1、2000 SIXTH CANMETIACI SIXIME CANMET/ACI INTERNATIONAL CONFERENCE INTERNATIONALE SUPERPLASTICIZERS AND OTHER CHEMICAL ADMIXTURES IN CONCRETE LES SUPERPLASTIFIANTS ET AUTRES ADJ UVANS CH I admixtures; concrete 1 2 Spiratos and Jolicoeur Nelu Spiratos president and CEO of Handy Chemicals Ltd., Candiac,
2、 Qu., Canada. He spearheaded the development of superplasticizers at Handy, and vigorously promoted the use of these admixtures in Canada and abroad, namely through sustained and supportive participation to international scientific events in concrete technologies. He was honored by the Canadian Boar
3、d of Trade in 1996 for his leadership in innovative technologies and received a Honorary Doctorate of Engineering from the Universit de Sherbrooke in 1997. Carmel Jolicoeur is Professor in the Department of Chemistry of the Universit de Sherbrooke since 1971; he also heads the Department since 1996.
4、 He specializes in solution and colloid chemistry, with applications in materials, particularly cementitious materials and chemical admixtures for the latter. He has Co-authored numerous papers in the area of concrete admixture chemistry, mode of action and application. He was recently recognized by
5、 ACI-CANMET for his contributions to the Chemistry of Superplasticizers INTRODUCTION Modern concrete materials and applications have come to rely strongly on chemical additives both, to enhance the properties of the fresh or final materials, and to broaden the scope of concrete technologies and appl
6、ications. Currently, quality concrete, and more so high-performance concrete (HPC), contain several chemical admixtures selected from the following partial list, and added individually or as pre-formulated combinations (in addition to clinker grinding aids frequently used in the cement production pr
7、ocess): set modifier: retarder or accelerator water reducer superplasticizer (high-range water-reducing admixture) air entraining agent corrosion inhibitor alkali-aggregate reaction control shrinkage-reducing, or shrinkage-compensating admixture anti-bleeding, -segregation or -washout admixture anti
8、-freeze additive defoamer As amply demonstrated elsewhere (13, each of these types of admixtures has shown specific benefits. Used in appropriate combinations, they have enabled, first, to meet with increasingly stringent demands on concrete strength and durability, and second, to promote greater us
9、e of secondary industrial products as supplementary materials. However, recognizing the inherent composition variability of cementitious binders, and the fact that each family of chemical admixtures comprises a variety of different chemical compounds, the Superplasticizers and Other Chemical Admixtu
10、res in Concrete 3 combined use of several types of chemical admixtures greatly increases the chemical complexity of the cementitious system. This has several immediately obvious consequences: - the design and preparation of concrete mixes must increasingly rely on adequate chemical information on th
11、e system; the probability of occurrence of chemical “incompatibility“ situations will be greatly increased. - In such a context, what trends can be anticipated in future requirements and in applications of concrete chemical admixtures? What will be the driving force underlying these trends? Any atte
12、mpt to predict the evolution of concrete admixtures must, of course, consider the current trends in all key aspects of modern concrete technologies. Given the scale, importance and impact of concrete construction technologies, “holistic“ approaches (6) must be adopted which aim the simultaneous opti
13、mization of: - material properties - - raw materials conservation - energy consumption - short- and long-term economic viability environmental impact of all components of the cementitious system and of the concrete. Many of these considerations derive, of course, from a global consensus to pursue su
14、stainable development of all technologies, as demonstrated, for example, through the recent Kyoto Protocol on carbon dioxide emissions into the atmosphere. TRENDS IN CEMENTITIOUS SYSTEMS In approximately the first two-thirds of the present century, considerable efforts were devoted to the chemical o
15、ptimization of portland cement (7,s). Through extensive studies on the hydration behavior of portland cement and its silicate (C$, C3S) and aluminate (C3A, C4AF) phases, and from detailed investigations on the role of sulfate in the hydration process, as well as on the importance of the form under w
16、hich sulfate is introduced, portland cement has evolved into a highly specified reactive mineral system. To achieve proper control of the early hydration phenomena, convenient lag phase and setting periods, adequate rate of early strength development and optimum long term consolidation behavior, the
17、 components of portland cement required extensive “fine tuning“, in accordance with selected key applications (cement types). 4 Spiratos and Jolicoeur In following developments however, various types of mineral additives were progressively introduced to supplement the reactive mineral phases in port
