Effect of Mixing Time in Fly Ash Block Property Essay Sample

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Development Alternatives, B-32, TARA Crescent, Qutab Institutional Area, New Delhi – 110016, India Abstract: This paper presents the effects of mixing time on the compressive strength of fly ash brick. Before designing the mix, sieve analysis of sand and fly ash was carried out and their fineness moduli were determined. Also the specific gravity and calorific value and ash content of fly ash were determined. The CaO content of lime also was determined by lime testing kit. The material batching is based on the composition derived from TARA laboratory report. Composition Sand Flay ash Virgin lime Phosphogypsum

50 50 9% of the total weight of sand and fly ash 3% of the total weight of sand and fly ash

The mixing duration was varying 6 minutes to 30 minutes. The strengths were measured at the curing ages of 28 days using compressive strength machine. The results revealed that the compressive strengths of fly ash bricks appreciably increased with increase in mixing duration from 6 minutes to 15 minutes but remained almost constant beyond 12 minutes for 28 days ages tested. For example, at 6, 8, 10, and 12 minutes mixing; the compressive strengths at 28 days 2 were 147.15, 149.59, 158.54, and 197.72 kg/cm respectively while at 14, 15, 20, 25 and 30 minutes mixing ; the compressive strengths at 28 days were 212.83, 2 216.54, 216.59, 221.34 and 227.12 kg/cm .

Keywords: Compressive strength, curing, machine mixing, mixing duration

Introduction Modern Construction prefers use of fly ash bricks. This one of the global go-green initiatives, and we are an active member of the same. This noble cause is supported by manufacturing and supplying bricks made from re-usable fly ash. Fly-Ash Bricks are an environment friendly cost saving building product. These bricks are 3 times stronger than conventional bricks with consistent strength. These bricks are ideally suited for internal, external, load bearing and non-load bearing walls. These Bricks with higher strength/weight ratio (about 3 to 4 times that of burnt clay bricks) aid in designing stronger, yet more economic structures. Fly Ash Bricks are durable, have Low water absorption, less consumption of mortar, Economical & ecofriendly, Low energy consumption and No emission of green house gases. These bricks are not affected environmental conditions and remain static longer building. wastages during thus life ensuring of the the ash and by

Also, in fly savings with regard to bricks are considerable unloading construction due to true shape and size, consistency in quality, and the workability of the fly ash bricks unlike traditional clay bricks. These bricks are very economical / cost effective, nil wastage while transporting and handling. Thus in recent times the use and production of fly ash bricks has demanded increasing significance. There are numerous machines and processes of fly ash brick production. Objectives This brief describes the optimum mixing time for improving the quality of existing fly ash brick properties without compromising on the technical qualities.

Experimental Raw materials Fly ash: Fly ash was obtained from Dadri Thermal Power Plant, Delhi from ESP in the dry form. The fly ash conform IS 3812: 1981. Sand: The sand collected from the Sindh river in Datia district, Madhya Pradseh, India and used as fine aggregate was clean, sharp, free from clay and organic matter and well graded in accordance with IS Lime: Lime was sourced from local Datia market, Madhya Pradesh, India. Gypsum: Gypsum was bought from a reputed supplier at Datia market, Madhya Pradesh, India and it conformed to IS Water: Tap water was used for mixing and it was properly examined to ensure that it was clean, free from contaminants either dissolved or in suspension and good for drinking as specified in BS 3148 Table 1: Particle size analysis of various raw materials Grain Size 6mm – 2mm 2mm – 0.5mm 0.5mm – 0.15mm Less than

0.15mm Sand 2.10% 95.20% 1.92% 0.78% Pulverized Fuel Ash 0.36% 11.93% 55.09% 36.62%

Equipment To ensure proper mixing of raw materials, a normal pan mixer was used. For block manufacturing a TARA MechRam – MV was used. The machine is an elegant machine with a twin molding mechanism powered by a hydraulic power pack. Table 1: Machine Specification Description Product Size (Standard/ Chamfered) Production Capacity Roller Pan mixer Electrical Power (3 Phase) Manpower Product Transfer Mechanism TARA MechRam-MV 230×110×75 mm 650 Blocks/ Hour 300 kg/ Charge 5+7.5 HP 8-10 Persons Pallet Trolley


Details of the machine can be had from www.taramachines.com.

