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Technical University of Civil Engineering
ABSTRACT. This paper reviews my previous research on hydraulic binding matrices
without cement. Pozzolanic- type reactions and various industrial wastes or local materials
have been used. Among the pozzolanic materials analysed, I am listing: fly ash, furnace slag,
diatomite, silica fume, while as Ca(OH)2 donor materials I mention: sludge of carbide and
slaked lime. Phosphogypsum was particularly used in the binding systems under research in
order to accelerate the initial hardening. Specific attention was given to the binding systems
with fly ash, outlining the existent possibilities for improving certain characteristics of
hardening structures by using various corrective admixtures for hydraulic and granulometric
purposes. The setting and hardening of the mixtures based on hydraulic admixture are
adequate criteria for assessing the overall pozzolanic activity of the hydraulic admixtures.
This paper presents optimal compositions, curing conditions, physical and mechanical
conditions of the hardening structures, pores distribution, as well as methods for the
acceleration of hardening processes in the binding systems without cement.
Keywords: Pozzolanic activity, Fly ash, Volcanic tuff, Phosphogypsum, Curing
Dr Ion Robu is a professor at the Technical University of Civil Engineering Bucharest Romania. Currently, he is the head of the chair of Chemistry and Building Materials within
the University. His research covers pozzolanic binders, possible utilisation of local materials
and industrial wastes, as well as various aspects related to concrete durability.
Modern Concrete Materials: Binders, Additions and Admixtures
332 Robu
This paper outlines major opportunities and limits in the utilisation of some hydraulic binding
matrices without cement. It is a synthesis of author's research in this area.
Hydraulic binding matrices without cement are based on pozzolanic type reactions between
active acid oxides (Si02, AI2O3) present in various industrial byproducts (fly ash, furnace
slag, silica fume) or local materials (volcanic tuffs, diatomite) and Ca(OH)2 from slaked lime
or lime substitutes (sludge of carbide [1]). These reactions give rise to cementitious calcium
hydro-aluminates and silicates with hydraulic properties.
Paper [2] details on the physical chemical and technological bases for the achievement of
these binding matrices without cement.
The utilisation of industrial byproducts as materials containing cementless binding matrices is
of a major economic and environmental interest both in Romania and abroad. The proportion
of industrial byproducts and / or local materials in the binding matrices exceeds 50% when
using hydrated lime as a base reagent and even 80% when using sludge of carbide as base
reagent (Table 1).
The binding compositions we have analysed are mainly fly ash binding compositions under
the condition of using as base reagent either slaked lime or sludge of carbide. Other similar
compositions using furnace slag, volcanic tuffs, diatomite or silica fume have also made the
object of our research.
The present paper outlines the optimal compositions of some binding matrices, the key
properties of the matrices within the reference binding system (fly ash - slaked lime or sludge
of carbide) and proposes ways for property improvement.
The materials we used during the research process have been mainly: fly ash (activity index
0.76 as per STAS 8819-88), slaked lime(hydrated lime in powder), sludge of carbide,
phosphogypsum, volcanic tuff, diatomite, silica fume, well grounded blast furnace slag,
granulated slag, poligranular quartz sand (0.08 - 2 mm), glass fibre wastes (ordinary glass)
and alkalis resistant glass fibres (zirconium glass).
Binding Systems
The process of obtaining hydraulic binding matrices required the analysis of several binding
systems, such as: fly ash-slaked lime-water, fly ash-sludge of carbide-water, fly ash-volcanic
tuff (diatomite)-slaked lime-water, fly ash-slaked lime (sludge of carbide) phosphogypsumwater. The real optimal compositions are disclosed in table 1 below.
Modern Concrete Materials: Binders, Additions and Admixtures
Hydraulic Binding Matrix
Technological Issues
The pozzolanic capacity of hydraulic additions mixed with hydrated lime has been assessed
on the basis of several physical and mechanical criteria (namely setting - hardening). The
setting time was determined on normal consistency pastes using Vicat's needle device
(diameter 3 mm and 1 mm), while the hardening capacity was estimated by the time
resolution of mechanical strengths (bending and compressive strengths).
