This paper investigated the effect of organic compounds with ester groups on the hydration and hardening of cement. The effects of five kinds of organic compounds with ester groups (ethyl acetate, dimethyl oxalate, glyceryl triacetate, trimethyl phosphate, and triethanolamine borate) on hydration heat, hydration degree, setting time, mechanical properties, microstructure, and pore structure of hardened cement slurry were studied. The test results showed that esters can make the end time of cement hydration induction longer and delay the occurrence of the second exothermic peak. Also, the effect of five kinds of esters on the hydration and hardening of cement was basically followed by TG> TB> DMO> EAC> TMP. In terms of molecular structure, for organic compounds containing only ester groups, the higher the number of ester groups, the greater the effect on the hydration of cement. The introduction of other functional groups (such as phosphate or borate) will influence the effect of the esters.
In recent decades, the new progress in the preparation and application of cement concrete materials has benefited from the development and effective application of chemical admixtures. At present, the chemical admixture has become a cement-based material, which is an important regulatory component or modification method. Addition of a small amount of admixture, especially organic admixture, can effectively adjust the performance of cement-based materials, such as improving work performance or rheology, speeding up or delaying the hydration rate and coagulation time, and improving the physical and mechanical properties and durability of materials [
In addition, it should be noted that the ester groups and phosphonate groups are also important functional groups. On the one hand, the ester-based organics can be used as antifoaming agents. On the other hand, many studies have reported that the esterification modification method is an important means for improving or modifying the performance of hydroxyl- or carboxyl-containing admixtures [
In order to explore whether the ester groups have an important effect on the hydration and hardening of cement, five common organic compounds, ethyl acetate, dimethyl oxalate, glyceryl triacetate, trimethyl phosphate, and triethanolamine borate, were selected for comparative experiments in this study. The effects of five kinds of organic compounds on hydration heat, hydration degree, setting time, mechanical properties, microstructure, and pore structure of hardened slurry were studied in this study.
Ordinary Portland cement, complying with the requirements of Chinese National Standard GB 175-2007 (common Portland cement), was used for all experiments here. The chemical composition and mineral composition of cement are shown in Table
The chemical compositions and mineral compositions of cement wt%.
CaO | SiO2 | Al2O3 | Fe2O3 | MgO | SO3 | R2O | f-CaO | LOI | C3S | C2S | C3A | C4AF |
---|---|---|---|---|---|---|---|---|---|---|---|---|
62.13 | 20.76 | 4.58 | 3.27 | 3.13 | 2.80 | 0.57 | 0.76 | 1.56 | 57.3 | 18.9 | 6.5 | 11.3 |
The physical properties of cement test results.
Specific surface area/(m2·kg−1) | Standard consistency, % | Setting time/min | Compressive strength/MPa | ||
---|---|---|---|---|---|
Initial | Final | 3 d | 28 d | ||
347 | 27.2 | 155 | 215 | 28.3 | 51.2 |
Basic information of five kinds of ester group.
Esters | Abbreviation | Chemical formula | Structure | Number of ester groups |
---|---|---|---|---|
Ethyl acetate | EAC | C4H8O2 |
|
1 |
Dimethyl oxalate | DMO | C4H6O4 |
|
2 |
Glyceryl triacetate | TG | C9H14O6 |
|
3 |
Trimethyl phosphate | TMP | C3H9O4P |
|
3 |
Triethanolamine borate | TB | C6H12BNO3 |
|
3 |
In this study, the hydration heat of cement paste was tested using an eight-channel micro-heat instrument (TAM-Air).
The hydration degree of cement was characterized by chemical-bound water and calcium hydroxide content. The prepared cement samples were maintained under the standard curing conditions to the corresponding age (3 d and 28 d), then the central portion was removed and the hydration was terminated with absolute ethanol. The samples were burned at 1000°C in the muffle furnace for 2 h to constant weight, and the mass loss of the samples was chemical-bound water. In addition, TG/DTA6300 thermogravimetric analyzer was used to test samples, and due to the decomposition of calcium hydroxide, the sample will have a significant mass loss at 400°C–500°C, and the content of calcium hydroxide in the sample can be calculated.
The morphology of cement hydration products was observed by JSM-6700F scanning electron microscopy, and the pore structure was measured by AutoPore IV 9500 mercury porosimeter.
The standard consistency water consumption and setting time of cement are in accordance with GB/T 1346-2001 “cement standard consistency of water consumption, setting time, stability test method,” and the cement mortar strength is in accordance with GB/T 17,671-1999 “cement mortar strength test method” (ISO method).
The amount of all kinds of esters was set to 0.05% by mass. Figure
Effect of ester on the rate of heat release of cement hydration.
Characteristic parameters of cement hydration.
