Question

Portland cement is a combination of limestone and clay, usually 80-90% limestone, leading to CaO:SiO₂ of approx. 2:1. There are a number of alternative cementitious materials now in use, e.g. slag, which form so-called ‘geopolymer’ pastes.

Find two example of alternative cement materials, compare their composition to that of Portland cement and outline the material and environmental advantages and importance of their use.

Cement Alternatives

Answer 1

One of the expected replacement for Cement in concrete are Lime & Surkhi .

According to the kind of binding material, the mortars are classified into the following five categories:

i. Lime mortar

ii. Surkhi mortar

iii. Cement mortar

iv. Gauged mortar

v. Gypsum mortar.

i. Lime mortar:

In this type of mortar, the lime is used as binding material. The lime may be fat lime or hydraulic lime. The fat lime shrinks to a great extent and hence it requires about 2 to 3 times its volume of sand. The lime should be slaked before use. This mortar is unsuitable for water-logged areas or in damp situations. It possesses good cohesiveness with other surfaces and shrinks very little. It is sufficiently
durable, but it hardens slowly. It is generally used for lightly loaded above-ground parts of buildings.

ii. Surkhi mortar:

This type of mortar is prepared by using fully surkhi instead of sand or by replacing half of sand in case of fat lime mortar. The powder of surkhi should be fine enough to pass BIS No. 9 sieve and the residue should not be more than 10% by weight. The surkhi mortar is used for ordinary masonry work of all kinds in foundation and superstructure. But it cannot be used for plastering or pointing since surkhi is likely to disintegrate after some time".

iii. Cement mortar:

In this type of mortar, the cement is used as binding material. Depending upon the strength required and importance of work, the proportion of cement to sand by volume varies from 1:2 to 1:6 or more. It should be noted that surkhi and cinder are not chemically inert substances and hence they cannot be used as adulterants with matrix as cement. Thus the sand only can be used to form cement
mortar. The proportion of cement with respect to sand should be determined with due regard to the specified durability and working conditions. The cement mortar is used where a mortar of high strength and water-resisting properties is required such as underground constructions, water saturated soils, etc.

iv. Gauged mortar:

To improve the quality of lime mortar and to achieve early strength, the cement is sometimes added to it. This process is known as the gauging. It makes lime mortar economical, strong and dense. The usual proportion of cement to lime by volume is about 1:6 to 1 :8. It is also known as the composite mortar or lime-cement mortar and it can also be formed by the combination of cement and clay. This mortar may be used for bedding and for thick brick walls.

v. Gypsum mortar:

These mortars are prepared from gypsum binding materials such as building gypsum and anhydrite binding materials.

- Uses and precautions for lime based concrete :

1.Lime concrete is very widely used for foundation bases of load bearing walls, columns, and under layers of floors.

2.Due to its flexibility it adjusts very well with the underneath base ground and upper construction of cement base.

3.For better quality of lime concrete it is important to compact & cure concrete properly. Lime causes rashes on human skin so the persons which are dealing lime concrete should be provided with suitable rubber gloves.

4.Persons should use oil on their skin to avoid rashes and cracking of their skin due to reaction of lime.

5. To achieve good quality lime concrete, certain admixtures, fibers etc can be used.

- Importance and use : Surkhi is used as a substitute for sand forconcrete and mortar, and has almost the same function as of sand but it also imparts some strength and hydraulicity. ... It is mixed with lime to produce mortar. It is manufactured artificially in India by burning some types of clays and grinding it very fine.

- Surkhi makes cement mortars and concretes more water proof, more resistant to alkalies and to salt solutions than those in which no surkhi is used. Surkhi mixed in cement concrete has been used in some of the big dams and other massive works in India. This admixture is known to reduce the temperature rise during hydration in a mass cement concrete and reduce cracking.

properties of cement-lime mortars:

1. Tensile Bond Strength - This is the strength of the mortar that holds the masonry units together. High tensile bond strength is developed by the following mortar characteristics:
   • Lime provides high water retention that allows for maximum early curing of the cementitious materials.
   • High initial flow which permits easy complete coverage of masonry units.
   • The low air content of cement-lime mortar increases bond strength.
   • The extent of bond is the percent of brick to which the mortar adheres. The low air content and the fineness and stickiness of hydrated lime particles increase the extent of bond of mortar to brick. These factors allow cement-lime mortars to penetrate deeply into the brick and seal the brick/mortar interface.

There are a number of studies that demonstrate the superior bond strength of cement-lime mortars.

Autogenous Healing - When hairline cracks develop in the mortar, hydrated lime reacts with carbon dioxide in the atmosphere. This reaction produces limestone which helps to seal the crack and fill voids in the mortar. This explains the increased moisture resistance noted after six months of curing in two studies.

- Elasticity : Research has shown that high lime content mortars were slow hardening and remained elastic or flexible. Lime, therefore, enhanced the ability of the assemblage to accommodate stresses caused by building movement and cyclical changes without excessive cracking.