This study investigates the effects of micropalm oil fuel ash (mPOFA) on compressive strength and pore structure of cement mortar. Various experimental techniques, such as compression test, isothermal calorimetry, mercury intrusion porosimetry, and X-ray diffraction, are performed to figure out the effect of using mPOFA as partial replacement of cement on the hydration of cement and determine its optimal replacement level to increase mechanical property of the mortar specimens. 10 wt.% of cement replacement with mPOFA is found to give the highest level of compressive strength, achieving a 23% increase over the control specimens after 3 days of curing. High K2O contents in mPOFA stimulate C3S in cement to form C-S-H at early ages, and high surface area of mPOFA acts as a nucleus to develop C-S-H. Also, small mPOFA particles and C-S-H formed by pozzolanic reaction fill the pores and lead to reduction in large capillary pores. In XRD analysis, a decrease in Ca(OH)2 and SiO2 contents with age confirmed a high pozzolanic reactivity of mPOFA.
The production of cement consumes an enormous amount of raw materials and energy [
As shown in Figure
Production of palm oil fuel ash (POFA).
Recent studies about POFA mostly focus on the effects of POFA on the durability and general properties of cement-based composite materials, such as the compressive strength of mortar and concrete [
Type I Ordinary Portland cement (OPC) was used in this study, which is defined by KS L 5201 [
Physical properties and chemical composition of cement and mPOFA.
Chemical composition | Cement (%) | mPOFA (%) |
---|---|---|
SiO2 | 19.47 | 70.9 |
Al2O3 | 5.24 | 5.63 |
Fe2O3 | 2.69 | 3.51 |
CaO | 61.8 | 3.78 |
MgO | 3.72 | 3.61 |
SO3 | 2.49 | — |
Na2O | 0.18 | 0.39 |
K2O | 0.87 | 5.66 |
LOI |
2.6 | 10.1 |
|
||
Specific gravity (kg/m3) | 3.15 | 2.3 |
Specific surface area (cm2/g) | 3578 | 6091 |
POFA was produced by the combustion of palm oil kernel shells and palm oil fibers at a palm oil mill, which is located in Johor, Malaysia. This POFA powder was dried in an oven at 110 ± 5°C for 24 h and then passed through a 300
Figure
SEM image of mPOFA.
The XRD result of the mPOFA is presented in Figure
XRD pattern of mPOFA.
Siliceous sand was used as fine aggregates. Its physical properties are shown in Table
Physical properties of fine aggregates.
Maximum size (mm) | Surface dry density (g/cm3) | Absolute volume (%) | Fineness modulus (F.M) | Absorption rate (%) | Specific weight (kg/m3) |
---|---|---|---|---|---|
5.0 | 2.6 | 61.2 | 2.87 | 1.02 | 1,590 |
Table
Mix proportion of control mortar and mortar specimens containing mPOFA.
Sample | W/B (%) | Replacement ratio (%) | Cement (g) | POFA (g) | Sand (g) | Water (g) |
---|---|---|---|---|---|---|
Control | 50 | 0 | 450 | — | 1350 | 225 |
POFA10 |
50 | 10 | 405 | 45 | 1350 | 225 |
POFA20 | 50 | 20 | 360 | 90 | 1350 | 225 |
POFA30 | 50 | 30 | 315 | 135 | 1350 | 225 |
POFA40 | 50 | 40 | 270 | 180 | 1350 | 225 |
The compressive strength was measured on prismatic mortar specimens with 40 mm × 40 mm × 160 mm in accordance with KS L ISO 679 using the universal testing machine (UTM) after 3, 7, 14, and 28 days of curing. Testing was carried out on six specimens at each age, and the average value was reported.
The heat of hydration was monitored for 72 h according to ASTM C1702 [
Small pieces (approx. 2 g in total) of mortar samples were soaked in acetone for 24 hours in a vacuum desiccator to stop hydration after 3 and 28 days of curing and then dried in an oven at 60°C for 24 h to remove any water. The pore structure of each sample was then examined using MIP (Autopore IV 9520) with maximum applied pressure of mercury, 400 MPa.
The crystal phases present in cement paste samples cured for 3 and 28 days were analyzed using XRD with a 2
Figure
Compressive strength of mortars containing mPOFA.
The results show that the optimal replacement rate of mPOFA is 10% by weight, and the maximum replacement rate of mPOFA is 20% by weight. When mPOFA is replaced with cement at certain levels, the small mPOFA particles fill the pores between cement particles, thereby affecting the development of compressive strength [
Figure
Heat evolution rate of mortars containing mPOFA.
The heat evolution of POFA10 was the highest in Stage 3, with the peak starting earlier than with the control. This is thought to be caused by the alkali ions of mPOFA (K+ and Na+) promoting hydrolysis by stimulating C3S and forming C-S-H during the initial period [
Figure
(a) Cumulative heat evolution of mortars containing mPOFA. (b) Relative cumulative heat evolution of mortars containing mPOFA.
Figure
Pore distribution of mortars containing mPOFA aged for (a) 3 and (b) 28 days.
After curing for 28 days, the amounts of pores larger than 0.1
Cumulative pore volume of mortars containing mPOFA aged for (a) 3 and (b) 28 days.
The pore size distribution of concrete is closely related to its permeability which governs durability. The pores can be categorized as gel, medium, large capillary pores, or air voids depending on their size [
The XRD results are shown in Figure
XRD patterns of pastes containing mPOFA aged for (a) 3 and (b) 28 days.
A schematic diagram of the role of mPOFA in a cement hydration is provided in Figure
Role of mPOFA in cement hydration.
As the elution of alkali ions is the ongoing process, the concentration of alkali ions in solution steadily increases and a Si-rich zone is formed around the surface of the mPOFA particle. Over time, SiO44− ions in the Si-rich zone are eluted by osmotic pressure and then react with Ca2+ to form C-S-H. It is thought that mPOFA particles play a role in this process by providing a nucleus for C-S-H growth due to their high specific surface area [
This study examines the effects of an agricultural waste, mPOFA, on changes in compressive strength and pore structure of cement mortar. An optimal replacement rate, 10 wt.% of cement with mPOFA is determined for maximizing its utility as a supplementary cementitious material. Based on various experimental results, the following conclusions are drawn.
A 10 wt.% replacement of cement with mPOFA yields the highest compressive strength at all ages, with a 33% increase in strength over the control at 28 days. This could be due to mPOFA filling the space between cement particles and promoting the formation of C-S-H.
Increasing the amount of mPOFA reduces the compressive strength, but the difference in strength between POFA40 and the control decreased by 27% with a prolonged curing of 28 days. This is attributed to the Si-rich mPOFA particles reacting with Ca(OH)2 formed during cement hydration to produce secondary C-S-H.
The rate of heat evolution generally decreased with increasing mPOFA content, but POFA10 exhibited a higher rate and earlier peak than the control in Stage 2. This could be the result of alkali ions eluted from mPOFA stimulating the hydrolysis of C3S.
The incorporation of 10% and 20% mPOFA produced fewer large capillary pores, which is most likely the result of filling the spaces between cement particles by mPOFA particles and newly formed C-S-H.
High levels of K2O in mPOFA stimulate the hydrolysis of C3S in cement, and mPOFA particles with high surface area acts as nuclei for hydration. Consequently, pores are probably filled with C-S-H formed by pozzolanic reaction, which makes a dense microstructure and refines the pore structure.
The data used to support the findings of this study are included within the article.
The authors declare no conflict of interest.
This research was supported by basic science research program through the National Research Foundation (NRF) of Korea funded by the Ministry of Science, ICT and Future Planning (No. 2015R1A5A1037548).