash in the Blended Cement ] N.B : The proportions of constituents of Mix-1 to Mix-4 are as per Table Properties of Activated Sintered Fly Ash Blended Cement Fly ash collected from thermal power plant is highly crystalline in nature, which contains quartz and mullites the major mineral phases. Hematite and magnetite are present as accessory mineral phases. Presence of some glassy (noncrystalline) particles are observed through microscope. Residual carbon present in the flyash is 6%. About 94% particles of fly ash is below 90-micron sizes. Activated fly ash prepared using this flyash in presence of lime bearing material by sintering at 1260 C temperature contains 56.30% SiC>2,23.45% AI2O3,5.58% Fe203,7.60% CaO, and 2.28% LOI. Mineralogical character of the lime-activated flyash is shown in Fig.6.1 (XRD pattern). 67

6 The activated fly ash contains mostly calcium bearing complex silicate mineral phases. It is seen that formation of calcium aluminosilicate and calcium iron silicate phases have increased where- as, free quarts and mullite phases have been decreased in the activated fly ash. Since the flyash is poor in lime (around 0.8%), the formation of lime bearing compounds in the activated fly ash is almost negligible as observed from XRD pattern (Fig.6.1 ). It is observed that mineralogical phases in activated fly ash have been influenced greatly by the addition of excess lime and high temperature. Generally lime and lime bearing complex silicate phases impart better pozzolanic property than crystalline quartz and mullite(3al2032si02). Blended cements of different composition MIX-1, MIX-2, MBC-3 and MIX-4 have been prepared replacing cement clinker by 50%, 40%, 30% and 20%, respectively, by activated fly ash (Table.6.1). 3% byproduct gypsum has been used as set retarder in all the samples. The control cement prepared from clinker contains 97% clinker and 3% gypsum. The physical properties and chemical characteristics of the 4 blended cement have been shown in Table-6.1 and Table It is observed that normal consistency and setting time of blended cement increases with increase of activated fly ash from 20 to 50%. However these values are well within the standard specification [76,78]. Le-Chatelier expansion of the blended cements are very much negligible showing less than 1mm expansion whereas the same of control cements show expansion up to 5mm. Compressive strength of blended cement cubes in 1:3 mortar ratio has been determined up to 90-day intervals, of curing. Fig.6.2.shows strength gain after 3 days, 7days, 28days, 60 days and 90days.lt shows that with increase of activated flyash addition from 20 to 50 % the strength gradually decreases. The blended cements achieve 16.5, 17.65, 19.5,20.5N/mm2 in 3 days and 31, 33.5, 35.0,36.5 N/mm2 strength in 28 days, in case of addition of 50%, 40%, 30% of activated flyash respectively. These strength values of blended cements are marginally lower than 68

7 tiie control cement without flyash. But it is interesting to note that all the blended cements shows remarkable increase in strength after 60 and 90 days of curing (Fig. 2). But in case of the control cement, the strength gain after 28 days is very much marginal. The blended cements of activated flyash show gradual increase in strength as observed up to 90 days of curing (Fig.6.2). Compressive strength of blended cement using 20% flyash (Mix O ) is shown in Fig It is observed that the strength values achieved in direct fly ash blended cement are much lower than activated flyash blended cements. The strength of blended cements with 40%-activated flyash is more than that with 20% flyash blended cement. It is clear that activated flyash contains contents lime activated silicate phase which induces better hydraulic property than the normal flyash. Use of activated flyash in making blended cement appears to be very much promising. It is also cleared from Fig.6.2 that blended cements contains 20%, 30%, 40% of activated flyash are quite satisfactory to meet the specification as recommended in BIS [76,78]. But 50% addition of activated flyash in cement gives marginal to 33 grade, from strength point of view. 69

8 INDEX' A Calcium alumino Silicate B- Quartz C- Calcium Iron Silicate D- Tridymite E- Cristoballite F- Mullite G- Di-Calcium silicate H - Hematite K- Magnetite CuKa 20 <0) Fig. 6.1: XRD Pattern of activated sintered fly ash and original Fly ash 70

9 Mix - 0 EE Mix - 1 E 3 M ix-2 H Mix- 3 HI Mix - 4 E3 pc I S Norm " f 25 c Number of days Fig. 6.2 : Comparison of compressive strength for 20 % of fly ash used as such ( M ix - 0 ) and different percentage of activated sintered fly ash used in blended cement 0 ' C0 w - I 3 71