505

Cement-based materials J Francis Young

Key advances toward a detailed understanding and modelling of the structure and properties of cement-based materials have been made in the past year. Significant topics that have been covered are the following: self-desiccation during curing; transport properties and modelling of transport processes; sulfate attack, including delayed ettringite formation; and the structure of calcium silicate hydrate.

Addresses Center for Cement Composite Materials, University of Illinois, 202 Ceramics Building, 105 S. Goodwin Avenue, Urbana, IL 61801, USA; e-maih jf-young@uiuc.edu

Current Opinion in Solid State & Materials Science 1998, 3:505-509

Electronic identifier: t 359-0286-003-00505

© Current Chemistry Ltd ISSN 1359-0286

Abbreviations CSH DEF ICCC IS ITZ MRS NMR rh w/c

calcium silicate hydrate delayed ettringite formation International Congress on the Chemistry of Cement impedance spectroscopy interfacial transition zone Materials Research Society nuclear magnetic resonance relative humidity water to cement mass ratio

I n t r o d u c t i o n Cement-based materials primarily cover concrete used in civil engineering structures; however, they also include materials with more advanced properties, such as fiber- reinforced cement and chemically bonded ceramics. This review concentrates on several key issues which underlie the performance of all cement-based materials regardless of their potential applications.

In 1997 the publication of several important proceedings in this area was seen, the 10th International Congress on the Chemistry of Cement [1"'] and two Materials Research Society Symposia [2°,3,4]. In addition a conference on the NMR studies of cements [5"] appeared early in 1998. Research in the field covers a broad range of topics ranging from the theoretical to the practical. This review covers selected topics which have either received considerable attention recently, or have shown particularly interesting advances in our understanding of the performances of cement-based systems in general. The review focuses on articles pub- lished in 1997 with the inclusion of some key references from earlier years.

S e l f - d e s s i c a t i o n As more cement-based materials are now being produced with lower water contents the issue of internal drying caused by water consumption during hydration under

sealed conditions (self-desiccation) becomes more impor- tant. Even when external water is available during the curing period the rapid formation of a discontinuous pore system may effectively provide sealed conditions inside the concrete, because water is removed by hydration more quickly than it can be replenished from the outside. A one day research seminar was held on this subject at the University of Lund in Sweden last June [6°]. Self- dessication can be followed by measurements of internal relative humidity (rh), which can drop to values below 80% rh [6°,7], at these low values hydration can be expected to cease. Concomitant bulk shrinkage (autoge- nous or chemical shrinkage) develops rapidly in the first 24 hours, and correlates well with decreasing rh. Self- dessication is important only at a w/c (water to cement mass ratio) <0.3 and it is greatly increased by the addition of silica fume [6°,7]. Internal tensile stresses develop around aggregates as a result of paste shrinkage and these stresses may be as high as 15 MPa, making internal microcracking a distinct possibility. Therefore the issues of self-dessication must be considered when the proper- ties of high performance mortars and concretes are under investigation. Physical and mathematical models to pre- dict the effects of self-dessication are under active devel- opment [6°,8].

T r a n s p o r t p r o p e r t i e s There is now an extensive research effort to quantify the rate at which deleterious chemicals enter concrete and ini- tiate degradation. Such quantification is a necessary step for rational service life prediction, which will be needed as the design of concrete structures becomes performance-based. The species of most interest is the chloride ion, which can initiate the corrosion of reinforcing steel. Research is focus- ing both on sophisticated mathematical modelling as well as experimental studies to improve the delineation of the links between microstructure and transport properties.

