English abstract
Infrastructure construction is continuously growing due to urbanization and population
growth. This sector produces huge amounts of cement and concrete annually,
contributing to greenhouse gases, energy consumption, and resource depletion. A large
amount of carbon dioxide (CO2) is emitted in ordinary Portland cement (OPC)
production by a calcination process that requires intensive energy consumption at
around 1500 °C. Hence, introducing the sustainability concept in this industry is
crucial in reducing the carbon footprint. Different researchers around the world are
investigating various sustainable materials that can be utilized as partial replacements
for traditional construction materials, which can preserve the environment. One of
these materials is supplementary cementitious materials (SCMs), which increase the
mechanical properties of concrete/mortar, enhance the durability performance, lower
carbon emissions, and are economical.
In this study, two raw clays labeled as HMR-C and HMR-1 were collected from Al
Hamra city in Oman. These materials were utilized as SCMs, and pozzolanic material
in a partial replacement for OPC. A preparation process of screening, grinding, sieving,
and thermal treatment was conducted for both clays. The various physical, chemical,
and microstructural properties of clays were obtained by material characterization. The
X-ray fluorescence (XRF) analysis showed that the combined total of (silica, alumina,
and iron) oxides in calcined HMR-C clay is 91%, and in HMR-1 is 81%. Hence, both
clays were classified as pozzolanic. Additionally, the thermogravimetric analysis
(TGA) and differential scanning calorimetry (DSC) findings revealed that the suitable
calcination temperature of HMR-C is (650 °C and 700 °C), and in HMR-1 is (1000 °C
and 1050 °C). The calcined clay was utilized as a replacement for OPC at different
replacement ratios by weight with a constant w/c ratio of 0.30 in different mortar
mixes. The potential of utilizing the HMR-C and HMR-1 calcined clays as pozzolana
in mortar was examined in two phases. In the first phase, binary mixes were designed
and produced with replacement percentages of (7%, 12%, 20%, and 30%). Depending
in the 28-day compressive strength of this phase, an optimum replacement level of
12% and optimum calcination temperature of 650 °C for HMR-C and 1050 °C for
HMR-1 was selected for the second phase. Different composite mixtures of calcined
clay and various conventional SCMs were designed and produced in this phase. All
the binary and composite mortar samples were experimentally tested to obtain their
various properties. Immediately after mixing, a flowability test, temperature, and fresh
density were performed. Also, the compressive strength and hardened density were
tested at 1, 7, 28, and 91 days. Additionally, the flexural strength of calcined clay
mortar samples was recorded at 28 days. Moreover, the ultrasonic pulse velocity
(UPV), water absorption, and porosity were tested at 91 days. Furthermore, the effect
of submerging mortar samples in a combined solution of (5% sodium sulfate and 5%
sodium chloride) and (5% magnesium sulfate and 5% sodium chloride) for 9 weeks
was tested. Similarly, acid resistance was conducted on mortar samples at 3 months
after 28 days of wet curing; these acids are sulfuric acid, citric acid, nitric acid,
hydrochloric acid, acetic acid, and phosphoric acid. The major outcomes of this study
indicated that as the percentage of HMR-C and HMR-1 clay replacement rises in the
blended paste, the normal consistency also increases, and the setting time is higher
than OPC. It was also observed that the flowability of HMR-C decreases as the
percentage of replacement of Portland cement with calcined clay increases. In contrast,
HMR-1 calcined clay increases the flowability compared with the OPC. In binary
mixes, a reduction in the early strength at 1 day is recorded up to around -45%, and an
enhancement in strength is observed at 7, 28, and 91 days (42.6%, 21.5%, and 8.6%),
respectively, while the composite mixes resulted in lower strength compared with the
plain OPC. An increase in flexural strength is recorded in some samples of HMR-C
and HMR-1 binary and composite mixes. The porosity and water absorption are lower
in HMR-C binary mixes compared to the HMR-1 mixes, and no significant difference
is recorded in UPV. The compressive strength test after exposure to high temperature
(800 °C) showed an increased range of (3.98 to 17.7) % in HMR-C that was calcined
at 700 °C. Moreover, the compressive strength after exposure to (5%Na2SO4
+5%NaCl) and wet/dry cycles resulted in an increase in strength in both clays. A loss
in strength is recorded in all mixes of both clays after exposure to (5%MgSO4
+5%NaCl) and wet/dry cycles. Weight loss, degradation, and color change are
observed after submerging all the HMR-C and HMR-1 mortar samples in various acids
for 9 weeks. Strength increase is detected in most of the HMR-1 samples after
exposure to H3PO4 and C2H4O2 compared with the plain OPC. An increase is also
observed after the HCl, HNO3, and H2SO4 acid attack for both clays.
