1. Introduction
Concrete hollow blocks applications are common in many coun-
tries. However, the manufacture of these blocks does not provide
satisfactory technology control due to the need for quick release
of the lots. Brazilian and international standards (ABNT NBR6136
[1], ASTM C55-11 [2], BS 6073/81 [3]) describe hollow blocks com-
pressive test at 28 days old, but this period can be considered long
for practical purposes. Therefore, the need for more rapid testing
enables the application of non-destructive testing (ultrasound) for
materials characterization.
Prassianakis & Prassianakis [4] evaluated the ability of non-destruc-
tive testing to determine the integrity of concrete specimens, and they
concluded that this methodology is feasible for blocks evaluation.
Hamid et al. [5] performed ultrasound tests on high performance
concrete (HPC), with the addition of silica fume, on cubic speci-
mens of 150 mm side. After that, the specimens were tested in
compression. Results confirmed that ultrasonic pulse velocity
(UPV) change was more evident between 1 and 14 days and that
UPV increases only slightly at older ages (28, 56 and 90 days old).
Moreover, there is an actual need to adopt practices to minimize
environmental impacts with the use of admixtures in Portland ce-
ment (SNIC [6]). The use of ash from agro-industrial and pozzo-
lanic materials allow reducing the Portland cement consumption,
thus making cement industry more sustainable. Additionally, ce-
ment replacement by ashes also improves the mechanical charac-
teristics and the durability of cementitious matrices.
According to Mehta and Monteiro [7], the highly reactive pozzolan,
even when present in small contents (30%) in a cementitious ma-
trix, is able to consume almost completely calcium hydroxide, thus
improving not only the mechanical strength but also the resistance
to acid and sulfate attacks.
Researchers studied the use of silica fume in concrete, comparing
the results of destructive and non-destructive testing (ultrasound).
They observed that the mixtures with higher percentage of silica
fume (30%) had smaller UPV when compared with mixtures with
10% and 20% silica fume. This can be justified because, with silica
fume utilization, concrete becomes less dense, despite showing
the same compressive strength (Hamid [5]).
Cement-based materials are heterogeneous mixtures, and the evo-
lution of its strength over time is not properly understood. However,
the use of non-destructive methods, such as the ultrasound moni-
toring of structural changes, could be a quick and efficient way to
predict material behavior (Beraldo [8], Teixeira [9], Beraldo [10]).
The aim of this study was to evaluate cementitious matrices prop-
erties by partial replacement of Portland cement with SF or RHA,
and to analyze their applications in nonbearing hollow blocks, test-
ed by destructive and non-destructive methods.
2. Materials and experimental program
2.1 Raw materials
It was employed a residual rice husk ash (RHA), obtained by non-
controlled burning process in rice industry located in São Paulo
State, Brazil. Silica fume (SF) was used as reference. These resi-
dues were used without any treatment.
Concrete hollow blocks were manufactured with ordinary Portland
cement (CPV-ARI-40, according to Brazilian standard NBR 5733
[11]), natural sand, gravel and a plasticizer additive.
2.2 Chemical and physical characterization
of RHA and SF
The chemical composition of residues was determined by X-ray
fluorescence (XRF), and the loss on ignition (LOI) was determined
by ASTM C-114 [12]. Mineralogical characterization of the ashes
was carried out by X-ray diffractions.
Granulometric size distributions of ashes were performed using a Mal-
vern Mastersizer 2000 apparatus, which allows an analysis of particles
by laser diffraction from 0.02 to 2000 μm, in liquid mode as dispersant,
with 10% - 15% of obscuration and ultrasonic agitation during 60 s.
2.3 Pozzolanic activity evaluation by electrical
conductivity
The development of the reaction of pozzolan/calcium hydroxide
produces insoluble products, therefore the concentration of CH
[Ca(OH)
2
] in the solution decreases. The decrease of Ca
2+
ions due
to the increase phase of CSH (calcium silicate hydrate) produces a
decrease in conductivity, which rate depends if the reactivity of the
pozzolan is higher or lower (Villar-Cociña [13]). Several authors
(Luxán [14], Tashiro [15], Payá [16]) applied the electrical conduc-
tivity test to evaluate the pozzolanic activity of a material. This test
involves taking measurements of the electrical conductivity of a so-
lution of calcium hydroxide and pozzolan, or those of RHA. Thus,
a qualitative analysis is obtained by means of an electrical conduc-
tivity curve as a function of the time for RHA and for SF.
It was added 200 mg of Ca (OH)
2
in deionized water to obtain an
unsaturated solution, at 60 °C. After stabilization of the solution’s
conductivity, it was placed 5.25 g of ash into the solution, and
electrical conductivity was obtained. Conductivity values were col-
lected every 20 s, and the test duration was 24 h.
2.4 Concrete hollow blocks manufacturing
For the hollow blocks production, three difference mixtures were
393
IBRACON Structures and Materials Journal • 2013 • vol. 6 • nº 3
M.S. RODRIGUES | G.C.S. FERREIRA | L. SHIROMA | A.L. BERALDO
Table 1 – Ratios used in the mixtures (by mass)
Mixtures
Portland cement
Water/binder
Plasticizer additive
Waste
Sand Gravel
Control
Cement/RHA
Cement/SF
1
0.9
0.9
0.5
0.6
0.7
–
0.03
–
–
0.1
0.1
–
7.6
–
–
6.1
–