In 걱정하지, why does the first ㅈ sound like ㅊ?

Question

I read that some consonants such as ㅈ are aspirated at the beginning of a sentence or phrase or something. However, I can't really find the explanation for this particular scenario where they sound aspirated.

For the first one ㅈ, I see a ㄱ before it, but I can't find the pronunciation rule for it.

I found this on Korean Phonology: When the second and third consonants are homorganic obstruents, they merge, becoming fortis or aspirate, and, depending on the word and a preceding |l|, might not elide: |lk-k| is [lk͈].

However, I don't really understand.

Answer 1

The standard pronunciation, which, unfortunately, is in Korean, covers it.

Korean language standard, Part Ⅱ (“Standard pronunciation”), §6 (“Tensation”), Clause 23.

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A “ᄀᅠ,” “ᄃᅠ,” “ᄇᅠ,” “ᄉᅠ,” or “ᄌᅠ” following a “ㄱ”-sounding final (“ᅟᅠᆨ”, “ᆩ”, “ᆿ”, “ᆪ”, or “ᆰ”), a “ㄷ”-sounding final (“ᆮ”, “ᆺ”, “ᆻ”, “ᆽ”, “ᆾ”, or “ᇀ”), or a “ㅂ”-sounding final (“ᆸ”, “ᇁ”, “ᆲ”, “ᆵ”, or “ᆹ”) gets tensed (unaspirated).

받침 ‘ᅟᅠᆨ(ᆩ, ᆿ, ᆪ, ᆰ), ᆮ(ᆺ, ᆻ, ᆽ, ᆾ, ᇀ), ᆸ(ᇁ, ᆲ, ᆵ, ᆹ)’ 뒤에 연결되는 ‘ᄀᅠ, ᄃᅠ, ᄇᅠ, ᄉᅠ, ᄌᅠ’은 된소리로 발음한다.

• 국밥 [국빱]
• 깎다 [깍따]
• 넋받이 [넉빠지]
• 삯돈 [삭똔]
• 닭장 [닥짱]
• 칡범 [칙뻠]
• 뻗대다 [뻗때다]
• 옷고름 [옫꼬름]
• 있던 [읻떤]
• 꽂고 [꼳꼬]
• 꽃다발 [꼳따발]
• 낯설다 [낟썰다]
• 밭갈이 [받까리]
• 솥전 [솓쩐]
• 곱돌 [곱똘]
• 덮개 [덥깨]
• 옆집 [엽찝]
• 넓죽하다 [넙쭈카다]
• 읊조리다 [읍쪼리다]
• 값지다 [갑찌다]

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The standard pronunciation follows the way Koreans find easier to pronounce. Though I can't quite explain why it is easier and how I learned those vast, overwhelming tons of rules. There must be some academic papers about it.

Answer 2

Roughly, almost rule on pronunciation is to find "easy pronunciation way"

When we pronounce 걱정 [걱정], 김밥 [김밥] (gimbap; making a rice roll in dried laver), it is stressful.

Hence we have 걱정[걱쩡], 김밥[김빱]. Here ㅉ may sound ㅊ.

Other similar examples are : 곡식 [곡씩] crops, 숙식[숙씩] board and lodging.