Mortar with fly ash as a partial cement replacement: analysing the compressive strength and heat of hydration

. Identifying sustainable alternatives and addressing the environmental impacts of cement production are becoming increasingly vital. Alternative materials, such as fly ash, can be used as a partial replacement for cement in concrete and mortar. This study will examine the impact of early-age heat of hydration on compressive strength of mortar when cement is partially replaced by fly ash. The mix proportion used in this study was 1:3 (cement: fine aggregate) with a w/b ratio of 0.4. Furthermore, the replacement of cement with fly ash was calculated based on weight percentage proportions, ranging from 10, 30, and 50% of the cement weight. Fly ash can be effectively used as a substitute for cement to reduce thermal hydration and maintain acceptable levels of compressive strength. As the fly ash substitution rate increased, the thermal hydration of the samples decreased. Nevertheless, the increased strength level may serve as a counterbalance for the initially reduced strength of the mortar that contains fly ash.


Introduction
In the construction industry, concrete is widely used to build dams, bridges, roads, buildings, and other types of infrastructure.Concrete is preferred in the construction industry due to its durability, cost-effectiveness, and versatility.Additionally, concrete is widely available and can be easily moulded into different shapes, making it a popular choice for various types of construction projects.It is common knowledge that cement is the primary component of concrete.However, cement is the primary contributor to the environmental impact of concrete [1].
There are significant environmental effects as a result of the high energy consumption and greenhouse gas emissions involved in cement production.The International Energy Agency (IEA) estimates that the production of cement is one of the major industrial sources of carbon dioxide (CO2) emissions, contributing roughly 7% of the world's CO2 emissions [2].Additionally, the IEA reported that the industry is responsible for about 8% of the world's energy consumption, which is a significant amount of energy consumed in the production of cement.Beyond greenhouse gas emissions and energy use, the production of cement also affects other environmental issues, such as water use and air pollution.Large amounts of water are used during the cement production process, which can have a negative effect on the local water supply, especially in dry areas.Moreover, cement production is a significant source of air pollution due to the release of particulate matter (PM), nitrogen oxides (NOx), and sulphur oxides (SOx) during the manufacturing process [3].In light of these factors and *Corresponding author: andi.prasetiyo@uajy.ac.id the fact that cement production is a major contributor to global warming, it is clear that we need more environmentally friendly options.
Identifying sustainable alternatives and addressing the environmental impacts of cement production are becoming increasingly vital.Reduce the amount of cement used in construction by substituting alternative materials or by optimising cement use through improved design and construction methods.Alternative materials, such as fly ash, can be used as a partial replacement for cement in concrete and mortar.The use of fly ash as a partial substitute for cement in concrete and mortar has been thoroughly researched in the past.The use of fly ash, a by-product of coal combustion, as a cement substitute material has many benefits, including less environmental impact, better workability, and increased durability of the final mixture.With varying outcomes, several studies have looked into how fly ash affects the durability and strength of mortar and concrete.Replacing cement with fly ash, a by-product of coal combustion, can be extremely beneficial [4].According to [5] fly ash have a potential to improve the development of the mechanical properties of concrete.Despite the fact that increasing amounts of fly ash have been linked to a decline in compressive strength [6].The fact that fly ash requires longer hydration and hardening times than cement may be the cause of the poor strength.
Past evidence suggests that fly ash can be used as a partial replacement for cement in the manufacture of concrete and mortar, leading to a number of positive results including increased durability, reduced environmental impact and improved workability.Factors such as fly ash particle size, amount used, and fly ash source can all influence the effects of fly ash on Fly ash can potentially be used as a replacement material for cement, but further research is needed to determine optimal mixing ratios for different uses.This investigation will examine the impact of earlyage heat of hydration on compressive strength of mortar when cement is partially replaced by fly ash (72 hours after mortar is made).This research is part of a larger series of studies conducted by the author into using fly ash to coat EPS beads for use as an alternative to lightweight aggregate in concrete.The results of this study are also expected to contribute to the knowledge of the behaviour and role of fly ash as an alternative to cement.

Methodology
A laboratory experiment is performed in this study.
Research was conducted at Coventry University's Faculty of Engineering, Environment and Computing, Concrete and Materials Laboratory in June 2022.As well as supporting the analysis and discussion, a literature review was also carried out.In this experiment, fly ash is used to partially replace cement in cement-based mortar and test its compressive strength and early-age temperature.A future study will build on this study to enhance the ongoing process.

