0 Introduction

Commercial concrete, also known as ready-mixed concrete, refers to the quality-qualified concrete mixture that is pre-mixed and sold to construction units in the form of products, and transported to the construction site for pouring. With the development of ready-mixed commercial concrete, concrete cracking has become one of the main issues leading to quality complaints from ready-mixed concrete companies. The causes of concrete cracking are quite complex, resulting from one or more improper factors such as raw materials, mix ratios, construction practices, and curing. This paper studies the improvement of cement performance to manufacture low-heat, low-alkali special cement for commercial concrete.P·O 42.5(hereinafter referred to as special cement) is used for concrete to reduce the occurrence of concrete cracking.

1 The impact of high cement hydration heat and high alkali content on concrete

High cement hydration heat can cause concreteto crack during the period of 1 to 10 days.High alkali content in cement can accelerate early cracking of concrete and cause alkali-silica reaction leading to concretecracking in the later stages from 2 to 20 years.Early cracking of concrete is believed by some to occur within 1 day, while others believe it occurs within 3 days, but most believe it occurs within 1 day. Most scholars believe that early cracking of concrete is caused by plastic shrinkage, self-shrinkage, and drying shrinkage. Previous studies only linked early cracking of concrete to free shrinkage, which does not align with actual engineering situations. In recent years, the concrete industry has adopted research methods focusing on cracking performance under restrained conditions, which are more in line with engineering realities. Huang Shiyuan analyzed the causes of early cracks from the earliest mechanical properties of concrete. He believes that for restrained concrete, the cracking condition is:ε=Rp/E>εLwhere:εis the strain of concrete;ε=Rp/E>εLis the tensile strength of concrete;is the elastic modulus of concrete;εLis the ultimate strain of concrete. That is, when the strain generated by concreteexceeds its ultimate strain,Rpthe concrete will crack. He believes that the parameter characterizing concrete cracking is not the shrinkage value, but the ultimate tensile strain value.ETemperature stress is currently a very important factor in early cracking of ready-mixed concrete. The stress in concrete comes from temperature changes,from drying shrinkage and wet expansion. Cement hydration heat is the main source of early temperature stress in concrete. To reduce early temperature strain in concrete, it is necessary to minimize cement hydration heat. Experts in concrete both domestically and internationally require that the compressive strength of concrete should not exceed12 MPais the ultimate strain of concrete. That is, when the strain generated by concrete, orfrom drying shrinkage and wet expansion. Cement hydration heat is the main source of early temperature stress in concrete. To reduce early temperature strain in concrete, it is necessary to minimize cement hydration heat. Experts in concrete both domestically and internationally require that the compressive strength of concrete should not exceed6 MPa at 12 hours,

which essentially aims to reduce the early hydration rate and hydration heat, thereby reducing the stress caused by temperature strain.2/3In recent years, the phenomenon of concrete cracking has become increasingly severe, prompting extensive research efforts in the cement and concrete industries to reduce cracking. The causes of modern concrete cracking, in addition to the use of water-reducing agents, low water-cement ratios, low aggregate ratios, and construction management issues, also include the impact of cement quality. Studies have shown that high alkali content in cement, high content of clinker, and high fineness are the main factors leading to increased early cracking in modern concrete.1/3Alkali-aggregate reaction refers to the reaction between alkali in the liquid phase of concrete and reactive aggregates, producing alkali-silica gel or other products that can cause expansion. These products absorb water and expand in wet or humid environments, generating significant internal stress in the concrete, which can lead to cracking in severe cases. The latency period for alkali-aggregate reaction is generally long, typically ranging from several years to several decades, and once the reaction occurs, there is almost no method for repair, hence it is referred to as the 'cancer' of concrete,ε=Rp/E>εLwhich severely affects the durability of concrete structures.2 Internal quality control standards for special cementWith the aim of reducing concrete cracking and improving the durability of concrete structures, combined with factors such as raw materials, equipment, and production technology from Anfu Southern, a quality internal control standard for low-heat, low-alkali special cement for commercial concrete has been established after more than a year of experimental research.See table.which severely affects the durability of concrete structures.TableQuality internal control indicators for low-heat, low-alkali special cement for commercial concrete

3 Production of special cement for commercial concreteC3ATo reduce the hydration heat of cement clinker and the alkali content of cement,C3Shigh-silica, low-calcium limestone from Anfu Southern Mine is used to manufacture high-silica, low

clinker, with a clinker rate value of-KH=0.90~0.92,-n=3.4~3.6.-骨料反应的潜伏期普遍较长，一般为几年到几十年之间，而且反应一旦发生，几乎没有修护的方法，因此被称作混凝土的“癌症”，严重影响混凝土结构耐久性。

