New cements for the 21st century: The pursuit of an alternative to Portland cement

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Abstract

Preparation of this article entailed authors analyzing the contents of quite a number of papers, although the main objective was never to review the state of the art of new cements. Rather authors intend to discuss why they believe alkaline activated cement can be positioned at the epicentre of a new and necessary transition from today's Portland cement to the new cements of the future. A brief history of alkaline cements serves as an introduction to the technology itself. The interest roused around calcium sulfoaluminate-based cements is also reviewed, albeit summarily. The greater part of the article focuses, however, on alkaline cements which are classified into five categories. The fundamental chemical and structural characteristics of aluminosilicate-based alkaline cements are also described, and the key advances made in the understanding of synthetic gels are discussed. The paper ultimately finds hybrid cements to be technologically viable materials for contemporary construction.

Introduction

In certain closed ecosystems herds of herbivorous species graze on pastures unchecked. Under these conditions, the herd size gradually increases to a point at which it can no longer be sustained because of scarcity of food. The ensuing death rates drive the herd to near extinction. When only a few feeding individuals are left, the plant cover revives. The renewed abundance of pasture leads to a second round of herd growth and a new cycle. What remains to be seen is whether the human race can do more than emulate the behaviour of these irrational animals, and how long it would take the planet to recover from the sophisticated pillage perpetrated by humanity. Is it reasonable to wait and find out?

Despite the failure of the Copenhagen summit (December 2009), it is tempting to think that the twenty-first century still has time to prove its sensitivity to environmental issues. Certain political initiatives would appear to point in that direction. Most of the pressure brought to bear to enhance environmental protection, however, is due to global awareness among the public at large of the severity of the problems that will have to be faced in a not-so-distant future as a result of unsustainable development: i.e., poorly managed technologically speaking and highly skewed geographically speaking.

In this context, water management and air pollution (in terms of CO2 concentration) are surely the areas where the need for action is most pressing, given the enormity of the consequences. Indeed, one item around which a certain consensus was reached at the Copenhagen climate summit held in late 2009 was the existence of key dates: the members of the international community agreed that the years 2020 and 2050 should be regarded to be points of reference on the climate change horizon and therefore milestones in the very difficult task of changing habits and lifestyles.

At the dawn of the industrial revolution in the mid-eighteenth century, the concentration of CO2 in the air came to around 280 ppm. By the outbreak of World War II 200 years later, CO2 levels had risen to 310 ppm, i.e., at a rate of 0.15 ppm per year (see Fig. 1). But between 1960 and 2000, CO2 concentration soared from 315 ppm to 365 ppm in just 40 years (1.25 ppm/year) [1].

Globally, cement companies are producing nearly two billion tonnes/year of their product and emitting nearly two billion tonnes of CO2 (or around 6 to 7% of the planet's total CO2 emissions) in the process. At this pace, by 2025 the cement industry will be emitting CO2 at a rate of 3.5 billion tonnes/year, more or less equal to the total emissions in Europe today (including the transport and energy industries) [2].

Section snippets

Portland cement: the debate around a historic material

In light of the foregoing facts, there would appear to be little doubt that alternative binders, less aggressive to the environment, must (at least partially) replace Portland cement. At the same time, however, serene reflection on the reasons why Portland cement has remained such a successful building material since its advent in the nineteenth century may prove to be helpful for the design of future strategies. Portland cement constitutes one of the most important technological advances in

Calcium sulfoaluminate cements

Calcium sulfoaluminate (CSA) cements were developed by the China Building Materials Academy in the 1970s, with the intention of manufacturing self-stressed concrete pipes to capitalise on the expansive properties of this material. These cements were produced by adding gypsum to CSA clinkers, which consist mainly of C4A3Ŝ (ye'elimite), belite and ferrite. Normally, 15 to 25 wt.% of gypsum is ground with the clinker to attain optimum setting time, strength development and volume stability [3]. The

Alkaline cements as an alternative in the transition

The first use of alkalis as a component of cementitious materials dates back to 1930, when Kuhl studied the setting behaviour of mixtures of slag ground to a powder and a KOH solution. Since that time many studies have been conducted on the role that alkalis may play in potential cementitious systems. One early milestone was reached in 1940, when Purdon conducted the first extensive laboratory study on clinkerless cements consisting of slag and NaOH [13].

In 1967 Glukhovsky made a significant

Hybrid alkaline cements: prospects

At this writing, immediate global replacement of Portland cement by any of the possible alkaline cements (or any other binder) is virtually impossible. Technical issues around alkaline cements such as paste, mortar and concrete rheology or the supply of universally available, standardised quality prime materials, are still outstanding viable solutions. But is total substitution truly desirable?

One of the possibilities being considered in the context of the aforementioned environmental issues is

Conclusions: Alkaline activation as a model for transition to the design of new alternative cements. Future research

The authors' intention in drafting this article was to discuss the environmental issues surrounding binders used in construction as a whole, and, especially, to provide a certain amount of elementary information on alkaline activation technology and alkaline cements. In particular, they stressed that such technology may shortly reach a stage of development in which it will serve as a link in the necessary transition from Portland cement to the cements of the future. No attempt has been made in

Acknowledgements

In the context of the special symbolism surrounding this issue of CCR, with guest contributions by 13th ICCC participants, the authors wish to address this acknowledgement to the many colleagues around the world who have encouraged us to continue to pursue a line of research undertaken many years ago now. Our sincere thanks to them all for reading our papers with the critical turn of mind that characterises scientific endeavour. C. Shi would like to thank the financial supports from Chinese

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