Inject html into thymeleaf template

I have thymeleaf templates lying in database,

First I retrieve template and process it:

String processedTemplate = templateEngine.process(databaseTemplate, context);

So now processedTemplate contains html as a String.

Then I retrieve another template and do basicly the same, but I want also inject previous template into it, so the java code should look like:

Context context = new Context(Locale.ENGLISH);
context.setVariable("htmlToInject", processedTemplated);
String result = templateEngine.process(mainTemplate, context);

So what should I put into my mainTemplate to be able to inject another html via Context into it?

I saw something like this:

<div th:replace="fragments/header :: header">Header</div>

But it works with templates from file, but not when they are lying in database.

It sounds that you want to insert text without HTML escaping, you do that with th:utext:

<div th:utext="${htmlToInject}"></div>

Or with inlining: