Experimental study on the effect of cement alkali content on concrete ASR

2.1. Raw materials

Three types of cement are used. C1 stands for P·I 42.5 portland cement. C2 stands for P·O 42 5 ordinary portland cement. C3 stands for P·MH 42.5 medium heat portland cement. Three types of sand are used as aggregate, including natural sand (S1), artificial sand (S2) and artificial sand (S3). The chemical composition of cement is shown in Table 1. The determination results of aggregate activity are shown in Table 2.

2.2. Test scheme

Three kinds of cement and three kinds of aggregate were used for the comparative test of rapid alkali silica reaction. The biggest difference between the two standards is whether to adjust the initial alkali content of cement. Therefore, the initial alkali content of cement without adjustment is called standard Ⅰ. If the initial alkali content of cement needs to be adjusted to 0.9 %±0.1 %, it is called standard Ⅱ. The comparative test scheme are shown in Table 3.

According to the scheme in Table 3, the alkali activity test is carried out by Accelerated Mortar Bar Test Method, and the expansion rate is calculated according to Eq. (1), accurate to 0.001 %:

where ${\epsilon }_t$ is the expansion rate (%) of the specimen at the age of $t$; $L_t$ is the length of the specimen at the age of $t$ (mm); $L_0$ is the reference length of the specimen (mm); $\mathrm{\Delta }$ is the probe length (mm). Take the average value of the measured values of each group of specimen as the expansion rate of this group. The evaluation criteria are as follows:

When the 14 d expansion rate is less than 0.1 %, it can be determined that there is no potential harm of alkali silica reaction in most cases;

When the 14 d expansion rate is greater than 0.2 %, it can be determined that there is potential harm of alkali silica reaction;

When the 14 d expansion rate is 0.1 %-0.2 %, the potential harm of alkali silica reaction cannot be finally determined.

3.1. Analysis of Influence of different test standards on ASR

According to the two kinds of test standards, the rapid alkali silica reaction comparative test is carried out for different kinds of cement and aggregate. The test results are shown in Table 4. Fig. 1 to Fig. 3 show the comparison of mortar expansion rates of aggregate S1 to S3 under different cement and test standards.

From Fig. 1 to Fig. 3, it can be seen that the type of cement has a significant impact on the expansion rate of mortar specimens, and the same type of cement has different effects on the expansion rate of mortar specimens under different aggregates. The test standards have little impact on the expansion rate test results of aggregates. That is, the alkali content of cement has a relatively small impact on the determination of aggregate alkali activity.

For natural sand S1, rapid alkali silica reaction was carried out according to standard Ⅰ, and the 14 day expansion rates of mortar specimens were 0.049 %, 0.042 %, and 0.046 %, respectively. According to standard Ⅱ, rapid alkali silica reaction was carried out, and the expansion rates of the mortar specimens after 14 days were 0.05 %, 0.044 %, and 0.045%, respectively. The maximum value of these 6 experimental results is 0.05 %, the minimum value is 0.042 %, the average value is 0.046 %, and the maximum difference from the average value is 8.7 % of the average value. This indicates that natural sand S1 is an aggregate without potential ASR.

For artificial sand S2, rapid alkali silica reaction was carried out according to standard Ⅰ, and the 14 day expansion rates of mortar specimens were 0.178 %, 0.183 %, and 0.160 %, respectively. According to standard Ⅱ, rapid alkali silica reaction was carried out, and the 14 day expansion rates of mortar specimens were 0.183 %, 0.18 %, and 0.164 %, respectively. The maximum value of these 6 experimental results is 0.183 %, the minimum value is 0.16 %, the average value is 0.175 %, and the maximum difference from the average value is 8.6 % of the average value. This indicates that artificial sand S2 cannot be determined as an aggregate with potential ASR hazards.

For artificial sand S3, rapid alkali silica reaction was carried out according to standard Ⅰ, and the 14 day expansion rates of mortar specimens were 0.281 %, 0.273 %, and 0.267 %, respectively. According to standard Ⅱ, rapid alkali silica reaction was carried out, and the 14 day expansion rates of mortar specimens were 0.293 %, 0.27 %, and 0.272 %, respectively. The maximum value of these 6 experimental results is 0.293 %, the minimum value is 0.267 %, the average value is 0.276 %, and the maximum difference from the average value is 6.2 % of the average value. This indicates that artificial sand S3 is a potentially hazardous aggregate for ASR.

Based on the above analysis, the influence of cement varieties on the alkali activity of aggregates is relatively small. When the alkali content of cement is adjusted to 1.0 %, the alkali activity test results of various aggregates are basically consistent. Therefore, when the alkali content does not exceed 1.0 %, the impact on the alkali activity of aggregates is relatively small and may not be considered in engineering.

Fig. 1Comparison of mortar expansion rates of aggregate S1

Fig. 2Comparison of mortar expansion rates of aggregate S2

Fig. 3Comparison of mortar expansion rates of aggregate S3

3.2. Influence of alkali content of cement on ASR

Combined with the research results in part A, the influence of different cement alkali content on the ASR of mortar specimens was studied for C1 kinds of cement. The test results of ASR of mortar specimens under different cement alkali content are shown in Table 5.

It can be seen from Table 5 that when the alkali content of cement exceeds 1.0 %, too much alkali in cement accelerates the reaction with the active components in the aggregate. The expansion rate of mortar bar specimen shows an increasing relationship with the alkali content of cement, and the increasing relationship becomes more and more obvious with the increase of alkali content of cement.

As shown in Fig. 4(a) and Fig. 4(b), the variation laws of expansion rate of S1 and S2 mortar bars at different ages under different cement alkali content are given respectively. For non-potential ASR hazard aggregates, ASR hazards will occur when the alkali content of cement exceeds 2.0 %. For aggregates cannot be judged to have potential alkali silica reaction, when the alkali content of cement exceeds 1.2 %, the harm of alkali silica reaction will occur.

Fig. 4Variation law of mortar bar expansion rate of sand with different cement alkali content at different ages

a) Natural sand S1

b) Artificial sand S2

This is because some alkali substances in concrete or mortar participate in the hydration reaction to generate cementitious materials, while others are adsorbed on the surface of particles, leaving a large amount of OH- present inside the concrete or mortar. After these OH- ions participate in the alkali silica reaction, there is still residual, and the formed osmotic pressure still exists, leading to the reaction of active ingredients in the aggregate all the time, which eventually leads to the increase of the generated expansive products, thus causing the expansion of concrete or mortar. This also explains why it is suspected that after increasing the alkali content of the cement, the expansion rate of the mortar rod increases significantly, until there is a significant harm of alkali aggregate reaction. However, non-reactive aggregates are not devoid of any active ingredients but cannot exhibit significant expansion in general environmental conditions. Due to the increase of alkali content in cement, a small amount of active ingredients continuously reacts with alkali under the action of Osmotic pressure until they are completely consumed. At this point, if the active ingredients in the non-active aggregate reach a certain amount, that is, if the generated expansive products reach a certain amount, it will also cause a more obvious alkali silica reaction. It is important that even under the action of Osmotic pressure, the internal structure of non-active ingredients will expand, so for non-active aggregates, obvious alkali aggregate reaction hazards can also occur under the condition of having a certain amount of active ingredients and a high alkali content.