Mohd Azrul Abdul Rajak*1, Zaiton Abdul Majid2 and Mohammad Ismail3
1 Preparatory Centre for Science & Technology, Universiti Malaysia Sabah, Kota Kinabalu, Sabah, MALAYSIA.
(E-mail: [email protected])
2 Department of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, Johor Bahru, Johor, MALAYSIA.
(E-mail: [email protected])
3 Faculty of Civil Engineering, Universiti Teknologi Malaysia, Johor Bahru, Johor, MALAYSIA.
(E-mail: [email protected])
*corresponding author ABSTRACT
The durability performance of mortar at the early age of hydration is also affected by the pores system of the hardened cement matrix. The inclusion of supplementary cementing material (SCM) in nano- sized particles might modify the durability behavior of mortars. Thus, the present study investigates the water absorption and sorptivity of mortar incorporates nano-sized palm oil fuel ash (nPOFA) at an early age of hydration. The Ordinary Portland Cement (OPC) cement was replaced with micro- sized palm oil fuel ash (mPOFA) and nPOFA in quantities ranging from 10% w/w to 30% w/w and 10% w/w to 60% w/w, respectively. The binder-to-sand ratio in mortar specimens was kept constant at 1:2.75. Water absorptivity and sorptivity tests were performed on the mortars at 28 days of curing age. It is observed that after 720 min of testing, the water absorption of 10nPOFA-40nPOFA mortars was found to be lower than that of 10mPOFA-30mPOFA mortars. Nonetheless, the water absorption for all mortars in the 720 min duration test was still above 10% by mass. According to the sorptivity test, the 20mPOFA mortar had the lowest sorptivity among the mPOFA mortars, while the 30nPOFA mortar had the lowest sorptivity among the nPOFA mortars. At this age, it is suggested that all mortar specimens are considered less durable due to the high-water absorption. It also suggested that as nPOFA is used in lieu of cement up to 40 % replacement by mass, the pores system of the hardened cement matrix improves when compared to mPOFA mortar.
Keywords: Nano-sized palm oil fuel ash, sorptivity, water absorption.
INTRODUCTION
Since 1990, palm oil fuel ash (POFA) has been studied as a supplementary cementing material (SCM) in the construction industry [1,2]. In the hardened cement matrix, POFA reacts with calcium hydroxide (CH) in the presence of water (known as a pozzolanic reaction) to generate secondary calcium silicate hydrate (CSH) and calcium aluminates hydrate (CAH) [3]. These two primary pozzolanic products could provide strength while also improving the mechanical and durability performance of cement-based products [2].
44 The water absorption and sorptivity properties of cement-based products might be used as an indication to define the durability performance of the products. The degree of capillary forces in the pores system of hardened cement matrix (mortars) to transport fluids into mortar is explained by water absorptivity [4]. Meanwhile, mortar sorptivity refers to a porous material’s ability to absorb and distribute water via its capillary system [5]. Sorptivity is determined by the rate of water absorption by mortar.
A nanoparticle is a particle having a diameter of less than 100nm [6]. Because of the high surface area of nano-sized particles, the application of nanotechnology in construction material contributes new perspectives to the products [7]. Nonetheless, there is no adequate literature on the use of nanosized-palm oil fuel ash (nPOFA) as SCM [6,8–10]. Therefore, the current work investigates the water absorption and sorptivity of mortar containing nPOFA at an early stage of hydration. At 28 days of curing age, the water absorption and sorptivity tests are performed on the mortar, including various percentage replacements of cement (up to 60% replacement) using nPOFA. The study is significant to explain the contribution of nPOFA in the development of the pores system of hardened cement matrix in the mortar at early curing ages.
METHODOLOGY Materials
Raw POFA was collected at Hadapan palm oil mill, Johor and labelled as raw-POFA (rPOFA).
The unnecessarily large were eliminated through a sieving process using a No. 100 (150 µm) sieve.
In line with ASTM C618 – 15, rPOFA was treated to a modified Los Angeles Abrasion test machine in order to get microsized POFA (mPOFA) particles. The POFA specimen was then subjected to 30 hours of ceramic ball milling in order to obtain nano-sized palm oil fuel ash (nPOFA). With a ball- to-specimen weight ratio of 10:1, the mPOFA specimen was placed into the ball mill [11]. The chemical composition of POFA and nanoparticles of nPOFA through TEM analysis were shown at Table 1 and Figure 1, respectively.
Figure 1. TEM analysis of nPOFA [3].
45 Table 1. Chemical composition of nPOFA and OPC [3].
Componen t
SiO2 CaO Al2O3 MgO Fe2O3 SO3 LOI POFA 54.8 14.0 7.24 4.14 4.47 0.71 8.5
OPC 21.45 60.98 3.62 1.22 4.89 2.30 -
Preparation of Mortar
The OPC cement was substituted with mPOFA and nPOFA in amounts ranging from 10% w/w to 30% w/w and 10% w/w to 60% w/w, respectively. These amounts were proposed based on a previous study that practised POFA in microsized particles and fly ash in nanosized particles [2,3,6,7].
The mixing, casting, and curing methods for mortars containing mPOFA and nPOFA were carried out in accordance with ASTM C109/C109M-12 [12]. Mix designs of mPOFA and nPOFA mortars for six mortar specimens per set are shown in Table 2. For mortar specimens, the binder-to- sand ratio was held constant at 1:2.75. The cement and POFA specimens were homogeneously mixed using a ceramic ball milling machine. Then, the gradient standard sand was added to the binder cement and mixed to make a homogeneous mixture. A flow table test was used to determine the workability of mortars. Water was added to the binder mixture to achieve a continuous flow of mortar in the range of 110 ± 5% by flow table test. The fresh mortars were manually mixed until they formed homogeneous pastes and subsequently casted in 50 mm × 50 mm × 50 mm cube moulds. After 24 hours of casting, the new mortars were demoulded before curing in saturated lime water at a temperature of 23 ± 1ºC until the test age.
Table 2: Mix design of OPC, mPOFA and nPOFA mortars.
Specimen Cement (g)
nPOFA (g)
mPOF A (g)
Sand (g)
OPC 500 - - 1375
10nPOFA 450 50 - 1375
20nPOFA 400 100 - 1375
30nPOFA 350 150 - 1375
40nPOFA 300 200 - 1375
50nPOFA 250 250 - 1375
60nPOFA 200 300 - 1375
10mPOFA 450 - 50 1375
20mPOFA 400 - 100 1375
30mPOFA 350 - 150 1375
Water Absorptivity Test
The water absorptivity test of mortars at 28 days of curing age was executed in compliance with BS 1881: Part 122 -1983 [13]. The mortar specimens were taken out from the saturated lime water and dried in an oven at a temperature of 85 ± 1ºC for 24 h before being placed at room temperature
46 to cool. The initial mass of mortar which is labeled as dry mass (MD) was measured. The mortar specimens were then submerged in distilled water in a container. The operation was stopped at 720 min to record the wet mass (MW) of the specimen. At testing time, the specimens were removed from the container and dried with a dry towel before being weighed. The water absorption (WA) percentage was calculated using Equation 1.
%WA =[(MW – MD) / MD] × 100 (1) Sorptivity Test
The sorptivity of mortar refers to a proclivity of porous material to absorb and diffuse water via its capillary system. The rate of water absorption by mortar is used to determine its sorptivity.
Sorptivity tests were performed on the mortar specimens after 28 days of curing. The evaluation of mortar sorptivity was executed by applying methods by Shah and Pitroda [5] and ASTM C1403 – 13 [14] standard. The dry mortar was weighed and recorded as the initial dry mass (MI) of the mortar specimens. The mortars were then immersed in distilled water up to 3 mm from the bottom of mortar specimens. The final weight (MF) of the mortar specimens were weighed after 30 min of the water uptake. The sorptivity (S) of mortar was computed based on Equation 2.
S = I / t1/2 (2)
Where I is the cumulative water absorption and t1/2 is the elapsed time between initial time and final time. The cumulative water absorption was computed using Equation 3.
I = (MF – MI) /Ad (3)
Where A is the surface area of mortars which was penetrated in water and d is the density of water.
RESULTS AND DISCUSSION Water Absorptivity of Mortars
Water absorption tests were performed to measure the degree of capillary forces in the pores system of hardened cement matrix (mortars) to transfer fluids into mortar, since this is important in determining mortar durability. According to previous research, better mortar mixes have water absorptivity of less than 10% by mass [4].
Figure 2 depicts the water absorption of OPC, mPOFA, and nPOFA mortars at 28 days of curing ages. The OPC mortar became the control specimen for the test and it recorded 10.6% at 720 min. At 720 min, 20nPOFA and 50nPOFA mortars show the lowest and the highest water absorption, which are 11.1% and 13.9%, respectively. However, during 720 min of the duration test, the water absorption of all mortars exceeds the water absorption of OPC mortar. This might be ascribed to the existence of high pores in the mortar matrix as well as the large connectivity between pores [15]. It also implies that the rate of pozzolanic reaction in mPOFA and nPOFA mortars is slow at this curing age. Nonetheless, the water absorption for all mortars in the 720 min duration test surpasses 10% by
47 mass. At this curing age, it is suggested that all mortar specimens are considered less durable due to the high-water absorption. Another finding was that the water absorption of 10nPOFA-40nPOFA mortars was lower than that of 10mPOFA-30mPOFA mortars after 720 min of testing. This might be linked to the fineness effect on hydration rate. Compared to micro-sized POFA particles, nanoparticles accelerate cement hydration and pozzolanic reaction. As a result, high hydration products and secondary CSH are produced in nPOFA mortars, enhancing the porosity system in the cement matrix.