Sample | The end time of the induction period (h) | The appear time of the second exothermic peak (h) | Second exothermic peak exothermic rate (J/g·h) | Total heat release (J/g) | ||
---|---|---|---|---|---|---|
24 h | 48 h | 72 h | ||||
Pure cement | 1.3 | 7.9 | 14.2 | 295.0 | 374.0 | 416.6 |
+0.05% EAC | 1.7 | 8.3 | 14.8 | 311.3 | 396.8 | 443.5 |
+0.05% DMO | 2.2 | 9.2 | 15.6 | 325.6 | 418.1 | 468.4 |
+0.05% TG | 1.8 | 10.6 | 17.7 | 330.0 | 430.1 | 482.2 |
+0.05% TMP | 4.2 | 10.4 | 13.2 | 266.7 | 345.2 | 389.0 |
+0.05% TB | 2.2 | 10.9 | 16.3 | 311.8 | 411.2 | 463.7 |
However, in addition to TMP, the other four kinds of esters improve the second exothermic peak of cement. When adding 0.05% TG to cement, the peak is raised from 14.15 to 17.74 J/g·h. The degree of effect was as follows: TG (3 ester groups)> TB (3 ester groups)> DMO (2 ester groups)> EAC (1 ester group). It indicates that the four esters will delay the early hydration reaction but will also promote the degree of response during the acceleration phase. TMP has a delayed effect on cement hydration and reduces the second exothermic peak, the reason may be that its molecular structure contains phosphorus.
The total amount of heat release in different hydration stages also shows a similarity to the second exothermic peak, as can be seen from Figure
Effect of ester on the total heat release of cement hydration.
Nonevaporable water content is usually used to characterize the degree of cement hydration. In the previous section, the hydration heat analysis speculated that, in addition to TMP, the other four kinds of organic compounds can promote the early hydration of cement. This section uses the results of nonevaporative hydration to verify that the above assumptions are correct. Figure
Effect of ester on nonevaporation water content of cement paste: (a) 3 d and (b) 28 d.
Compared with the molecular structure of five kinds of organic compounds, EAC, DMO, and TG belong to the same series of organic matter with increasing number of ester groups, while TB and TMP contain three ester groups as in TG, but both contain boronic acid groups and phosphoric acid groups, respectively. It can be inferred that the higher the number of ester groups contained in the organic matter containing only the ester functional groups, the greater the effect on the degree of hydration of the cement. Also, the introduction of other functional groups (such as phosphate or borate) will have some influence on the effect of ester organic matter.
The effects of ester on the morphology of hydrated products of cement at 3 d and 28 d are shown in Figures
The hydration products morphology of different hardened cement at 3 d. (a) Blank. (b) +0.05% EAC. (c) +0.05% DMO. (d) +0.05% TG. (e) +0.05% TMP. (f) +0.05% TB.
The hydration products morphology of different hardened cement at 28 d. (a) Blank. (b) +0.05% EAC. (c) +0.05% DMO. (d) +0.05% TG. (e) +0.05% TMP. (f) +0.05% TB.
It can be seen from Figure
SEM can only qualitatively observe the density of cement hydration product structure, and MIP can visually analyze the pore structure of cement hydration products quantitatively. The effect of ester on the pore size distribution of cement paste at 28 d is shown in Figure
Pore volume distribution at different ages of hardened cement. (a) Differential distribution. (b) Cumulative distribution.
It can be seen from Figure
According to Renhe et al. [
Pore structural parameters of hardened cement.
Sample | Total pore volume (mL/g) | Distribution ratio of holes (%) | |||
---|---|---|---|---|---|
<20 nm | 20–50 nm | 50–200 nm | >200 nm | ||
Blank | 0.17 | 40.9 | 27.6 | 24.1 | 7.4 |
EAC | 0.16 | 42.0 | 34.9 | 17.7 | 5.4 |
DMO | 0.15 | 46.5 | 30.7 | 13.7 | 9.1 |
TG | 0.12 | 42.7 | 32.4 | 19.4 | 5.5 |
TMP | 0.17 | 44.8 | 38.5 | 10.2 | 6.5 |
TB | 0.14 | 44.6 | 35.6 | 15.0 | 4.8 |
The setting time of the cement slurry with different ester-based organic materials is shown in Figure
Effect of ester organics on the setting time of cement. (a) Initial setting time. (b) Final setting time.
The effect of different content of ester on the compressive strength of cement mortar is shown in Figure
Effect of esters on the compressive strength of cement mortar. (a) 3d. (b) 28d.
The five kinds of esters have improved the hydration degree of cement and promoted the formation of hydration products and optimized the pore structure. Therefore, it can be concluded that the esters will increase the strength of cement at 28 d. The results in Figure
Based on the experimental results, the following conclusions can be drawn: Ester can make the end time of cement hydration induction longer and delay the occurrence of the second exothermic peak, indicating that it has a delayed effect on the hydration reaction rate of cement at early stage (hydration induction period and acceleration period). TG, TB, DMO, and EAC increased the second exothermic peak and hydration degree of cement and promoted the formation of hydration products and the optimization of the pore structure. Macroscopic performance is that mechanical properties of cement at 3 d and 28 d improved significantly. TMP has a retarding effect on cement and reduces the second exothermic peak of cement and inhibits the formation of early hydration products, which reduces the strength of cement at 3 d, but it improves the strength of cement at 28 d. The effect of five kinds esters on the hydration and hardening of cement was basically followed by TG> TB> DMO> EAC> TMP. In terms of the molecular structure, for organic compounds containing only ester groups, the higher the number of ester groups, the greater the effect on the hydration of cement. The introduction of other functional groups (such as phosphate or borate) will influence the effect of the esters.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
The work described in this paper was fully supported by the scientific and technological projects of Shanxi Province, most through the Research Institute of Concrete and Ecomaterials in China University of Mining and Technology (Beijing) (Project no. MC2014-04).