The energy required for the heating and cooling of buildings is strongly dependant on the thermal properties of the construction material. Cement mortar is a common construction material that is widely used in buildings. The main aim of this study is to assess the thermal properties of cement mortar in terms of its thermal conductivity, heat capacity and thermal diffusivity in a wide range of grades (cement: sand ratio between 1:2 and 1:8). As there is insufficient information to predict the thermal conductivity and diffusivity of a cement mortar from its physical and mechanical properties, the relationships between thermal conductivity and diffusivity and density, compressive strength, water absorption and porosity are also discussed. Our results indicate that, for a cement mortar with a 28-day compressive strength in the range of 6–60 MPa, thermal conductivity, specific heat and thermal diffusivity are in the range of 1.5–2.7 W/(m.K), 0.87–1.04 kJ/kg.K and 0.89–1.26 (x10^{-6} m^{2}/s), respectively. The scanning electron microscope (SEM) images showed that pore size varied from 18 μm to 946 μm for samples with different cement-to-sand ratios. The porosity of cement mortar has a significant effect on its thermal and physical properties. For this reason, thermal conductivity and thermal diffusivity was greater in cement mortar samples with a higher density and compressive strength.

^{-6} m^{2}/s), respectivamente. Las imágenes del microscopio electrónico de barrido (SEM) mostraron que el tamaño de poro variaba de 18 µm a 946 µm para muestras con diferentes proporciones de cemento:arena. La porosidad del mortero de cemento tiene un efecto significativo en sus propiedades térmicas y físicas. Por esta razón, la conductividad térmica y la difusividad térmica fueron mayores en las muestras de mortero de cemento con mayor densidad y resistencia a la compresión.

The amount of energy required for the heating and cooling of buildings is dependent on the thermal conductivity (k-value), specific heat capacity (C-value) and thermal diffusivity (α) of the building envelope (

Building mortar is prepared by mixing cementitious material, fine aggregate and water in appropriate proportions. Depending on its various applications, it can be categorised as either masonry or plastering mortar. Masonry mortar is used to bind bricks, stones and blocks in the construction process. Plaster mortar is applied to the walls of the building to create a smooth, durable surface.

Cement mortar contains cement as a binder, sand as a fine aggregate, water and also admixture in some cases. Changing the type and amount of each component causes variations in the mortar’s thermal and mechanical properties, affecting its suitability for different applications. Kim et al. (

Olmeda et al. (

Widodo et al. (

Different researchers have selected various cement-to-sand ratios (C/S) for different applications. Based on the available literature, most researchers selected a cement-to-sand ratio of between 1:0.5 and 1:8 (

In this study, we used ordinary Portland cement (OPC) meeting the specifications of MS522 part 1:2003 (^{2}/g, respectively. The chemical properties of the OPC are shown in

The chemical compositions and LOI of OPC (% by mass).

Chemical Composition | SiO_{2} |
CaO | Al_{2}O_{3} |
MgO | Fe_{2}O_{3} |
P_{2}O_{5} |
MnO | K_{2}O |
TiO_{2} |
SO_{3} |
SrO | LOI |
---|---|---|---|---|---|---|---|---|---|---|---|---|

OPC | 20.14 | 60.82 | 3.89 | 3.10 | 3.35 | 0.064 | 0.14 | 0.24 | 0.16 | 2.25 | 0.02 | 2.23 |

LOI: Loss on ignition

Sieve analysis for sand.

Seven different mixes were prepared, each with a different cement-to-sand (C/S) ratio (1:2, 1:3, 1:4, 1:5, 1:6, 1:7 and 1:8) and good workability (flow of 190 ± 5 mm). This range of C/S ratios was selected as these are the most commonly found in mortars used in practice and other research. The proportions for all mixes are shown in

Mixture composition.

Mix No | Cement (kg/m^{3}) |
Sand (kg/m^{3}) |
Water (kg/m^{3}) |
C:S | W/C | Flow (mm) |
---|---|---|---|---|---|---|

M1:2 | 639.5 | 1290.0 | 270.3 | 1:2 | 0.42 | 190 ± 5 |

M1:3 | 488.8 | 1466.6 | 244.4 | 1:3 | 0.50 | |

M1:4 | 389.8 | 1559.4 | 250.6 | 1:4 | 0.64 | |

M1:5 | 326.4 | 1626.6 | 246.9 | 1:5 | 0.75 | |

M1:6 | 273.2 | 1639.7 | 286.9 | 1:6 | 1.05 | |

M1:7 | 239.5 | 1676.6 | 283.8 | 1:7 | 1.18 | |

M1:8 | 212.4 | 1715.8 | 271.7 | 1:8 | 1.27 |

A cylinder with a diameter of 100 mm and a height of 200 mm was cast for the thermal conductivity, specific heat capacity and porosity tests. A cube with dimensions 100 mm × 100 mm × 100 mm was prepared for use in the density, compressive strength and water absorption tests. The samples were removed from the moulds after 24 h. The specimens were cured in normal water with a temperature of 23 ± 3 °C until reaching testing age. All properties were evaluated at 28 days of curing, except compressive strength which was measured at both 7 and 28 days.

