Effects of Particle of BA, CMA and FA on Water Absorption of Concrete

The porosity of concrete is the root cause of durability problems in concrete. Water absorption rate and water absorption speed can be used to detect the pore structure distribution of concrete. Based on the principle of permeability coefficient method, the water absorption method was improved in this study. This paper conducted an experimental study on how BA (biomass ash), CMA (cattle manure ash), and FA (fly ash) with different particle sizes affect the water absorption speed and coefficient of concrete. The results showed that among BA, FA, and CMA, the water absorption speed and coefficient of FA are the smallest among all admixtures in concrete, and the water absorption speed and coefficient of BA are the smaller. The water absorption coefficient is greater than that of FA, and the water absorption speed and coefficient of CMA are greater than those of BA. With the same dosage (15% in this test), the smaller the water cement ratio, the smaller the water absorption rate of CMA concrete; The water absorption rate of CMA concrete decreases with the increase of age. When the dosage is the same, the influence of particle size on the water absorption coefficient of concrete is the same as the influence of particle size on the water absorption speed of concrete. Factors affecting the water absorption coefficient of CMA materials include particle size distribution, shape, specific surface area, and so on. The results provide a reference for studying the durability of CMA concrete.


Introduction
According to the website of China Concrete, China's concrete output is about 3.293 billion cubic meters in 2021, a year-on-year increase of 6.9%.As the largest building material now, concrete has made important contributions to the development of China's economy and infrastructure construction.The reason why concrete can be widely used is that it has low cost, excellent performance, and many raw materials such as cement, sand, stone, and water can be obtained locally.
However, the mass production of concrete needs to consume a large amount of cement.The existing data [1] shows that each ton of Portland cement production needs to consume about 1 ton of limestone ore and emit about 1 ton of carbon dioxide.Some data also pointed out that the carbon dioxide emissions caused by the cement industry accounted for 90% of the industrial carbon dioxide emissions [2,3].It can be seen that the consumption of large quantities of cement to produce concrete is contrary to the new concept of environmental protection; In addition, the production of concrete also requires the consumption of large amounts of non-renewable natural resources such as sand and stone.Therefore, how to minimize the consumption of cement, sand, and stone in the production of concrete has become an urgent problem in the sustainable development of concrete industry.
In order to reduce the amount of cement, many researchers use BA (CMA, etc.), FA, silica fume, slag and some agricultural by-products (wood ash, straw ash, palm ash) as mineral admixtures to replace some cement in the concrete industry [4][5][6][7].
Engineering practice has proved that the utilization of the above mineral by-products can not only reduce the consumption of cement and sand, but also improve the durability of concrete [8,9].
The root cause of concrete durability problems lies in the compactness of concrete, that is, the effect of porosity on concrete.The amount of porosity and the distribution of the pore structure are extremely important for resisting the permeability of concrete.The closed pores may have little water absorption, and have a lower water absorption rate.Connected holes with small pore size may have high water absorption rate and fast water absorption rate.Therefore, the distribution of the pore structure of concrete can be detected through water absorption rate and water absorption speed, and then the compactness of concrete can be inferred.A large number of studies have proved that the permeability, water absorption and carbonization resistance, frost resistance, chloride ion permeability of concrete have shown good correlation, and the durability characteristics of concrete have begun to be characterized by permeability and water absorption indicators [10].
In this study, experimental research is conducted on how BA, CMA and FA with different particle sizes affect the water absorption rate and water absorption coefficient of concrete, which provides a reference for studying the durability of CMA concrete.

Cement
The cement used in this experiment is Common Portland cement P. O42.

CMA
In this study, muffle furnace is used to simulate fixed combustion conditions.Before constant temperature combustion, it is preheated for 20 minutes and kept at constant temperature for 2 hours, then stopped heating, cooled naturally, opened the oven door and collected CMA.
The materials in the test are abbreviated as follows: (1) Cattle manure ash obtained at constant temperature 500C, referred to as CMAa.
(2) Cattle manure ash obtained at constant temperature 650C, referred to as CMAb.
(3) Cattle manure ash obtained at constant temperature 800C, referred to as CMAc.
The physical properties of the materials and their chemical composition are shown in table 8

Admixture
Admixtures used in the study include high-efficiency water reducing agent and air entraining admixture, which meet the requirements of "Concrete Admixtures" (GB8076-2017).β-Naphthalene sulfonic Acid-Formaldehyde Condensate (NA), solid powder, produced by Shaanxi Longhui Company, was selected in the test, with the dosage of 1-1.4% of the quality of the cementitious material and the water-reducing rate of 24.7%.

