Tag Archives: Portland cement

Hydration Mechanisms of Portland Cement: Topochemical or Through-Solution Reaction?

When we simulate the hydration process of Portland cement, it is of significant importance to know the hydration mechanisms. Basically, there are two mechanisms for the hydration of Portland cement, viz. Topochemical and Through-Solution Reaction.

1 Topochemical Reaction Concept

Immediately after the first contact of the cement with water a calcium-rich silicious clinker would liberate Ca2+ ions into the solution. A calcium-poor skeleton is left which reacts with the calcium-rich solution which is accompanied by swelling of the hydration products compared with the original volume of the anhydrous cement.

The topochemical concept was launched by Michaelis at the beginning of last century. Since then topochemical phenomena have been reported for the hydration of both C3S and Portland cement.

Schematic representation of proposed hydration mechanisms

2 Through-Solution Concept

In the through-solution concept dissolution of the anhydrous grain after contact of the cement with water is considered to be followed by hydration in the solution. The hydration products then precipitate on the grain surface.

The through-solution concept was first formulated by Le Chatelier for plaster and Portland cement in the first decade of this century. The concept has been supported by many reserachers, Regourd et al., Brunauer and Gauglitz, Williamson and Dron et al.

The debate on whether cement hydration proceeds topochemically or according to a through-solution mechanism dates back to the very beginning of cement chemistry. The subject is of significant importance in view of mathematical modeling of both the hydration process and structural formation. As such, a so-called Simultaneously Operating Mechanisms is proposed to account for the debate.

3 Simultaneously Operating Mechanisms

According to Neville the controversy between the topochemical and through-solution concept can largely be reduced to a matter of terminology. Shebl et al. explains that hydration of C3S involves both through-solution reactions and topochemical reactions (solid-state reaction). The water/solid ratio would be an important factor in this respect. For low water/solid ratios the reaction would be predominantly topochemical, whereas for high water/solid ratios the through-solution mechanism would be more important. Both reactions could occur simultaneously.

The concept of simultaneously operating mechanisms is plausible indeed if the outer products, i.e. the products which are formed outside the original grain boundaries in a relatively water-rich environment, are formed by a through-solution mechanism, while the inner products, formed inside the original grain boundaries, are the result of a topochemical reaction.

From a literature survey on this topic Daimon concluded that it is very difficult to determine which mechanism has marred. The fact that we are dealing with a poly-size system significantly contributes to the complexity of the subject. Probably the effect of the particle size distribution also explains why Kalousek found many contradictions between different authors on this point.

The conclusion is that the mechanism of cement hydration depends on the water/solid ratio, viz. low water/solid ratios lead to topochemical reaction, while through-solution mechanism is more important at high water/solid ratios, and both reactions could occur simultaneously.

Reference: Simulation of hydration and formation of structure in hardening cement-based materials, by K. Van Breugel, TuDelft.

How GGBS is used and its typical substitution rates

How is GGBS used?

GGBS (Ground Granulated Blastfurnace Slag) is used all over the world as a direct replacement for Portland cement. It is added to the concrete mixer along with ordinary cement, aggregates and water.

The normal ratios and proportions of aggregates and water to cementitious material in the mix remain unchanged. Mixing times are the same as for ordinary cement. Both wet mixing and dry mixing processes can be used for making concrete with GGBS.

What is the substitution rates of GGBS in concrete?

Usually, replacement rates for GGBS vary from 30% to up to 85%, depending on the application and technical requirements. Typically 50% is used in most instances. Higher replacement rates up to 85% are used in specialist applications such as in aggressive environments and to reduce heat of hydration.

According to the EU code “EN-197-1-2000 Cement. Composition, specifications and conformity criteria for common cements“, slag in Blastfurnace cement CEM III/C can be used at a substitution rate as high as 95%.

Below are some typical Substitution Rates

Ready-mix Concrete:
50% GGBS is used for most mixes.

Precast Concrete:
30% to 50% GGBS primarily depending on concrete curing conditions.

Special Concrete:
Mass concrete typically has at least 70% GGBS for temperature control. Concrete in aggressive environments usually contains from 50% to 70% GGBS for enhanced durability performance.

Overview of global cement price 2012

Overview of cement price in the world 2012

Several days ago, I posted that the global cement output is decreasing in the first half of 2012. So how about the price trend of cement worldwide? Is it increasing or not?

Information from different sources shows the prices of cement varies worldwide. For instance, prices of cement in South Africa had been retailing at US $186-230/ton in March of 2012. Due to an acute shortage of cement in Ghana, cement price has rapidly risen over the latest 12 months by 25% in the capital Accra and by up to 35% in rural areas.

Compared with the expensive price in Africa, cement price in Asia except China is more or less similar to that of United States. e.g. the price in Pakistan is US $96/ton, while it is US$101/ton in US.

In the biggest construction market China, cement price has been decreasing in the first two quarters of 2012, now varying at US $55/ton and still much cheaper than that in other places.

The price above is for Portland cement, and conversion of US dollar from local currencies is based on the rates at time of publication.

There is a possibility that cement price will increase soon because of increased public sector spending in the coming fiscal year that normally starts on 1 July 2012, in the meanwhile, the shrank production of cement in 2012 may account for the risen price in the coming months as well.