Saratov State Technical University,

Institute of Architecture and Road Construction

«Influence of Isomeric Carbohydrates On the Structure of Cement Composite»

By Roman PetrovResearch supervisor:

PhD, docent Evgeniy Shoshin

Saratov, 2013

IntroductionMany chemical admixtures have influence with hydration and rheology and

microstructure of cement pastes. The mechanism of these effects are yet poorly understood. The modifying effect of organic additives on the rheological and operational properties of cement systems is usually studied from the point of adsorption of organic molecules on the surface of hydrate formations, which affiliate the cement gel. The following parameters of the organic additive molecules are considered to be essential: the carbon chain length, the functional structure, the periodicity of the functional groups in the molecular chain, i.e. the parameters which characterize the molecular profile of the corresponding geometric structure of the surface and determine the possibility of adsorption and bonding strength. However, the adsorption of organic molecules on the hydrated mineral surface suggests the presence of water and its possible participation in the adsorption processes. In this case, the surface water is an element of the surface structure of a mineral particle, and therefore, it is necessary to take into account the spatial arrangement of the functional groups of the adsorbed molecules, i.e. its spatial isomerism.

Therefore, the isomers of organic molecules adsorbed on hydrosilicate gel particles will have a different influence on the pattern of the formed gel phases, their stability and the pattern of the products of their condensation.

Materials and research methods

In order to confirm the hypothesis of present research a study was carried out which involved an X-ray diffraction of cement paste samples modified by isomeric carbohydrates (arabinose, ribose, xylose). In a comparative analysis we also used control samples of the cement stone without carbohydrate content

Arabinose Ribose Xylose

Materials and research methods

Carbohydrates were chosen as a research object for several reasons:1. Carbohydrates can form stable complexes with the nuclei phase of cement;2. Carbohydrates can form hydrogen bonds, which creates the possibility of

their interaction with the surface hydrated structures;3. Carbohydrates do not change the surface tension of the solution, thus

eliminating possible distortion of hydration processes resulting from surface activity symptoms of the admixture;

4. Carbohydrate have a high variance in spatial distribution of OH-groups, which makes them a convenient modeling object.

The dosage of carbohydrates was 0.5%, which is close to the limit of the critical concentration of carbohydrates, when the changes of hydration processes are catastrophic in nature.

Fragments of diffraction patterns of unmodified samples

(1) - 1 day hydration, (2) - 7 day hydration, (3) - 28 days of hydration. * - Alito-belite phase (ABP), P - portlandite; F - СаF2 (internal standard)

Fragments of diffraction patterns of unmodified samples

A day after mixing cement with water a diffraction pattern of cement stone reveals only the initial phase of clinker: alite and belite, and a small amount of portlandite, and, at the same time, there are significant amounts of weakly-crystallized phases, which are manifested in the form of wide reflections in the areas of interplanar distance (i.d.) d = 1,47 – 0,80 nm (gel phase 1) and d = 0,33 – 0,24 nm, the latter being characteristic for cement C-S-H-(I) gel, which volume defines the strength properties of the stone.

Over time the redistribution of the reflection intensities of weakly-crystallized phases takes place: the intensity of reflection gel phase 1 is reduced (d = 1,47-0,80 nm), while increasing the reflections intensity of portlandite (d = 0.49 nm) and cement C-S-H (I) gel (d = 0,33-0,24 nm)

Diffractograms of modified samples

From left to right: fragments of diffraction patterns of samples modified by xylose, arabinose,

ribose. (1) – 1 day hydration; (2) – 7 day hydration; (3) – 35 day hydration

Diffractograms of modified samples

In the presence of carbohydrates general pattern remains the same, i.e. on the first day of hydration diffraction patterns show mainly the reflections of weakly-crystallized phases with the same diffraction angles, but the picture of the spectrum of i.d. d = 1.47-0.80 nm. in the presence of carbohydrates is significantly different from that of the unmodified sample: within this range of m.d. there are strong reflections typical for crystalline phases and textures, and each carbohydrate has its own specific set of reflexes in this range of i.d.: for xylose - 0.94 nm, 1.07 nm and a split reflection of 1.30 nm, for arabinose - 0.95 nm, 1.12 nm, 1.28 nm and 1.40 nm; for ribose - 0.95 nm, 1.12 nm. Of these reflexes only the 0.95 nm reflex is unambiguously identified as an ettringite phase, and with a slightly enlarged interplanar distance, which is apparently due to involvement of carbohydrates into the composition of the ettringite phases

