The Ghanaian concrete industry is really a booming industry due to many infrastructural developments and the surge in residential development. However, many developmental projects that utilize concrete do suffer from the negative impact of moisture rise including paint peeling-off, bacterial and fungi growth, and microcracks as well as unpleasant looks on buildings. Such negative outlook resulting from the effects of moisture rise affects the longevity of concrete and hence makes concrete less sustainable. This study seeks to develop materials that could minimize the rise of moisture or ions through concrete medium. The experimental works performed in this study included pozzolanic strength activity index, water sorptivity, and shrinkage test. Calcined clay produced from clay was used as pozzolan to replace Portland cement at 20%. The strength activity test showed that the cement containing the calcined material attained higher strength activity indices than the control. The thermal gravimetric analysis showed that the pozzolan behaved partly as a filler material and partly as a pozzolanic material. The sorptivity results also showed that the blended mix resulted in lower sorptivity values than the control mortar. The study recommends that calcined clay and Portland cement mixtures could be used to produce durable concrete to maximize sustainability.
Rapid urbanization and population growth in this century have resulted in the high consumption of concrete which is used for road pavement constructions as well as residential and office buildings. Concrete is known to be the second highest consumed product after water [
Ghana has a growing economy and therefore there is a surge in the building and infrastructure development. The construction industry in the country depends hugely on concrete for almost every infrastructure and building development especially in the major cities. However, in recent times, many engineers and developers have been struggling with the negative impact of moisture or ions rising through the base of many structures from soils. Moisture rise also known as rising damp through concrete medium is usually not realized during the early periods of structural development. Long periods after completion of projects are rather seen sometimes. The impact of rising damp or moisture rise in building projects especially at the completion stage could be very unpleasant, worrying, and very expensive to fix.
Moisture rise in concrete is caused fundamentally by capillary suction in porous building materials [
The extent of moisture rise through concrete medium could be related to the type of materials used for concrete production. Proper and intelligent use of Portland cement and pozzolans are known from literature to be one of the possible ways to control moisture rise through concrete [
The materials used for the study included Portland cement, powdered calcined clay, sand, a high range water reducer (HRWR), and potable water. The Portland cement was obtained from Ashgrove in Chanute, Kansas, the United States, which satisfied the ASTM type I/II type cement. Clay used was sourced from the Nyamebekyere area in the Ashanti region of Ghana. The HRWR used was a polycarboxylate liquid, Glenium 7500 obtained from BASF Company in the United States. The sand used was in conformity with the ASTM C778 standards. The potable water was from the running tap of University of Missouri-Kansas City (UMKC). The cement and the clay chemical compositions are illustrated in Table
Chemical compositions of Portland cement and clay.
Property | ASTM type I/II | Raw clay |
---|---|---|
| ||
Fineness (m2/kg) | 401.7 | |
Specific gravity | 3.13 | |
| ||
SiO2 (%) | 20.49 | 59.70 |
Al2O3 (%) | 4.26 | 25.53 |
Fe2O3 (%) | 3.14 | 5.22 |
CaO (%) | 63.48 | 0.16 |
MgO (%) | 2.11 | 1.37 |
SO3 (%) | 2.9 | 0.07 |
Na2O + K2O (%) | 0.49 | 2.41 |
LOI (%) | 2.2 | 4.50 |
| ||
C3S (%) | 56 | |
C2S (%) | 15 | |
C3A (%) | 6 | |
C4AF (%) | 9 |
The clay material was calcined in a laboratory furnace (Thermolyne 8000) at a temperature of 800°C. The calcined material was sieved through a 75
Mortar mix proportion for determining strength activity index.
Temp (°C) | Mix name | Mass (g) | w/b | HRWR (%) | Flow (%) | |||
---|---|---|---|---|---|---|---|---|
Cement | Clay | Sand | Water | |||||
Control | Control | 500 | 0 | 1375 | 242 | 0.485 | 0.0 | 106 |
800 | 20P800 | 400 | 100 | 1375 | 242 | 0.485 | 0.4 | 110 |
The degree of hydration was determined using the thermal gravimetric analyzer. Approximately 35 mg of the hydrated samples was used in a Mettler Toledo TGA/SDTA 851e analyzer heated to 750°C, ramping at 10°C per minute in N2 gas. The Ca(OH)2 content was deduced from the equation provided by Yu et al. [
The sorptivity test was performed in accordance with ASTM C1585. 25 mm cube mortar samples cured after 1 and 7 days were conditioned. For the conditioning process, samples were placed in a desiccator containing saturated solution of potassium bromide (KBr) placed at the bottom part inside the dessicator without making any contact with the specimen. KBr was used to maintain a high humidity environment (about 98%). The desiccator and content were placed in an oven at a temperature of 50°C for 3 days. After the three days, the dessicator and content were removed from the oven. Mortar samples were placed in sealable transparent plastic containers and placed in an environmental chamber maintained at 50% humidity and 23°C for at least 15 days. The sealed plastic bags were removed from the chamber after the conditioned period and samples taken out from the sealed plastic bags for their weight measurement. The sides and the top part of the specimens were covered with a black duct tape leaving the side opposite to the top part uncovered. This was done to allow water to flow in one direction. The mass of the covered specimen was recorded as the initial mass of water absorption. A support device was placed at the bottom of a rubber container (shoe box) and filled with tap water up to the height of the support device. The uncovered portion of the mortar specimen was placed on the support device while the water level was increased to about 2 mm above the specimen from the bottom.
The equation used for sorptivity is given below
An average of three mortar specimens was used for the absorption calculations. The gain in mass per unit area over the density of water was plotted against the square root of the elapsing time. The slope of the line of best fit of these points was taken as the sorptivity value.
Figure
Strength activity indices of Portland cement (control) and blended cement mortars.
Figure
Degree of hydration.
At 28 days of curing, the Ca(OH)2 generated from the control (Con) paste was 20% whereas that of pozzolan paste (20P800) was 16%. The decrease in the content of Ca(OH)2 at 28 days is an indication of pozzolanic reaction [
Figure
Sorptivity results of control and blended calcined clay/Portland cement mortars.
The study analyzed the performance of clay calcined at 800°C with unblended Portland cement matrix using the PSAI, TGA, and sorptivity techniques. From the results, the following conclusions were drawn: Clay calcined at 800°C and used to replace 20% by weight of Portland cement gave a higher pozzolanic strength activity than plain Portland cement. The calcined material behaved as partly a filler material at the early period of cement hydration and partly as a pozzolanic material at the late period of cement hydration. The filler effect improved the 7 days’ strength whereas the pozzolanic effect improved the 28 days’ strength. The sorptivity studies showed that mortar containing 20% calcined clay pozzolan at 800°C has the ability to refine the pore structures of cement paste. This inhibits the transport of moisture through concrete medium.
The study recommends the utilization of calcined clay pozzolan for the Ghanaian concrete industry. This could enhance concrete durability and consequently the sustainability of concrete.
The authors declare that they have no competing interests.