The situation is complicated by the fact that concrete will often be below the fully saturated state so that transport will be a combination of vapor and liquid diffusion. This has been addressed mathematically by Johannesson [9 °] and experimentally by Hedenblad [10], using mercury intrusion porosimetry. Johannesson has addressed the problem of nonlinear transient phenomena for porous media in general, but his interest is in the transport of C1- and CO~-, which can lead to the depassivation of steel sur- faces. He has developed equations to account for mass balance and energy conservation under concomitant con- vection and diffusion, allowing for variable pore water frac- tions. Unfortunately, the derived equations are complex and will require the development of numerical methods for their solution. Experimental confirmation of their predic- tions is still needed.

506 Ceramics, composites and intergrowths

T h e penetrat ion of chlorides through concretc is affected by a variety of material factors, which makes quanti tat ive model l ing of this process particularly difficult. Andrade and co-workers [11"] have a t t empted to model the effect of a surface skin on the diffusion of chloridc through con- crete. This skin forms because concrete at the surface of a structure is subjected to a different micro-environment during curing from the bulk concrete, resulting in changes to its microstmcture. T h e concrete was treated as a two layered material (skin and bulk) by Andrade e ta / . [11 "] and it was concluded that ignoring the presence of a skin can introduce error in the chloride diffusion profile if the skin is ei ther very thick (>0.5 cm) or very thin. T h e at,thors also concluded that a difference in binding capacity of chloride be tween skin and bulk must be considered.

Chemical binding of chloride by cement paste is of critical importance as it affects the value of the measured diffusion coefficient. It is known that the formation of calcium chh)roa luminate hydra te (Ca4AIzO7CIe . l lHzO) is an important mechanism for binding chlorides in Portland cement . Recent studies (L Tang, I , -O Nilsson, personal communicat ion; [12,13]) show that chloride binding fol- lows a Freundl ich isotherm indicating that the amount of bound chloride depends on the concentration of chloride in solution. Binding capacities relate to the total ahu-ainate content of the cement i t ious mixture, but there is still con- troversv as to whether chlorides are adsorbed onto calcium silicate hydrate (CSH).

T h e influence of the interf:acial transition zone ( ITZ) has been considered to play an important role in transport properties. Compute r simulation model l ing has shown [14] that the I T Z in a mortar becomes percolating at sand con- tents above 40% and that this should increase diffusivity and permeabi l i t> However, exper imenta l studies [15"] show that ch lor ide di f fus ivi t ies are actual ly s l ight ly r educed with increasing sand concentra t ions . Similar results have been found by Mason and co-workers at Northwestern University in studies that will soon be pub- lished. This behavior appears to be due to the fact that the influence of a percolat ing ITZ, which should increase dif- fllsivity, is more than offset by a decreasing diffusivity of the cement paste, and its dilution by the sand.

Impedance spectroscopy (IS) to de te rmine relative con- ductivi t ies has proven to be a powerful method for the quanti tat ive measurement of transport properties. Relative conduct ivi ty can be related to ionic diffusivity through the N e r n s t - E i n s t e i n equa t ion [16] and to pe rmea b i l i t y through the Ka tzThompson equat ion [17]. Using the latter approach it has been shown that a percolat ing I T Z has a large influence on permeabilit~y, in contrast to diffusivity. IS also allows calculations of the apparent dielectric con- stant (~). Moist cement pastes have values of e as high as 10 s, indicat ing that some amplification process is taking place because E for dry. cement paste is less than 10 [18] and e for water is 80. A recent paper [19 "°] has provided a

model to explain this phenomenon, which is a t t r ibuted to the formation of barriers to conduct ivi ty formed bv thc hydration products. Even though the capillary pore svstcm remains percolated, pore volumes are connccted by very small pathways which block current tim< and this phe- nomenon can lye s imulated in model systems.