Materials
In this study, Hanson 52.5N High Strength Cement, a type-1 cement, was used.It weighed in at around 1370 Kg/m3.Sand with a fineness modulus of 1.95 is used as fine aggregate.It weighed 1350 kg/m3, 1650 kg/m3, and 1850 kg/m3 in loose, compacted, and wet conditions, respectively.
A cubic metre of Pulverized Fuel Ash (PFA) or fly ash derived from Cornish Lime U.K. weighs approximately 1,070 kg, and 83.96 percent of its chemical composition is SiO2 + Al2O3 + Fe2O3.

Samples and test procedures
In accordance with the standard Specification for Cement Mortar ASTM C1329 [7], the mix proportion utilized in this study was 1:3 (cement: fine aggregate), with a w/b ratio of 0.4.While, the targeted strength for the mortar is M-type with a minimum expected strength of 20MPa at 28 days.
The replacement of cement with fly ash was calculated based on weight percentage proportions, ranging from 10, 30, and 50% of the cement weight.Low volume fly ash (LVFA) concrete and high volume fly ash (HVFA) concrete are the abbreviations used for concrete that contains up to 30% and above 50% fly ash (FA).Therefore, concrete that has had at least half of the Portland cement replaced with fly ash is known as highvolume fly ash concrete (HVFAC) [8].The terms LVFA concrete and HVFA high volume concrete have been recognized and utilized in numerous prior scholarly studies [9][10].In fact, another publication, which reviews around 180 studies, concludes that replacing more than 40% of cement with fly ash qualifies as HVFAC [11].The utilization of concrete with a composition of 50-60% fly ash as the entire cementitious materials is a prevalent approach in attaining sustainable development in the field of concrete [12].The information derived from the studies mentioned earlier serves as the foundation for establishing the proportion of fly ash replacement in the present investigation.The objective of this study is to enhance the understanding of the application of fly ash as an alternative to ordinary cement.The test specimens were made by filling the moulds with mortar while being vibrated using a vibrating machine.The compressive strength was evaluated through a total of 12 samples, with each sample code having three corresponding samples.Regarding internal temperature measurements, the experimental procedure involves conducting a singular trial for each of the PFA concentrations of 10%, 30%, and 50%, utilizing a single sample for each trial.

Compressive strength of mortar
Three mixtures of mortar were prepared as shown in Table 1, with three specimens for each type.After 24 h, the specimens were removed from the moulds and cured by immersing them in a water bath until the day of the test.The mortar specimens were then taken for compressive strength testing at 7, 14 and 28-day.50×50×50mm cube specimens were prepared for compressive strength.Table 1.Mix composition.

Early-age temperature of mortar
Temperature measurements in the initial condition of the hardened mortar were carried out using the following tools: • 3x Cube mould (100x100x100mm) material: plywood with lid -thickness: 17mm • Inner insulation (layer 1) Polyisocyanurate (PIR) insulation (XtraTherm) with lid -thickness: 50mm • Outer Insulation (layer 2) (Styrofoam) with lidthickness: 25mm Fig. 1 illustrates the container used for specimens.Additional tools utilised include the following: • Thermocouple type-K (Fig. 2) • DataLogger -DataTaker (Fig. 3) The temperature sensor is embedded within a 100mm cube-shaped specimen.One sensor is used to measure the room temperature for comparison.All the sensors are then linked to a data logger, which is in turn connected to a computer that has been loaded with data processing software, which will store the reading data from the logger, analyse it, and create a graph for easy    3 Result and discussion

Compressive strength
The compressive strength of mortar cement samples with various fly ash substitutes at different curing times can be observed form Fig. 4. The compressive strength of the samples decreased as the proportion of fly ash substitute increased.After 7 days of curing, the compressive strength of the control group was 24.84 MPa, while the compressive strength of the samples containing 10%, 30%, and 50% fly ash substitute was 16.93 MPa, 12.03 MPa, and 11.85 MPa, respectively.Similarly, the compressive strength of the control group after 28 days of curing was 31.17MPa, whereas the compressive strength of the 10%, 30%, and 50% fly ash substitute samples was 23.02 MPa, 19.73 MPa, and 19.25 MPa, respectively.