The pastes used in the determination of mechanical strength and of other characteristics have
been stronger than normal as they contained less water (by 15 %) than normal consistency
pastes; the pieces used in the determination test were 40 x 40 x 160 mm prisms or 100 mm
sided cubes. The target compactness was obtained by vibration or vibro-pressing techniques,
making full use of the tixotropic properties of fresh mix.
The curing conditions of test pieces after striking have varied widely: air, moist air or mix
conditions. The overall maximum binding potential has been assessed based on the
compressive strength after 90 - 180 days of moist air curing.
The reference binding system has been fly ash - slaked lime - water. Sludge of carbide has
been used as a lime replacement, while the other hydraulic admixtures (volcanic tuffs,
diatomite, silica fume, slag) have been used for enhancing some pozzolanic and
granulometric fly ash properties; phosphogypsum has been equally used for increasing the
early strength of the binding matrices.
Table 1 Real optimal compositions
(Kg/m )
Fly ash
Volcanic tuff
Slaked lime
Sludge of carbide
(45 % humidity)
T h e C h a r a c t e r i s t i c s of t h e H y d r a u l i c B i n d i n g M a t r i c e s W i t h o u t C e m e n t
The reference binding matrix has been on fly ash and slaked lime. An equivalent matrix
based on fly ash has been obtained by substituting slaked lime with sludge of carbide. The
core characteristics we have analysed in optimal composition in both matrices are the
following: absolute and apparent density, porosity and pores distribution, water absorption
capacity, bending and compressive strength, thermal conductivity, wear resistance and
hardening shrinkage, mortar adhesion. Generally, the values recorded depend on the initial
moist air curing period.
Modern Concrete Materials: Binders, Additions and Admixtures
The absolute density varies within 2.19 to 2.22 g/cm . The apparent density after striking
reached 1525 Kg/m for pastes made of fly ash and slaked lime and 1485 Kg/m for
pastes containing fly ash and sludge of carbide. However, after reaching the hygrometric
equilibrium, the apparent density decreased to 1200 - 1300 Kg/m , respectively to 1150 1250 Kg/m (depending on the initial curing in moist air).
The overall porosity parameter ranged within 48 - 50 % for matrices based on ash and
slaked lime and within 50 - 54 % for those based on fly ash and sludge of carbide. The
submicron pores distribution was verified by means of a porosimeter with mercury, by
analysing the integral and differential distribution curves; we have found that the
maximum pores distribution corresponds to a 0.05 um radius (as in case of cement
pastes). However, the overall submicron porosity is 4 times higher than the normal figure
for cement pastes (320 mm /g as compared to 80 mm /g), that provides also an
explanation of the strength differences which exist among these types of matrices.
Water absorption capacity depends directly on the duration of the initial moist air curing.
The values obtained have been of 29.5 % (for a 90 day period) and of 38.8 % (for a 14
day period) in case of ash and slaked lime matrices and of 35.2 % and 45.9 % in case of
ash and sludge of carbide matrices.
The bending strength was assessed using 40 x 40 x 160 mm prisms. After a 28 day curing
in moist air, the bending strength reached 3.3 Mpa for fly ash and slaked lime pastes and
only 3.0 MPa for fly ash and sludge of carbide pastes.
The compressive strength depends mainly on the composition and hardening conditions
(Figure 1 and Figure 2). For instance, pastes containing fly ash and 25 % slaked lime and
3 % CaCh hardened in moist air for 180 days and tested at 3 year age, have indicated
compressive strengths of 40 MPa.
c o n d i t i o n s ( w i t h 14
d a y s in m o i s t air)
in m o i s t air
Figure 1 The time variation of compressive strength for ash and slaked lime pastes
Modern Concrete Materials: Binders, Additions and Admixtures
Hydraulic Binding Matrix 3 3 5
¥•> days
ordinary conditions
(with 14 days in moist air)
^ <^days
I, 1 8 Ndavs
y i >
in moist air
w £
> S
^ a y s ^ ^ . .
Figure 2 The dependence of compressive strength on the proportion of sludge of carbide
and on the hardening time in variable conditions
The hardening shrinkage at 90 days (including a 14 day moist air curing) was of 2.1
mm/m for fly ash and slaked lime pastes and of 2.3 mm/m for fly ash and sludge of
carbide pastes. As no cracks have been noticed during shrinkage, it results a good
deforming characteristic of these matrices.
