Cement-based building materials
Concrete is a cement-based building material used in construction industry on very large scale. However there are many other cement-based materials used in to improve the economy, conserve materials, energy and to reduce the carbon footprint of construction. This article focuses on the following four cement-based building materials: dry mixed mortar (DMM) plasters, cement-based fly ash bricks, autoclaved aerated concrete (AAC) blocks, and micro-concrete for concrete repair work.
The cement-based DMM plaster is different from job-site mortar plaster. It is manufactured in a factory with dedicated facilities for batching and blending of all the necessary ingredients in the controlled process. In this way, DMM plaster with well-defined properties and performance to meet specific requirements and applications can be produced.
DMM plaster provides excellent technical properties to meet the stringent performance requirements which are common in the current construction scenario, such as crack free surface, no leaching and aesthetic look. The use of DMM plaster is cost effective, reducing potential construction problems with the long-term integrity of structures with a simple materials approach. The advantages of DMM plaster are wuality controlled and factory blended to maintain consistently high quality, excellent adhesion, no cement and sand storage required at site, reduces wastage, better workability, suitable for wide range of masonry/concrete backgrounds, fibre reinforced for shrinkage crack resistance, aesthetic look due to better finish, and no leaching. Most DMM plasters require only the addition of potable water and mixed with a simple mixer to produce high-quality fresh mortar for wall application. Normal curing process is followed. Most of the high-performance plasters are usually based on extensive development process and tests in order to achieve the desired materials properties. The basic raw materials are: cement, filler and fine aggregate.
The gradation of aggregate and the choice of the filler are critical. Desirable properties of DMM plaster in fresh and hardened state are as follows.
Mixing time: Mixing time of DMM plaster is one of the important parameters to define its ease of application for the mason. Dry mortar powder should quickly mix with water to get the desired workability.
Workability retention (pot Life): Workability retention is the time taken by fresh mortar/concrete to lose its plasticity. Once the mortar is mixed with water it has to maintain its workability till application, for a reasonable period of time: minimum 60 m in peak summer noon and maximum 90 m in the morning/evening or winter season. Workability Retention can be measured from the time of adding water to dry mix till it loses its plasticity i.e. its nature to stick to wall, when mason applies. Loss of workability before application encourages meson to add water to obtain desired workability and such plaster develops cracks after hardening.
Drying time: Plaster should get surface-dried after application, within certain period of time, to start surface finishing and curing. During the process of curing, plaster attains its early strength and binds properly to the substrate (wall/roof top). Addition of polymers can delay surface drying. Polymer mixed DMM may also stick to trowel and the float used for surface finishing, making the whole process difficult and time-consuming
Coverage area: Good coverage area of a plaster offers cost saving to the customer. Coverage area can be measured by calculating the spread area for constant thickness. It depends on the bulk density of plaster. Higher is the density of plaster lower is the spread area. Density of DMM also affects porosity. Optimum bulk density should be obtained balancing the two factors. Typical coverage can be expressed for 10 mm thickness as: m2/kg
Rebound loss: Rebound loss of a plaster shows its capacity to stick to the wall. Lesser is the rebound loss, lesser the wastage of plaster during application. Rebound loss depends on many factors, irrespective of the nature of plaster.
Firstly, it varies from mason to mason. Sometimes the masons' handling makes difference in the rebound loss.
Second factor is the water content of a plaster mortar. If water is higher than recommended, mortar applied on the wall slides and does not stick properly. If water is lesser than recommended, mortar gets brittle and falls down immediately. Third factor is "saturation of backing surface". Any readymade plaster product should be used only with recommended water content. Water content fixed by manufacturer is enough to prepare a workable mix. It is very important to make backing surface (substrate) wet till it gets saturated and surface dry. When the surface is not saturated, it absorbs water from the plaster and makes it brittle. Similarly, when the surface is over saturated, excess water makes plaster flowing down the wall. The surface of application should be saturated-surface-dry.
Binding property: The binding of DMM to the backing surface (wall with red clay bricks, fly ash bricks or AAC blocks and roof top) must be tested before application.
Compressive strength: No standards specifically mentions about the compressive strength of cement wall plaster. However, experience shows it should have strength of at least 7 MPa at three days.
Cement-based fly ash bricks
The IS 16720: 2018 gives the specification of fly ash-cement bricks. Pulverized fuel ash or fly ash (FA) is a byproduct from thermal power stations, which use pulverised coal as fuel. This national resource can be gainfully utilised for manufacture of FA-cement bricks as an alternative to common burnt clay bricks, leading to conservation of natural resources and improvement in environment quality. The FA-cement bricks are made from materials consisting of FA in major quantity, cement and aggregate. These bricks are manufactured by mixing of all ingredients, which are then moulded into bricks and are de-moulded when sufficiently hardened and then subjected to curing.