Sul fa te a t tack External sulfates T h e at tack of concrete by sulfates which enter from the envi ronment has been s tudied for over 60 years and prac- tical solutions are now available for controll ing its adxcrsc effects in the field. Never the less therc is still considcrablc deba te as to exactly how sulfate at tack proceeds. In a 1995 MRS Sympos ium [2"] several papers addressed this ques- tion. "lhylor and Gollop [20 °'] reviewed the microstructur- al and chemical issues involved. T h e relativc impommce of localized expans ion versus general microstructural weakening and disintegrat ion is still a subject of debate . An earlier review by Mehta [21], which supports the latter concept is worthwhile reading. Ferraris et al. [221 have repor ted on an exper imenta l investigation that empha- sizes the progressive spatial nature of deteriorat ion as sul- fate ions diffuse inwards. Progressive forlnation of gypsunl and et t r ingi te occur leading to cracks which form roughly parallel to the exterior surf:ace. However, the authors sug- gest that chemical reaction occurs before the bulk expan- sion which causes cracking, which may support Mehta 's point of view.

T h e formation of thaumas i t e [(]aSiO,~,.(]aCO.~.(]aS() 4. 15HeO], has been implicated through laboratory studies of deter iorated concrete in some foundations. Thaumas i te is isostructural with et tr ingite and so is expected to generate similar expansive forces. It has been found that thaumasi tc can easily be formed in the laboratory [23], at low temper- atures (<15°C) in the presence of l imestone aggregates. Even sulfate-resist ing cements seem to be suscept ible to the formation of thaumasite, but addit ion of granulated blast furnace slag can inhibit its formation.

Delayed ettringite formation A current hot topic is the deter iorat ion of concrete caused by et t r ingi te formation later in the life of the structure. T h e problem involves internal cemen t sulfates onl> This si tuation was first encounte red in s team-cured precast ele- ments and can be avoided by controll ing the SO.~ content of the cemen t used and the curing temperature . Recent studies [24,25",26] confirm earlier conclusions that this reaction does not occur if curing tempera tures are below 70°C. Above this t empera tu re e t t r ingi te that forms during initial hydrat ion becomes unstable and decomposes only to reform at room tempera tu re later on. T h e stabil i ty of e t t r ingi te also depends on alkali concentrat ions in solution [27] such that the alkali content and t empera tu re are inter-related. "]'here also appears to be a close connect ion be tween D E F (delayed et t r ingi te formation) and incipi- ent alkali-silica reactions [26-28].

Cement-based materials Young 507

The mechanisms of DEF are not fully understood; one sug- gestion is that aluminate and sulfate, which would normally be used to form ettringite, are adsorbed into CSH [29,30"]. The aluminate and sulfate-rich gel is unstable if the AIzO3:SO 3 ratio is close to three and desorption occurs with subsequent formation of ettringite. Desorption is accompa- nied by expansion, and a reduction of the A1203:SO 3 ratio in CSH to a value near one.

An important question is whether DEF does occur in site- cast concrete. It has been suggested (WG Him•, personal communication; [31]) that DEF can occur at near ambient temperatures when sulfates in the clinker phases are slow- ly released during later hydration. But this claim is not sup- ported by a careful analysis of current US clinkers [32"] and by studies in the UK on correlations between cement composition and expansion [25"]. The opinion in the main is that DEF is unlikely to occur in concretes that have not been exposed to temperatures above 70°C. Concretes that reach these internal temperatures in the field due to heat of hydration are likely to show internal damage due to thermal cracking, or incipient reactions of alkali reactive aggregates. Indeed ettringite formation is usually observed in gaps around aggregates or internal microcracks and even air-voids [28,33] indicating that long-range transport of alu- mina to these moisture-rich zones is an important aspect of DEF [28]. It has been pointed out [34] that when ettrin- gite fills air-voids, concrete pavements can suffer acceler- ated freeze-thaw damage that could be erroneously attributed to DEE

