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Use of Blastfurnace Cement for Sulphate Resisting Properties

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1) Sulphate Resisting Portland Cement (SRPC) & ASTM Type V Cement

SRPC is a costly and special type of cement where in the manufacturing process, one of the major compound i.e. tricalcium aluminate (C3A), is limited to < 3.5 %. Whereas for ASTM Type V cement, the maximum C3A content allowed is 5%.

This C3A component, typically 8-12% in Portland Cement, is highly susceptible to sulphate attack. The sulphates in ground water or soil attack the concrete, in particular the C3A hydrates, leading to expansion and eventual disintegration. Hence, SRPC provides sulphate resisting property by limiting the amount of C3A available for the sulphate to attack. Anyhow, SRPC with low C3A content is not performing in order to resist chloride attack under marine environment as the presence of C3A can act as a ‘Binder’ to bind the incoming chlorides as such to delay the chloride attack.

2) Blastfurnace Cement

In the case of Blastfurnace Cement, the mechanism in providing protection against sulphate attack is different from conventional SRPC.

Sulphate-Chloride Resisting Property

Fundamentally, there are two mechanisms that contribute to the high sulphate and chloride resisting properties of Blastfurnace Cement:-

(a) The reduction in the C3A component in the blend with the replacement of OPC with 50~70% GGBS (Normal recommending % of GGBS). For example, our OPC has a typical C3A of 8.5% and after blending with GGBS to produce Blastfurnace Cement, this C3A level would be reduced to 4.25 and 2.55% accordingly, thus meeting the requirements to reduce the C3A content to delay the sulphate attack.

(b) The reduced permeability of concrete containing GGBS. This important approach is to reduce the quantity of Ca(OH)2 in hydrated cement paste by the use of high % of GGBS or pozzolana. This is attributed to the pozzolanic reaction of the slag with lime and other alkalies released during the hydration of OPC. The silicate hydrates thus formed occupies the pore spaces/voids reducing the porosity and hence, the permeability of the concrete. This significantly slows down the ingress and diffusion of aggressive chemical like sulphates, chlorides and other liquids/gases.

As such, the addition of GGBS will decrease the C3A available for reaction but more importantly the uniquehydration reactions in concrete containing GGBS have a pore-filling capacity that reduced the permeability of concrete which significantly inhibits the ingress of sulphates.

In accordance with the recommendations of the UK Building Research Establishment (BRE) Digest 363/ BRE Special Digest 1 , the use of High GGBS content cement is now extended to the most severe Class ‘4’ and ‘5’ sulphate conditions. This guideline has since been adopted in the latest BS 8500-1:2006: Concrete-Complementary British Standard to BS EN 206-1.

In all cases, whether chloride and simultaneous sulphate attacks, high slag cements (> 50% GGBS content) are proven to withstand such aggressive environment. In fact in some countries such as Australia, it is mandatory to use slag cement for structures located within 5km from the coastline to resist sulphate and chloride attack. This project comes under an aggressive sulphate environment where the use of slag cement is highly beneficial for the production of high strength and low permeability concrete.

The above measures are helpful to improve the sulphate resisting properties of concrete, but more important is the prevention of ingress of sulphates and chlorides into the inner part of concrete structures by making the concrete as dense as possible and with as low permeability as possible.

As such, the use of the Blastfurnace Cement will provide good sulphate resisting properties as compared to the conventional OPC & SRPC concrete structures. The use of Blastfurnace Cement shall be enforced in order to ensure the durability requirement of the concrete structures (Bore piles, pile caps, slab at grade & etc.) can be well implemented.

Related Topic: What is High Alumina Cement?



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