FA and cement together should be considered as binder. IS specifies, FA content should not be less than 35%. However, FA could be as high as 65 per cent depending upon quality of both cement and FA. It will be worthwhile to find the strength of FA+ cement mixture, before deciding proportions. Sand or bottom ash from boiler can be used as aggregate. Nominal maximum size of aggregate should be passing 6.3 mm sieve. The typical dimensions of FA-cement bricks are given in Table 1.
The mixing of ingredients should be done in suitable mechanical mixer. The uniformity of mixture should be tested in terms of color and consistency. The mixture thus prepared may be compacted in moulds by hydraulic or vibratory press or hydraulic-cum-vibratory press and finished to proper size without broken edges. After demoulding, the bricks should be protected till they develop sufficient strength, before curing. Curing can be done with water as per IS 456, mist or steam, so as to develop sufficient strength as required by the designated category. Table 2 gives classification of FA-cement bricks on the basis of 28-day wet compressive strength. The average drying shrinkage is limited to 0.05 per cent (max). The water absorption should be below 20 per cent (mass) for Class up to 10 and below 15 per cent (mass) for higher classes. Typical FA-Cement bricks and red clay bricks are shown in Plate 1.
Advantages of FA-cement bricks over conventional red clay bricks:
- The strength of common red clay bricks lies in the range of 3.5 to 5 MPa; whereas that of FA-Cement bricks goes up to 15 MPa. Strength also increases over a period of time.
- Lesser water absorption hence requires less water for curing.
- Uniform dimensions and more dimensional stability.
- Lesser transit waste.
They are also known as cellular blocks. Specification is given in IS 2185 (Part 3). Autoclaved aerated concrete (AAC) is a versatile lightweight construction material and usually used as blocks. Compared to normal dense concrete, AAC has low density and excellent sound and heat insulation properties. The density of AAC is in the range of 450-1000 Kg/m3 as against 2300-2500 Kg/m3 for that of the dense concrete. Plate û 2 shows typical AAC blocks. The common raw materials used while making AAC are given in the Table - 3
The above proportions may vary subject to different plant practices and requirement of AAC. Quartz-rich sand and gypsum is also be used in the raw mix. Aluminium is added as a pore forming agent. Instead, suitable foaming agent can also be added; however, that method is out of the scope of the present paper. The aluminium reacts with soluble alkalies from cement and calcium hydroxide to form hydrogen bubbles according to chemical reaction: Al + 2OH- + 2H2O ? Al(OH)4- + H2 Hydrogen bubbles formed in reaction are responsible for the pore formation in AAC blocks. The raw mix is poured in the moulds, after mixing. The mixture rises in the moulds after formation of bubbles. It is cured at ambient temperature for about 45 minutes and cut into block pieces of required unit size, with wires. The blocks are further cured in the autoclave with high pressure steam, which also improves their compressive strength. Typical conditions in the curing chamber are steam pressure of 4-16 MPa and curing duration of 8-16 hours.
AAC blocks contain more than 80 per cent air by volume and its mass is about one-fourth of the red clay bricks, making it the lightest building material. The comparison of AAC blocks and burnt (red) clay bricks is given in Table 4.
Micro-concrete for concrete repair work
Micro concrete is a proportionate mixture of Portland cement, graded aggregate of 10 mm down size or 6 mm down size. Micro-concrete also has a non-shrink additive in the mix to limit the plastic shrinkage up to 0.4 per cent.
It is generally used in sections which are inaccessible and where there is thick reinforcement. Generally, micro-concreting is done as a repair job in structures. The distressed concrete section or spalled concrete is removed and after application of suitable bonding agent over the existing surface, micro-concrete is poured or applied. Micro-concrete is dimensionally stable and compatible to the existing structural material and section. It is to be noted that shuttering to be done leak proof while micro-concreting and proper curing methods to be followed since the heat of hydration of micro-concrete is higher than normal concrete mixes. Micro-concrete is useful for the following areas of application:
Repair of damaged reinforced concrete elements, like slabs, beams, columns, wall, etc., where access is restricted and compaction is not possible.
To jacket RCC columns, to increase load-bearing capacity (Plate - 3)
- Can be pumped or poured into restricted locations
- Flowable mortar, hence does not require compaction
- Develops high initial and ultimate final strength
- Offers excellent resistance to moisture ingress
- Makes repaired sections durable
- Rapid strength gain to facilitate early reinstatement
Note: Fine, sharp washed sand from zone III to IV, as per IS 383 - 2016 May also contain a non-shrink additive to limit plastic shrinkage < 0.4%
ABOUT THE AUTHORS:
Dr J D Bapat is with the Development Professional for Cement and Concrete. Email Email: email@example.com | Web: www.drjdbapat.com
Kalpana Karthikeyan is R&D Manager, Sanghavi Industries