Fig. 4. Compressive strength of mortar
As expected, the compressive strength of mortar mixtures containing fly ash as a cement substitute decreased as the proportion of substitute increased.It can also be stated that the compressive strength decreases with increasing fly ash content.Other studies that mention the contribution of fly ash to strength support this fact due to the slow pozzolanic reaction, this only occurs at later ages [8].
Results show that the compressive strength of mortar cement is significantly affected when fly ash is used as a replacement.There was a negative correlation between the percentage of fly ash replacement and the compressive strength of the samples across all curing times.Using these findings, the strength and sustainability of mortar cements made with fly ash can be optimised.
According to the findings of this study, replacing cement with fly ash significantly affects the compressive strength of mortar cement.Several factors contribute to the decrease in compressive strength with increased fly ash replacement.First, because fly ash is less reactive than cement, it can result in slower strength growth and lower overall strength.Increasing the ash replacement rate reduces the cementitious material's overall reactivity and compressive strength.Second, fly ash particle size and shape can affect the compressive strength of cementitious materials.Compared to cement particles, fly ash particles are generally smaller and less angled, reducing inter-particle intermeshing and bonding and weakening the cementitious material.Third, the water required for optimal fly ash hydration is typically more significant than that required for cement, resulting in a higher water-to-cementitious material ratio and weaker cementitious materials.
It is important to note that using fly ash in mortar cement has several advantages, despite the decrease in compressive strength with increasing fly ash replacement.Fly ash is a by-product of coal combustion.Its use in cement can reduce the environmental impact of coal combustion by diverting waste from landfills and reducing greenhouse gas emissions.In addition, using fly ash in cement also reduces costs, as fly ash is generally cheaper than cement.The optimum fly ash replacement rate for mortar cement depends on many factors, including the specific application, required compressive strength, and fly ash availability.
According to the findings of this study, replacing cement with fly ash does not improve the compressive strength of mortar.However, mixtures containing 30% and 50% fly ash substitutes show a significant increase in compressive strength after 28 days, with 53.05% and 62.43% increases in strength, respectively, compared to the control mix (Fig. 5).It should be noted that the early strength of the fly ash replacement mixture is lower than that of the control mixture.This is most likely because fly ash develops less initial strength than cement.On the other hand, the gains in the long-term strength of the blends containing fly ash substitutes are more significant than those of the control mix.

Fig. 5. Percentage mortar compressive strength increases
These findings align with previous research on the use of fly ash in cementitious materials.According to a study conducted by Chindaprasirt et al., the use of fly ash in mortar increased the compressive strength of concrete, particularly at later ages [14].Moreover, fly ash addition is more advantageous to enhance the flexural strength [15].Nevertheless, the effectiveness of the cement mixture with fly-ash is highly dependent on the quality of the type of fly-ash used as well as the process by which it is obtained [16].It is important to note, however, that the optimal fly ash replacement rate will vary depending on the application and fly ash properties.As a result, more research is required to determine the optimal fly ash replacement rate for a specific application.
A likely limitation of this study is using one type of fine aggregate.Different types of fine aggregate might interact differently with fly ash, which can influence the development of mortar strength.In addition, in this study, only the compressive strength of the mortar was investigated; other properties, such as durability and workability, were not considered.Therefore, future studies could also investigate the impact of fly ash on these properties.
Overall, the findings suggest that fly ash may be a promising substitute for cement in improving mortar strength, particularly at later ages.Fly ash replacement rates may differ depending on the specific application and fly ash properties.More research is needed to determine the best practices for using fly ash in mortar cement.

