This book is number 10 in a series of 12 under the heading Modern Concrete Technology Series. The book consists of nine chapters and includes 1069 references in 213 pages. This is some five references per page, better than most academic papers! In this regard the book is an excellent source of focused reference.
Sulphate attack is a difficult topic that is multi faceted embracing both chemical and physical changes. The authors seem somewhat apologetic for their efforts, but they most certainly should not be so. On the contrary they should be applauded. The book brings together a wealth of information and opinion that would otherwise be difficult to amass. The point is well made at the beginning of the book that concrete is abused both by man and nature and one can forgive the thoughtlessness of nature but not that of man. The information in this book reduces the problem to a manageable engineering issue, since most of the solutions to the problems are known but not always acted upon.
The point is made several times that we should design concrete for particular end uses and acknowledges the role of the environment. In 1999 1.603 billion tons of cement were made globally resulting in concrete being the most widely used material in construction. China is by far the largest producer of cement, being five to six times that of the USA.
The resulting concrete is subject to many abuses and sulphate attack is just one. Sulphate attack is complex and tends to be divided into internal or external forms. This duality runs throughout the book with both explanations and recommendations reflecting the origination of the sulphate attack. Having introduced the subject, Chapter 2 is concerned with Portland clinker based cements coupled with aggregates, mineral additives and admixtures. At long last there is recognition of the role of water and the merits of low water : cement ratios. Emphasis is on the chemically active/reactive nature of hydrated cement, resulting in dismissing the myth that concrete can be considered as inert.
Chapter 3 deals with concrete deterioration and identifies the physical and chemical attacks that exist—it is surprising that concrete lasts at all. The means of ingress and the presence of moisture are key components. In the absence of both of these there would be no deterioration, although abrasion is an exception.
There is a clear emphasis on concrete being made correctly (mix design) and processed correctly (compacted and finished). The message is clear that in the majority of cases sulphate attack can be avoided.
Chapter 4 is the heart of the book and is entitled ‘Sulphate Attack’. It is by far the longest chapter, some 84 pages out of 217 (39%) with 13 pages of references totalling 114 in number.
In the last 50 years the fineness of cement has increased as has the sulphate content of ASTM cements. Sulphate may also arise from sulphides in the aggregate; again the split between internal and external sulphate attack. Dealing first with internal. Sulphate attack has been recognised for over 100 years. Detailed descriptions are given of its various forms. Expansion is preceeded by shrinkage. Low w/c concrete may be more susceptible to sulphate attack since the binding matrix is denser and has less residual volume to accommodate expansion. Delayed ettringite formation (DEF) is covered but is rather a special case being associated with heat induced sulphate attack. It should be remembered that ettringite formation is not expansive per se, it is the conditions under which it is formed that causes the resulting damage.
In cases of external sulphate attack, such as from groundwater, the sourcing is important. The presence of sodium and potassium (alkali sulphates) can interact with the monosulphate that was formed originally from the ettringite to produce more ettringite. In the case of sulphate being associated with magnesium, this reacts with the lime to produce brucite (magnesium hydroxide) and calcium sulphate plus the decomposition of the calcium silicate hydrates to amorphous hydrous silica.
One should also not forget the role of sulphuric acid, both in acid attack as well as sulphate. This can arise from waste water, oxidation of pyrites, sewers and industrial environments. Associated with sulphuric acid attacks are those of amonium sulphate giving rise to similar products to sulphuric acid producing calcium sulphate and amorphous hydrosilica.
The reaction of sulphate with both silica and carbonate—thaumasite formation is covered under the acronym TSA (thuaumasite sulphate attack) and results in a non-cohesive mass.
Sea water is also a source of sulphates giving rise to both brucite and aragonite (calcium carbonate) formation. The chapter ends with sulphate resisting cements, with a discussion on fly ash's role in improving resistance due to its ability to reduce the C3A content. Slag may also be used to reduce sulphate attack. All these aspects are dealt with in considerable detail.
The consequences of sulphate attack are dealt with in Chapter 5 describing swelling, spalling and cracking and all these result in reduced mechanical properties. Again, internal and external attack are distinguished. Much data from both laboratory and field experience is presented. High concentrations of tricalcium aluminate in the cement can result in sulphate rapid attack—a matter of months. Specifying an acceptable limit of expansion is very variable. One of the problems that one has to acknowledge is relating laboratory based data with that obtained in the field. There are echoes of this concern throughout this book.
The chapter dealing with the prevention of sulphate attack, that one might have thought was a significant item, extends to only five pages with no figures or tables. However, the message is very clear: to produce durable concrete initially and that inadequacy reflects on policy in the mixer and not the concrete itself. There is an element of idealism here, since ambient conditions may not be that controllable and therefore while the concrete may be adequate to begin with, over time external conditions may change giving rise to consequential attack.
The use of sulphate resisting binders, the proper placing and curing of the concrete with water/total cementitious ratios of 0·4 together with admixtures, including the use of fly ash and slag are regarded as practical and usable options. There is some confusion between what is an additive and an admixture. It is interesting to note that barriers to prevent sulphate attack are not recommended.
At this point the book concerns itself with modelling of the deterioration processes. There are empirical, mechanistic and numerical models all with various levels of relevance and practical application. Overall the conclusion seems to be that we have some way to go to link the microstructural changes with the mechanical changes that occur in structures subject to this form of degradation. Naturally the book then moves on to case histories and again to make criticisms of not using the existing knowledge correctly in order to avoid the problem to begin with. The merits of low permeability, low w/c ratio, adequate thickness, increased cement contents and the use of sulphate resisting cement offer solutions. Several anecdotal examples highlighting the problems in southern California that could, it would seem, have been avoided with good practice. Considerable detail on the recent example in the UK of thaumasite sulphate attack at the Tredington–Ashchurch bridge in Gloucestershire. This was a particularly severe example of thaumasite attack giving rise to surface softening and the manifestation of visible white halos of thaumasite around the dolomitic coarse aggregate.
The final chapter deals with the assessment of sulphate attack suspected cement and concrete performance. Again procedures developed to address the issues of internal or external sulphate attack and table 9·1 based on BS 5328 is presented in some detail and includes cement types with and without PFA and GGBS as well as various types of exposure condition and form of construction. This chapter also includes acid attack and covers full and partial immersion testing as well as wetting and drying cycles. Unfortunately, test procedures can be very prolonged and linking the laboratory data with field responses can be obscure. Solutions are to be found, allegedly, in ASTME632.
This very concise and compact volume has a useful if somewhat limited index. It has been written for students of materials, practitioners and researchers and I have to say is not for the faint hearted. As well as being prolifically supplied with references the volume contains some 68 figures and 17 tables of data. A necessary volume for the serious concrete researcher and technologist. Other volumes in this Modern Concrete Technology Series are also worthy of our attention.