The thermal conductivity coefficient at 28 day age (including 14 in moist air) has been of
0.343 W/m.K for tests made of fly ash and slaked lime and of 0.327 for those containing
fly ash and sludge of carbide; good insulation properties are thus revealed.
The wear resistance has been measured by means of a Bohme device using 10 mm sided
cubes dried at 105° C, following a 90 day curing period in moist air. The values obtained
are summarised in table 2 and are in accordance with the mechanical strengths.
Table 2 Wear resistance
W, (g/cm )
W (mm)
W (cm )
The capillary absorption shows values which are mainly dependent on the duration of
moist air curing (Figure 3). Pastes made of fly ash and sludge of carbide present an
absorption trend similar to the one illustrated in Figure 3, but with higher values for the
tests cured in moist air.
Modern Concrete Materials: Binders, Additions and Admixtures
The mortar adhesion of products based on ash and slaked lime has been measured using
plastering mortars M10 and masonry M25. The separation strength has been of 0.10 0.14 MPa in the former case and of 0.15 - 0.25 MPa in the latter. The hydraulic binding
systems without cement present a crystalline-gellic structure (revealed by electronic
microscopy) and contain cementitious hydrated compounds [2].
TIME, hours
Figure 3 The influence of moist air curing period on the capillary absorption
C h a r a c t e r i s t i c s I m p r o v e m e n t of Fly Ash B i n d i n g M a t r i c e s
The hydraulic binding matrices without cement containing fly ash and slaked lime (or
alternatively, sludge of carbide) present a series of limitations (disadvantages) that must be
taken into consideration: slow setting and hardening process, initial 1 0 - 1 4 day curing in
moist air, high strain, limited binding potential and smaller tensile and impact strength
compared to Portland cement.
A way to improve the mechanical strength of the matrices consist in mixing fly ash with
activators of pozzolanic nature (furnace slag, volcanic tuffs, silica fume, diatomite) and
slaked lime. In this way, some other properties (technological features, setting and water
stability) are also improved.
The active hydraulic admixtures react consistent with the pozzolanic and granulometric
effect. Figure 4 illustrates the role of volcanic tuff in the binding system fly ash-volcanic
tuff-slaked lime.
The maximum strength in various binary systems (fly ash: hydraulic admixture) depends on
the hydraulic activity and on the fineness of the admixture involved.
The admixture proposition in the mix containing fly ash (value relative to the maximum
strength) has been of 20 % in case of volcanic tuff, 30 % in case of diatomite and 15 % in
case of both silica fume and furnace slag.
Modern Concrete Materials: Binders, Additions and Admixtures
Hydraulic Binding Matrix 337
Therefore, based on the data obtained and the underlying principle, we consider that
structurally integrated composite matrices can be further obtained; such matrices present
superior characteristics as compared to the reference system (fly ash- slaked lime) values.
Curing in moist air [45 days]
- Fly ash (F)
- Volcanic tuff (T)
- Slaked lime (L)
> ^
L/(F+T)= 1 5 %
90 0 J 40 SO 40 70 80 90
10 20 »
Figure 4 The influence of volcanic tuff on the compressive strength
of some ash composite matrices
We have also noticed that a 10 - 15 % content of phosphogypsum may further enhance the
early strength (7 days). The favourable influence of phosphogypsum can be explained by the
formation of ettringite-type compound which can be revealed by X-Ray diffraction [3].
Similarly, in order to reduce the hardening shrinkage lean materials such as quartz sand or
granulated slag may be used. Figure 5 illustrates the influence of 0,08 - 2 m m poligranular
quartz sand on the overall shrinkage. Thus, the compressive strength of test pieces containing
sand decreased by 30 - 50 % after a 90 day cure in moist air. Granulated slag has been used
as an aggregate in the achievement by vibro-pressing of some experimental lots of fly ash and
sludge of carbide masonry elements. As mentioned, the main limitation of the hydraulic
binding matrices without cement consists in the need of an initial moist air curing period of at
least 1 0 - 1 4 days. Such limitation can be overcome through thermal treatments, such as:
steaming, autoclaving, greenhouse effect, high frequency current etc. [4]. However, the best
results have been obtained in case of curing in high frequency field [5] or under steaming
treatment [6].