Calcium silicate hydrate (CSH) The formation and structure of CSH continues to be a topic of great interest, because it is the major hydration product and binding phase in Portland cement-based composites. Being an amorphous phase of variable composition it is very difficult to study. A seminal review by Taylor and Gollop [20 °°] appeared a few years ago, but since then additional studies have been reported, z9Si NMR spectroscopy and other spectroscopic studies have been particularly fruitful [5°]; Cong and Kirkpatrick [35,36] have proposed a defect model of the tobermorite layer structure based on their NMR spectroscopy studies of semicrystalline CSH. Variations in the CaO:SiO z molar ratio are attributed to omission of bridging tetrahedra from the dreierketten units of the silicate chains leading to depolymerization. Above a CaO:SiO z ratio o f -1 .3 loss of whole segments of chains occur forming jennite-like regions. These ideas have been further confirmed by Raman and X-ray adsorption studies of the same set of samples [37°°,38°]. However, Noma et al.

[39"] have proposed an alternate scheme whereby dreier- ketten units can be interrupted by the insertion of Ca 2÷ ions into the chain. These studies were made on preparations synthesized hydrothermally at 130°C, using z9-Si NMR spectroscopy.

NMR spectroscopy has also been used to probe the more complicated structures o f CSH formed from hydration

reactions of cementitious systems. Richardson and Groves [40"] have studied CSH-phases formed in cement-slag blends. Products formed by normal hydration and alkali activation seem to be structurally similar, although the lat- ter are semicrystalline. Their results are in agreement with the defect-toberomorite structure. Aluminum is confirmed to substitute for silicon only in bridging tetrahedra and alkaline cations can substitute in the interlayer [41,42].

The above papers indicate that CSH is a 'dynamic' phase, which can vary its composition depending on the presence of pozzolans (finely divided, amorphous, aluminosilicate materials that react with lime) and temperature. Silicate chain lengths increase as the main Ca:Si molar ratio of the CSH decreases as the pozzolanic reaction proceeds [42,43]. This is in accord with a similar correlation between composition and degree of silicate polymerization with precipitated CSH. The effect of temperature on polymer- ization can be modelled using an activated Arrenhius model [43], where activation energies of 35 kJ/mole for dimerization and 100 kJ/mole for subsequent polymeriza- tion were calculated.

Conclusions This selected group of papers describes an active field in which modern materials characterization techniques are being used to help solve long standing problems. Accurate quantitative models for predicting transport behavior are critical for a realistic lifetime assessment of concrete struc- tures and considerable progress is being made on this front. Impedance spectroscopy should become an impor- tant test method for assessing transport properties of cement-based materials. The variability of the CSH struc- ture is a particularly challenging materials problem, but a comprehensive and unified conceptual model is now emerging as a result of several sophisticated studies. The relevance of delayed ettringite formation in highway struc- tures is still in dispute. This is a critical issue which requires relevant science to provide objective answers; however, the current evidence seems to suggest that DEF is playing a secondary role, and that internal damage initi- ated by another process is the primary cause.

References and recommended reading Papers of particular interest, published within the annual period of review, have been highlighted as:

• of special interest 00 of outstanding interest

1. Justness H (Ed): Proceedings of the 120th International Congress on =e the Chemistry of Cement (ICCC): 1997 June; Gothenburg, Sweden

(4 vols). Gothenburg: Congrex Got•borg AB; 1997. International Congresses are held every 6 -8 years and provide a snap-shot of current research worldwide. Review papers on selected topics are included.

2. Scrivener K, Young J F (Eds). Mechanisms of Chemical Degradation • of Cement-Based Systems: Proceedings of an MRS Symposium:

1995 Nov; Boston MA. London: E & FN Spon; 1997:1-455 This focused group of papers covers all aspects of durability from general and mechanistic aspects, to detailed studies of particular degradations (cor- rosion of steel, sulfate attack, DEF, chloride ingress). Papers on cementitious waste-forms are included.

508 Ceramics, composites and intergrowths

3. MRS Symposium on Structure-Property Relationships in Hardened Cement Paste and Composites: 1996 Dec; Boston MA. Cement Concr Res 1997, 27:issue 10.

