Environmental Engineering Reference
In-Depth Information
and a diverse composition. When they come from
a basic magma (of basaltic, basanitic, or tephritic
composition) they are usually related to strombo-
lian-type eruptions and appear either by forming
volcanic cones or extensive blankets spread around
close examination reveals that individual par-
ticles of lapilli are sharp edged and angular, dark
coloured (dark grey to black, or dark red to orange
when the iron has oxidized), and have many inter-
connected vesicles (originally air bubbles in the lava)
grain size, depending only on the force of the wind
and on the distance from the volcano vent.
These lapilli have cryptocrystalline to micropor-
phidic textures and sometimes are hypocrystalline
(containing a mixture of glass, microcrystals, and
cryptocrystals), but pure glassy texture (obsidian)
is uncommon. Moreover, the lapilli used in the
cement industry are low weathered and have a low
content of fines.
2.2
Geochemical composition and non reactivity
The lapilli used as aggregate in the canaries do
not contain reactive silica as a result of the basic
composition, which is mainly basaltic, basanitic,
and nephelinitic (with low content of sio
2
), and
because the texture is not glassy. They come from
ultrabasic geochemical composition magmas (<45
percent content of silica, sio
2
, and >3 percent of
alkali, nao+k
2
o), with alkaline trends.
Thus, the lack of reactive silica eliminates the
problems of alkali-silica reactivity (asR) with
Portland cement. in contrast, certain acidic vol-
canic rocks from other regions of the world, such
as rhyolite, dacite, or andesite pyroclast or tuff, are
able to produce this expansive process. it occurs
when the following factors coincide: an aggregate
with a high content of silica and glassy or poorly
crystalline texture; a wet atmosphere with 80 per-
cent or greater relative humidity, and an alkaline
source, normally present in Portland cement.
The asR process produces a very characteristic
external appearance that usually appears between 2
and 5 years; in the canary islands there has never
been a registered case of a concrete structure with
this problem, even though basaltic lapilli have been
extensively used for the last 50 years in the produc-
tion of mass concrete and concrete blocks.
3
PoRosiTY oF laPilli PaRTicles
3.1
Types of porosity
natural layers of basaltic lapilli show high poros-
ity ranging between 35 and 60 percent, which
mainly corresponds to interconnected voids (i.e.
open pores). This high porosity is the cause of
the low unit weight, but basaltic lapilli particles
also include abundant closed voids (i.e. vesicles or
closed pores) not interconnected and generated by
gas bubbles trapped in the original magma.
closed (V
cp
) and open pores (V
op
), have to be
considered because they strongly determine
important properties of this material such as dura-
bility, mechanical strength, permeability, absorp-
tion properties, etc. (de santiago & Raya, 2008).
helium pycnometry and Mercury porosimetry
are adequate techniques for such a porosity deter-
mination. santana et al. (2008) have defined four
types of pore structures (reticular, vacuolar, mixed
and matrix) and among them the most common in
ultrabasic pyroclastic rocks, which in the case of
basaltic lapilli tuffs are vacuolar and mixed pore
structures. although the consideration of a basal-
tic lapilli layer as a rock would need some degree
of welding or consolidation among lapilli parti-
cles, which are not always present, the irregular
Figure 2. Volcanic basaltic tephra cones are frequent
in the canary islands and they have abundant basaltic
lapilli. Timanfaya national Park in lanzarote island.
Figure 3. Basaltic lapilli are sharp edged and angular
particles, with a predominant grain size of 2-64 mm.