Fig. 2. Fly ash brick making machine Process

Fig. 3. Pan mixer

All materials are collected in dry state. Before batch preparation all the materials were weighed. Laboratory tests on the fly ash, sand and lime for the purposes of characterization and classification include determination of particle size analysis, natural moisture content, calorific value, ash content, specific gravity and CaO content were carried out. The flow chart given below shows the process followed for fabrication of the fly ash bricks: Lime + Water Slaked for 24 hrs Lime slurry prepared Pour sand, lime slurry & gypsum Mix for 1 minute Add Fly ash Mix for 2 minutes Sprinkle required amount water Mix for 3,5,7,9, 11, 12, 17, 22, and 27 minutes Check moisture content by drop test Clean the pan mixer

Mixture prepared

Fed in the mould of MV machine Press in 2200 Psi pressure Fly ash block

Drying For 2 days Curing For 28 days Testing


Testing The fully cured paving blocks were tested for compressive strength as specified in IS 15658: 2006. The process is as follows: Take minimum five cured blocks and put it in water for 24 ± 4 hours maintaining at a temperature  of 20 ± 5 C. After 24 ± 4 hrs the blocks are taken out from the water and soaked  the water with a cotton cloth. Bearing plates of the testing machine shall be wiped clean.  Align the block with those bearing plates.  Apply the load without shock  Observe the pointer of the machine.  Take the breaking load of that brick.  Put it in the following formula to get the strength of the brick Strength = [ (Breaking load*1000)/{(L/10)*(B/10)*9.8}] Where: L= Length of the bricks B= Breadth of the bricks Fig.4. Compressive strength machine 0

Composition The compositions selected for the above experimentation is given below. All values are on weight ratio basis. Table 1: Batch composition list Sample Code Fly ash (by weight) 6PFA 8PFA 10PFA 12PFA 14PFA 15PFA 20PFA 25PFA 30PFA 50 50 50 50 50 50 50 50 50 Sand (by weight) 50 50 50 50 50 50 50 50 50 Lime (in % of the total weight of sand and fly ash) 9 9 9 9 9 9 9 9 9 Gypsum (in % of the total weight of sand and fly ash) 3 3 3 3 3 3 3 3 3 Mixing duration ( in minutes) 6 8 10 12 14 15 20 25 30


Results and discussion The results of compressive strength are given in Table 3. Table 2: Compressive strength results of PFA block of different compositions Composition details 6PFA 8PFA 10PFA 12PFA 14PFA 15PFA 20PFA 25PFA 30PFA Graph 1: Compressive strength chart Test value of compressive strength in kg/cm 147.12 149.59 158.74 197.72 212.83 216.54 216.59 221.34 227.12 2

Block Quality Analysis


Compressive strength (in kg/cm2)






0 6 8 10 12 14 16 18 20 22 24 26 28 30

Mixing duration (in minutes)

Table 3: Water absorption test values of PFA block of different compositions Composition details 6PFA 8PFA 10PFA 12PFA Test value of water absorption in % 13.71 12.09 12.35 12.37



12.84 12.04 13.44 12.91 12.36

Graph 2: Water absorption analysis chart

Water Absorption Analysis


Water absorption (in %)




8 6 8 10 12 14 16 18 20 22 24 26 28 30

Mixing duration (in minutes)

Results of compressive strength test produced by machine mixing are presented in Table 2. The degree of mixing was measured in terms of mixing duration inside the pan mixer. The compressive strengths of PFA blocks appreciably increased with increase in mixing duration, but remained almost constant beyond 12 minutes for all the 28 days ages tested. It can be observed too that low strength was recorded at 6 minutes mixing duration. This indicates that below 12 minutes mixing duration, satisfactory strength of PFA blocks may not be achieved. The time of the mixing duration was recorded from the time when sand (the very first feeding raw material) was poured inside the pan mixer to the ejection of mixture from the pan mixer.

Conclusion 1. Compressive strength of PFA block increased with increase in degree of mixing recorded in terms of mixing duration. The maximum strength was recorded at 20 minutes mixing duration and there was no significant increase in compressive strength beyond 12 minutes mixing duration. 2. Adequate compressive strength of 197.72 kg/cm2 was achieved at 28 days for 12 minutes mixing duration concluding that a minimum of 12 minutes mixing duration is required to produce uniform mixture in composition with satisfactory strength.


3. 4.

Low compressive strengths were recorded at 28 days curing ages for mixing duration below 12 minutes, concluding that mixing below 12 minutes is not satisfactory. There was normal increase in compressive strengths of PFA blocks with age of curing for all the mixing duration by machine mixing.

Acknowledgement The author is grateful to Shri. Sugar Singh and Shri. Puran Kumar Kuswaha for help rendered in mixture preparation. Thanks are also due to Dr. Soumen Maity, Programme Director, Development Alternatives, New Delhi for technical support. TARA Nirman Kendra, for supplying raw materials and workers, is also gratefully acknowledged.

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