Figure 2. Water absorption of mortars in 720 min period test at 28 days curing age
Figure 3. Sorptivity of OPC, mPOFA and nPOFA at 28 days curing age.
Sorptivity of the Mortars
Sorptivity refers to a mortar’s ability to absorb and transmit water through porous solids by capillary suction [5]. According to ASTM C1403 – 13 [14] and Shah and Pitroda [5], the acceptability limits for mortar sorptivity shall not exceed 0.77 mm/min(1/2).
Figure 3 depicts the sorptivity of OPC, mPOFA, and nPOFA mortars at 28 days of curing ages.
At 28 days curing age, the OPC mortar was used as control specimens for the sorptivity test, as the specimen recorded 0.42 mm/min(1/2). The 20mPOFA mortar has the lowest sorptivity value of 0.24 mm/min(1/2) at 28 days of curing age compared to the other mPOFA mortars. Meanwhile, the sorptivity of 30nPOFA and 60nPOFA mortars, which are 0.24 mm/min(1/2) and 0.38 mm/min(1/2), are the lowest and greatest in the nPOFA mortars, respectively. In addition, it has been observed that the sorptivity of all mPOFA and nPOFA mortars at 28 days curing age is lower compared to the sorptivity of OPC mortar. The high sorptivity of OPC mortar at 28 days curing age is due to the low degree of cement hydration process, as it led to a low progress in microstructure of cement matrix. At this point, the cement matrix still possesses the high capillary pores and empty voids. The high sorptivity of OPC mortar at 28 days curing ages is attributed to the low degree of cement hydration process, which resulted in low degree in the refinement of cement matrix microstructure. The cement matrix still has significant capillary pores and unfilled voids during this age.
As shown in Figure 3, the 20mPOFA mortar has the lowest sorptivity among the mPOFA mortars, while the 30nPOFA mortar has the lowest sorptivity among the nPOFA mortars at 28 days
10.6
12.1 12.313.0
11.4 11.1 11.3 11.7 13.9
11.2
0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0
Water Absorption (%)
Mortars specimens
0.42
0.3
0.24 0.25 0.3
0.26 0.240.25 0.31
0.38
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
Sorptivity, mm/min(1/2)
Mortar specimens
48 curing age. It has been proposed that the fineness of POFA affects the sorptivity of mortar. In addition, it is suggested that a 30% w/w cement replacement with nPOFA could provide the optimum cement hydration and pozzolanic reaction. However, it can be considered to undergo up to 40% replacement [6]. At this amount of cement replacement, nPOFA could improve the microstructure of the cement- based product’s hardened cementitious matrix [6]. It is also proposed that the sorptivity of 30nPOFA mortar is due to the high pozzolanic reactivity and filler effect. As a result, the sorptivity value of 30nPOFA is the lowest when compared to the other mortar sorptivity values. This agrees with other researchers [16–18], as they claimed that the nanosized SCM accelerates the pozzolanic reactivity and filler effect due to its small diameter particles and high specific surface area. The high formation of CSH and the secondary CSH in the hardened cementitious matrix is responsible for the pore size refinement.
CONCLUSION
Based on the findings, it can be concluded that the presence of POFA as SCM in the mortar at 28 days curing ages does not completely enhance the durability behavior of the mortar since the water absorption of all mortars is higher than OPC mortar. Yet, the nPOFA mortar shows lower water absorption compared to mPOFA mortars. It is suggested the nanoparticles produce the compact microstructure of a hardened cement matrix. In addition, a high cement replacement using nPOFA up to 40% by mass enhances the sorptivity of mortars at an early stage of hydration. The nano-sized particles improve the responsibility of POFA as the filler and pozzolanic material. As a result, the pores system and microstructure of the hardened cementing matrix are refined.
ACKNOWLEDGEMENT
The authors express their gratitude to Universiti Malaysia Sabah and Universiti Teknologi Malaysia.
The author also wishes to thank the staff of the Department of Chemistry, Faculty of Science and Department of Structure and Materials, School of Civil Engineering, Faculty of Engineering, Universiti Teknologi Malaysia.
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