4. MRS Symposium on Structure-Property Relationships in Hardened Cement Paste and Composites: 1996 Dec; Boston MA. Adv Cement-Based Mater 1997, 6:issues 3/4.

5. Colombet P, Grimmer A-R, Zanni H, Sozzani P (Eds): Nuclear • Magnetic Resonance Spectroscopy of Cement-Based Materials.

Berlin: Springer-Verlag; 1997:1-430. Proceedings of a Conference held in Bergamo, June 1996, presenting the latest work on NMR spectroscopy of cementitious systems.

6. Persson B, Fagerlund G (Eds): Self-Dessication and Its Importance in • Concrete Technology. Rpt. TVBM-30?5. Lund, Sweden: Division of

Building Materials, Lund University; 1997:1-265. Proceedings of a one-day seminar, which presents papers covering all aspects of self-dessication and autogeneous shrinkage of concrete. This collection of papers gives a good perspective of experimental and modelling studies of the effects of self-dessication, including volumetric strain and the development of internal stresses and microcracking.

?. Persson B: Long-term effect of silica fume on the principal properties of low-temperature-cured ceramics. Cement Concr Res 1997, 27:1667-1180.

8. Koenders EAB, van Breugel K: Numerical modelling of autogeneous shrinkage of hardening cement paste. Cement Concr Res 199"7, 27:1489-1500.

9. Johannsesson BF: Nonlinear transient phenomena in porous • media with special regard to concrete and durability. Adv Cement-

Based Mater 1997, 6:71-75. Provides a general quantitative framework for modelling transport processes in complex porous materials.

10. Hedenblad G: The use of mercury intrusion porosimetry or helium porosimetry to predict the moisture transport propert ies of hardened cement paste. Adv Cement-Based Mater 199'7, 6:123- 129.

11. Andrade C, Diez JM, Alonso C: Mathematical modelling of a • concrete surface "skin effect" on diffusion in chloride

contaminated media. Adv Cement-Based Mater 1997, 6:39-44. Discusses the origin of a 'surface skin' on concrete and its importance in controlling the ingress of chlorides.

12. Tang L, Nilsson L-O: Chloride binding capacity and binding isotherms of • P C pastes and mortars. Cement Concr Res 1993, 23:247-253.

13. Delagrave A, Marchand J, OIliver J-P, Julien S, Hazrati K: Chloride binding capacity of various hydrated cement phase systems. Adv Cement-Based Mater 1997, 6:25-35.

14. Garboczi E J, Schwartz LM, Bentz DP: Modelling the influence of the interfacial zone on the DC electrical conductivity of mortar. Adv Cement-Based Mater 1995, 2:169-181.

15. Delagrave A, Bigas JP, Olliver JP, Marchand J, Pigeon M: Influence of • the interfacial zone on the chloride diffusivity of mortars. Adv

Cement-Based Mater 1997, 5:86-92. An experimental study that links experimental measurements of diffusivity with microstructural characterization using mercury intrusion porosimetry to determine how increasing sand fractions to cement paste influence the properties of the composite.

16. Christensen B J, Coverdale RT, Olsen MA, Ford S J, Garboczi E J, Jennings HM, Mason TO: Impedance spectroscopy of hydrating cement-based materials: measurement, interpretation and application. J Am Ceram Soc 1994, 77:2?89-2804.

17. Christensen B J, Mason TO, Jennings HM: Comparison of measured and calculated permeabilities for hardened cement paste. Cement Concr Res 1996, 26:1325-1334.

18. Keddam N, Takenouti M, Novoa XR, Andrade C, Alonso C: Impedance measurements on cement paste. Cement Concr Res 1997, 27:1191-1202.

19 Ford SJ, Hwang J-H, Shane JD, Olsen RA, Moss GM, Jennings HM, • • Mason TO: Dielectric amplif ication in cement pastes. Adv Cement

Based-Mater 199"7, 5:41-48. Describes the development and testing of a conceptual model to show how restriced electrical flow within a cementitious microstructure can lead to ana- mously high dielectric constants.