2　专用水泥内控质量标准

以减少混凝土开裂、提高混凝土建筑结构物耐久性为目的，结合安福南方原材料、装备、生产技术等因素，经过一年多的试验研究，制定了安福南方低热低碱商品混凝土专用水泥P·O 42.5质量内控标准，见表1。

表1　低热低碱商品混凝土专用水泥P·O 42.5质量内控指标

 

3　商品混凝土专用水泥生产

3.1　降低水泥熟料水化热、水泥碱含量

用安福南方矿山高硅低钙石灰石制造高硅率、低C3A熟料，熟料率值为KH=0.90~0.92、n=3.4~3.6、P=1.2~1.4Clinker minerals: silicate mineralsC3S+C2S=80.0%Approximately, tricalcium aluminateC3A 5.5%Approximately, clinkerC3AThe content ratio is usually lower than that of typical enterprise clinkerby 30% or more, and the hydration heat of this clinker is low. The alkali in cement clinker mainly comes from raw materials such as limestone, clay, and iron powder, with clay generally having a higher alkali content. To produce clinker, the clay content must be reduced. Anfu Southern uses low-calcium high-silica aggregate stone to replace clay in raw material batching, which reduces the alkali content of the clinker.Special cement grinding production

3.2　Special cement uses clinker with low hydration heat, low alkali content, and high early strength but not high late strength (

compressive strengthεR3=29 MPaapproximately,28 dR28=59 MPaR3=29 MPaapproximately), the material ratio for grinding cement is: clinker84%, gypsum5%, limestone, slag, limestone6%;The specific surface area of the cement grinding is controlled at (340±20).Technical indicators for special cement productionm2/kgThe hydration heat of special cement meets the requirements for medium heat cement (

3.3　C3A≤6.0%

);Hydration heat≤251 kJ/kgε,7 d≤293 kJ/kg). The alkali content of the cement is,approximately 0.25%,which meets the requirements for low alkali cement and has a relatively low alkali content.4 Comparison of physical and chemical performance indicators of different brands of cementMainly selected local concrete mixing station commonly used cement:

brand,

brand compared with special cement, relevant testing data can be found in the tableAHLPhysical and chemical performance testing data of different brands of cementBHSFrom the table, it can be seen that special cement has lower hydration heat than ordinary cement, lower alkali content, slightly lower early strength, longer setting time, but greater late strength growth. This special cement belongs to high-silica cement, with a total amount of silicate minerals about 4% higher than that of ordinary enterprise cement.2。

表2　This results in a high late strength growth rate, with strengths at six months and one year significantly higher than ordinary cement, making structures built with this special cement more durable.

 

5 Comparison of C30 concrete performance using different brands of cement2Comparison of physical properties of concreteCement brand,Cement brand, special cement mixed according to the same mix ratioR28=59 MPaConcrete (see table

), physical performance comparison can be seen in table

5.1　3 Concrete mix ratios

AHLDifferent brands of cement mixedBHSConcrete physical propertiesC30It can be seen that using special cement results in lower hydration heat compared to ordinary enterprise cement, slower temperature rise in concrete, longer setting time, but higher strength.3Comparison of admixture dosage in concrete4。

表Admixture dosage for different brands of cement concrete can be seen in the table

表4　, from the table, it can be seen that to achieve the same water reduction effect, the amount of admixture used with special cement can be reduced by about 10%.C30Comparison of admixture dosage for different brands of cement

5 Comparison of C30 concrete performance using different brands of cement4Comparison of crack resistance performance of concrete). The alkali content of the cement isComparison of crack resistance performance of concrete using different brands of cement can be seen in the table.R28=59 MPaThrough the investigation of crack conditions at the construction site, it was found that concrete made with special cement has fewer cracks than that made with other cements, and its crack resistance performance is much better.

5.2　Comparison of crack resistance performance of concrete using different brands of cement

6 Conclusion5Relying on the unique raw materials of Anfu Southern to produce high-silica cement clinker, optimizing the physical and chemical performance indicators of cement, and manufacturing low heat, low alkali commercial concrete special cement. The concrete produced using low heat, low alkali commercial concrete special cement can significantly reduce cracking compared to ordinary cement and also lower the cost of commercial concrete admixtures.10%左右。

表5　不同品牌水泥混凝土外加剂掺量

5.3　混凝土抗开裂性能对比

不同品牌水泥混凝土抗开裂性能对比见表6。通过工地裂纹情况调查，专用水泥比其他水泥制造的混凝土裂纹少，混凝土抗开裂性能要优良得多。

 

表6　不同品牌水泥混凝土抗开裂性能对比

6　结束语

依托安福南方特有的原材料生产高硅率水泥熟料，优化水泥物理化学性能指标，制造低热低碱商品混凝土专用水泥。使用低热低碱商品混凝土专用水泥生产的混凝土，比一般水泥可以大大减少混凝土开裂，也可降低商品混凝土外加剂成本10%左右。