18、land cement (9). Such supplementary materials, typically, silica fume, blast furnace slag and fly ash, are included in the cementitious system, both to improve its mechanical and durability properties of the concrete, and to take advantage of secondary industrial products that are widely available i
19、n large quantities. Hence, in addition to drawing benefits from these secondary industrial products, the use of supplementary materials reduces the consumption of portland cement, with concomitant benefits in raw materiais conservation, energy consumption and release of COZ, i.e., from the productio
20、n of cement clinker. However, because of the inherent composition variability of the supplementary cementitious materials, the chemical complexity of the reactive system is increased and the chemical optimization developed initially in the normal portland cement system is lost. In principle, this si
21、tuation could be addressed through at least two routes: - an ad hoc reformulation of the portland cement: alternative combinations of silicates and aluminates could possibly be better suited (overall) than normal portland cement for systems containing high volumes of supplementary materials, such as
22、 high volume fly ash (HVFA) concrete; addition of appropriate chemical admixtures: the latter can be introduced in the dry cementitious system, or in the fresh concrete, to compensate for changes in the chemical composition of the supplementary cementitious materials. - In the foreseeable future, it
23、 may be expected that the second approach will be emphasized since optimization using chemical admixtures is considerably more versatile than reformulating cement compositions, particularly to correct local situations on a case-by-case basis. The addition of chemical admixtures directly into the dry
24、 cementitious system is thus likely to gain increasing acceptance. On the other hand, as chemical knowledge and practical experience accumulate on the design and application of mixed cementitious systems, the first approach (alternatives to normal portland cement), may also prove rewarding. TRENDS I
25、N CONCRETE TECHNOLOGIES In the final third of the present century, the development of concrete technologies has been mostly driven by the need to increase the performance of concrete materials and the necessity to optimize concrete production, handling, placing and curing operations in order to acce
26、lerate construction rates (1 O- 12). Superplasticizers and Other Chemical Admixtures in Concrete 5 Concrete Material Performance With respect to improvements in concrete properties, considerable progress has been achieved in several areas through a combination of experimental approaches. Some of the
27、 most significant challenges which were successfully met include: 0 increase of compressive strength and reduction of concrete permeability through reduction of pore volume which was made possible by water reducers and superplasticizers, and the addition of ultra-fine supplementary or pozzolanic mat
28、erials; improvement in the freezing and thawing resistance of concrete through the controlled introduction of a air void network using air entraining admixtures; minimization of deleterious alkali-aggregate reactions (AAR) through AAR- preventing admixture allowing the use of marginal aggregate. red
29、uction in the corrosion of concrete steel reinforcement through addition of corrosion inhibitors, and the use of sealers or water-proofing membranes; 0 minimization of microcracking due to drying shrinkage through shrinkage- compensating or shrinkage-reducing admixtures, curing compounds and more ap
30、propriate curing methods; minimization of early thermal micro-cracking in large concrete elements, by improved control of reaction rate and heat generation, using both supplementary cementitious materials and chemical admixtures. increase in concrete service life resulting from enhanced durability,
31、corrosion resistance, freezing and thawing resistance, etc. Concrete Fabrication, Handline and Placinp Techno1oe;ies In line with the “just-in-time approach adopted in many other industries, the production of concrete has been increasingly carried out using high-speed, high-shear batching systems, w
32、ith addition of chemical admixtures in the batching process. Ideally, the fresh concrete properties (slump, homogeneity, air entrained, air void system) should remain constant for sufficient time to eliminate all “on-site“ adjustments. This aspect will remain a major challenge for chemical admixture
33、s in years to come. A possible extension of concrete fabrication technologies may be in “continuous flow mixes“, allowing constant on-line monitoring and adjustments 6 Spiratos and Jolicoeur of the mix composition and properties. Such approaches would likely generate additional requirements in the p
