Thermal treatment of desert sand to produce construction material.

Abstract

The concrete and reinforced concrete industry has refrained from using desert sand. The reasons for this are the rounded form of the particle, its grading and its chemical composition. This article provides an overview of the background, the constructability and technological challenges that the construction industry faces regarding construction sand’s shortage. Also, construction aggregates consumption and its influence on fresh and hardened concrete properties are described. The paper concludes by presenting several feasible approaches and provides a feasible technological solution.

Full text

Thermal treatment of desert sand to produce construction material.
Frank Neumann1, Manfred Curbach2
1, 2 Technische Universität Dresden, Institute of Concrete Structures, Dresden, Germany
Abstract. The concrete and reinforced concrete industry has refrained from using desert sand. The reasons for
this are the rounded form of the particle, its grading and its chemical composition. This article provides an
overview of the background, the constructability and technological challenges that the construction industry faces
regarding construction sand’s shortage. Also, construction aggregates consumption and its influence on fresh and
hardened concrete properties are described. The paper concludes by presenting several feasible approaches and
provides a feasible technological solution.
1 Demand for construction aggregates
The most common construction material in the world is
concrete. About 8 billion m³ of concrete were produced in
2015 [1]. This means that nearly 5 billion tons of the fine
aggregate fraction, denominated as sand was consumed for
concrete construction in 2015. In Germany, the
consumption is estimated at 253 million tons of sand and
gravel [2] per year. In addition to its use as a construction
material, sand is often used for artificial land reclamation,
especially in China, Singapore and the United Arab
Emirates (UAE). The worldwide amount of sand used,
along with concrete production included is calculated
between 15 and 40 billion tons per year [3, 4].
2 Aggregates for concrete casting
The shortage of construction sand, the economic
disadvantage of sand imports and the regional
unavailability of sufficient construction sand had led
several researchers involved in concrete and mortar
production to investigate the feasibility of substituting fine
aggregates by desert sands.
Concrete consists of 20 % water, cement and 80 %
aggregates. Aggregates with different diameters (Figure 1)
are chosen for concrete production to achieve a stable
granular structure. The appropriate selection of the
material is important especially for fresh concrete as it
prevents aggregate separation and sedimentation. To
reduce the amount cement and water needed, to minimize
pores and to reduce shrinkage and the risk of carbonation
of the hardened concrete [5], appropriate grain curve has
to be chosen. Because of this, nearly a third up to a half of
the used aggregates have a maximum diameter of 2 mm.
Figure 1 compares different packing rates and illustrates
the effect of stabilization by using different grain sizes for
concrete production.
Fig. 1: Structures packed with different tightness to ensure
stability within fresh concrete: homogeneous (left) and poorly
graded grain (right). (Graphic: F. Neumann)
The specific surface and the shape of aggregates
influence the water consumption and workability of fresh
concrete. For this reason, the above-mentioned grain
curves and aggregates with corresponding surface and
shape have to be chosen according to the requirements and
properties of the concrete. Desert sand grains are mostly
distributed in a monomodal manner. This means grains
with one specific diameter are dominating. Thereof only a
single fraction of fine aggregates in the ordinary concrete
mixture can be substituted to obtain the required grading
curve. The shape of desert sand grains, in comparison to
aqueous grains, is mostly round. It is caused by aeolian
transport effects and varies from equant over elongated to
flaky and between very angular and well-rounded. This
affects the mechanical interlock between aggregates and
thereof the stability of the mixture as well as fresh
concretes properties.
MATEC Web of Conferences 149, 01030 (2018) https://doi.org/10.1051/matecconf/201814901030
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© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution
License 4.0 (http://creativecommons.org/licenses/by/4.0/).

