Civil Engineering Reference
In-Depth Information
19
Reactive magnesia cement
A. Al-TAbbAA, University of Cambridge, UK
DOI
: 10.1533/9780857098993.4.523
Abstract
:
This chapter presents an overview of reactive magnesia cements
which have recently emerged as a viable alternative to Portland cement, with
both technical and sustainability advantages. Details of research work to date
on the characterisation and properties of a range of different origin materials
as well as a range of applications are presented.
Key words
: reactive magnesia cement, construction products, environmental
applications, properties.
19.1 Introduction
Magnesium is the eighth most abundant element in the earth's crust, at ~2.3%
by weight, present in a range of rock formations such as dolomite, magnesite
and silicate. Magnesium is also the third most abundant element in solution
in seawater, with concentrations of ~1300ppm. Magnesia (magnesium oxide,
MgO) is mainly produced from the calcination of magnesite in a process
similar to the production of lime from limestone. A smaller proportion of
the world's MgO production comes from seawater and brine sources. The
principal phases of magnesium in seawater are chlorides and sulphates and
the production process is initiated by the addition of a strong base to facilitate
the precipitation of magnesium hydroxide (Mg(OH
2
)), followed by thermal
decomposition (e.g. Gilbert, 1951). The current global production of MgO is
~14 million tonnes annually (USGS, 2012), compared with that of Portland
cement (PC) of over 2.6 billion tonnes, with current costs of around ~£200/
tonne for reactive MgO (calcined), compared to ~£70/tonne for PC.
MgO is manufactured in three main relevant grades (Shand, 2006):
∑
dead burned MgO, or periclase: manufactured at temperatures of ~1400-
2000 °C, constitutes ~60% of MgO production, has the least reactivity,
highest crystallinity and lowest surface area, is used extensively in
refractory applications and is the type that is problematic in Portland
cement;
∑
hard burned MgO: manufactured at temperatures of ~1000-1400 °C,
has intermediate properties and is used in animal feeds and fertilizers
and has been extensively used as expansive additive in concrete dams
in China;
523