In this study, we tried to prepare the mixtures with an approximate fixed workability. For this reason, the W/C ratio varied depending on the cement-to-sand ratio (from 1:2 to 1:8). The workability of fresh mortar was evaluated using the flow table test according to ASTM C 1437-1 (

The flow table test.

At 28 days, three cylindrical specimens (100 mm x 200 mm) were selected to measure thermal conductivity. The samples were dried in an oven at 105 °C for 24 h to remove all moisture. The k-value of the specimens was determined with a KD2-PRO analyser using a TR1 needle. A TR1 sensor (2.4 mm in diameter and 100 mm in length) is capable of measuring thermal conductivity in the range of 0.1 W/(m·K) to 4 W/(m·K) (

The thermal conductivity measurement process: (a): prepared sample in fresh state, (b): wrapping the dried samples with plastic and (c): the k-value measurement.

The KD2-PRO analyser works by heating the needle for a certain period of time (t_{h}) and monitoring the temperature during heating and cooling. The temperature during heating and cooling is calculated using Equation [

Where _{0} and _{1} are the initial temperature, _{2} is the rate of background temperature drift and _{3} is the gradient of a line relating temperature rise to logarithm of temperature.

Consequently, thermal conductivity can be calculated using the following Equation [

The influence of ambient temperature and humidity on the samples should be kept to a minimum, in order to achieve a more accurate value while using the KD2-PRO. Therefore, the specimens were wrapped in a plastic sheet to minimise the effect of ambient moisture and keep the sample in a dry condition.

The specific heat capacity of cement mortar in dry conditions can be calculated using the law of mixtures. Given a well-hydrated sample, the law of mixtures is a reasonable predictor for the heat capacity of mortar, and accounts for both the ‘bound water’ and ‘physical water’ heat capacity (

Where C_{p} is the specific heat of mixture (kJ/kg.K), C_{pi} is the specific heat of each component and F_{i} is the weight fraction of each component.

Differential scanning calorimetry (DSC) at a heating rate of 10 °C/min was used to measure the specific heat of cement and sand. The specific heat capacity of water is 4.18 (kJ/kg·K) in 23 °C. The measured specific heat capacity of cement and sand in the range of 20°C to 50 °C is shown in

Specific heat capacity of cement and sand.

The following Equation [

Where ^{2}/s), ^{3}) and C is specific heat (J/kg·K).

A compressive strength test was carried out on 100-mm^{3} cubic specimens. Like the cylindrical specimens, the cubic specimens were cured in water and tested at the ages of 7 and 28 days. A water-absorption test was also carried out on the 100-mm^{3} cubic samples at the age of 28 days. The specimens were dried in an oven for 24 h prior to the test. The initial and final water absorption of the specimen were determined after full immersion in water for 30 min and 72 h, respectively.

The porosity test was carried out on three cylindrical specimens, each with a diameter of 100 mm and a height of 200 mm, after 28 days of curing. One piece 5 cm thick was cut from each sample. Next, the samples were dried in an oven at 105 °C for 24 h to remove moisture. The vacuumed samples were then filled with water. The amount of water held by the sample is a measure of its porosity, as follows (

Where P is porosity (%), _{a} is oven-dry weight (kg) and _{b} is saturated surface dry weight (kg).

Compressive strength value.

Thermal properties of cement mortars.

Sample ID | Thermal conductivity (k-value) (W/(m·K)) | Specific heat capacity (C-value) (kJ/kg·K) | Thermal diffusivity (* 10^{-6} m^{2}/s) |
Oven-dry density (kg/m^{3}) |
Porosity (%) |
---|---|---|---|---|---|

M1:2 | 2.43 | 1.04 | 1.03 | 2233.2 | 7.1 |

M1:3 | 2.79 | 0.98 | 1.26 | 2247.4 | 6.3 |

M1:4 | 2.40 | 0.94 | 1.18 | 2138.4 | 8.4 |

M1:5 | 2.23 | 0.91 | 1.18 | 2053.1 | 8.9 |

M1:6 | 1.99 | 0.89 | 1.09 | 2023.5 | 10.7 |

M1:7 | 1.67 | 0.88 | 0.95 | 1984.7 | 11.3 |

M1:8 | 1.54 | 0.87 | 0.89 | 1973.4 | 11.7 |

The k-value indicates the cement mortar’s capacity for steady-state conduction heat transfer. Low thermal conductivity results in good thermal performance, indicating that the mortar is suitable for use as a heat-resistant material. As can be seen from

The k-value for M1:3is greater than for M1:2despite its higher water-to-cement ratio (W/C) and its lower cement-to-sand ratio (C/S). This may be related to the different proportions of cement and sand in each mix. The larger amount of sand in M1:3 in comparison with M1:2reduces the potential porosity between cement and sand and results in homogenous slurry. In addition, Mix M1:2, with a higher cement content (about 31%) and lower W/C ratio (16%), is a stickier mixture in its fresh state compared to mix M1:3. This means that given the same vibration method and time, more trapped air will remain inside the mixture, which can be seen in the porosity test results of the hardened mortars (