(a) 0.5 mm to 1.0 mm fraction from Libya.
(b) 0.5 mm to 1.0 mm fraction from Algeria.
Fig. 2: ESEM analysis of desert sand grains. (Photos: Simone Hempel, TU Dresden)
Up to a certain extent, the workability is improved
with benefits to sedimentation. Figure 2a shows well-
rounded desert sand grains from Libya with a diameter
between 0.5 mm and 1.0 mm studied via environmental
scanning electron microscopy (ESEM). Figure 2b shows
grains with the same diameter from Algeria with a slightly
edged shape and thereof a higher surface. Concrete
properties are also affected by desert sand’s chemical
composition. The solubility of certain salts contained in
aggregates and cement phases depend on the pH value of
the pore solution which is affected by adsorption and
desorption of dissolved carbonates, chlorides and sulfates
[6]. On the one hand, dissolved chloride ions are leaching
the portlantide. On the other hand, chloride or sulfate ions
that are present dissolute in the mixing water and react
with the tricalcium aluminate to give Friedel’s salt and the
ettringite, respectively [7].
Some researchers investigated concretes made with
desert sand aggregates and compared the mechanical
properties with those of standard concrete. Although the
workability was improved and the variation in air content
was negligible while w/c-ratio was constant, a reduction in
compressive and tensile strength of 16 % [8, 9, 10] was
found. Depending on the substituted amount of fine or very
fine aggregates, a reduction of about 21 % was measured.
By substituting 100 % of the fine aggregates with dune
sand, the compressive and tensile strength of self-
compacting concrete (SCC) were reduced to 50 %
compared to an SCC made of crushed sand [11].
Another approach to meet the challenge of a
construction sand shortage comes along with the idea of
reducing the needed amount of concrete to prevent steel
reinforcement from corrosion. The layers of concrete
surrounding the steel are thicker than statically necessary.
The objective of the new material called carbon reinforced
concrete, based on substitution of common steel
reinforcements with carbon fiber reinforcements, is to
reduce the amount of concrete and thereof the amount of
cement [12, 13, 14] and aggregates. Along with that, the
concrete composition has to be adjusted in order to meet
the requirements of carbon reinforced concrete. Typical
mesh sizes of textile reinforcements are between
10 mm and 14 mm which prevent ordinary aggregates
from penetrating trough the grid. Mostly, the used concrete
is based on a grain curve with a maximum grain size of
2 mm.
Fig. 3: Comparison of mass fractions between common
concrete and fine aggregate concrete. (Graphic: F. Neumann)
In Figure 3, a common concrete mixture for steel
reinforced concrete with a maximum grain size of 63 mm
is compared to an optimized fine graded concrete mixture
for carbon fiber reinforcements with a maximum grain size
of 2 mm. The substitution of 20 % of common steel
reinforced concrete with carbon fiber reinforced concrete
will consequently raise the overall consumption of fine
aggregates by about 1 billion tons per year.
However, the substitution of fine aggregates by
desert sand comes along with a loss in strength which is
acceptable in an economical point of view unless the
strength does not fall below a certain value. Nevertheless,
most previous studies focus on fine or very fine grains
only. Shape as well as chemical composition mostly
remain unconsidered although they are affecting fresh and
hardened concrete and are varying from deposit to deposit
even among single grain sizes.
1 mm 1 mm
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3 Approaches for a technical solution
In the past, some technical solutions where
developed to modify or reshape desert grains in order to
substitute concrete’s ordinary aggregates. These technical
approaches are mainly based on a melting process
followed by a crushing process [15, 16] or on pre-shaping
the aggregate with a high voltage electrostatic field [17].
One of the challenges of the 21st century is the
construction of buildings on celestial bodies. At this
moment the use of local available resources is essential.
Several investigations focused on the available aggregates
on the moon, called regolith. Such an aggregate compares
well with terrestrial ashes [18] with grain sizes between
0.05 mm and 0.1 mm [19]. Lunar base construction
materials may be produced by direct sintering of the
regolith [20].
The method, patented by Technische Universität
Dresden [21], is based on the same process which allows
the reshaping of solids by combining single particles
(Figure 4). The sintering process, which is known in the
field of powder metallurgy, connects two or more particles
with each other without melting them.
Fig. 4: Concept model. Comparison of a round shaped single
grain with a combination of grains. (Graphic: F. Neumann)
The process occurs below the melting point meaning that
the crystalline structure of grains is kept and amorphous
states are avoided.
In Figure 5, untreated and sintered samples from two
different sites in Namibia are compared. Sample I contains
