The mechanical properties of sand-lime bricks produced by autoclaving under different conditions and incorporation of granulated slag have been investigated previously. In this study, the relationship is established between the structure, the phases formed and the strength. Based lime and granulated slag a new binder is developed. This is cured at saturated vapour pressures of 1.0 and 1.8MPa. The results showed a decrease in compressive strength due to the substitution. The microstructure analysis showed that reaction products consist mainly of 11Å tobermorite and xonotlite. Also, when increasing the autoclave temperature, it results in an increase in xonotlite relative to tobermorite. The X-ray diffractions of these phases are very low hardly visible, they are masked by the presence of quartz. Their intensities increase with the presence of slag.
Calcium silicate brick is considered to be one of the advanced building materials in world and is made by hydrothermal reaction between sand or siliceous materials and lime. Despite many previous studies, several aspects of the reaction are still incompletely understood [1-3]. Under saturated vapour pressure and a temperature varying between 170 and 200 °C, fine-grained quartz (which is insoluble at ambient temperature) becomes chemically active and reacts with hydrated lime “Ca (OH)2”. This gives hydrated calcium silicate that is solid, resistant and insoluble in water . These phases are often regrouped in the tobermorite appellation. The best known being the 14Å, 11Å or 9Å tobermorite; some are termed ‘normal’ while the others are called ‘anomalous’ with a Ca/Si ratio of 0.8-1.0 [5-7].
By virtue of its chemical composition, which is close to that of cement, the blast furnace slag can also be used instead of lime in the sand-lime materials. The rapid cooling of the slag results in a metastable glassy structure that favors its reaction with lime under specific conditions . The blast furnace slag is known to possess latent hydraulic activity; i.e., it shows cementitious properties when in contact with water over a long period of time . A chemical activation is necessary to start germination . In addition, slag can also be reactive by thermal activation (in steam room and autoclave) .
Granulated blast furnace slag (GBFS): The granulated blast furnace slag was provided by ArcelorMittal steel factory
(situated in the east of Algeria). The chemical characteristic of slag is shown in Table 1. The slag was ground to a fineness of about 350 m2/kg (by Blain’s method) in a laboratory ball mill. Figure 1 Indicates the XRD patterns of the BFWS used. The crystal structure of GBFS is almost entirely amorphous since the XRD peaks are hardly identified. The angular band between 25° and 35° (2θ) could be attributed to a significant proportion of amorphous structure (glass). However, traces of mellilite, merwinite and probably monticellite at 2θ = 27.7°, can well be distinguished on more crystallized portions of granulated slag. It is likely that some slag agglomerates underwent partial crystallization during the cooling process. In addition to these minerals, traces of quartz, calcite and iron oxide are present [12-14].
Hydrated lime: The quick lime is also collected from the ArcelorMittal Steel plant. Extinction and grinding were carried out at the Civil Engineering Research Laboratory, Annaba University. The chemical composition of the hydrated lime is given
in Table 1. The X-ray diffraction diagram (Figure 2) shows a mineralogical
composition of the hydrated lime primarily composed
of portlandite Ca (OH)2. The presence of calcite is also noted.
This is likely to be the result of carbonation of the portlandite.
The brucite Mg (OH)2 (magnesium hydrate) is also identified: the
limestone being slightly dolomitic.
Sand: The sand used in this work is collected from the siliceous
dune sand of El-Kala area, eastern Algeria. It was ground
to a specific area of 232.5m2/Kg and its chemical composition is
given in Table 1.
Testing: To characterize the autoclaved sand-lime bricks,
the cylindrical samples of 50mm in diameter and slenderness
ratio of 2 were made by pressing in a hydraulic machine at a
pressure of 20MPa. On the basis of previous study (Arabi 1988),
a mixture of 20% hydrated lime and 80% ground sand was then
humidified to 10% water. The granulated slag replaces hydrated
lime partially. The samples were then put in a laboratory autoclave
apparatus with 1.0 and 1.8MPa saturated vapour pressure
corresponding to temperatures of 176° and 204 °C respectively.
The cyclic treatment in autoclaving was 10 hours: 2 hours of progressively
rising temperatures followed by 6 hours conservation
at constant temperature then 2 hours of cooling by ventilation.
The average of three tests results for each mixture was taken to
characterize the compressive strength.
The mechanical strength is observed to improve significantly
when the saturated vapor pressure, inside the autoclave,
ranges from 1.0 to 1.5MPa. On the other hand, for the case of
1.5 to 1.8MPa saturated vapour pressure a slight improvement
is noticed Figure 3. A significant increase of curing temperature
can result in the phase breakdown followed by recrystallisation
of other phases with weaker properties at microscopic scale (or
at crystal scale). In the work reported by Black et al. . concerning
xonotlite and was not general to hydrothermally formed
phases, an increasing synthesis temperature led to larger crystals,
but the crystals also appeared to have split along their
length. This case is also observed in this study: there is a conversion
of the tobermorite to xonotlite. Figure 4 shows well-crystallized
tobermorite in the form of dense and entangled platelets,
whereas xonotlite is in the form of very fine interwoven needles
Figure 5. However, at material scale the interleaving of xonotlite
needles insures an improvement of mechanical strength.
The X-ray powder diffraction patterns Figure 6 shows a significant
presence of portlandite in the sample without slag. The
decrease of portlandite is induced by pozzolanic effect when
the slag partially replaces hydrated lime, is also due to dilution.
Portlandite disappears completely for mixtures to 60 and 100%
granulated slag. The XRD patterns show at used temperatures
and in the presence of slag, the poorly crystallized phases appear.
The spectral line more visible is located at 7.8° 2θ; the XRD
patterns from sample without slag do not show clearly.
It should be underlined the complexity of the CaO-SiO2-H2O
system under hydrothermal reaction and saturated vapour pressure
is widely discussed for many years. The presence of slag
with almost a complete amorphous structure under such conditions,
does not release new hydrate phases other than those
known in similar conditions. The relatively high concentration of
calcium ions is required for hydrate formation; the presence of
lime is found to be essential as an activator for slag. The mechanical
strengths are closely related to composition of mixtures. It
was estimated that the increment of temperature has not an influence
significant in improving resistance (in this study). However,
the temperature acts much more on the morphology and
crystalline degree of C-S-H. The usual forms of tobermorite and
xonotlite are distinct and are not disturbed by the presence of
slag. The investigation of phases by X-ray diffractograms is complicated
by the presence of quartz (well crystallized and intensities
that mask other phases).
Barnett SJ, Soutsos MN, Millard SG, Bungey JH (2006) Strength development of mortars containing ground granulated blast-furnace slag: Effect of curing temperature and determination of apparent activation energies. Cem Concr Res 36(3): 434-440.