xxiii πανελληνιο ÏƒÏ Î½ÎµÎ´ÏÎ¹Î¿ Ï†Ï ÏƒÎ¹ÎºÎ·Ï‚ στερεας καταστασης & επιστημης ...

Modification of Perlite Cementitious Samples with Nanostructured Titania
A.I. Kontos 1 , A.G. Kontos 1,2 , M. Fardis 3 , S. Kourkoulis 2 , V. Tsitsias 1 , G. Papavassiliou 3 and P. Falaras 1*
1 Institute of Physical Chemistry, NCSR Democritos, 15310, Aghia Paraskevi, Athens, Greece
2
Faculty of Applied Sciences, National Technical University, 157 80 Athens, Greece
3 Institute of Materials Science, NCSR Democritos, 15310, Aghia Paraskevi, Athens, Greece
*papi@chem.demokritos.gr
Modified cementitious construction materials such as paints, mortars, concrete etc. have recently attracted much interest,
taking advantage of the photoinduced properties of semiconducting nanoparticulate additives [1]. TiO 2 has proved to be very
effective in the reduction of pollutants such as NO x , aromatics, ammonia, and aldehydes as well as bacteria both in air-solid
and liquid–solid interface [2]. For light irradiation of TiO 2 in the near UV, with energy above its band gap, E g =3.2 eV, an
electron is injected from the valence band to the conduction band, leaving a hole in the valence band. The photogenerated
electrons then react with molecular oxygen (O 2 ) to produce superoxide radical anions (٠O 2 - ), and the photogenerated holes
react with water to produce hydroxyl (٠OH) radicals [3]. These highly reactive species are able to decompose a variety of
pollutants and kill microorganisms (bacteria and viruses). The new technology of TiO 2 modified cements aims in the
maintenance of the aesthetic characteristics of concrete structures, particularly those based on white Portland Cement (PC) by
degrading the accumulation of colored organic species which are responsible for discoloration and dirt.
Expanded Perlite (EP) is a light, insulating and fire-resisting material that used as an additive to cement, and Portland cement
with many construction applications. One of the most important is the production of insulating roof decks. Furthermore,
unexpanded crude perlite (CP) is also used as a mineral additive of cement [4]. For the first time, in this report, we present
the photocatalytic modification of perlite cementitious materials and test them for their efficiency in maintaining clean
surfaces. Properties, such as the type of the perlite-cement mixture slurries and the compression strength, as well as
processes, like the hydration and photocatalysis, have been investigated.
The cementitous samples were prepared mixing CP and EP perlite (S&B Industrial minerals S.A.) with PC (Hercules
Cements Co, 42.5N) and were photocatalytically modified using TiO 2 (Degussa P25). The proportions of materials used for
the preparation of CP (P091) concrete were water/PC=0.4, CP/PC=0.4 [4] and TiO 2 /(TiO 2 +PC)=0.05 [5]. For the preparation
of the lightweight expanded perlite concrete, the water/PC content ratio was changed to 0.96; all the rest proportions were
kept the same as before. This water/PC proportion is usually applied in commercial products [6]. Two EP samples were
examined, with grain size 0.3 (P072) and 4.75 mm (P082). The hydration process of the cement samples was followed by the
nuclear magnetic resonance (NMR) relaxometry method. 1 H NMR measurements were carried on a Brucker MSL
spectrometer operating at 200 MHz. T 1 relaxation measurements were done by applying a single 90 o pulse and recording the
relaxed magnetization at various time steps, after.
Compressive strength tests were performed on mortar discs of aspect ratio 2, height 100 mm, under quasi static displacement
control conditions. Results concerning the fracture stress are given in Table I. Addition of Degussa P25 and CP in the
cement blend samples improves the mechanical strength, while the light cementetious samples present an order of magnitude
lower compressive strength than the others. The conclusion concerning the other mechanical properties, like Poisson’s ratio,
modules of elasticity and strain energy density are of similar nature.
Comparison of the different hydration kinetic graphs can give important information on the hydration and physical properties
of the materials as well as the influence of perlite and TiO 2 on the hydration process. The dependence of T 1 upon the
hydration time for Portland Cement (PC) and the modified sample (PC+P25) is shown in fig. 1. All graphs are of similar
shape and present a step like progression where the three periods of hydration (starting, acceleration and hardening period)
are well distinguished. The critical parameters which can be extracted from the graphs are a) the starting (t s ) and the ending
time (t e ) of the acceleration period b) the plateau T 1 value in the initial period (T 1 in ) and c) the T 1 (T 1 fin ) level of steady state
response in the hardening period.
Sample σ (MPa) t s (hours) t e (hours) T in 1 (msec) T fin 1 (msec)
PC 20.36 2.4 28 230 3.1
PC +TiO 2 22.30 1.3 23 140 2.5
CP+PC 25.38 2.1 32 195 3.5
CP+PC+TiO 2 16.20 1.5 19 140 2.5
EP(P072)+PC 2.36 1.7 37 430 4
EP(P072)+PC+TiO 2 1.37 2.4 21 450 5
Table 1. Compression strength and hydration kinetic characteristics of various cementitious samples with portland cement,
expanded or crude perlite and TiO 2 . Kinetic results deduced by NMR T 1 relaxation measurements.
The hydration characteristics of the reference PC and the CP+PC samples, shown also in Table 1, are quite similar. The
addition of TiO 2 in PC or the CP+PC mixture significantly accelerates the whole process and shifts the hydration curve in the
log time scale (t s and t e decrement). Similar effect was observed in PC+TiO 2 samples [2] and indicates interactions between
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