Early-age temperature of mortar
A three-day period was used to determine the internal temperatures of the mortar samples shown in Fig. 6.The K-Type 1, 2, 3 indicate the internal temperature measurements of mortar with sample codes PFA10%, PFA30%, and PFA50%, respectively.The heat produced from mortar mixtures decreased as the fly ash replacement increased.Using fly ash as a replacement for cement resulted in a decrease in the highest temperature increase during adiabatic curing circumstances.A direct relationship exists between the exchange rate of fly ash and the temperature rise, whereby an increase in the former results in a decrease in the latter [13].
The temperature measurements associated with the initial stages of heat development that occur during the hydration process of the mortar samples.The study reports the highest temperatures observed within the initial 72 hours of casting for concrete mixes containing fly ash replacements of 10%, 30%, and 50% to be 42°C, 41°C, and 38°C, respectively.The maximum temperatures were attained within the initial 12-hour period for all the mixtures.The specimens' initial temperature was approximately 23°C, and it experienced a gradual increase during the initial hours of hydration.After the 72-hour observation period, the temperature reached a state of equilibrium at approximately 28°C across all mixtures.The percentage of fly ash replacement impacted the mortar samples' hydration heat.As the percentage of fly ash replacement increased, there was a decrease in the peak temperature of the mixes.The mortar specimens with 10%, 30%, and 50% replacement showed peak temperatures of 42°C, 41°C, and 38°C, respectively.The observed phenomenon can be defined as the comparatively reduced reactivity of fly ash compared to cement, leading to a decelerated pace of heat generation throughout the hydration process.
The findings of this research suggest that the substitution of cement with fly ash has a noteworthy impact on the initial phases of thermal energy release in the process of mortar hydration.The study observed a negative correlation between the replacement level and the maximum temperature attained within the initial 72hour period following the mixing process.The recorded maximum temperatures were 42°C, 41°C, and 38°C for the 10%, 30%, and 50% replacement levels, respectively.The results presented in this study align with prior research that has documented a decrease in the heat of hydration when incorporating fly ash into cement-based systems, as reported in previous study [17].Moreover, the use of fly ash results in a reduction of the drying shrinkage of mortars [18].
The study indicates that the decrease in the heat of hydration can be ascribed to the pozzolanic reaction between the fly ash and cement.This reaction consumes a considerable amount of calcium hydroxide generated during cement hydration, leading to a slower rate of heat evolution.Therefore, the utilisation of fly ash has significant implications for the performance and durability of the mortar due to its association with lower temperature rise and heat release rate.
The findings of this study indicate that replacing up to fifty percent of the cement with fly ash is a viable strategy for regulating the initial phases of heat generation and extending the durability of mortar while maintaining its strength and workability.The ideal replacement level is subject to variation based on the application and mixed design.Further research is required to examine the enduring impacts of fly-ash substitution on the mechanical and durability characteristics of the mortar.
In summary, the findings of this research exhibit the possibility of utilising fly ash as a substitute for cement in mortar blends and offer significant perspectives into the impact of fly ash on the exothermic reaction that occurs during the initial phases of solidification.The results mentioned above may be useful in improving the performance and durability of mortar in various contexts by optimising its blend composition and curing conditions.

Conclusion
Based on the results of experiments conducted with mortar cement, it can be concluded that fly ash can be effectively used as a substitute for cement to reduce thermal hydration and maintain acceptable levels of compressive strength.As the fly ash substitution rate increased, the thermal hydration of the samples decreased, with maximum temperatures measured at around 42 degrees Celsius for PFA10 and about 41 and roughly 38 degrees Celsius for PFA30 and PFA50, respectively.
In the early stages of testing, the samples containing the fly ash showed a reduced compressive strength compared to the control group.However, the compressive strength growth of the fly ash mortar group caught up with the control group and, in some cases, improved over time.The 30% and 50% fly ash replacement samples showed significant improvement in strength compared to the control group, with PFA50 increases of 28.23% and 53.05% at 14 days and 62.43% at 28 days.The addition of fly ash to concrete may result in an increase in compressive strength that exceeds the standard 28-day duration, thereby surpassing the strength of mortar or concrete that does not have fly ash.
Therefore, using fly ash in mortar cement can provide a sustainable solution to reduce thermal hydration and increase mortar strength, which is beneficial for construction purposes.Subsequently, the increased level of strength can compensate for the initial drop in compressive strength of the mortar with fly ash.The advantages of using fly ash in mortar make them a promising alternative to cement.In this way, incorporating fly ash into mortar cement can contribute to developing a sustainable construction industry.

Fig. 6 .
Fig. 6.The internal temperature of the mortar specimens in the first 72 hours of ageing.
The mortar was recorded for 72 hours after its creation.This temperature measurement technique is based on and adapted from the semi-adiabatic container utilised in the investigation conducted byChung et al.,  2016 [13].