20. Taylor HFW, Gollop RS: Some chemical and microstructural 00 aspects of concrete durability. In Mechanism of Chemical

Degradation of Cement Based Systems. Edited by Scrivener KL,

Young JE London: E&FN Spon; 1997:177-192. An elegant, condensed review summarizing the current status regarding the critical issues for concrete subjected to various forms of sulfate attack.

21. Mehta PK: Sulfate attack on concrete-a review. In Materials Science of Concrete III. Edited by Skalny J. Westerville, Ohio: American Ceramic Society; 1992:105-130.

22. Ferrarris CF, Clifton JR, Stutzman PE, Garboczi E J: Mechanisms of degradation of portland cement-based systems by sulfate attack. In Mechanism of Chemical Degradation of Cement Based Systems. Edited by Scrivener KL, Young JE London: E&FN Spon; 1997:185- 192.

23. Crammond NJ and Halliwell MA: Assessment of the conditions required for the thaumasite form of sulfate attack. In Mechanism of Chemical Degradation of Cement Based Systems. Edited by Scrivener KL, Young JE London: E&FN Spon; 1997:193-200.

24. Kelham S: The effect of cement composition and fineness on expansion associated with delayed ettringite formation. Cement Concr Comp 1996, 18:171-179.

25. Kelham S: The effect of cement composition and hydration • temperature on volume stability of mortar. In Proceedings of the

l Oth /CCC: 1997 June; Gothenburg, Sweden. Edited by Justness H. Gothenburg: Congrex Goteborg AB; 199?, 4:Paper 4iv060.

An extensive study correlating expansion of mortars made with cements of varying characteristics and cured at various temperatures.

26. Meland I, Justnes H, Lingard J, Smeplass S: Durability problems related to delayed ettringite formation and/or alkali aggregate reactions. In Proceedings of the 10th/CCC: 1997 June; Gothenburg, Sweden. Edited by Justness H. Gothenburg: Congrex Goteborg AB; 199?, 4:Paper 4iv064.

2?. Michand V, Nonat A, Sorrentino D: Experimental simulation of the stability of ettringite in alkali silica solutions. Produced by alkali- silicate reaction. In Proceedings of the lOth /CCC: 1997 June; Gothenburg, Sweden. Edited by Justness H. Gothenburg: Congrex Goteborg AB; 199?, 4:Paper 4iv065.

28. Bonen D, Diamond S: Characteristics of delayed ettringite deposits in asr-affected steam-cured concretes. In Mechanisms of Chemical Degradation of Cement-Based Systems. Edited by Scrivner K, Young JE London: E&FN Spon, 1997:243-251.

29. Scrivener K, Lewis M: A microstructural and microanalytical study of heat cured mortars and delayed ettringite formation. In Proceedings of the l Oth ICCC: 1997 June; Gothenburg, Sweden. Edited by Justness H. Gothenburg: Congrex Geteborg AB; 199"7, 4:Paper 4iv061.

30. Lewis MC, Scrivener KL: Microchemical effects of elevated • temperature curing and delayed ettringite formation. In

Mechanisms of Chemical Degradation of Cement-Based Systems. Edited by Scrivener K, Young JE London: E&FN Spon, 199?:243- 251.

A similar study to reference [28] which reaches similar conclusions. Both papers emphasise the detailed painstaking studies required with such com- plex materials.

31. Diamond S: Delayed ettringite formation-processes and problem. Cement Concr Comp 1996, 18:205-15.

32. Klemm WA, Miller FM: Plausibility of delayed ettringite formation - • considerations at ambient and elevated temperatures. In

Proceedings of l Oth ICCC: 1997 June; Gothenburg Sweden. Edited by Justness H. Gothenburg: Congrex Goteborg AB; 199?, 4:Paper 4iv059.