flaky, elongated angular grains (a) whereas sample II
contains more equant and rounded grains (c) which may
have led to the less porous sintered aggregate (d) compared
to the porous structure from sintered sample I (b).
(a) Sample I: Sieved grains from Namibia with a diameter
between 0.5 mm and 1.0 mm.
(b) Sample I: Sintered product from Namibia.
(c) Sample II: Sieved grains from Namibia with a diameter
between 0.5 mm and 1.0 mm.
(d) Sample II: Sintered product from Namibia.
Fig. 5: ESEM analysis of different desert sand samples from Namibia.
(Photos: Simone Hempel, TU Dresden)
0.2 mm
0.2 mm
0.2 mm 0.2 mm
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To proof the concept, desert sand samples were
gathered and analysed by grading curve, chemical
composition, melting point and pH value. The grading
curves of unsintered samples from Algeria, Libya and
Namibia are given in Figure 6 and are compared with a
standard grading curve B 63 according to the German
standard DIN 1045-2 [22].
The preliminary tests were conducted with grain size
fractions passing the 100 µm sieve. The chemical analysis
was conducted to predict the melting point and to ensure
that a protective atmosphere while processing is
unnecessary (Table 1). Then the melting point was
examined via a differential calorimetry scan (DSC)
analysis. From all samples, only the ones from Namibia
could be analysed due to the maximum operational
temperature of 1500 °C of the DCS device. Taking the
chemical composition and the melting point into account,
the parameters for the sintering process were determined
to 1250 °C and one hour under atmospheric pressure
without any protective inert gases.
Fig. 6: Desert sand grain size distribution depending on their
origin compared to standard grading curve B 63.
(Sieve analysis by Simone Hempel, TU Dresden)
After the sintering process, the chemical composition
and pH value from produced grains were analysed again to
compare them with those from the untreated material.
Identified elements and the pH value from raw material
grains (a), sintered grains (b) and a reference are compared
in Table 1. The reference grain (R) is an unsintered dune
sand from the Kharga Oasis in Egypt [8].
Table 1: EDX analysis of sintered and unsintered samples from
site I and II in Namibia including pH value comparison.
(EDX analysis by Simone Hempel, Institute for Construction
Materials and Michal Pejko, Institute of Manufacturing Science
and Engineering, both TU Dresden)
No. Elements [Atomic %] pH [-]
O Al Si S K Ca Fe
Ia 52.6 11.0 24.2 - 3.1 1.3 3.6 5.7
Ib 58.9 7.0 20.4 - 4.0 0.3 2.0 9.0
IIa 58.8 1.5 4.6 13.0 0.5 19.6 1.0 8.1
IIb 53.8 1.8 11.0 na 0.1 28.1 1.4 11.5
R 51.6 5.16 40.3 - 0.1 0.7 0.8 7.5
4 Summary
The Technische Universität Dresden has successfully
tested a process, with which different types of desert sand
were sintered to produce aggregates. The production of
aggregates with varying but distinct grain sizes allows the
entire substitution of conventional aggregates with those
made from desert sand. Within preliminary tests, the
chemical composition of different desert sands was
observed via SEM micrograph and EDX analysis before
and after the sinter process. It was found, that the pH value
of sieved, cleaned and dried samples has changed from
acidic to neutral or even alkaline. The process of sintering
in combination with the chemical composition of different
desert sands affects the porosity. In general, desert sand
can be used as a substitute for fine aggregates. The
produced aggregates with different porous structures are
shown in Figure 7.
Fig. 7: Sintered aggregates produced from desert sand by TU Dresden.
(Photo: F. Neumann)
5 Outlook
Based on the presented, first positive results, the
Technische Universität Dresden and the German
Aerospace Centre (DLR) want to conduct a research
project along with participating partners from Argentina,
Brazil, Chile and Portugal taking into account the method
of physical transformation of sand, patented by TU
Dresden. Due to the high energy consumption of the
sintering process, the project focuses on a solar sintering
technology provided by the DLR. In the project, the
partners want to focus firstly on desert sand deposit
evaluation and analysis of mono- and multimodal grain
sintering. Afterwards, a solar based sintering technology
has to be developed. Concrete and mortar development,
feasible alternative concrete made materials,
developmental and social and techno-economic studies are
also conducted.
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The authors like to thank all colleagues, especially Simone
Hempel, Michal Pejko, Johannes Trapp (TU Dresden) and
Karin Weimann (BAM Berlin) for supporting the idea with
trial testing.
Dipl.-Ing. (FH)
Frank Neumann
Research associate
Institute of Concrete Structures
Technische Universität Dresden
01062 Dresden, Germany
frank.neumann1@tu-dresden.de
Prof. Dr.-Ing. Dr.-Ing. E.h.
Manfred Curbach
Professor, institute’s director
Institute of Concrete Structures
Technische Universität Dresden
01062 Dresden, Germany
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