Method
As shown in figure 1, the Initial Surface Absorption Test [11,12] is improved according to the principle of the permeability coefficient method, and the surface water absorption speed and surface water absorption coefficient of concrete are tested.The size of test block is 100mm × 100mm × 100mm cube.

Main Test Instruments and Equipment
Drying oven, airtight containers, balance, scraper, epoxy resin, water storage containers.

Test Steps
(1) Prepare concrete according to table 10, and the standard curing period is 7,28 days.
(2) Water absorption speed test: the specimen is maintained to the test age, the specimen is dried in a drying oven at 50 °C for 3 days, and then cooled in a airtight containers for 15 days, leaving one side uncoated with epoxy resin, and covering the other sides with epoxy resin.Weigh the initial weight of the concrete specimen coated with epoxy resin (m0).After weighing the initial weight, put the specimen into Concrete Specimens 5mm 100mm 100mm Epoxy Resin Epoxy Resin the water, as shown in figure 1, the specimen is immersed in water for 5mm, take it out of the water according to the following time (1, 5, 10, 20, 30, 60, 110 and 120min) [13], dry the excess water on the surface with a towel, and weigh the mass (mn) of concrete, put it into the water after each weighing and soak it for the above time until the next weighing.
(3) Water absorption coefficient experiment: When the cured concrete reaches the experiment age period, the concrete is dried in a drying oven at 110°C for 7 days until constant weight, and then cooled in an airtight container for 1 day.
Seal the other sides of the specimen with epoxy resin, and after drying, weigh its initial weight (m0), and then soak one end of the specimen in water for 5mm, as shown in figure 2. After 60 minutes, the specimen is removed from the water, the surface is wiped dry, the weight (mn) of the concrete is weighed, and its water absorption coefficient is calculated.
(4) Calculation of the experiment results: The water absorption speed of the concrete is calculated according to the equation ( 1).
( ) The equations ( ) is the water absorption mass in the interval time , the unit is kg/s; a is the area of the water-immersed part of the specimen, the unit is m 2 ; d is the density of water , the unit is kg/m 3 ; The unit of I is m/s.The water absorption coefficient of the concrete is calculated according to the equation ( 2). 2 1 In the equations, a k is the water absorption coefficient, the unit is (m 2 /s); Q-The unit of water absorption of Q-dried specimen within 3600 seconds is (m 3 ), Q=mn-m0; A is the surface area (m 2 ) of the water-immersed part of the concrete, the unit is m 2 ;

Proportioning of Test Blocks
The mix proportion of concrete is shown in table 10.

Water Absorption Velocity
The water absorption rate of CMA, FA and BA concrete with different water cement ratios (W/C=0.70,0.55, 0.50) for 7 days and 28 days is shown in figure 2, figure 3, and figure 4. Figure 2 shows that when W/C=0.70 and the content of each mineral admixture is 15% of the cement mass, the water absorption rate of CMA, FA, and BA concrete decreases with the increase of curing age.The reason why the water absorption rate of 28 days is lower than that of 7 days is that the cement hydrates, and the hydration products continuously fill the cement paste, gradually increasing the density and reducing the porosity.
The literature shows that the secondary hydration reaction between pozzolanic mineral admixtures and Ca (OH) 2 in cement generally occurs after 28 days.Therefore, when the mineral admixture content is the same, the water absorption rate of mineral admixture concrete depends on the physical effect of the mineral admixture and cement concrete.
The results in figure 2 also show that the water absorption rate of FA is the smallest among all the concrete admixtures, which is closely related to the particle characteristics of FA.Spherical FA plays the role of both ball bearing and dense filling in concrete, and the combination of water reduction and dense filling forms a pore structure that is not conducive to water absorption.Compared with the irregular shape of CMA, the water absorption rate of FA concrete is low.The three kinds of CMAs are all irregular in appearance, and their water absorption rate is increased compared with the spherical particles of FA.It can be seen that the irregular shape of CMAs is not conducive to the material's resistance to water absorption.