34、erformance of chemical admixtures. Developments in other aspects of the production and handling of concrete are also largely driven by turnover rates and minimum labor requirements. In the future, these considerations will continue to favor such concrete technologies as: flowable, or pumpable concre
35、te; self-compacting concrete; “dry”, zero-slump concrete (e.g. roller compacted concretes, dry cast); anti-washout concrete for under-water concreting; shotcrete. In all of these applications, stringent requirements are imposed on the properties of the fresh concrete (rheology, segregation), and the
36、se can only be controlled through appropriate combinations of chemical admixtures. OTHER INFLUENTIAL TRENDS Several other observable trends are likely to influence the development course of concrete chemical admixtures and their applications; these trends may either be driven directly by requirement
37、s from the “field”, or indirectly, by various external factors. Some of these are identified below. Developing countries will continue to require huge quantities of “normal strength-good quality” concrete (e.g., 30-40 MPa) for housing and industrial infrastructure. Because of economic pressures, the
38、 production of such concrete will aim to minimize cement contents, using all supplementary cementitious materials locally available (natural pozzolans, fly ash, calcined clay, etc.); the admixtures required in conjunction with the supplementary materials will be selected for optimal costperformance
39、ratio, for these specific concrete materials and applications. The environmental regulations on emissions from coal-, or crude oil- burning plants are likely to become increasingly severe, for example with regard to NOx emissions. The changes in the operational conditions of the plants to meet such
40、regulations can markedly influence the physical and chemical properties of the combustion ashes (fly ash). In some cases, this results in fly ash with high levels of carbon; the latter can interact with chemical admixtures (for example, air-entraining admixtures), interfering with their action. Chem
41、ical admixtures must be able to accommodate, or even compensate for, changes in the properties of the supplementary materials. The ongoing globalization of markets for raw materials enhances the worldwide trading of clinker and supplementary cementitious materials. Hence, cement producers can grindm
42、ix materials from various sources, a practice which may tend to introduce greater variability in the properties of the resulting blends. Quite probably, chemical admixtures, added in the dry mix or in the concrete Superplasticizers and Other Chemical Admixtures in Concrete 7 batching process, will b
43、e increasingly called upon to alleviate any shortcomings of the blended cements. Recent climatic perturbations have shown unprecedented variations in temperature which may cause concern for appropriate protection of exposed concrete; this should also lead to increasing use of chemical admixtures, fo
44、r example, air entraining agents to increase the freezing and thawing resistance of concrete. Finally, the behavior of concrete exposed to intense heating, such as may occur in fires, has recently attracted considerable interest, particularly with high performance concretes( 13); the extremely low p
45、orosity of these materials results in fewer mechanisms to relieve localized expansive thermal stresses, and the escape of gaseous products formed within the mass. These situations may require development of new types of admixtures, or innovative combinations of admixtures and concrete technologies.
46、TRENDS IN CHEMICAL ADMIXTURES From the foregoing considerations, it is quite apparent that the future of concrete chemical admixtures will be subjected to many influences from within the construction industry, as well as from external sources, that may not be easily predictable; some of these influe
47、nces will, moreover, be in direct opposition. However, the direction of future trends in the development of chemical admixtures may become evident through the following simple question: Which of the various types of admixtures listed in the introductory section are absolutely essential for the produ
48、ction of high quality concrete? i.e., by opposition to those which are highly beneficial, convenient and cost-effective, but not strictly essential. For example, set modifiers, curing compounds, or corrosion inhibitors are extremely valuable admixtures; however, what they achieve could, at least in
49、part, be obtained by changing other components, or parameters, of the system ( e.g., adjusting the mixing temperature to control the setting time; modifjing the curing conditions to minimize shrinkage cracking; and using galvanized steel or stainless steel to avoid corrosion). In this perspective, there appears to be relatively few chemical admixtures that are truly indispensable for making HPC (high performance referring here to strength and durability); three such admixtures would be: high-range water- reducers (superplasticizers), air entraining admixtures and colloidal admixtures.