An extensive survey of the forms of sulfate that occur in US Portland cements.

33. Thaulow N, Johanssen Y, Jakobsen UH: What causes delayed ettringite formation? In Mechanisms of Chemical Degradation of Cement-Based Systems. Edited by Scrivener K, Young JE London: E&FN Spon; 1997:219-226.

34. Stark J, Bollman K: Ettringite formation - a durabil ity problem of concrete pavements. In Proceedings of the l Oth ICCC: 1997 June; Gothenburg, Sweden. Edited by Justness H. Gothenburg: Congrex Goteborg AB; 1997, 4:Paper 4iv062.

37. Cong X-D, Kirkpatrick R J: 29-Sl MAS NMR study of the structure of calcium silicate hydrate. Adv Cement-Based Mater 1996, 3:144- 156.

36. Con 9 X-D, Kirkpatrick RJ: 17-O MAS NMR investigation of the structure of calcium silicate hydrate gel. J Am Ceram Soc 1996, 79:1585-1592.

Cement-based materials Young 509

37. Kirkpatrick R J, Brown GE, Xu N, Cong X-D: Calcium X-ray • . adsorption edge spectroscopy of C-S-H and some model

compounds. Adv Cement Res 1997, 9:31-36. An excellent example of how modern materials characterization techniques can be applied to cementitious systems. Ca K-edge adsorption is used to examine tobermorite, jennite and precipitated CSH.

38. Kirkpatrick R J, Yarger JL, McMillan PF, Yu P, Cong X-D: Raman • spectroscopy of C-S-H tobermorite and jennite. Adv Cement-

Based Mater 199?, 5:93-99. Raman spectroscopy in the range 100-1200 cm -1 is used to interpret structural features of tobermorite, jennite and precipitated CSH.

39. Noma H, Adachi Y, Yamada H, Nishino T, Matsuda Y, Yokoyama T: • 2s'Si MAS NMR spectroscopy of poorly crystalline calcium silicate

hydrates (C-S-H). In Nuclear Magnetic Resonance Spectroscopy of Cement-Based Materials. Edited by Colombet P, Grimmer AR, Zanni H, Sozzani P. Berlin: Springer-Verlag; 1998:158-168.

The authors propose an interesting variant to the CSH structural model to explain a careful NMR study of hydrothermal CSH. They suggest that Ca 2+ separates silicate tetrahedra in dreierketten chains.

40. Richardson IG, Groves GW: The structure of the calcium silicate • • hydrate phases present in hardened pastes of white portland

cement/blast-furnace slag blends, J Mater Sci 1997, 32:4793- 4802.

A carefully coordinated study using TEM, mioroanalysis and NMR spec- troscopy to investigate CSH in cement paste microstructures. Results ana- lyzed in terms of earlier structural models.

41. Lognot I, Klur I, Nonat A: NMR and infrared spectroscopies of C-S-H and AI-substituted C-S- synthesized in alkaline solutions. In Nuclear Magnetic Resonance Spectroscopy of Cement-Based Materials. Edited by Colombet P, Grimmer AR, Zanni H, Sozzani P. Berlin: Springer-Verlag; 1998:189-196.

42. Justnes H: Kinetics of reaction in cementiUous pastes containing silica fume as studied by 29-SI MAS NMR. In Nuclear Magnetic Resonance Spectroscopy of Cement-Based Materials. Edited by Colombet P, Grimmer AR, Zanni H, Sozzani P. Berlin: Springer-Verlag; 1998:245-268.

43. Brough AR, Richardson IG, Groves GW, Dobson CM: 29-Si enrichment and selective enrichment for study of the hydration of model cements and blended cements. In Nuclear Magnetic Resonance Spectroscopy of Cement-Based Materials. Edited by Colombet P, Grimmer AR, Zanni H, Sozzani P. Berlin: Springer-Verlag; 1998:269-2?6.