Analysis of Water Absorption Coefficient Test Results of CMA, FA, BA Concrete
In principle, the water absorption coefficient and the water absorption rate have a certain correlation, and the water absorption coefficient acts on a certain area.
Figure 5 shows that when W/C=0.70 and the content of mineral admixtures is 15% of the cement mass, the water absorption coefficient of CMA, FA and BA concrete decreases with the increase of curing age, and the water absorption coefficient of 28-day-old concrete is lower than that of 7-day-old concrete.The reason is that the cement is hydrated, the hydration products are constantly filled with cement paste, and the compactness gradually increases and the porosity decreases.Literature shows that the secondary hydration reaction between mineral admixture with pozzolanic activity and Ca(OH) 2 in cement usually takes place after 28 days.Therefore, the water absorption coefficient of mineral admixture concrete depends on the physical effect of the interaction between mineral admixture and cement concrete at the same dosage.
The results in figure 5 show that the water absorption coefficient of FA is the smallest among all the concrete admixtures.
The reason why FA has the smallest water absorption coefficient is closely related to the particle characteristics of FA.Spherical FA plays the role of both ball bearing and filling compactness in concrete.The combination of water reducing and compactness is not conducive to water absorption.Compared with the irregular shape of CMA, the water absorption coefficient of FA concrete is small.Figure 7 shows that the development trend of water absorption coefficient of mineral admixtures is similar to that of W/C=0.70 and W/C=0.55,but the value of water absorption coefficient decreases.

Conclusion
By studying the effects of particle sizes of BA, CMA and FA on the water absorption rate and coefficient of concrete, the main conclusions are as follows: (1) At the same dosage (15% in this study), the smaller the water-cement ratio, the greater the particle packing density of concrete, and the denser the concrete, the smaller the water absorption speed of CMA concrete.The water absorption rate of CMA concrete decreases with the increase of age.
The water absorption rate of CMA concrete is increasing with the increase of particle size volume fraction of 3-32 m and 32-65 m.The water absorption rate of CMAa is lower than that of CMAc and CMAb.The reason for the high water absorption rate of CMAa also includes its high ignition loss.
(2)When the dosage is the same, the effect of particle size on the water absorption coefficient of concrete is the same as the effect of particle size on the water absorption rate of concrete.

Figure 1 .
Figure 1.Experiment on water absorption rate and water absorption coefficient of concrete.

Figure 2 .
Figure 2. Water absorption rate of CMA, FA and BA concrete at W / C=0.70.

Figure 3 .
Figure 3. Water absorption rate of CMA, FA and BA concrete at W / C=0.55.

Figure 3
Figure3shows that the development trend of water absorption rate of mineral admixtures is similar to that of W/C=0.70.

Figure 4 .
Figure 4. Water absorption rate of CMA, FA and BA concrete at W / C=0.50.

Figure 4
Figure4shows that the development trend of water absorption rate of mineral admixtures is similar to that of W/C=0.70 and W/C=0.55,but the water absorption rate value decreases.The test results in figure2, figure3, and figure4show that the water absorption rate of CMA concrete decreases with the decrease of W/C.The three kinds of CMAs are all irregular in appearance, and their water absorption rate is increased compared with the spherical particles of FA.It can be seen that the irregular shape of CMAs is not conducive to the material's resistance to water absorption.

Figure 5 .
Figure 5. Water absorption coefficient of CMA, FA and BA concrete at W / C=0.70.

Figure 6
Figure6shows that the development trend of water absorption coefficient of mineral admixtures is similar to W/C=0.70.

Table 1 .
5, which comes from Xinxing Cement Co., Ltd., Tangshan City, Hebei Province.Its physical properties, chemical composition and mineral composition are shown in table 1 and table 2. The mechanical performance indicators are shown in table 3, and OPC is the code of cement.Chemical composition and mineral composition of cement (%).

Table 2 .
Physical properties of cement.

Table 3 .
Physical and mechanical properties of test cement.

Table 4 .
Physical performance table of FA.

Table 5 .
FA chemical composition table (%).The selected BA comes from the remaining industrial by-products produced by Kaidi biomass power Plant in Yiyang, Hunan province.The fuel of Kaidi Biomass power plant is mainly the local straw straw, bamboo, abandoned wood, old formwork, rice husks, reed and other agricultural and forestry wastes.The main physical properties and chemical components of BA are shown in table 6 and table 7.

Table 6 .
Physical performance indicators of BA.

table 9 :Table 8 .
and Physical performance indicators of CMA.

Table 9 .
Chemical composition of CMA (%).The fineness modulus of Tianjin sea sand is 2.39, Zone II sand, clay content, clay lumps and friable particles content, soundness and chloride ion content meet the requirements of the "Standard of technical requirements and test method of sand and crushed stone (or gravel) for ordinary concrete" (JGJ 52-2019).The apparent density of the sand is 2.60 g/cm3.2.5.2.Stone.It is crushed stone with an average particle size of less than 30mm and an apparent density of 2.91g/cm3.The stone meets the "Standard of technical requirements and test method of sand and crushed stone (or gravel) for ordinary concrete" (JGJ 52-2019).The stone is the basalt.

Table 10 .
Mix proportion scheme for water absorption test of CMA concrete.