Addition of sand 0.08 - 2 mm
AGE, days
Figure 5 Decreasing the hardening shrinkage by addition of quartz sand
Modern Concrete Materials: Binders, Additions and Admixtures
338 Robu
Within the optimal matrix (fly ash - volcanic tuff - slaked lime - water) we have aimed at
increasing the tensile - bending strength and the impact strength by using ordinary glass fibre
wastes and alkalis resistant glass fibres ( 1 . 5 - 3 % compared to the fresh mix and 3 - 5 cm
length). Please note that significant improvements have been obtained only when using
alkalis resistant glass fibres (Figure 6); in case of using ordinary glass fibres wastes, the
compressive strength has been inferior to that witness, due to the alkaline attack.
Zi.5-Matrix with 1.5 %
fibres of zirconium glass.
Z - Matrix with 3 % fibres
of zirconium glass.
N - Matrix with 3 % fibres
of ordinary glass.
M - Witness - matrix
TIME, days
Figure 6 The bending strength of glass fibres reinforced matrices
Based on the above data and our related comments, several conclusions can be drawn up:
1. The hydraulic binding matrices without cement available for different hardening
structures offer real opportunities for an efficient use of some industrial byproducts or
local materials in the field of construction and building materials. The basic compounds
are one or more pozzolanic-type materials and one Ca(OH) donor material, such as
slaked lime or sludge of carbide.
2. In order to obtain durable hardening struc4ftures with adequate features, certain
conditions regarding the mix composition and the hardening process should be met. An
optimal ratio between the composition with pozzolanic properties (fly ash, furnace slag,
volcanic tuffs, diatomite, silica fume or a combination of these) and the base reagent
(slaked lime or sludge of carbide) should exist, as well as an initial moist air curing of 1014 days or another replacement treatment for the acceleration of hardening.
3. The sludge resulted from the acetylene manufacturing out of carbide may be used both as
a lime substitute in mortars or as a base reagent in case of pozzolanic activity materials.
The slaked lime or sludge of carbide content should be chosen according to technical economic criteria and not based on maximal performance parameters, as the additional
strength benefit is low after a certain threshold is met (20 % hydrated lime or 25 % sludge
of carbide - the equivalent in dry powder - relative to the fly ash or mixed addition
Modern Concrete Materials: Binders, Additions and Admixtures
Hydraulic Binding Matrix 339
4. The features of the reference matrices based on fly ash and slaked lime (or sludge of
carbide) may be improved by adding more active pozzolanic materials (slag, silica fume,
volcanic tuffs, diatomite) in ratios up to 10 - 30 % according to the hydraulic and
granulometric effect. An enhance in the initial strength can be achieved by adding 1 0 - 1 5
% phosphogypsum as compared to the dried binding mix; for reducing the hardening
shrinkage and for improving the technological properties either quartz sand or granulated
slag can be used. In a similar manner, the parameters quantifying the bending and impact
strength have been improved by using (3 % as compared to fresh mix) alkalis resistant
glass fibres.
5. The hardening structures obtained based on hydraulic binding matrices without cement
have proved to have features compatible to the requirements imposed by masonry
elements or other building elements such as polymer coated slabs for irrigation canals.
1. ROBU, I. Hardening properties of fly ash and sludge of carbide, Rev. Materiale de
15(1), 1985, pp 51 - 53.
2. ROBU, I. Binding compositions with fly ash, Ph. D. Thesis, Civil Engineering Institute,
Bucharest, 1986.
3. ROBU, I., ROBU, M. Opportunities and limits in the use of phosphogypsum, Rev.
Materiale de Constructii, 1986, pp 225 -228.
4. ROBU, I. Aspects regarding the acceleration of hardening in fly ash binding systems,
Rev. Materiale de Constructii, 21(2 - 3), 1991, pp 89 - 93.
5. ROBU, I. Improving characteristics of hardening structures in fly ash binding systems,
Proceedings of The National Conference on Chemistry and Chemical Engineering,
Bucharest, 1995, pp 362 - 366.
6. NICOLESCU, L. Fly ash in construction, Ed. CERES, Bucharest, 1978.
Modern Concrete Materials: Binders, Additions and Admixtures
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