As can be seen from

The heat capacity (C-value) of cement mortar indicates its capacity to store heat. Mortar with a high C-value is not affected by sudden changes in temperature. M1:8, with a C-value of around 0.87 kJ/kg·K, has the lowest specific heat capacity of all mortar types tested. The highest C-value was for mix M1:2,at around 20% greater than that of mix M1:8_{.}

The thermal diffusivity of a cement mortar indicates its transient heat conductivity. Cement mortars with low thermal diffusivity are considered heat insulators in transient heat transfer conditions. The thermal diffusivity of M1:3is greater than those of the other mortars due to its higher k-value. M1:8has the lowest thermal diffusivity, at around 30% lower than M1:3.

Knowing the thermal properties of cement mortar is essential for analysing energy consumption in buildings. To measure thermal properties, special tools are required. Preparing the samples correctly, setting up the test and the testing procedure itself take time. Therefore, using equations to predict the thermal properties of cement mortar is essential to save time and costs. The high coefficient of determination (R^{2}) shows the strength of the correlation (

The relationship between oven-dry density and the thermal properties of cement mortar.

The relationship between compressive strength and the thermal properties of cement mortar.

The water absorption of cement mortar is the flow of fluid inside the porosities of unsaturated cement mortar specimens when there is no external pressure on the samples. Water absorption is used as a quantifying factor when evaluating the durability of cementitious systems (

Initial and final water absorption.

As represented in ^{2} values of 0.93 and 0.95, respectively. The k-value of cement mortar decreased with each increment in initial and final water absorption.

The relationship between water absorption and the thermal properties of cement mortar.

The volume of cement mortar that is not composed of solid material is called porosity. Porosity may affect the mechanical and thermal properties of cement-based materials.(

The relationship between porosity and thermal properties of cement mortar.

The available voids inside cement-based materials have a significant effect on their thermal conductivity and diffusivity (

Void size in cement mortars with a C/S ratio of a) 1:2, b) 1:3, c) 1:4, d) 1:5, e) 1:6, f) 1:7 and g) 1:8.

This study was carried out to assess the thermal properties of cement mortars with different cement-to-sand (C/S) ratios. Thermal conductivity and thermal diffusivity are important factors when considering the amount of heat transfer in steady-state and transient conditions, respectively. Furthermore, the correlation between thermal conductivity and diffusivity with oven-dry density, compressive strength, water absorption and the porosity of specimens was analysed to derive equations to predict the mortar’s thermal properties. From the test results of this experimental work it can be concluded that:

Generally, the thermal conductivity (k-value) of a cement mortar declined when the mixture’s sand content and water-to-cement ratio increased. The k-value of the lowest-quality cement mortar (mix M1:8) was about 45% lower than the thermal conductivity of the highest-quality cement mortar (mix M1:3)_{.}

The specific heat capacity (C-value) of the cement mortar with a C/S ratio of 1:8, with a value of around 0.87 kJ/kg·K, was the lowest of all types of cement mortar tested. The C-value of mix M1:2 was about 20% greater than the C-value of mix M1:8_{.}

The cement mortar with a C/S ratio of 1:3 proved to have the highest thermal diffusivity value among all the tested mixes, due to its higher k-value and lower C-value. Mix M1:8 had the lowest thermal diffusivity, about 30% lower than that of mix M1:3.

The average pore size varied between 18.4 μm for mix M1:2 and 946 μm for mix M1:8. The variation in thermal conductivity and diffusivity between different mixes can be attributed to their different pore sizes.

Both the k-value and thermal diffusivity of cement mortar increased with dry density, and these properties can be correlated using the following equations:

k = -2E-05ρ^{2} + 0.0672ρ - 72.475 (R² = 0.92), α = -9E-06ρ^{2} + 0.0397ρ - 41.529 (R² = 0.66).

The k-value and thermal diffusivity of cement mortar increased with compressive strength. These values can be estimated based on compressive strength with a good degree of accuracy as follows:

k = -0.0007fc2 + 0.0631fc + 1.1464 (R² = 0.95), α = -0.0004fc2 + 0.0271fc + 0.7391 (R² = 0.92).

There is a correlation between initial and final water absorption and the k-value and thermal diffusivity of cement mortars. The k-value and thermal diffusivity of cement mortar decreased as water absorption increased. The thermal properties of mortar can be estimated based on the final water absorption as follows:

k = -0.0021Wa2 - 0.1159Wa + 3.2024 (R² = 0.95), α = -0.0064Wa2 + 0.0656Wa + 1.0123 (R² = 0.67).

Both the k-value and thermal diffusivity of a cement mortar decreased with greater porosity. These thermal properties can be estimated using the following equations:

k = -0.0145ϕ2 + 0.0605ϕ + 2.8856 (R² = 0.95), α = -0.0142ϕ2 + 0.2115ϕ+ 0.4008 (R² = 0.66).

The authors gratefully acknowledge financial support from a University of Malaya postgraduate research grant (PPP), with grant no. PG217-2016 A.