The relative intensity of phase reflectionsAdditive/duration of hydration, days

Intensity ratio of analytical phase reflections, J / Alito-belite phase (ABP)

Portlandite C-S-H (I) Gel phase 1 Gel phase 2 Calcium hydro-aluminate

Ettringite

—/1 0.61 5.75 3.01 0.40 0.13 0.13

—/3 1.00 7.21 1.17 — 0.11 0.13

—/28 0.96 6.72 — — 0.17 0.10

Arabinose/2 — 1.34 0.80 0.50 0.10 0.16

Arabinose/7 0.04 3.00 1.67 0.53 0.06 0.19

Arabinose/35 0.23 4.87 0.96 1.02 0.15 0.11

Ribose/2 — 2.07 0.93 0.22 0.05 0.05

Ribose/7 0.09 2.08 0.99 0.23 0.09 0.16

Ribose/35 0.24 5.20 0.66 0.34 0.19 0.16

Xylose/2 0.06 2.43 1.00 — 0.09 0.11

Xylose/7 0.07 3.35 1.00 0.44 0.08 0.14

Xylose/35 0.18 6.78 0.52 — — 0.21

The relative intensity of phase reflections of a control sample

Quantitative evaluation of changes in relative intensities of this reflections allows us to suggest that the reflection within the range of i.d. d = 1,47 – 0,8 nm corresponds to the initial highly hydrated gel form, which eventually hardens and becomes the basic form of C-S-H (I)-gel with the reflection of i.d. d = 0.33 – 0.24 nm: so in two days of hydration the intensity of the reflections of the primary gel phase 1 is reduced by 60%, whereas the intensity of the reflections of condensed gel phase C-S-H (I) for the same period increased by 20%, and the intensity reflection of the portlandite increased by almost 40%. Reflections intensities of other phases in a given period show a considerably less alternation. Thus, major changes in three days of hydration occurred in the pattern weakly-crystallized cement phases: there is a condensation of a gel phase 1 with the formation of the C-S-H-(I)-gel structure, portlandite is crystallized out simultaneously

The relative intensity of phase reflections of a modified samples

It is significant that the content of ettringite phases in the early period of structure formation, measured by the relative reflection intensity, for all the three carbohydrates is also different - the maximum reflection intensity of the ettringite phase is typical for the sample modified by arabinose. The rest of these reflexes cannot be identified, and define an own structure of a highly hydrated gel phase 1.

Observation of the change in the profile of a gel phase 1 signal curve (i.d. 1,47-0,80 nm) also shows distinctive differences from the control sample, if in a gel phase 1 reflection intensity of the control decreases by 1.3 times sample in three days of hydration, in the modified samples during 7 days the reflection intensity either does not decrease, or even increases.

The latter indicates the stabilization of highly hydrated gel form by hydrocarbons. Such stabilization hampers the condensation of the gel phase 1, and therefore the rate of increase in the reflection intensity of the C-S-H (I)-gel is significantly lower: the intensity growth in 7 days amounted 2.24 times for arabinose, 0 for ribose, 1.37 times for xylose; for the control sample the increase in intensity of a corresponding reflection during 3 days was 1.77 times.

Conclusion

Thus, optical isomers of organic compounds are capable of influencing to varying degrees the kinetics of hydration processes and of the formation of C-S-H-(I) gel, as well as the composition of crystalline phases of the cement. The presence of changes in the phase composition of cement stone, modified by optical isomers of carbohydrates, indicates the crucial role of hydration shells surrounding the gel particles not only in the rheology of cement systems, but also in the phase formation of the stone.